,illustration3 ,a special color reconstruction of the eruption of the volcano ,loki on the ,jovian satellite ,io4 ,the picture was taken by ,voyager #a from a range of about half a million kilometers4 ,p-#baccd;,c ,,nasa ,,sp-#dci ,illustration3 ,voyage ,to ,jupiter ,voyage ,to ,jupiter ,david ,morrison and ,jane ,samz ,illustration3 ,,nasa ,scientific and ,technical ,information ,branch #aihj ,national ,aeronautics and ,space ,administration ,washington1 ,,dc ,for sale by the ,superintendent of ,documents ,u4,s4 ,government ,printing ,office1 ,washington1 ,d4,c4 #bjdjb ,library of ,congress ,catalog ,card ,number #hj-fjjabf ,,foreword ,few missions of planetary exploration have provided such rewards of insight and surprise as the ,voyager flybys of ,jupiter4 ,those who were fortunate enough to be with the science teams during those weeks will long remember the experience2 it was like being in the crow's nest of a ship during landfall and passage through an archipelago of strange islands4 ,we had known that ,jupiter would be remarkable1 for man had been studying it for centuries1 but we were far from prepared for the torrent of new information that the ,voyagers poured back to ,earth4 ,some of the spirit of excitement and connection is captured in this volume4 ,its senior author was a member of the ,imaging ,team4 ,it is not common that a person can both 8do science0 at the leading edge and also present so vivid an inside picture of a remarkable moment in the history of space exploration4 ,april #cj1 #aihj ,thomas ,a4 ,mutch ,associate ,administrator ,office of ,space ,science ,,introduction ,the two ,voyager encounters with ,jupiter were periods unparalleled in degree and diversity of discovery4 ,we had1 of course1 expected a number of discoveries because we had never before been able to study in detail the atmospheric motions on a planet that is a giant spinning sphere of hydrogen and helium1 nor had we ever observed planet-sized objects such as the ,jovian satellites ,ganymede and ,callisto1 which are half water-ice4 ,we had never been so close to a ,moon-sized satellite such as ,io1 which was known to be dispersing sodium throughout its ,jovian neighborhood and was thought to be generating a one-million-ampere electrical current that in some way results in billions of watts of radio emission from ,jupiter4 ,the closer ,voyager came to ,jupiter the more apparent it became that the scientific richness of the ,jovian system was going to greatly exceed even our most optimistic expectations4 ,the growing realization among ,voyager scientists of the wealth of discovery is apparent in their comments1 discussions1 and reports as recounted by the authors in their descriptions of the two encounters4 ,although many of the discoveries occurred in the few weeks around each encounter1 they were1 of course1 the result of more than those few weeks of effort4 ,in fact1 planning started a decade earlier1 and the ,voyager team of engineers and scientists had been designing1 building1 and planning for the encounters for seven years4 ,the ,pioneer spacecraft made the first reconnaissance of ,jupiter in #aigc-aigd1 providing key scientific results on which ,voyager could build1 and discoveries from continuing ground-based observations suggested specific ,voyager studies4 ,voyager is itself just the second phase of exploration of the ,jovian system4 ,it will be followed by the ,galileo program1 which will directly probe ,jupiter's atmosphere and provide long-term observations of the ,jovian system from an orbiting spacecraft4 ,in the meantime1 the ,voyager spacecraft will continue their journey to ,saturn1 and possibly ,uranus and ,neptune1 planets even more remote from ,earth and about which we know even less than we knew of ,jupiter before #aigi4 ,as is clearly illustrated in this recounting of the voyage to ,jupiter1 scientific endeavors are human endeavors2 just as ,galileo could not have foreseen the advancement in our knowledge initiated by his discoveries of the four ,jovian moons in #afaj1 neither can we fully comprehend the scientific heritage that our exploration of space is providing future generations4 ,april #aihj ,edward ,c4 ,stone ,voyager ,project ,scientist ,,acknowledgement ,the authors are grateful to the many members of the ,voyager ,project team who not only made this historic mission of exploration possible but also took time from their busy schedules to offer us assistance1 information1 and encouragement in the preparation of this book4 ,among many too numerous to name individually1 we particularly thank ,e4 ,c4 ,stone1 ,a4 ,l4 ,lane1 ,c4 ,h4 ,stembridge1 ,r4 ,a4 ,mills1 ,e4 ,montoya1 ,m4 ,a4 ,mitz1 and ,b4 ,a4 ,smith1 ,l4 ,soderblom1 and their colleagues on the ,voyager ,imaging ,team4 ,we are grateful to ,f4 ,e4 ,bristow1 ,d4 ,l4 ,bane1 ,l4 ,j4 ,pieri and especially ,j4 ,j4 ,van der ,woude for their assistance in obtaining optimum versions of the photographs printed here4 ,c4 ,b4 ,pilcher and ,i4 de ,pater kindly made available their groundbased pictures of the ,jovian magnetosphere4 ,many colleagues have read and provided helpful comments on parts of the manuscript1 among them ,g4 ,a4 ,briggs1 ,s4 ,a4 ,collins1 ,s4 ,cruikshank1 ,j4 ,doughty1 ,a4 ,l4 ,guin1 ,a4 ,l4 ,lane1 ,r4 ,a4 ,mills1 ,e4 ,montoya1 ,e4 ,c4 ,stone1 ,j4 ,l4 ,ward1 and especially ,c4 ,r4 ,chapman4 ,,contents ,foreword v ,introduction vii ,acknowledgment ix ,chapter #a4 ,the ,jovian ,system #a ,chapter #b4 ,pioneers to ,jupiter #aa ,chapter #c4 ,the ,voyager ,mission #bc ,chapter #d4 ,science and ,scientists #cc ,chapter #e4 ,the ,voyage to ,jupiter--,getting ,there #dg ,chapter #f4 ,the ,first ,encounter #fc ,chapter #g4 ,the ,second ,encounter3 ,more ,surprises ,from the 8,land0 of the ,giant #ic ,chapter #h4 ,jupiter--,king of the ,planets #aag ,chapter #i4 ,four ,new ,worlds #aci ,chapter #aj4 ,return to ,jupiter #afi ,appendix ,a4 ,pictorial ,maps of the ,galilean ,satellites #agg ,appendix ,b4 ,voyager ,science ,teams #aie ,appendix ,c4 ,voyager ,management ,team #aig ,additional ,reading #aii ,illustration3 ,jupiter is the largest planet in the solar system--a gaseous world as large as #acjj ,earths1 marked by alternating bands of colored clouds and a dazzling complexity of storm systems4 ,the ,voyager mission gave us our first close look at this spectacular planet4 ,p-#bajhe ,,chapter #a ,,the ,,jovian ,,system ,introduction ,in the ,sun's necklace of planets1 one gem far outshines the rest3 ,jupiter4 ,larger than all the other planets and satellites combined1 ,jupiter is a true giant4 ,if intelligent beings exist on planets circling nearby stars1 it is probable that ,jupiter is the only member of our planetary system they can detect4 ,they can see the ,sun wobble in its motion with a twelve-year period as ,jupiter circles it1 pulling first one way1 then the other with the powerful tug of its gravity4 ,if astronomers on some distant worlds put telescopes in orbit above their atmospheres1 they might even be able to detect the sunlight reflected from ,jupiter4 ,but all the other planets--including tiny inconspicuous ,earth--would be hopelessly lost in the glare of our star1 the ,sun4 ,jupiter is outstanding among planets not only for its size1 but also for its system of orbiting bodies4 ,with fifteen known satellites1 and probably several more too small to have been detected1 it forms a sort of miniature solar system4 ,if we could understand how the ,jovian system formed and evolved1 we could unlock vital clues to the beginning and ultimate fate of the entire solar system4 ,ancient peoples all over the world recognized ,jupiter as one of the brightest wandering lights in their skies4 ,only ,venus is brighter1 but ,venus1 always a morning or evening star1 never rules over the dark midnight skies as ,jupiter often does4 ,in ,greek and ,roman mythology the planet was identified with the most powerful of the gods and lord of the heavens--the ,greek ,zeus2 the ,roman ,jupiter4 ,as befits the king of the heavens1 the planet ,jupiter moves at a slow and stately pace4 ,twelve years are required for ,jupiter to complete one orbit around the ,sun4 ,for about six months of each year1 ,jupiter shines down on us from the night sky1 more brightly and steadily than any star4 ,during the late #aigjs it was a winter object1 but in #aihj it will dominate the spring skies1 becoming a summer 8star0 about #aihb4 ,early ,discoveries ,even seen through a small telescope or pair of binoculars1 ,jupiter looks like a real world1 displaying a faintly banded disk quite unlike the tiny1 brilliant image of a star4 ,it also reveals the brightest members of its satellite family as starlike points spread out along a straight line extended east-west through the planet4 ,there are four of these planet-sized moons2 with their orbits seen edge-on from ,earth1 they seem to move constantly back and forth1 changing their configuration hourly4 ,in ,january #afaj1 in his first attempt to apply the newly invented telescope to astronomy1 ,galileo discovered the four large satellites of ,jupiter4 ,he correctly interpreted their motion as being that of objects circling ,jupiter--establishing the first clear proof of celestial motion around a center other than the ,earth4 ,the discovery of these satellites played an important role in supporting the ,copernican revolution that formed the basis for modern astronomy4 ,a few decades later the satellites of ,jupiter were used to make the first measurement of the speed of light4 ,observers following their motions had learned that the satellite clock seemed to run slow when ,jupiter was far from ,earth and to speed up when the two planets were closer together4 ,in #afge the ,danish astronomer ,ole ,roemer explained that this change was due to the finite velocity of light3 ,the satellites only seemed to run slow at large distances because the light coming from them took longer to reach ,earth4 ,knowing the dimensions of the orbits of ,earth and ,jupiter and the amount of the delay 7about fifteen minutes71 ,roemer was able to calculate one of the most fundamental constants of the physical universe--the speed of light 7about #cjj #jjj kilometers per second74 ,illustration3 ,galileo's notes summarizing his first observations of the ,jovian satellites ,io1 ,europa1 ,ganymede1 and ,callisto in ,january #afaj were made on a piece of scratch paper containing the draft of a letter presenting a telescope to the ,doge in ,venice4 ,these observations were the result of ,galileo's first attempt to apply the telescope to astronomical research4 ,the four great moons of ,jupiter are called the ,galilean satellites after their discoverer4 ,their individual names--,io1 ,europa1 ,ganymede1 and ,callisto--were proposed by ,simon ,marius1 a contemporary and rival of ,galileo4 7,marius claimed to have discovered the satellites a few weeks before ,galileo did1 but modern scholars tend to discredit his claim47 ,io1 ,europa1 ,ganymede1 and ,callisto are names of lovers of the god ,jupiter in ,greco-,roman mythology4 ,since ,jupiter was not at all shy about taking lovers1 there are enough such names for the other eleven ,jovian satellites1 as well as for those yet to be discovered4 ,in the century following ,galileo's death1 improvements in telescopes made it possible to measure the size of ,jupiter and to note that it bulged at the equator4 ,the equatorial diameter is known today to be #adb #hjj kilometers1 while from pole to pole ,jupiter measures only #acc #ejj kilometers4 ,for comparison1 the diameter of the ,earth is #ab #ijj kilometers1 only about one-tenth as great1 and the flattening of ,earth is also much smaller 7less than one percent74 ,by measuring the orbits of the satellites and applying the laws of planetary motion discovered by ,johannes ,kepler and ,isaac ,newton1 astronomers were also able to determine the total mass of ,jupiter--about #b #aj tons1 or #cah times the mass of the ,earth4 ,once the size and mass are known1 it is possible to calculate another fundamental property of a planet--its density4 ,the density1 which is the mass divided by the volume1 provides important clues to the composition and interior structure of a planetary body4 ,the density of ,earth1 a body composed primarily of rocky and metallic materials1 is #e4f times the density of water4 ,the mass of ,jupiter is #cah times that of ,earth2 its volume is #acag times that of ,earth4 ,thus ,jupiter's density is substantially lower than ,earth's1 amounting to #a4cd times the density of water4 ,from this low density1 it was evident long ago that ,jupiter was not just a big brother of ,earth and the other rocky planets in the inner solar system4 ,rather1 ,jupiter is the prototype of the giant1 gas-rich planets ,jupiter1 ,saturn1 ,uranus1 and ,neptune4 ,these giant planets must1 from their low density1 have a composition fundamentally different from that of ,mercury1 ,venus1 ,earth1 ,moon1 and ,mars4 ,jupiter ,through the ,telescope ,jupiter is a beautiful sight seen with the naked eye on a clear night1 but only through a telescope does it begin to reveal its magnificence4 ,the most prominent features are alternating light and dark bands1 running parallel to the equator and subtly shaded in tones of blue1 yellow1 brown1 and orange4 ,however1 these bands are not the planet's only conspicuous markings4 ,in #affd the ,english astronomer ,robert ,hooke first reported seeing a large oval spot on ,jupiter1 and additional spots were noted as telescopes improved4 ,as the planet rotates on its axis1 such spots are carried across the disk and can be used to measure ,jupiter's speed of rotation4 ,the giant planet spins so fast that a ,jovian day is less than half as long as a day on ,earth1 averaging just under ten hours4 ,during the nineteenth century1 observers using increasingly sophisticated telescopes were able to see more complex detail in the band structure1 with wisps1 streaks1 and festoons that varied in intensity and color from year to year4 ,furthermore1 observations revealed the remarkable fact that not all parts of the planet rotate with the same period2 near the equator the apparent length of a ,jovian day is several minutes shorter than the average day at higher latitudes4 ,it is thus apparent that ,jupiter's surface is not solid1 and astronomers came to realize that they were looking at a turbulent kaleidoscope of shifting clouds4 ,although the face of ,jupiter is always changing1 some spots and other cloud features survive for years at a time1 much longer than do the largest storms on ,earth4 ,the record for longevity goes to the ,great ,red ,spot4 ,this gigantic red oval1 larger than the planet ,earth1 was first seen more than three centuries ago4 ,from decade to decade it has changed in size and color1 and for nearly fifty years in the late eighteenth century no sightings were reported1 but since about #ahdj the ,great ,red ,spot has been the most prominent feature on the disk of ,jupiter4 ,illustration3 ,this ground-based photograph of ,jupiter showing the ,great ,red ,spot in the southern hemisphere was taken with the ,catalina ,observatory's #fa-;inch telescope in ,december #aiff4 ,it was not until the twentieth century that the composition of the atmosphere of ,jupiter could be measured4 ,in #aije spectra of the planet revealed the presence of gases that absorb strongly at red and infrared wavelengths2 thirty years later these were identified as ammonia and methane4 ,these two poisonous gases are the simplest chemical compounds of hydrogen combined with nitrogen and carbon1 respectively4 ,in the atmosphere of ,earth they are not stable1 because oxygen1 which is highly active chemically1 destroys them4 ,the existence of methane and ammonia on ,jupiter demonstrated that free oxygen could not be present and that the atmosphere was dominated by hydrogen--a reducing1 rather than oxidizing1 condition4 ,subsequently1 hydrogen was identified spectroscopically4 ,although much more abundant than methane or ammonia1 hydrogen is harder to detect4 ,in the #aidjs and #aiejs the ,german-,american astronomer ,rupert ,wildt used all the available data to derive a picture of ,jupiter that is still generally accepted4 ,he noted that both the low total density and the observed presence of hydrogen-rich compounds in the atmosphere were consistent with a bulk composition similar to that of the ,sun and stars4 ,this 8cosmic composition0 is dominated by the two simplest elements1 hydrogen and helium1 which together make up nearly #ii percent of all the material in the universe4 ,wildt hypothesized that the giant planets1 because of their large size1 had succeeded in retaining this primordial composition1 whereas the hydrogen and helium had escaped from the smaller inner planets4 ,he also used his knowledge of the properties of hydrogen and helium to calculate what the interior structure of ,jupiter might be like1 concluding that the planet is mostly liquid or gas4 ,wildt suggested that there probably was a core of solid material deep in the interior1 but that much of ,jupiter is fluid--extremely viscous and compressed deep below the visible atmosphere1 but still not solid4 ,the atmosphere seen from above is just the thin1 topmost layer of an ocean of gases thousands of kilometers thick4 ,recent ,earth-,based ,studies of ,jupiter ,in the past1 a great deal of planetary research was basically descriptive1 consisting of visual observations and photography4 ,beginning in the #aifjs1 a new generation of planetary scientists began to apply the techniques of modern astrophysics and geophysics to the study of the solar system4 ,inspired in part by the developing space programs of the ,united ,states and the ,soviet ,union1 scientists began to ask more quantitative questions3 ,what are the surfaces and atmospheres made of8 ,what are the temperatures and wind speeds8 ,exactly what quantities of different elements and isotopes are present8 ,and how can these new data be used to infer the origin and evolution of the planets8 ,illustration3 ,the major features of ,jupiter are shown in schematic form4 ,the planet is a banded disk of turbulent clouds2 all its stripes are parallel to the bulging equator4 ,large dusky gray regions surround each pole4 ,darker gray or brown stripes called belts intermingle with lighter1 yellow-white stripes called zones4 ,many of the belts and zones are permanent features that have been named4 ,one feature of particular note is the ,great ,red ,spot1 an enigmatic oval larger than the planet ,earth1 which was first seen more than three centuries ago4 ,during the years the spot has changed in size and color1 and it escaped detection entirely for nearly fifty years in the #agjjs4 ,however1 since the mid-nineteenth century the ,great ,red ,spot has been the most prominent feature on the face of ,jupiter4 #bice ,n ,north polar region ,north north north temperate belt ,north north temperate zone ,north north temperate belt ,north temperate zone ,north temperate belt ,north tropical zone ,north equatorial belt ,equatorial zone equatorial band ,south equatorial belt ,n4 component ,s4 component ,south tropical zone ,great ,red ,spot ,south temperate belt ,south temperate zone ,south south temperate belt ,south south temperate zone ,south polar region ,s ,wildt had already suggested the basic gases in the atmosphere of ,jupiter3 primarily hydrogen and helium1 with much smaller quantities of ammonia and methane4 ,undetected but possibly also present were nitrogen1 neon1 argon1 and water vapor4 ,the abundance of helium was particularly a problem2 although it was presumably the second-ranking gas after hydrogen1 it has no spectral features in visible light and its presence remained only a hypothesis1 unconfirmed by observation4 ,although the presence of a gas can usually be inferred from spectroscopy1 solids or liquids cannot normally be detected in this way4 ,thus the composition of ,jupiter's clouds could not be determined directly4 ,however1 the presence of ammonia gas provided an important clue4 ,at the temperatures expected in the upper atmosphere of the planet1 ammonia gas must freeze to form tiny crystals of ammonia ice1 just as water vapor in the ,earth's atmosphere freezes to form cirrus clouds4 ,most investigators agreed that the high clouds covering much of ,jupiter must be ammonia cirrus4 ,but ammonia crystals are white1 so the presence of this material provides no explanation for the many colors seen on ,jupiter4 ,additional materials must be present--perhaps colored organic compounds1 produced in small amounts by the action of sunlight on the atmosphere4 ,because ,jupiter is five times farther from the ,sun than is the ,earth1 a given area on ,jupiter receives only about four percent as much solar heating as does a comparable area on ,earth4 ,thus ,jupiter is colder than ,earth2 even though it may be warm deep below its blanket of clouds1 ,jupiter presents a frigid face4 ,illustration3 ,these blue filter photographs of ,jupiter were taken at ,mauna ,kea ,observatory1 ,hawaii4 ,they show changes on ,jupiter's surface between #aigc and #aigh1 with the dates of the observations4 ,illustration3 ,july #be1 #aigc ,illustration3 ,october #e1 #aigd ,illustration3 ,october #b1 #aige ,illustration3 ,november #bj1 #aigf4 ,illustration3 ,january #bh1 #aigh4 ,illustration3 ,december #ai1 #aigh4 ,the development of a new science1 infrared astronomy1 in the #aifjs made it possible to measure these low temperatures directly4 ,in the case of a cloudy planet like ,jupiter1 the infrared emission evident at various wavelengths originates at different depths in the atmosphere4 ,it is a general property of any mixed1 convecting atmosphere that the temperature varies with depth2 the rate of variation depends only on the composition of the atmosphere1 the gravity of the planet1 and the presence or absence of condensible materials to form clouds4 ,on ,jupiter it is about #a4i ,c warmer for each kilometer of descent through the atmosphere4 ,thus1 although the ammonia clouds are very cold1 a little above -#agc ,c1 if one goes deep enough one can reach temperatures that are quite comfortable4 ,with a variation of #a4i ,c per kilometer1 terrestrial 8room temperature0 would be reached about #ajj kilometers below the clouds4 ,to measure the total energy radiated by a planet1 it is necessary to utilize infrared radiation at wavelengths more than one hundred times longer than the wavelengths of visible light4 ,even when detectors were developed that could measure such radiation1 it was impossible to observe celestial sources such as ,jupiter because of the opacity of the terrestrial atmosphere4 ,even a tiny amount of water vapor in our own atmosphere can block our view of long-wave infrared4 ,to make the required measurements1 it is necessary to carry a telescope to very high altitudes1 above all but a fraction of a percent of the offending water vapor4 ,in the late #aifjs a ,lear-,jet airplane was equipped with a telescope and made available by ,,nasa to astronomers to carry out long-wave infrared observations from above #ii percent of the terrestrial water vapor4 ,in #aifi ,frank ,low of the ,university of ,arizona and his colleagues used this system to make a remarkable discovery3 ,jupiter was radiating more heat than it received from the ,sun6 ,repeated observations demonstrated that between two and three times as much energy emanated from the planet as was absorbed4 ,thus ,jupiter must have an internal heat source2 in effect1 it shines by its own power as well as by reflected sunlight4 ,theoretical studies suggest that the heat is primordial1 the remnant of an incandescent proto-,jupiter that formed four and one-half billion years ago4 ,illustration3 ,images of ,jupiter in visible light 7below7 and five-micrometer infrared light show the planet's characteristic belts and zones4 ,the infrared image reveals areas that emit large amounts of thermal energy4 ,the source of the energy is thought to be breaks in the ,jovian cloud cover1 which allow investigators a glimpse of the deep regions of the atmosphere4 ,one of the mysteries of ,jupiter concerns its heat balance3 ,the planet appears to radiate more heat than it receives from the ,sun4 ,p-#bjieg ,illustration3 ,jupiter in visible light4 ,at the same time that the internal heat source on ,jupiter was being revealed with long-wave airborne infrared telescopes1 a new discovery was being made from short-wave infrared observations4 ,the clouds of ,jupiter are too cold to emit any detectable thermal radiation at a wavelength of #e micrometers 7about ten times the wavelength of green light74 ,nevertheless1 images of ,jupiter at #e micrometers revealed a few small spots where large amounts of thermal energy were being emitted4 ,the sources of the energy appeared to be holes or breaks in the clouds1 where it was possible to see deeper into hotter regions4 ,the discovery of these hot spots opened the possibility of probing deep regions of the ,jovian atmosphere that had previously been beyond the reach of direct investigation4 ,the ,jovian ,magnetosphere ,the discovery of planetary magnetospheres began in #aiei when the first ,u4,s4 ,explorer satellite detected the radiation belts around the ,earth4 ,named for ,james ,van ,allen of the ,university of ,iowa1 whose geiger-counter instrument aboard ,explorer #a first measured them1 these belts are regions in which charged atomic particles--primarily electrons and protons--are trapped by the magnetic field of the ,earth4 ,they are one manifestation of the terrestrial magnetosphere--a large1 dynamic region around the ,earth in which the magnetic field of our planet interacts with streams of charged particles emanating from the ,sun4 ,at almost the same time that the terrestrial magnetosphere was being discovered by artificial satellites1 astronomers were finding evidence to suggest similar phenomena around ,jupiter4 ,radio astronomy is a branch of science that measures radiation from celestial bodies at radio frequencies1 which correspond to wavelengths much longer than those of visible or infrared light4 ,all planets emit weak thermal radio radiation1 but in the late #aiejs investigators found that ,jupiter was a much stronger long-wave radio source than would be expected for a planet with its temperature4 ,this radiation bore the signature of higher-energy processes4 ,physicists had seen similar emissions produced in synchrotron electron accelerators1 huge machines in which electrons are whirled around at nearly the speed of light so that they can be used for experiments in nuclear physics4 ,the ,russian theorist ,i4 ,s4 ,shklovsky identified the ,jovian radio radiation as also resulting from the synchrotron process1 due to spiraling electrons trapped in the planet's magnetic field4 ,from the intensity and spectrum of the observed synchrotron radiation1 it was clear that both the magnetic field of the planet and the energy of charged particles in its ,van ,allen belts were much greater than was the case for ,earth4 ,using radio telescopes of high sensitivity1 astronomers determined the approximate strength and orientation of the magnetic field of ,jupiter4 ,although they were able to measure synchrotron radiation only from the innermost parts of the ,jovian magnetosphere1 they could infer that the total volume occupied by the magnetosphere was enormous4 ,if our eyes were sensitive to magnetospheric emissions1 ,jupiter would look more than twice the diameter of the full moon in the sky4 ,all four ,galilean satellites orbit within the magnetosphere of ,jupiter2 in contrast1 our ,moon lies well outside the terrestrial magnetosphere4 ,striking evidence of the interaction of the satellites and the magnetosphere was provided when it was found that the innermost large satellite--,io--actually affects the bursts of radio static produced by ,jupiter4 ,only when ,io is at certain places in its orbit are these strong bursts detected4 ,theorists suggested that electric currents flowing between the satellite and the planet might be responsible for this effect4 ,the ,jovian ,satellites ,for nearly three centuries after their discovery in #afaj1 the only known moons of ,jupiter were the four large ,galilean satellites4 ,in #ahib ,e4 ,e4 ,barnard1 an ,american astronomer1 found a much smaller fifth satellite orbiting very close to the planet1 and between #aijd and #aigd eight additional satellites were found far outside the orbits of the ,galilean satellites4 ,the outer satellites are quite faint and presumably no more than a few tens of kilometers in diameter1 and all have orbits that are much less regular than those of the five inner satellites4 ,four of them revolve in a retrograde direction1 opposite to that of the inner satellites and ,jupiter itself4 ,in #aige the ,international ,astronomical ,union assumed the responsibility for assigning names to the non-,galilean satellites of ,jupiter4 ,following tradition1 they named the inner satellite ,amalthea for the she-goat that suckled the young god ,jupiter4 ,the outer eight were named for lovers of ,jupiter3 ,leda1 ,himalia1 ,lysithea1 ,elara1 ,ananke1 ,carme1 ,pasiphae1 and ,sinope4 ,for the non-,galilean satellites1 the 8e0 ending is reserved for satellites with retrograde orbits2 those with normal orbits have names that end in 8a40 ,because they are so large1 the ,galilean satellites have attracted the most attention from astronomers4 ,more than fifty years ago large telescopes were used to estimate their sizes1 and a careful series of measurements of their light variation showed that all four always keep the same face pointed toward ,jupiter1 just as our ,moon always turns the same face toward ,earth4 ,also1 the subtle gravitational perturbations they exert on each other were used to determine the approximate mass of each4 ,illustration3 ,the pattern of the ,galilean satellites changes from hour to hour1 as seen from ,earth4 ,viewed edge-on1 the nearly circular orbits produce an apparent back and forth motion with respect to ,jupiter4 ,these images recreate the kinds of observations first made by ,galileo in #afaj4 ,callisto1 the outermost ,galilean satellite1 is larger than the planet ,mercury4 ,it also has the lowest reflectivity1 or albedo1 of the four1 suggesting that its surface may be composed of some rather dark1 colorless rock4 ,callisto takes just over two weeks to orbit once around ,jupiter4 ,ganymede1 which requires only seven days for one orbit1 is the largest satellite in the ,jovian system1 being only slightly smaller than the planet ,mars4 ,its albedo is much higher than that of ,callisto1 or of the rocky planets such as ,mercury1 ,mars1 or the ,moon4 ,in #aiga astronomers first measured the infrared spectrum of reflected sunlight from ,ganymede and found the characteristic absorptions of water ice1 indicating that this satellite is partially covered with highly reflective snow or ice4 ,europa1 which is slightly smaller than the ,moon1 circles ,jupiter in half the time required by ,ganymede4 ,its surface reflects about sixty percent of the incident sunlight1 and the infrared spectrum shows prominent absorptions due to water ice2 ,europa appears to be almost entirely covered with ice4 ,however1 its color in the visible and ultraviolet part of the spectrum is not that of ice1 so some other material must also be present4 ,io1 innermost of the ,galilean satellites1 is the same size as our ,moon4 ,it orbits the planet in #db hours1 half the period of ,europa4 ,like ,europa1 it has a very high reflectivity1 but1 unlike ,europa1 it has no spectral absorptions indicative of water ice4 ,before ,voyager1 identification of the surface material on ,io presented a major problem to planetary astronomers4 ,when the sizes and masses of these satellites were measured1 astronomers could calculate their densities4 ,the inner two1 ,io and ,europa1 both have densities about three times that of water--nearly the same as the density of the ,moon1 or of rocks in the crust of the ,earth4 ,callisto and ,ganymede have densities only half as large1 far too low to be consistent with a rocky composition4 ,the most plausible alternative to rock is a composition that includes ice as a major component4 ,calculations showed that if these satellites were composed of rock and ice1 approximately equal quantities of each were required to account for the measured density4 ,thus the two outer ,galilean satellites were thought to represent a new kind of solar system object1 as large as one of the terrestrial planets1 but composed in large part of ice4 ,in #aigc the attention of astronomers was dramatically drawn to ,io when ,robert ,brown of ,harvard ,university detected the faint yellow glow of sodium from the region of space surrounding it4 ,it seemed that this satellite had an atmosphere1 composed of the metal sodium6 ,continued observations showed1 however1 that this was not an atmosphere in the usual sense of the word4 ,the gas atoms were not bound gravitationally to ,io1 but continuously escaped from it to form a gigantic cloud enveloping the orbit of the satellite4 ,fraser ,fanale and ,dennis ,matson of the ,caltech ,jet ,propulsion ,laboratory suggested that bombardment of ,io by high-energy particles from the ,jovian ,van ,allen belts was knocking off atoms of sodium by a process called sputtering1 releasing these atoms and allowing them to expand outward to form the observed sodium cloud4 ,no one anticipated then that powerful volcanic eruptions on ,io might also be contributing to this remarkable gas cloud4 ,illustration3 ,the ,galilean satellites in orbit around ,jupiter1 along with the outer satellites1 constitute a miniature solar system4 ,here they are shown relative to the size of ,mercury and that of the ,moon4 ,the portrayal of their internal and external composition is based on theoretical models that preceded the ,voyager flybys4 ,,pc-#agjed;,,ac ,illustration3 ,this image of ,io's extended sodium cloud was taken ,february #ai1 #aigg1 at the ,jet ,propulsion ,laboratory's ,table ,mountain ,observatory4 ,a picture of ,jupiter1 drawings of the orbital geometry1 and ,io's disk 7the small circle on the left7 are included for perspective4 ,the sodium cloud image has been processed for removal of sky background1 instrumental effects1 and the like4 ,this photograph demonstrates that the cloud is highly elongated and that more sodium precedes ,io in its orbit than trails it4 ,p-#bjjdg ,this picture of the satellites was developed just as the first space probe reached the ,jovian system4 ,in the next chapter we describe the ,pioneer program by which scientists reached out across nearly a million kilometers of space to explore ,jupiter1 its magnetosphere1 and its system of satellites4 ,illustration3 ,pioneer #aj was launched on ,march #b1 #aigb1 at #h3di p4m4 from ,cape ,canaveral1 ,florida4 ,a powerful ,atlas-,centaur rocket served as the launch vehicle1 which propelled the space probe to its goal nearly a billion kilometers away4 ,the beauty of the night launch was enhanced by the rumbling thunder and flashing lightning of a nearby storm4 ,,chapter #b ,,pioneers ,,to ,,jupiter ,reaching for the ,outer ,planets ,since the beginning of the ,space ,age1 scientists had dreamed of sending probes to ,jupiter and its family of satellites4 ,initially1 robot spacecraft were limited to studying the ,earth and its ,moon4 ,in #aifb1 however1 the first true interplanetary explorer1 ,mariner #b1 succeeded in escaping the ,earth-,moon system and crossing #ajj million kilometers of space to encounter ,venus1 studying ,earth's sister planet at close range using half a dozen scientific instruments4 ,by the mid #aifjs a ,u4,s4 planetary spacecraft had also flown to ,mars1 there had been a second flyby of ,venus1 and an ambitious program was under way for two more flybys of ,mars in #aifi1 followed by a ,mars orbiter in #aiga4 ,based on this success with the inner planets1 ,,nasa scientists and engineers began to plan seriously to meet the challenge of the outer solar system4 ,not only ,jupiter1 but ,saturn and even ,uranus and ,neptune1 were considered as possible targets4 ,however1 the distances between the outer planets are so vast that many years of flight would be required for a spacecraft to reach them1 even using the most powerful rocket boosters then contemplated4 ,if a cautious exploration program were followed1 investigating one planet at a time before designing the next mission1 it would be well into the twenty-first century before even a first reconnaissance of the solar system could be achieved4 ,a way to bridge the space between planets in a more efficient1 economical manner was needed4 ,in the late #aifjs celestial mechanicians--scientists who study the motions of planets and spacecraft--began to solve problems posed by the immensity of the outer solar system4 ,if a spacecraft is aimed to fly close to a planet in just the right way1 it can be accelerated by the gravity of the planet to higher speeds than could ever be obtained by direct launch from ,earth4 ,if a second1 more distant planet is in the correct alignment1 the gravity boost given by the first encounter can speed the craft on to the second4 ,jupiter1 with its huge size and strong gravitational pull1 could be used as the fulcrum for a series of missions to ,saturn1 ,uranus1 ,neptune1 and even distant ,pluto4 ,in addition1 the early #aihjs would offer an exceptional opportunity1 one repeated only about once every two centuries4 ,at that time1 all four giant planets would be in approximate alignment1 so that gravity-assist maneuvers could be done sequentially4 ,a single spacecraft1 after being boosted from ,jupiter to ,saturn1 could use the acceleration of ,saturn to continue to ,uranus1 and in turn could be accelerated all the way out to ,neptune4 ,such an ambitious1 multiplanet mission was named the ,grand ,tour4 ,the first essential step in the ,grand ,tour was a flyby of ,jupiter4 ,however1 this planet is ten times farther away from ,earth than ,venus or ,mars4 ,in addition1 there were two potentially lethal hazards that had not been faced before in interplanetary flights3 the asteroid belt and the ,jovian magnetosphere4 ,the first danger was presented by the many thousands of asteroids that occupy a belt between the orbits of ,mars and ,jupiter4 ,the largest asteroid1 ,ceres1 was discovered in #ahja and was initially thought to be the 8missing planet0 sometimes hypothesized as lying between ,jupiter and ,mars4 ,however1 ,ceres is only #ajjj kilometers in diameter1 too small to deserve the title of planet4 ,hundreds more of these minor planets were discovered during the nineteenth century1 and by the #aifjs more than #cjjj had well-determined orbits4 ,most were only a few tens of kilometers in diameter1 and astronomers estimated that #ej #jjj existed that were #a kilometer or more in diameter4 ,any spacecraft to ,jupiter would have to cross this congested region of space4 ,even #ej #jjj minor bodies spread through the volume of space occupied by the asteroid belt would present little direct danger1 although a chance collision with an uncatalogued object was always possible4 ,much more serious was the possibility that these larger objects were accompanied by large amounts of debris1 from the size of boulders down to microscopic dust1 that were undetectable from ,earth4 ,collisions with pebble-sized stones could easily destroy a spacecraft4 ,the only way to evaluate this danger was to go there and find out how much small debris was present4 ,a second danger was posed by ,jupiter itself4 ,in order to use the gravity boost of ,jupiter to speed on to another planet1 a spacecraft would have to fly rather close to the giant4 ,but this would mean passing right through the regions of energetic charged particles surrounding the planet4 ,some estimates of the number and energy of these particles indicated that the delicate electronic brains of a spacecraft would be damaged before it could penetrate this region4 ,again1 only by going there could the danger be evaluated properly4 ,the ,pioneer ,jupiter ,mission ,in #aifi the ,u4,s4 ,congress approved the ,pioneer ,jupiter ,mission to provide a reconnaissance of interplanetary space between ,earth and ,jupiter and a first close look at the giant planet itself4 ,the ,project was assigned by ,,nasa to the ,ames ,research ,center in ,mountain ,view1 ,california4 ,the primary objectives were defined by ,,nasa3 ,explore the interplanetary medium beyond the orbit of ,mars4 ,investigate the nature of the asteroid belt1 assessing possible hazards to missions to the outer planets4 ,explore the environment of ,jupiter1 including its inner magnetosphere4 ,the ,pioneer spacecraft was designed for economy and reliability1 based on previous experience at ,ames with ,pioneers #f through #i1 all of which had proven themselves by years of successful measurement of the interplanetary medium near the ,earth4 ,unlike the ,mariner class of spacecraft being used to investigate ,venus and ,mars1 the ,pioneer craft rotated continuously around an axis pointed toward the ,earth4 ,this spinning design was extremely stable1 like the wheels of a fast-moving bicycle1 and required less elaborate guidance than a nonspinning craft4 ,in addition1 the spin provided an ideal base for measurements of energetic particles and magnetic fields in space1 since the motion of the spacecraft itself swept the viewing direction around the sky and allowed data to be acquired rapidly from many different directions4 ,the only major disadvantage of a spinning spacecraft is that it does not allow a stabilized platform on which to mount cameras or other instruments that require exact pointing4 ,thus the spacecraft design was optimized for measurements of particles and fields in interplanetary space and in the ,jovian magnetosphere1 but had limited capability for observations of the planet and its satellites4 ,as finally assembled1 the ,pioneer ,jupiter spacecraft had a mass of #beh kilograms4 ,one hundred forty watts of electrical power at ,jupiter were supplied by four radioisotope thermoelectric generators 7,r,t,gs71 which turned heat from the radioactive decay of plutonium into electricity4 ,the launch vehicle for the flight to ,jupiter was an ,atlas-,centaur rocket1 equipped with an additional solid-propellant third stage4 ,this powerful rocket could accelerate the spacecraft to a speed of #ea #ejj kilometers per hour1 sufficient to escape the ,earth and make the billion-kilometer trip to ,jupiter in just over two years4 ,the specific scientific investigations to be carried out on ,pioneer were selected competitively in #aifi from proposals submitted by scientists from ,u4,s4 universities1 industry1 and ,,nasa laboratories1 and also from abroad4 ,eleven separate instruments would be flown1 in addition to two experiments that would make use of the spacecraft itself4 ,three complete ,pioneer spacecraft1 with their payloads of #be kilograms of scientific instruments1 were built3 one as a test vehicle and two for launch to ,jupiter4 ,one of these--the test vehicle--is now on display at the ,national ,air and ,space ,museum in ,washington4 ,the first opportunity to launch--the opening of the 8launch window0--was on ,february #bg1 #aigb4 ,however1 it was not until shortly after dark on ,march #b that all systems were ready1 and ,pioneer #aj began its historic trip to ,jupiter4 ,pioneer #aj was the first human artifact launched with sufficient energy to escape the solar system entirely4 ,fittingly1 the craft carried a message designed for any possible alien astronauts who might1 in the distant future1 find the derelict ,pioneer in the vastness of interstellar space4 ,a small plaque fastened to the spacecraft told the time and planet from which it had been launched1 and carried a symbolic greeting from humanity to the cosmos4 ,illustration3 ,two identical ,pioneer spacecraft were designed and fabricated by ,,trw ,systems ,group at their ,redondo ,beach1 ,california facility4 ,each weighed only about #bfj kilograms1 yet carried eleven highly sophisticated instruments capable of operating unattended for many years in space4 ,data systems on board controlled the instrumentation1 received and processed commands1 and transmitted information across the vast distance to ,earth4 ,the second ,pioneer was to wait more than a year before launch4 ,by following far behind ,pioneer #aj1 its trajectory--in particular how deeply it penetrated the radiation belts during ,jupiter flyby--could be modified1 depending on the fate of the first spacecraft4 ,at dusk on ,april #e1 #aigc1 ,pioneer #aa blasted from the launch pad at ,cape ,canaveral and followed its predecessor on the long1 lonely journey into the outer solar system4 ,flight to ,jupiter ,within a few hours of launch1 each ,pioneer spacecraft shed the shroud that had protected it and unfurled booms supporting the science instruments and the ,,rtg power generators4 ,after each craft had been carefully tracked and precise orbits calculated1 small onboard rockets were commanded to fire to correct its trajectory for exactly the desired flyby at ,jupiter4 ,pioneer #aj was targeted to fly by the planet at a minimum distance of #c ,jupiter radii 7,r,j7 from the center1 or #b ,r,j 7about #adj #jjj kilometers7 above the clouds4 ,this close passage1 inside the orbit of ,io1 allowed the craft to pass behind both ,io and ,jupiter as seen from ,earth1 so that its radio beam could probe both the planet and its innermost large satellite4 ,pioneer #aa was intended to fly even closer to ,jupiter1 but the exact targeting options were held open until after the ,pioneer #aj encounter4 ,on ,pioneer #aj1 all instruments appeared to be working well as the craft passed the orbit of ,mars in ,june #aigb1 just #ig days after launch4 ,at this point1 as it headed into unexplored space1 it truly became a pioneer4 ,in mid-,july it began to enter the asteroid belt1 and scientists and engineers anxiously watched for signs of increasing particulate matter4 ,pioneer #aj carried two instruments designed to measure small particles in space4 ,one1 with an effective area of about #j4f square meters1 measured the direct impact of dust grains as small as one-billionth of a gram4 ,the other looked for larger1 more distant grains by measuring sunlight reflected from them4 ,to the surprise of many1 there was little increase in the rate of dust impacts recorded as the craft penetrated more and more deeply into the belt4 ,at about #djj million kilometers from the ,sun1 near the middle of the belt1 there appeared to be an increase in the number of larger particles detected optically1 but not to a level that posed any hazard4 ,in ,february #aigc the spacecraft emerged unscathed from the asteroid belt1 having demonstrated that the much-feared concentration of small debris in the belt did not exist4 ,the pathway was open to the outer solar system6 ,on ,november #bf1 #aigc1 the long-awaited encounter with ,jupiter began4 ,on that date1 at a distance of #f4d million kilometers from the planet1 instruments on board ,pioneer #aj detected a sudden change in the interplanetary medium as the spacecraft crossed the point--the bow shock--at which the magnetic presence of ,jupiter first becomes evident4 ,at the bow shock1 the energetic particles of the solar wind are suddenly slowed as they approach ,jupiter4 ,at noon the next day1 ,pioneer #aj entered the ,jovian magnetosphere at a distance of #if ,r,j from the planet4 ,as the spacecraft hurtled inward toward regions of increasing magnetic field strength and charged plasma particles1 the instruments designed to look at ,jupiter began to play their role4 ,a simple line-scan camera that could build up an image from many individual brightness scans 7like a newspaper picture transmitted by wire7 obtained its first pictures of the planet1 and ultraviolet and infrared photometers prepared to observe it also4 ,by ,december #b1 when the spacecraft had crossed the orbit of ,callisto1 the outermost of the large ,galilean satellites1 the line-scan images were nearly equal in quality to the best telescopic photos taken previously1 and as each hour passed they improved in resolution4 ,near closest approach1 ,pioneer #aj transmitted partial frames of ,jupiter that represented a threefold improvement over any ,earth-based pictures ever taken4 ,illustration3 ,the asteroid belt is a region between ,mars and ,jupiter that is populated by thousands of minor planets1 most only a few kilometers in diameter4 ,before #aigj1 some theorists suggested that large quantities of abrasive dust might damage spacecraft passing through the asteroid belt4 ,pioneer #aj proved that this danger was not present1 thus opening the way to the outer solar system4 ,earth at encounter ,earth at launch ,jupiter at encounter ,jupiter at launch ,asteroid belt ,tension increased as the spacecraft plunged deeper into the radiation belts of ,jupiter4 ,would it survive the blast of x-rays and gamma-rays induced in every part of the craft by the electrons and ions trapped by the magnetic field of ,jupiter8 ,several of the instruments measuring the charged particles climbed to full scale and saturated4 ,others neared their limits but1 as anxious scientists watched the data being sent back1 the levels flattened off4 ,meanwhile1 the spacecraft itself began to feel the effects of the radiation1 and occasional spurious commands were generated4 ,several planned high-resolution images of ,jupiter and its satellites were lost because of these false signals4 ,but again the system stabilized1 and no more problems occurred as1 just past noon on ,december #c1 #aigc1 ,pioneer #aj reached its closest point to ,jupiter1 #acj #jjj kilometers above the ,jovian cloud tops4 ,pioneer had passed its most demanding test with flying colors1 and at a news conference at ,ames1 ,,nasa ,planetary ,program ,director ,robert ,kraemer pronounced the mission 8#ajj percent successful40 ,he added1 8,we sent ,pioneer off to tweak a dragon's tail1 and it did that and more4 ,it gave it a really good yank1 and it managed to survive40 ,the ,project ,science ,chief pronounced it 8the most exciting day of my life10 and most of the hundreds of scientists and engineers who participated in the encounter probably agreed with him4 ,pioneer #aa continued to follow steadily1 emerging from the asteroid belt in ,march #aigd4 ,based on the performance and findings of ,pioneer #aj1 it was decided to send ,pioneer #aa still closer to ,jupiter1 but on a more inclined trajectory4 ,on ,april #ai thrusters on the spacecraft fired to move the ,pioneer #aa aimpoint just #cd #jjj kilometers above the clouds of ,jupiter4 ,in using the ,pioneer #aj data to assess the hazard to ,pioneer #aa1 scientists had to consider three aspects of the charged particle environment4 ,first was the energy distribution of the particles3 ,the most energetic presented the most danger4 ,second was the flux1 the rate at which particles struck the craft4 ,third was the total radiation dose4 ,one can make an analogy with a boxing match4 ,the energy distribution tells you how hard the blows of your opponent are4 ,the flux is a measure of how many times a minute he hits you1 and the total dose measures how many blows land4 ,the spacecraft reacts just like a boxer2 the crucial question is how much total dose it absorbs4 ,enough radiation blows1 and the system is knocked out4 ,the trajectory chosen for ,pioneer #aa resulted in higher flux1 since the craft probed more deeply into ,jupiter's inner magnetosphere than had ,pioneer #aj4 ,but by moving at a high angle across the equatorial regions where the flux is highest1 the total dose could be kept below that experienced by ,pioneer #aj4 ,pioneer #aa entered the ,jovian magnetosphere on ,november #bf1 #aigd1 just a year after its predecessor4 ,closest approach took place on ,december #b4 ,as with ,pioneer #aj1 the radiation dose taxed the spacecraft to its limit4 ,again spurious commands were issued1 this time affecting the infrared radiometer more than the imaging system4 ,the craft flew at high latitude over the north polar region of ,jupiter1 an area never seen from ,earth1 and returned several excellent high-resolution pictures4 ,once more the little ,pioneers had succeeded against the odds in opening the way to the giant planets4 ,illustration3 ,pioneer #aj and #aa encounters with ,jupiter are shown as viewed from the celestial ,north ,pole4 ,pioneer #aj swung around the giant planet in the counterclockwise direction1 while ,pioneer #aa followed a clockwise approach4 ,in this view1 ,jupiter rotates counterclockwise4 ,pioneer #aa ,pioneer #aj ,callisto ,ganymede ,europa ,io ,amalthea ,following their encounters with ,jupiter1 both ,pioneer spacecraft returned to their normal routine of measuring the interplanetary medium4 ,pioneer #aj had gained speed from the gravity field of ,jupiter and became the first craft to achieve the velocity needed to escape from the solar system4 ,pioneer #aa1 however1 had used the pull of ,jupiter to bend its trajectory inward1 aiming it across the solar system toward ,saturn4 ,following the successes at ,jupiter1 ,,nasa announced that ,pioneer #aa would be targeted for a close flyby of ,saturn five years later1 which was successfully carried out in ,september #aigi4 ,in early #aihj1 far beyond their design lifetimes1 both spacecraft were still performing beautifully4 ,illustration3 ,in this view of ,jupiter1 the ,great ,red ,spot is prominent and the shadow of ,io traverses the planetary disk4 ,the gross morphology of the belts and zones1 with structures showing turbulence and convective cells in the middle latitudes1 is clearly seen4 ,the small white spots surrounded by dark rings1 seen mainly in the southern hemisphere1 indicate regions of intense vertical convective activity1 somewhat similar to cumulonimbus or thunderclouds4 ,jupiter ,results ,the scientific results of the ,pioneer flybys of ,jupiter were many and varied4 ,as is always the case1 some old questions were answered and new problems were raised by the spacecraft data4 ,highlights of these results are summarized below4 ,photographs of ,jupiter4 ,the line-scan imaging systems of ,pioneer returned some remarkable pictures of the planet during the two encounters1 showing individual features as small as #ejj kilometers across4 ,in addition1 the ,pioneers were able to look at ,jupiter from angles never observable from ,earth4 ,one of the discoveries made from these pictures was the great variety of cloud structures near the boundaries between the light zones and dark belts4 ,many individual cloud patterns suggested rising and falling air4 ,the convoluted swirls evident in these regions appeared to be the result of dynamic motions2 unfortunately1 with only the few 8snapshots0 obtained during the hours of the flyby1 the actual motions of these clouds could not be followed4 ,thermal ,emission4 ,in the year the ,pioneer ,project was begun1 astronomers on ,earth had measured that ,jupiter emitted more heat than it absorbed from the ,sun4 ,from the ,earth these measurements could be made only of the sunlit part of the planet2 neither the night side nor the poles could be seen4 ,one of the main objectives of the ,pioneer flybys was to determine the heat budget accurately from temperature measurements at many points on both the sunlit and the night sides4 ,the ,pioneer data confirmed the presence of a heat source in ,jupiter and supplied a quantitative estimate of its magnitude4 ,the global effective temperature was found to be -#adh ,c1 to a precision of #c degrees4 ,this temperature implies that ,jupiter radiates #a4i times as much heat as it receives from the ,sun4 ,the corresponding internal heat source is #aj watts4 ,surprisingly1 the poles were as warm as the equator2 apparently1 the atmosphere is very efficient at transferring solar heat absorbed near the equator up to high latitudes1 or perhaps the internal component of the heat comes preferentially from the polar regions4 ,helium in the ,atmosphere4 ,the ,pioneer infrared experiment made the first measurement of the amount of helium on ,jupiter4 ,the ratio of the number of helium atoms to the number of hydrogen atoms was found to be ,he_/,h #j4ad #j4jh4 ,this is consistent with the known solar ratio of ,he_/,h #j4aa4 ,measurements of helium in the upper atmosphere were also made by the ultraviolet experiment4 ,illustration3 ,one of the best ,pioneer images of ,jupiter was obtained at a range of #ede #jjj kilometers by ,pioneer #aa4 ,structure within the ,great ,red ,spot and the surrounding belts and zones can be seen4 ,there was much less turbulent cloud activity round the spot at the time of the ,pioneer flybys than was seen five years later by the ,voyager cameras4 ,illustration3 ,pioneer #aj confirmed theoretical models of ,jupiter that suggest the planet is nearly all liquid1 with a very small core and an extremely deep atmosphere4 ,the liquid interior seethes with internal heat energy1 which is transferred from deep within the planet to its outer regions4 ,the temperature at the center may be #cj #jjj ,k4 ,since the temperature at the cloud tops is around -#abc ,c1 there is a large range of temperatures within the planet4 ,distance 7km7 ,visible clouds ,hydrogen gas ,cloud tops ,ammonia crystals ,ammonium hydrosulfide crystals ,ice crystals ,water droplets -#abc ,c ,transparent atmosphere ,hydrogen_/,helium gas #gj #jjj ,transition zone #aihj ,c #fj #jjj ,liquid hydrogen #ej #jjj #aa #jjj ,c ,transition zone #c million atmospheres pressure #dj #jjj ,liquid metallic hydrogen #cj #jjj #bj #jjj ,possible 8sea0 of helium #aj #jjj #cj #jjj ,k ,possible solid core #j ,,pioneer ,,science ,,investigations ,project ,scientist3 ,j4 ,h4 ,wolfe1 ,,nasa ,ames ,investigation ,principal ,investigator ,primary ,objectives ,magnetic fields ,e4 ,j4 ,smith1 ,,jpl ,measurement of the magnetic field of ,jupiter and determination of the structure of the magnetosphere4 ,magnetic fields ,n4 ,f4 ,ness1 ,,nasa ,measurement of the magnetic 7,pioneer #aa only7 ,goddard field of ,jupiter and determination of the structure of the magnetosphere4 ,plasma analyzer ,j4 ,h4 ,wolfe1 ,,nasa ,ames ,measurement of low-energy electrons and ions1 determination of the structure of the magnetosphere4 ,charged particle ,j4 ,a4 ,simpson1 ,u4 ,determination of the composition ,chicago number1 energy1 and composition of energetic charged particles in the ,jovian magnetosphere4 ,cosmic ray energy ,f4 ,b4 ,mc,donald1 ,,nasa ,measurement of number and spectra ,goddard energy of very high energy charged particles in space4 ,jovian charged ,j4 ,a4 ,van ,allen1 ,u4 ,measurement of number and particles ,iowa energy distribution of energetic charged particles and determination of magnetospheric structure4 ,jovian trapped ,r4 ,walker ,fillius1 ,,uc ,measurement of number and radiation ,san ,diego energy distribution of energetic charged particles and determination of magnetospheric structure4 ,asteroid-meteoroid ,r4 ,k4 ,soberman1 ,observation of solid astronomy ,general ,electric particles 7dust and larger7 in the vicinity of the spacecraft4 ,meteoroid detection ,w4 ,h4 ,kinard1 ,,nasa ,detection of very small ,langley solid particles that strike the spacecraft4 ,celestial mechanics ,j4 ,d4 ,anderson1 ,,jpl ,measurement of the masses of ,jupiter and the ,galilean satellites with high precision4 ,ultraviolet photometry ,d4 ,l4 ,judge1 ,u4 ,measurement of ultraviolet ,southern ,california emissions of the ,jovian atmosphere and from circumsatellite gas clouds4 ,imaging ,t4 ,gehrels1 ,u4 ,arizona ,reconnaissance imaging of photopolarimetry ,jupiter and its satellites2 study of atmospheric dynamics4 ,jovian infrared ,g4 ,mnch1 ,caltech ,measurement of ,jovian thermal structure temperature and heat budget2 determination of helium to hydrogen ratio4 ,s-,band occultation ,a4 ,j4 ,kliore1 ,,jpl ,probes of structure of ,jovian atmosphere and ionosphere4 ,atmospheric ,structure4 ,several ,pioneer investigations yielded information on the variation of atmospheric temperature and pressure in the regions above the ammonia clouds4 ,near the equator1 at a level where the atmospheric pressure is the same as that on the surface of the ,earth 7#a bar71 the temperature is -#ajh ,c4 ,about #aej kilometers higher1 where the pressure drops to #j4a bar1 is the minimum atmospheric temperature of about -#afe ,c4 ,above this point the temperature rises again1 reaching about -#abc ,c near a pressure level of #j4jc bar4 ,presumably this temperature rise is due to absorption of sunlight by a thin haze of dust particles in the upper atmosphere of ,jupiter4 ,internal ,structure4 ,the measurements of the amount of helium1 of the gravitational field1 and of the size of the internal heat source on ,jupiter greatly clarified scientists' understanding of the deep interior of the planet4 ,calculations showed that the core of ,jupiter must be so hot that hydrogen cannot become solid1 but must remain a fluid throughout the interior4 ,even at great depths1 therefore1 ,jupiter does not have a solid surface4 ,the theory that the ,great ,red ,spot was the result of interactions with a surface feature below the clouds thus became untenable4 ,whatever its exact nature1 the ,red ,spot must be a strictly atmospheric phenomenon4 ,magnetic ,field4 ,pioneer data showed that the magnetic field of ,jupiter has a dipolar nature1 like that of the ,earth1 but #bjjj times stronger4 ,the calculated surface fields measured about #d gauss1 compared to a field of about #j4e gauss on the ,earth4 ,the axis of the magnetic field was tilted #aa degrees with respect to the rotation axis1 and it was offset by about #aj #jjj kilometers 7#j4a ,r,j7 from the center of the planet4 ,illustration3 ,pioneers #aj and #aa did not obtain very detailed pictures of the satellites of ,jupiter4 ,the best view was of ,ganymede1 which showed a surface of contrasting light and dark spots of unknown nature4 ,illustration3 ,a less detailed image of ,europa clearly reveals the illuminated crescent but supplies little information about the surface of this ice-covered satellite4 ,satellite ,atmospheres4 ,two experiments yielded exciting new information on possible atmospheres of the ,galilean satellites4 ,first was an occultation1 in which the ,pioneer #aj spacecraft was targeted to pass behind ,io as seen from ,earth4 ,at the moments just before the spacecraft disappeared1 and just after reemergence from behind the satellite1 the radio signal was influenced by a thin layer of ionized gas1 in which the electrons had been stripped from the atoms by absorbed sunlight or by other processes4 ,the ionosphere thus discovered had a peak density of about #fj #jjj electrons per cubic centimeter4 ,in addition1 a very extended far-ultraviolet glow1 probably due to atomic hydrogen1 was found near the orbit of ,io by the ultraviolet photometer4 ,masses of ,jupiter and ,its ,satellites4 ,precise radio tracking of the ,pioneers as they coasted past ,jupiter and its satellites revealed that ,jupiter is about one percent heavier than had been anticipated1 and several satellites were found to have masses that differ by more than ten percent from values determined previously4 ,these improvements in knowledge of the masses were required to achieve the close satellite flybys being planned for later missions4 ,illustration3 ,the ,pioneer spacecraft carried this plaque on the journey beyond the solar system1 bearing data that tell where and when the human species lived and that convey details of our biological form4 ,when ,pioneer #aj flew by ,jupiter it acquired sufficient kinetic energy to carry it completely out of the solar system4 ,some time between one and ten billion years from now1 the probe may pass through the planetary system of a remote stellar neighbor1 one of whose planets may have evolved intelligent life4 ,if the spacecraft is detected and then inspected1 ,pioneer's message will reach across the eons to communicate its greeting4 ,the ,inner ,magnetosphere4 ,a great deal of the scientific emphasis of the ,pioneer missions was directed at characterizing the particles and fields in the inner magnetosphere1 the region in which charged particles are trapped in stable orbits4 ,the ,pioneers found that it extended to about #be ,r,j1 well beyond the orbit of ,callisto4 ,within this region1 instruments on the spacecraft recorded the numbers and energy of electrons1 protons1 and ions4 ,the electrons reached a maximum concentration near #c ,r,j1 and their numbers remained almost constant from there in toward the planet4 ,the maximum concentration of protons observed by ,pioneer #aj was at #c4d ,r,j1 a little inside the orbit of ,io4 ,pioneer #aa penetrated deeper and found another maximum1 about twenty times higher1 at #a4i ,r,j2 at this distance1 #aj million energetic protons hit each square centimeter of the spacecraft every second4 ,it was believed that the gap between these two peaks was due to tiny ,amalthea1 the innermost satellite1 which orbits at #b4e ,r,j4 ,apparently this satellite sweeps up the particles as it circles ,jupiter4 ,another large dip in the proton distribution was attributed to sweeping by ,io1 with smaller effects seen near the orbits of ,europa and ,ganymede4 ,there was an additional small effect at #a4h ,r,j1 later found by ,voyager to be due to ,jupiter's ring and its fourteenth satellite4 ,the ,outer ,magnetosphere4 ,from about #be ,r,j outward to its boundary near #ajj ,r,j1 the ,jovian magnetosphere is a complex and dynamic place4 ,beyond about #fj ,r,j1 both ,pioneers found the boundary to be highly unstable1 apparently blown in and out by variations in the pressure of the solar wind1 which consists of charged particles flowing outward from the ,sun4 ,in this region concentrations of ,jovian particles are sometimes seen that rival the inner magnetosphere in intensity4 ,between #fj ,r,j and #be ,r,j1 a region sometimes called the middle magnetosphere1 the particle motions are more ordered1 and for the most part electrons and protons are carried along with the planet's rotation by its magnetic field4 ,near the equatorial plane1 the flow of these particles produces an electric current circling the planet1 and this current in turn generates its own magnetic field1 which approaches in strength that of ,jupiter itself4 ,occasionally the outer magnetosphere collapses down to about #fj ,r,j1 and energetic particles are squirted from the middle region into space2 these bursts of ,jovian particles can sometimes be detected as far away as ,earth4 ,at the same time that ,pioneer scientists were analyzing their results and developing new concepts of the ,jupiter system1 a new team of investigators had been selected for the next mission to ,jupiter4 ,from about #aige on1 attention shifted from ,pioneer to its successor--,voyager4 ,illustration3 ,the ,voyager spacecraft are among the most sophisticated1 automatic1 and independent robots ever sent to explore the planets4 ,each craft has a mass of one ton and is dominated by the #c4g-meter-diameter white antenna used for radio communication with ,earth4 ,here ,voyager undergoes final tests in a space simulator chamber4 #cgc-gafb;,,ac ,,chapter #c ,,the ,,voyager ,,mission ,genesis of ,voyager ,voyager had its origin in the ,outer ,planets ,grand ,tour1 a plan to send spacecraft to all the planets of the outer solar system4 ,in #aifi1 the same year in which the ,pioneer ,project received ,congressional approval1 ,,nasa began to design the ,grand ,tour4 ,at the same time1 the ,space ,science ,board of the ,national ,academy of ,sciences completed a study called 8,the ,outer ,solar ,system10 chaired by ,james ,van ,allen of the ,university of ,iowa1 which recommended that the ,united ,states undertake an exploration program3 #a4 ,to conduct exploratory investigations of the appearance1 size1 mass1 magnetic properties1 and dynamics of each of the outer planets and their major satellites2 #b4 ,to determine the chemical and isotopic composition of the atmospheres of the outer planets2 #c4 ,to determine whether biologically important organic substances exist in these atmospheres and to characterize the lower atmospheric environments in terms of biologically significant parameters2 #d4 ,to describe the motions of the atmospheres of the major planets and to characterize their temperature-density-composition structure2 #e4 ,to make a detailed study for each of the outer planets of the external magnetic field and respective particle population1 associated radio emissions1 and magnetospheric particle-wave interactions2 #f4 ,to determine the mode of interaction of the solar wind with the outer planets1 including the interaction of the satellites with the planets' magnetospheres2 #g4 ,to investigate the properties of the solar wind and the interplanetary magnetic field at great distances from the ,sun at both low and high solar latitudes1 and to search for the outer boundary of the solar wind flow2 #h4 ,to attempt to obtain the composition1 energy spectra1 and fluxes of cosmic rays in interstellar space1 free of the modulating effects of the solar wind4 ,the report also noted that 8exceptionally favorable astronomical opportunities occur in the late #aigjs for multiplanet missions10 and that 8professional resources for full utilization of the outer-solar-system mission opportunities in the #aigjs and #aihjs are amply available within the scientific community1 and there is a widespread eagerness to participate in such missions40 ,an additional ,academy study1 chaired by ,francis ,s4 ,johnson of the ,university of ,texas at ,dallas and published in #aiga1 was even more specific3 8,an extensive study of the outer solar system is recognized by us to be one of the major objectives of space science in this decade4 ,this endeavor is made particularly exciting by the rare opportunity to explore several planets and satellites in one mission using long-lived spacecraft and existing propulsion systems4 ,we recommend that ,mariner-class spacecraft be developed and used in ,grand ,tour missions for the exploration of the outer planets in a series of four launches in the late #aigjs40 ,thus the stage was set to initiate the ,outer ,planets ,grand ,tours4 ,,nasa's timetable called for dual launches to ,jupiter1 ,saturn1 and ,pluto in #aigf and #aigg1 and dual launches to ,jupiter1 ,uranus1 and ,neptune in #aigi1 at a total cost over the decade of the #aigjs of about #gej million4 ,a necessary step was to obtain from the scientific community the best possible set of instruments to fly on the spacecraft4 ,following its initial internal studies1 ,,nasa turned for its detailed scientific planning to an open competition in which any scientist or scientific organization was invited to propose an investigation4 ,in ,october #aigj ,,nasa issued an 8,invitation for ,participation in the ,mission ,development for ,grand ,tour ,missions to the ,outer ,solar ,system10 and a year later it had selected about a dozen teams of scientists to formulate specific objectives for these missions4 ,at the same time1 an advanced spacecraft engineering design was carried out by the ,caltech ,jet ,propulsion ,laboratory 7,,jpl71 and studies were also supported by industrial contractors4 ,in fiscal year #aigb1 plans called for an appropriation by ,congress of #cj million to fund these developments1 leading toward a first launch in #aigf4 ,even as the scientific and technical problems of the ,grand ,tour were being solved1 however1 political and budgetary difficulties intervened4 ,the ,grand ,tour was an ambitious and expensive concept1 designed in the enthusiasm of the ,apollo years4 ,in the altered national climate that followed the first manned lunar landings1 the ,united ,states began to pull back from major commitments in space4 ,the later ,apollo landings were canceled1 and in fiscal year #aigb only #aj million of the #cj million needed to complete ,grand ,tour designs was appropriated4 ,it suddenly became necessary to restructure the exploration of the outer planets to conform to more modest space budgets4 ,illustration3 ,the original plan for the ,outer ,planets ,grand ,tour envisaged dual launches to ,jupiter1 ,saturn1 and ,pluto in the mid-#aigjs1 and dual launches to ,jupiter1 ,uranus1 and ,neptune in #aigi4 ,however1 political and budgetary constraints altered the plan1 and the ,voyager mission to ,jupiter and ,saturn1 with an optional encounter with ,uranus1 was formulated to replace it4 ,here the original ,grand ,tour trajectories from ,earth to the outer planets are shown4 ,p-#ajfab;,,ac ,redesign of the ,mission ,the new mission concept that replaced the ,grand ,tour dropped the objectives of exploring the outer three planets--,uranus1 ,neptune1 and ,pluto4 ,in this way the lifetime of the mission was greatly shortened1 placing less stringent demands on the reliability of the millions of components that go into a spacecraft4 ,limiting the mission to ,jupiter and ,saturn also relieved problems associated with spacecraft power1 and with communicating effectively over distances of more than #b billion kilometers4 ,the total cost of the new mission was estimated at #bej million1 only a third of that previously planned for the ,grand ,tour4 ,because it was based on the proven ,mariner spacecraft design1 the new mission was initially named ,mariner ,jupiter ,saturn1 or ,,mjs2 in #aigg the name was changed to ,voyager4 ,in ,january #aigb the ,president's proposed fiscal year #aigc budget included #aj million specifically designated for ,voyager2 after authorization and appropriation by ,congress1 the official beginning of ,voyager was set for ,july #a1 #aigb4 ,with approval of the new mission apparently assured in the ,congress1 ,,nasa issued an 8,announcement of ,flight ,opportunity0 to select the scientific instruments to be carried on ,voyager4 ,seventy-seven proposals were received2 #ca from groups of scientists with designs for instruments1 and #df from individuals desiring to participate in ,,nasa-formed teams4 ,of these #gg proposals1 #bd were from ,,nasa laboratories1 #dh were from scientists in various ,u4,s4 universities and industry1 and #e were from foreign sources4 ,after extensive review1 #bh proposals were accepted3 #i for instruments and #ai for individual participation4 ,the newly selected ,principal ,investigators and ,team ,leaders met for the first time at ,,jpl just before ,christmas1 #aigb4 ,to coordinate all the science activity of the ,voyager mission1 ,,nasa and ,,jpl selected ,edward ,stone of ,caltech1 a distinguished expert on magnetospheric physics1 to serve as ,project ,scientist4 ,the team assembled in #aigb by ,,jpl and its industrial contractors included more than a thousand highly trained engineers1 scientists1 and technical managers who assumed responsibility for the awesome task of building the most sophisticated unmanned spacecraft ever designed and launching it across the farthest reaches of the solar system4 ,at the head of the organization was the ,project ,manager1 ,harris 7,bud7 ,schurmeier4 ,later1 ,schurmeier was succeeded by ,john ,casani1 ,robert ,parks1 and ,ray ,heacock4 ,this team had only four years to turn the paper concepts into hardware1 ready to deliver to ,kennedy ,space ,center for launch in the summer of #aigg4 ,illustration3 ,project ,scientist ,edward ,c4 ,stone ,illustration3 ,project ,manager ,harris 7,bud7 ,schurmeier ,the ,objectives of ,voyager ,voyager is one of the most ambitious planetary space missions ever undertaken4 ,voyager #a1 which encountered ,jupiter on ,march #e1 #aigi1 was to investigate ,jupiter1 its large satellites ,io1 ,ganymede1 and ,callisto1 and tiny ,amalthea2 ,saturn1 its rings1 and several of its satellites--including ,titan1 the largest satellite in the solar system4 ,voyager #b1 which arrived at ,jupiter on ,july #i1 #aigi1 was to examine ,jupiter1 ,europa1 ,ganymede1 ,callisto1 and ,saturn and several of its satellites1 after which it was to be hurled on toward an encounter with the ,uranian system in #aihf4 ,both spacecraft were also designed to study the interplanetary medium and its interactions with the solar wind4 ,scientific objectives as well as the orbital positions of the planets and satellites influenced the choice of spacecraft trajectories1 which were designed to provide close flybys of all four of ,jupiter's ,galilean satellites and six of ,saturn's satellites--featuring a very close approach to ,titan--and occultations of the ,sun and ,earth by ,jupiter1 ,saturn1 ,titan1 and the rings of ,saturn4 ,however1 all these objectives could not be accomplished by a single spacecraft4 ,no single path through the ,jupiter system1 for instance1 can provide close flybys of all four ,galilean satellites4 ,specifically1 a trajectory for ,voyager #a that included a close encounter with ,io precluded a close encounter with ,europa and did not allow the spacecraft the option of being targeted from ,saturn to ,uranus without having to travel through the rings of ,saturn4 ,on the other hand1 a trajectory that would send ,voyager #b on to ,uranus precluded not only a close encounter with ,io1 but also a close encounter with ,titan and with ,saturn's rings4 ,the alignment of the planets and their satellites was such that encounter with ,jupiter on or before ,april #d1 #aigi was necessary for optimum investigation of ,io1 but encounter with ,jupiter after ,june #ae1 #aigi was mandatory for a trajectory that would maintain the option to go on to ,uranus4 ,the latter trajectory also allowed ,voyager #b to maintain a healthier distance from ,jupiter1 avoiding the full dose of radiation that would be experienced by ,voyager #a4 ,although the two ,voyager trajectories do have certain fundamental differences1 they also have a great deal in common3 ,both spacecraft were designed to study the interplanetary medium1 and both would investigate ,jupiter and ,saturn and have close flybys with several of their major satellites4 ,also1 if it were decided not to send ,voyager #b on to ,uranus1 this spacecraft could be retargeted to a ,saturn trajectory similar to that of ,voyager #a1 providing a close flyby of ,titan and a closer look at the rings4 ,the flight paths of ,voyager #a and ,voyager #b complement each other--allowing the planets and some of the satellites to be viewed from a number of angles and over a longer period of time than would be possible with only one spaceprobe--yet the trajectories were also designed to be more or less capable of duplicating each other's scientific investigations4 ,this redundancy helps to ensure1 so far as is possible1 the success of the mission4 ,the redundancy built into the mission is evident1 not only in the design of the trajectories1 but in the spacecraft themselves4 ,voyagers #a and #b are identical spacecraft4 ,in addition1 many crucial elements are duplicated on each3 ,for example1 the computer command subsystem 7,,ccs71 the flight data subsystem 7,,fds71 and the attitude and articulation control subsystem 7,,aacs7--which function as onboard control systems--each have multiple reprogrammable digital computers4 ,in addition1 the ,,ccs1 which decodes commands from ground control and can instruct other subsystems from its own memory1 contains duplicates for all its functional units4 ,the ,,aacs1 which controls the spacecraft's stabilization and orientation1 has duplicate star trackers and ,sun sensors4 ,the communications system contains two radio receivers and four transmitters1 two each to transmit both ,s-band 7frequency of about #bbie megahertz7 and ,x-band 7frequency of about #hdah megahertz74 ,illustration3 ,project ,manager ,john ,casani ,the ,spacecraft ,the ,voyager spacecraft are more sophisticated1 more automatic1 and more independent than were the ,pioneers4 ,this independence is important because the giant planets are so far away that the correction of a malfunction by engineers on ,earth would take hours to perform4 ,even at 8nearby0 ,jupiter1 radio signals take about forty minutes to travel in one direction between the spacecraft and ,earth4 ,saturn is about twice as far away as ,jupiter1 and ,uranus is twice as far as ,saturn1 slowing communication even more4 ,illustration3 ,about the same size and weight of a subcompact car1 the ,voyager spacecraft carry instruments for eleven science investigations of the outer planets and their satellites4 ,power to operate the spacecraft is provided by three radioisotope thermoelectric generators mounted on one boom2 other booms hold the science scan platform and the dual magnetometers4 ,high-gain directional antenna ,magnetometer ,extendable boom ,planetary radio astronomy and plasma wave antenna ,radioisotope thermoelectric generators ,plasma detector ,cosmic ray detector ,wide angle ,,tv ,narrow angle ,,tv ,,tv electronics ,ultraviolet spectrometer ,photopolarimeter ,infrared interferometer spectrometer and radiometer ,low energy charged particles ,thrusters ,electronic compartments ,science instrument calibration panel and shunt radiator ,propulsion fuel tank ,planetary radio astronomy and plasma wave antenna ,perched atop the ,centaur stage of the launch rocket1 each ,voyager spacecraft has a mass of #b tons 7#bjff kilograms71 divided about equally between the spacecraft proper and the propulsion module used for final acceleration to ,jupiter4 ,the ,voyager itself1 with a mass of #hae kilograms and typical dimensions of about #c meters1 is almost the size and weight of a subcompact car--but enormously more complex4 ,a better analogy might be made with a large electronic computer--but no terrestrial computer was ever asked to supply its own power source and to operate unattended in the vacuum of space for up to a decade4 ,each ,voyager spacecraft carries instruments for eleven science investigations covering visual1 infrared1 and ultraviolet regions of the spectrum1 and other remote sensing studies of the planets and satellites2 studies of radio emissions1 magnetic fields1 cosmic rays1 and lower energy particles2 plasma 7ionized gases7 waves and particles2 and studies using the spacecraft radios4 ,each ,voyager has a science boom that holds high and low resolution ,,tv1 photopolarimeter1 plasma and cosmic ray detectors1 infrared spectrometer and radiometer1 ultraviolet spectrometer1 and low energy charged particle detector2 in addition1 each spacecraft has a planetary radio astronomy and plasma wave antenna and a long boom carrying a low- and a high-field magnetometer4 ,communication with ,earth is carried out via a high-gain antenna #c4g meters in diameter1 with a smaller low-gain antenna as backup4 ,the large white dish of the high-gain antenna dominates the appearance of the spacecraft1 setting it off from its predecessors1 which were able to use much smaller antennas to communicate over the more modest distances that separate the planets of the inner solar system4 ,although the transmitter power is only #bc watts--about the power of a refrigerator light bulb--this system is designed to transmit data over a billion kilometers at the enormous rate of #aae #bjj bits per second4 ,data can also be stored for later transmission to ,earth2 an onboard digital tape recorder has a capacity of about #ejj million 7#e #aj7 bits1 sufficient to store nearly #ajj ,voyager images4 ,illustration3 ,project ,manager ,ray ,heacock ,illustration3 ,during the early assembly stage1 technicians at ,caltech's ,jet ,propulsion ,laboratory equip ,voyager's extendable boom with low- and high-field magnetometers that measure the intensity and direction of the outer planets' magnetic fields4 #cgc-gagi;,,bc ,radioisotope thermoelectric generator 7,r,t,gs71 rather than solar cells1 provide electricity for the ,voyager spacecraft4 ,the ,,rtg use radioactive plutonium oxide for this purpose4 ,as the plutonium oxide decays1 it gives off heat which is converted to electricity1 supplying a total of about #dej watts to the spacecraft at launch4 ,this power slowly declines as the plutonium is used up1 with less than #djj watts expected at ,saturn flyby five years after launch4 ,hydrazine fuel is used to make mid-course corrections in trajectory and to control the spacecraft's orientation4 ,since the ,voyagers must fly through the inner magnetosphere of ,jupiter1 it was imperative that the hardware systems be able to withstand the radiation from ,jovian charged particles4 ,the electronic microcircuits that form the heart and brain of the spacecraft and its scientific instruments are especially susceptible to radiation damage4 ,three techniques were used to 8harden0 components against radiation3 #a4 ,special design using radiation-resistant materials2 #b4 ,extensive testing to select those electronic components which come out of the manufacturing process with highest reliability2 and #c4 ,spot shielding of especially sensitive areas with radiation-absorbing materials4 ,,voyager',s ,,greetings ,,to ,,the ,,universe ,the ,voyager spacecraft will be the third and fourth human artifacts to escape entirely from the solar system4 ,pioneers #aj and #aa1 which preceded ,voyager in outstripping the gravitational attraction of the ,sun1 both carried small metal plaques identifying their time and place of origin for the benefit of any other spacefarers that might find them in the distant future4 ,with this example before them1 ,,nasa placed a more ambitious message aboard ,voyager #a and #b--;a kind of time capsule1 intended to communicate a story of our world to extraterrestrials4 ,the ,voyager message is carried by a phonograph record--a #ab-;inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on ,earth4 ,the contents of the record were selected for ,,nasa by a committee chaired by ,carl ,sagan of ,cornell ,university4 ,dr4 ,sagan and his associates assembled #aae images and a variety of natural sounds1 such as those made by surf1 wind and thunder1 birds1 whales1 and other animals4 ,to this they added musical selections from different cultures and eras1 and spoken greetings from ,earthpeople in sixty languages1 and printed messages from ,president ,carter and ,u4,n4 ,secretary ,general ,waldheim4 ,each record is encased in a protective aluminum jacket1 together with a cartridge and needle4 ,instructions1 in symbolic language1 explain the origin of the spacecraft and indicate how the record is to be played4 ,the #aae images are encoded in analog form4 ,the remainder of the record is in audio1 designed to be played at #af revolutions per second4 ,it contains the spoken greetings1 beginning with ,akkadian1 which was spoken in ,sumer about six thousand years ago1 and ending with ,wu1 a modern ,chinese dialect4 ,following the section on the sounds of ,earth1 there is an eclectic #ij-minute selection of music1 including both ,eastern and ,western classics and a variety of ethnic music4 ,once the ,voyager spacecraft leave the solar system 7by #aiij1 both will be beyond the orbit of ,pluto71 they will find themselves in empty space4 ,it will be forty thousand years before they come within a light year of a star1 called ,,ac #gi #chhh1 and millions of years before either might make a close approach to any other planetary system4 ,as ,carl ,sagan has noted1 8,the spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space4 ,but the launching of this bottle into the cosmic ocean says something very hopeful about life on this planet40 ,,languages ,,on ,,voyager ,,record ,sumerian ,akkadian ,hittite ,hebrew ,aramaic ,english ,portuguese ,cantonese ,russian ,thai ,arabic ,roumanian ,french ,burmese ,spanish ,indonesian ,kechua ,dutch ,german ,bengali ,urdu ,hindi ,vietnamese ,sinhalese ,greek ,latin ,japanese ,punjabi ,turkish ,welsh ,italian ,nguni ,sotho ,wu ,korean ,armenian ,polish ,netali ,mandarin ,gujorati ,ila 7,zambia7 ,nyanja ,swedish ,ukrainian ,persian ,serbian ,luganada ,amoy 7,min dialect7 ,marathi ,kannada ,telugu ,oriya ,hungarian ,czech ,rajasthani ,,sounds ,,of ,,earth ,,on ,,voyager ,,record ,whales ,planets 7music7 ,volcanoes ,mud pots ,rain ,surf ,crickets1 frogs ,birds ,hyena ,elephant ,chimpanzee ,wild dog ,footsteps and heartbeats ,laughter ,fire ,tools ,dogs1 domestic ,herding sheep ,blacksmith shop ,sawing ,riveter ,morse code ,ships ,horse and cart ,horse and carriage ,train whistle ,tractor ,truck ,auto gears ,jet ,lift-off ,saturn #e rocket ,kiss ,baby ,life signs--,,eeg1 ,,ekg ,pulsart ,,voyager ,,record ,,photograph ,,index ,calibration circle ,solar location map ,mathematical definitions ,physical unit definitions ,solar system parameters ,the ,sun ,solar spectrum ,mercury ,mars ,jupiter ,earth ,egypt1 ,red ,sea1 ,sinai ,peninsula and the ,nile ,chemical definitions ,,dna structure ,,dna structure magnified ,cells and cell division ,anatomy 7eight7 ,human sex organs ,diagram of conception ,conception ,fertilized ovum ,fetus diagram ,fetus ,diagram of male and female ,birth ,nursing mother ,father and daughter 7,malaysia7 ,group of children ,diagram of family ages ,family portrait ,diagram of continental drift ,structure of ,earth ,heron ,island 7,great ,barrier ,reef of ,australia7 ,seashore ,snake ,river and ,grand ,tetons ,sand dunes ,monument ,valley ,forest scene with mushrooms ,leaf ,fallen leaves ,sequoia ,snowflake ,tree with daffodils ,flying insect with flowers ,diagram of vertebrate evolution ,seashell 7,xancidae7 ,dolphins ,school of fish ,tree toad ,crocodile ,eagle ,waterhold ,jane ,goodall and chimps ,sketch of ,bushmen ,bushmen hunters ,man from ,guatemala ,dancer from ,bali ,andean girls ,thailand craftsman ,elephant ,old man with beard and glasses 7,turkey7 ,old man with dog and flowers ,mountain climber ,cathy ,rigby ,sprinters ,schoolroom ,children with globe ,cotton harvest ,grape picker ,supermarket ,underwater scene with diver and fish ,fishing boat with nets ,cooking fish ,chinese dinner party ,demonstration of licking1 eating and drinking ,great ,wall of ,china ,house construction 7,african7 ,construction scene 7,amish country7 ,house 7,africa7 ,house 7,new ,england7 ,modern house 7,cloudcroft1 ,new ,mexico7 ,house interior with artist and fire ,taj ,mahal ,english city 7,oxford7 ,boston ,,un ,building1 day ,,un ,building1 night ,sydney ,opera ,house ,artisan with drill ,factory interior ,museum ,x-ray of hand ,woman with microscope ,street scene1 ,asia 7,pakistan7 ,rush hour traffic1 ,india ,modern highway 7,ithaca7 ,golden ,gate ,bridge ,train ,airplane in flight ,airport 7,toronto7 ,antarctic expedition ,radio telescope 7,westerbork1 ,netherlands7 ,radio telescope 7,arecibo7 ,page of book 7,newton1 ,system of the ,world7 ,astronaut in space ,titan ,centaur launch ,sunset with birds ,string quartet 7,quartetto ,italiano7 ,violin with music score 7,cavatina7 ,,music ,,on ,,voyager ,,record ,bach ,brandenberg ,concerto ,number ,two1 ,first ,movement 8,kinds of ,flowers0 ,javanese ,court ,gamelan ,senegalese ,percussion ,pygmy girls initiation song ,australian ,horn and ,totem song 8,el ,cascabel0 ,lorenzo ,barcelata 8,johnny ,b4 ,goode0 ,chuck ,berry ,new ,guinea ,men's ,house 8,depicting the ,cranes in ,their ,nest0 ,bach ,partita ,number ,three for ,violin2 ,gavotte et ,rondeaus ,mozart ,magic ,flute1 ,queen of the ,night 7,aria ,number #ad7 ,chakrulo ,peruvian ,pan ,pipes ,melancholy ,blues ,azerbaijan ,two ,flutes ,stravinsky1 ,rite of ,spring1 ,conclusion ,bach ,prelude and ,fugue ,number ,one in ,c ,major from the ,well ,tempered ,clavier1 ,book ,two ,beethoven's ,fifth ,symphony1 ,first ,movement ,bulgarian ,shepherdess ,song 8,izlel ,delyo ,hajdutin0 ,navajo ,indian ,night ,chant ,the ,fairie ,round from ,pavans1 ,galliards1 ,almains ,melanesian ,pan ,pipes ,peruvian ,woman's ,wedding ,song 8,flowing ,streams0--,chinese ,ch'in music 8,jaat ,kahan ,ho0--,indian ,raga 8,dark ,was the ,night0 ,beethoven ,string ,quartet ,number #ac 8,cavatina0 ,illustration3 ,each ,voyager carries a message in the form of a #ab-;inch gold-plated phonograph record4 ,the record1 together with a cartridge and needle1 is fastened to the side of the spacecraft in a gold-anodized aluminum case that also illustrates how the record is to be played4 ,p-#aigbh ,,the ,,brains ,,of ,,the ,,voyager ,,spacecraft#a ,the ,voyager spacecraft had greater independence from ,earth-based controllers and greater versatility in carrying out complex sequences of scientific measurements than any of their predecessors4 ,these capabilities resulted from three interconnected onboard computer systems3 the ,,aacs 7attitude and articulation control subsystem72 the ,,fds 7flight data subsystem72 and the ,,ccs 7computer command system74 ,operating from 8loads0 of instructions transmitted earlier from ,earth1 these computers could issue commands to the spacecraft and the science instruments and react automatically to problems or changes in operating conditions4 ,the complex sequence of scientific observations and the associated engineering functions were executed by the spacecraft under the control of an updatable program stored in the ,,ccs by ground command4 ,at appropriate times1 the ,,ccs issued commands to the ,,aacs for movement of the scan platform or spacecraft maneuvers2 to the ,,fds for changes in instrument configuration or telemetry rate2 or to numerous other subsystems within the spacecraft for specific actions4 ,the two identical 7redundant7 #djif-word memories within the ,,ccs contained both fixed routines 7about #bhjj words7 and a variable section 7about #abij words7 for changing science sequencing functions4 ,a single #abij-word science sequence load could easily generate #cjj #jjj discrete commands1 thus providing significantly more sequencing capability than would be possible through ground commands4 ,a #abij-word sequencing load in the ,,ccs controlled both the science and engineering functions of the spacecraft for a period lasting for day at closest approach and for up to #ajj days during cruise4 ,each #abij-word program 7or load7 was built from specific science measurement units called links4 ,some links were used repeatedly in a looping cyclic 7like a computer ,,do loop7 to perform the same observation numerous times2 other links that involved special measurement geometry or critical timing occurred only once4 ,about #age science links were defined for the ,voyager #a ,jupiter encounter4 ,it took almost two years to convert the desired science objectives and measurements first into links1 then into a minute-by-minute timeline for the #ih-;day encounter period1 and finally into the specific computer instructions that could be loaded into the ,,ccs memory for that portion of the encounter time represented by a particular load4 ,the total ,voyager #a ,jupiter encounter period used eighteen sequence memory loads1 supplemented by about #ajjj ground commands to modify the sequences because of changing conditions or calibration requirements4 ,for the ,voyager #b encounter1 concern about the ailing spacecraft receiver limited the number of loads that could be transmitted1 particularly while the spacecraft was deep within the ,jovian magnetosphere1 where radiation effects caused the receiver frequency to drift unpredictably4 ,however1 a careful redesign of the planned sequences permitted the accomplishment of very nearly the original set of observations even with these constraints4 ,all three approaches were required on the ,voyager craft1 especially after ,pioneer #aj and #aa demonstrated that the radiation at ,jupiter was even more intense than had been assumed in early design studies4 ,the steady streams of engineering and scientific data received on ,earth are transmitted from the receiving stations of the ,deep ,space ,network 7,,dsn7 to ,,jpl1 where the ,voyager control functions are centered4 ,there1 dozens of technicians check and recheck every subsystem to search for the slightest hint of malfunction4 ,in case of problems1 there are thick notebooks of instructions and racks of precoded computer tapes ready to be used to correct any apparent malfunction4 ,normally ,voyager runs itself4 ,detailed instructions are programmed into its onboard computers and command systems for dealing with such potential emergencies as a stuck valve in the fuel system1 loss of orientation in the star trackers1 erratic gyroscope functions1 failure of radio communications1 or a thousand and one other nightmares4 ,the instructions for operating the scientific instruments are also stored on board1 with new blocks of commands sent up once every few days to replace those for tasks already completed4 ,whether in the calm of cruise mode or the intense excitement of a planetary encounter1 the ,voyager craft is alone in space1 continuously sensing and reacting to its environment1 tied by only a tenuous thread of radio communication to the anxious watchers back on ,earth4 ,illustration3 ,project ,manager ,robert ,parks ,,nasa ,,planetary ,,missions ,spacecraft ,launch ,destination ,encounter ,type of ,date ,date ,encounter ,mariner #b #h_/bf_/fb ,venus #ab_/ad_/fb flyby ,mariner #d #aa_/bh_/fd ,mars #g_/ad_/fe flyby ,mariner #e #f_/ad_/fg ,venus #aj_/ai_/fg flyby ,mariner #f #b_/be_/fi ,mars #g_/ca_/fi flyby ,mariner #g #c_/bg_/fi ,mars #h_/je_/fi flyby ,mariner #i #e_/cj_/ga ,mars #aa_/ac_/ga orbiter ,pioneer #aj #c_/jc_/gb ,jupiter #ab_/jd_/gc flyby ,pioneer #aa #d_/jf_/gc ,jupiter #ab_/jc_/gd flyby ,saturn #i_/ja_/gi flyby ,mariner #aj #aa_/jc_/gc ,venus #b_/je_/gd flyby ,mercury #c_/bi_/gd flyby ,viking #a #h_/bj_/ge ,mars #f_/ai_/gf orbiter #g_/bj_/gf lander ,viking #b #i_/ji_/ge ,mars #g_/jg_/gf orbiter #i_/jc_/gf lander ,voyager #a #h_/bj_/gg ,jupiter #c_/je_/gi flyby ,voyager #b #i_/je_/gg ,jupiter #g_/ji_/gi flyby ,pioneer ,venus #e_/bj_/gh ,venus #ab_/jd_/gh orbiter #h_/h_/gh ,venus #ab_/ji_/gh probe ,illustration3 ,the ,voyager scan platform contains sophisticated instruments that gather data for ,voyager's remote sensing investigations4 ,five of the remote-sensing instruments--two ,,tv cameras1 the infrared spectrometer1 the ultraviolet spectrometer1 and the photopolarimeter--are mounted together on the scan platform1 which can be pointed to almost any direction in space1 allowing exact targeting of the observations4 #cgc-gadf;,,bc ,,chapter #d ,,science ,,and ,,scientists ,introduction ,there are many reasons for sending spacecraft to the planets1 but in the final analysis1 we send our robot messengers across the vastness of space for the sake of scientific exploration4 ,science and exploration have always gone hand in hand1 whether in the transcontinental journey of ,lewis and ,clark or the ,pacific voyages of ,captain ,james ,cook4 ,in this century1 as exploration has become more and more dependent on advances in technology1 the scientific element has attained increased prominence4 ,the greatest legacy of the ,,nasa ,planetary ,program is the knowledge it has provided of the other worlds that share our corner of the universe4 ,despite the central role of science in motivating missions to the planets1 specific scientific considerations are not dominant during most of the development of a mission such as ,voyager4 ,the problem of building a spacecraft and getting it to the outer solar system is too demanding4 ,of the ton of mass in a ,voyager spacecraft1 the scientific instruments make up only #aae kilograms 7about eleven percent74 ,similarly1 the cost of these instruments amounts to only about #aj percent of the cost of the spacecraft and launch vehicle4 ,unless the launch and operation of the spacecraft are nearly perfect1 there can be no scientific return in any case2 even the most sophisticated package of scientific instruments will not tell much about ,jupiter if1 following launch1 it rests at the bottom of the ,atlantic ,ocean4 ,but it is equally true that the ultimate purpose of the mission is scientific discovery1 and ,,nasa makes every effort to ensure that the very best instruments are flown and that a broad scientific community is given the opportunity to participate in each mission4 ,a decade before the #aigg launch1 many astronomers and space scientists began their involvement with the ,voyager mission through participation in study groups convened by ,,nasa and by the ,national ,academy of ,sciences4 ,they came primarily from universities1 but also in significant numbers from ,,nasa laboratories1 from industry1 and from abroad4 ,in #aiga the ,outer ,planets ,grand ,tours mission definition group carried out a one-year final study of the mission that was to become ,voyager4 ,a competition was held in #aigb to select the ,voyager flight instruments and science teams1 and a third review stage followed a year later to confirm this selection4 ,out of this process emerged eleven science investigations1 with which more than one hundred scientists were associated4 ,in this chapter we look at the instruments and the persons who designed them for this challenging task4 ,direct and ,remote ,measurements ,the diverse measurements made by ,voyager of a planet and its environment can be divided into two broad categories1 usually called direct or in situ measurements and remote sensing measurements4 ,a direct measurement involves the analysis of the immediate environment of the spacecraft2 remote measurements can be made by analyzing radiation from distant objects4 ,the direct measurement instruments on ,voyager measure cosmic ray particles1 low energy charged particles1 magnetic fields1 plasma particles1 and plasma waves4 ,their activity began immediately after launch1 monitoring the ,earth environment and then interplanetary space until the magnetosphere of ,jupiter was reached a few days before the actual flyby4 ,long after the flybys of ,jupiter and ,saturn are completed1 particles and fields data can continue to be acquired by the ,voyagers as they speed outward into previously unexplored regions of space4 ,the remote sensing investigations are essentially astronomical in nature1 measuring the light reflected from or emitted by the planet and its satellites4 ,on ,voyager1 however1 these instruments far outstrip their terrestrial counterparts in capability4 ,primarily1 they derive their advantage from their proximity to what they observe--at its closest1 ,voyager was more than a thousand times nearer to ,jupiter than are ,earth-based telescopes2 and for the still closer satellite encounters1 ,voyager was nearly ten thousand times nearer than astronomers on ,earth4 ,in addition1 ,voyager provided perspectives1 such as views of the night side of ,jupiter1 that are impossible from ,earth4 ,finally1 these instruments could exploit the full spectrum of electromagnetic radiation without concern for the opacity of the terrestrial atmosphere1 which restricts ground-based astronomers to certain spectral windows and blocks all observation at other wavelengths4 ,five of the remote sensing instruments--the two ,,tv cameras1 the infrared spectrometer1 the ultraviolet spectrometer1 and the photopolarimeter--are mounted together on a scan platform4 ,this platform can be pointed to almost any direction in space1 allowing exact targeting of the observations4 ,one remote sensing instrument1 the planetary radio astronomy receiver1 is not on the scan platform4 ,it measures long-wave radio emission without requiring special pointing4 ,illustration3 ,the fully deployed ,voyager spacecraft is capable of a wide variety of direct and remote sensing measurements4 ,the instruments and their objectives were selected many years before the first ,jupiter encounter4 ,because of the exploratory nature of the ,voyager mission1 every effort was made to fly versatile instruments that could yield valuable results no matter what the nature of the ,jovian system4 ,p-#ahhaa;,,ac ,narrow angle ,,tv ,wide angle ,,tv ,steerable platform ,science instrument boom ,plasma detector ,cosmic ray detector ,high-gain antenna ,low-gain antenna ,extendable magnetometer boom ,photopolarimeter ,low energy charged particles detector ,infrared interferometer spectrometer and radiometer ,ultraviolet spectrometer ,thrusters ,star trackers ,radioisotope thermoelectric generators ,science instrument calibration panel and shunt radiator ,thrusters ,planetary radio astronomy and plasma wave antenna ,illustration3 ,the science instrument boom supports the plasma particle detector1 the cosmic ray detector1 and the low energy charged particle detector4 ,these instruments began collecting data immediately after launch1 monitoring the ,earth environment and then interplanetary space until the magnetosphere of ,jupiter was reached a few days before the actual flyby4 #cec-biib;,,bc ,a final ,voyager investigation did not fit into this pattern of direct versus remote sensing instruments4 ,in fact1 it required no special instrument at all4 ,this investigation deals with radio science1 and it utilizes the regular communications link between the spacecraft and ,earth to derive the masses of ,jupiter and its satellites1 to probe the atmosphere of ,jupiter1 and to study properties of the interplanetary medium4 ,illustration3 ,the ,voyager ,imaging ,science ,team ,imaging ,the eyes of ,voyager are in its imaging system4 ,two television cameras1 each with a set of color filters1 look at the planets and their satellites and transmit thousands of detailed pictures to ,earth4 ,the imaging system is probably the most versatile and therefore the most truly exploratory of the ,voyager instruments4 ,no matter what is out there1 the imaging system will let us see it and hence1 we hope1 begin to understand its nature4 ,,voyager ,,science ,,investigations ,project ,scientist3 ,e4 ,c4 ,stone1 ,caltech ,investigation ,principal ,investigator ,primary ,objectives at or ,team ,leader ,jupiter ,imaging science ,b4 ,a4 ,smith1 ,u4 ,arizona ,high resolution reconnaissance over large phase angles2 measurement of atmospheric dynamics2 determination of geologic structure of satellites2 search for rings and new satellites4 ,infrared radiation ,r4 ,a4 ,hanel1 ,,nasa ,determination of 7,,iris7 ,goddard atmospheric composition1 thermal structure1 and dynamics2 satellite surface composition and thermal properties4 ,ultraviolet ,a4 ,l4 ,broadfoot1 ,kitt ,measurement of upper spectroscopy ,peak ,observatory atmospheric composition and structure2 auroral processes2 distribution of ions and neutral atoms in the ,jovian system4 ,photopolarimetry ,c4 ,f4 ,lillie_/,c4 ,w4 ,measurement of atmospheric ,hord1 ,u4 ,colorado aerosols2 satellite surface texture and sodium cloud4 ,planetary radio ,j4 ,w4 ,warwick1 ,u4 ,determination of astronomy ,colorado polarization and spectra of radio frequency emissions2 ,io radio modulation process2 plasma densities4 ,magnetic fields ,n4 ,f4 ,ness1 ,,nasa ,goddard ,measurement of plasma electron densities2 wave-particle interactions2 low-frequency wave emissions4 ,plasma particles ,h4 ,s4 ,bridge1 ,,mit ,measurement of magnetospheric ion and electron distribution2 solar wind interaction with ,jupiter2 ions from satellites4 ,plasma waves ,f4 ,l4 ,scarf1 ,,trw ,measurement of plasma electron densities2 wave-particle interactions2 low-frequency wave emissions4 ,low energy charged ,s4 ,m4 ,krimigis1 ,johns ,measurement of the particles ,hopkins ,u4 distribution1 composition1 and flow of energetic ions and electrons2 satellite-energetic particle interactions4 ,cosmic ray particles ,r4 ,e4 ,vogt1 ,caltech ,measurement of the distribution1 composition1 and flow of high energy trapped nuclei2 energetic electron spectra4 ,radio science ,v4 ,r4 ,eshleman1 ,measurement of atmospheric ,stanford ,u4 and ionospheric structure1 constituents1 and dynamics2 satellite masses4 ,unlike other ,voyager instruments1 the imaging system is not the result of a competition among proposals submitted by groups of scientists4 ,,nasa assigned the development of the cameras directly to ,,jpl1 to be integrated from the beginning with the design of the ,voyager spacecraft and its subsystems4 ,the members of the ,voyager ,imaging ,science ,team were selected1 as individuals1 on the basis of the scientific studies they proposed to carry out4 ,initially1 ten members of the ,imaging ,science ,team were selected1 but by the time of the ,jupiter encounters1 the team had been expanded to #bb scientists4 ,illustration3 ,bradford ,a4 ,smith1 imaging science ,team ,leader ,the ,team ,leader is ,bradford ,a4 ,smith1 a professor in the ,department of ,planetary ,science at the ,university of ,arizona4 ,smith was involved in imaging science on several previous missions1 including ,mariners #f and #g and ,viking4 ,he was also active in ground-based photography of ,jupiter at both ,new ,mexico ,state ,university and ,university of ,arizona1 and he is a member of the team developing a planetary camera for the ,space ,telescope1 scheduled for operation in ,earth orbit in the mid-#aihjs4 ,originally1 the ,deputy ,team ,leader was ,geoffrey ,a4 ,briggs1 a young ,british-born physicist from ,,jpl4 ,however1 in #aigg ,briggs took a position at ,,nasa ,headquarters in ,washington and was replaced by geologist ,laurence ,a4 ,soderblom of the ,u4,s4 ,geological ,survey in ,flagstaff1 ,arizona4 ,an energetic and articulate scientist1 ,soderblom1 with his interest in satellite geology1 complemented ,smith1 whose personal scientific interests are directed more toward the atmosphere of ,jupiter4 ,the objectives of imaging involved multicolor photography of ,jupiter and its satellites4 ,both wide- and narrow-angle cameras were needed to obtain the highest possible resolution while retaining the capability to study global-scale features on ,jupiter and the satellites4 ,in normal photographic terms1 both cameras used telephoto lenses4 ,for the wide-angle camera1 a focal length of #bjj millimeters was selected1 giving a field of view of about #c degrees4 ,this field is similar to that obtained with a #djj-millimeter telephoto lens on a #ce-millimeter camera4 ,the narrow-angle ,voyager camera has a focal length of #aejj millimeters and field of view of #j4d degrees4 ,the camera optics are combinations of mirrors and lenses1 designed for extreme stability of focus and for freedom from distortion4 ,each camera has a rotating filter wheel that can be used to select the color of the light that reaches the camera4 ,for the wide-angle camera1 these colors are clear1 violet1 blue1 green1 orange1 and three special bands for selective observation in sodium light 7#ehi-nanometer wavelength71 and in methane spectral lines at #eda nanometers and #fah nanometers4 ,for the narrow-angle camera1 the filters are clear1 ultraviolet1 violet1 blue1 green1 and orange4 ,to create a color picture1 the cameras are commanded to take1 in rapid succession1 pictures of the same area in blue1 green1 and orange light4 ,these three pictures can then be reconstructed on ,earth into a 8true0 color image4 ,other combinations of colors are used to investigate particular scientific problems and to determine the spectrum of sunlight reflected from features on ,jupiter and its satellites4 ,the detector in the cameras is not photographic film but the surface of a selenium-sulfur vidicon television tube1 #aa millimeters square4 ,unlike most commercial ,,tv cameras1 these tubes are designed for slow-scan readout1 providing #dh seconds to acquire each picture4 ,the shutter speed can be varied from a fraction of a second 7for ,jupiter and ,io7 to many minutes 7for searches for faint features1 such as aurorae on the night side of ,jupiter74 ,each picture consists of many numbers1 each of which represents the brightness of a single picture element1 or pixel1 on the image4 ,there are #fdj #jjj pixels in each ,voyager image1 representing a square image of #hjj #hjj points4 ,this information content is more than twice that of an ordinary television picture1 which has only #ebj lines4 ,for each pixel1 eight binary numbers are required to specify the brightness2 the total information in a single image is thus #h #fdj #jjj #e #abj #jjj bits4 ,even at a transmission of one frame per #dh seconds1 the 8bit rate0 is more than #ajj #jjj bits per second4 ,for comparison1 the bit rate from the first spacecraft to ,mars 7,mariner #d7 was about #aj bits per second1 requiring a week to transmit #ba pictures1 with a total information content equivalent to a single ,voyager picture4 ,altogether1 ,voyager took nearly #bj #jjj pictures at each ,jupiter encounter1 representing #aj--;a hundred billion--bits of information4 ,illustration3 ,rudolph ,hanel1 infrared spectrometer ,principal ,investigator ,infrared ,spectrometer ,the infrared investigation on ,voyager is based on one of the most sophisticated instruments ever flown to another planet4 ,in the past1 most infrared instruments on planetary spacecraft measured at only a few wavelengths1 but ,voyager carries a true spectrometer1 capable of measuring at nearly #bjjj separate wavelengths1 covering the spectrum from #d to #ej micrometers4 ,twelve scientists1 led by ,principal ,investigator ,rudolph ,hanel of the ,,nasa ,goddard ,space ,flight ,center at ,greenbelt1 ,maryland1 proposed this infrared instrument4 ,hanel is an acknowledged world leader in infrared spectroscopy from space4 ,with his co-workers at ,goddard1 he has pioneered in adapting the extremely complex art of interferometric spectroscopy to the rigors of space flight4 ,his spectrometers have made many studies of the ,earth's atmosphere from meteorological satellites1 and a ,hanel interferometer flew successfully to ,mars on ,mariner #i4 ,the primary goals of the infrared spectrometer investigation are directed toward analysis of the composition and structure of the atmosphere of ,jupiter4 ,among the molecules to be searched for on ,jupiter were hydrogen 7,h71 helium 7,he71 methane 7,,ch71 ammonia 7,,nh71 phosphine 7,,ph71 water 7,h,o71 carbon monoxide 7,,co71 simple compounds of silicon and sulfur1 and a variety of organic compounds consisting of atoms of carbon and hydrogen 7e4g41 ,c,h1 ,c,h1 ,c,h74 ,in addition to indicating the abundance of these constituents of the atmosphere1 the infrared spectra also contain information on atmospheric structure1 that is1 on the variation of temperature and pressure with altitude4 ,the presence of clouds or dust layers can also be inferred from the shapes of spectral lines4 ,in addition to its spectroscopy of ,jupiter1 the ,voyager instrument could be used as a heat-measuring device to map the temperatures of both the satellites and the atmosphere of ,jupiter4 ,particularly interesting for the satellites are measurements of the surface cooling and heating rates1 since these rates reveal the physical compactness of the surface1 easily distinguishing between rock and sand or dust4 ,the infrared instrument is a ,michelson interferometer at the focus of a gold-plated telescope of #ea-;centimeter aperture4 ,the spectrum is not obtained directly1 by moving a prism or grating1 but indirectly through the interference effects of light of different wavelengths3 hence its name1 an interferometer4 ,the complex interference pattern generated by the motion of one of the mirrors in the light path is transmitted to ,earth1 where computer analysis is required to transform it into a recognizable spectrum4 ,the entire instrument1 which has a mass of #bj kilograms1 is called ,,iris1 for infrared interferometer spectrometer4 ,the development of the infrared system for ,voyager posed many problems4 ,,iris was designed to cover the optimum spectral region for studies of the atmospheres of ,jupiter and ,saturn4 ,however1 when it was decided in #aigd that an extended ,voyager mission might also allow a visit to ,uranus1 ,hanel and his colleagues realized that their instrument had serious deficiencies for investigation of that colder and more distant planet1 and they proposed that a modified ,,iris 7,,miris7 be substituted for the original design4 ,a crash program was authorized to develop ,,miris in parallel with ,,iris1 and in early #aigg1 as launch approached1 it appeared that the improved instrument would be ready4 ,problems occurred during testing1 however1 and for several weeks in ,june and ,july the decision hung in the balance4 ,unfortunately1 there simply was not enough time to solve all the problems2 both ,voyagers were launched carrying the original ,,iris1 and the final flight qualification of ,,miris came in ,october1 about six weeks after launch4 ,with no other missions planned beyond ,saturn1 no alternate use for ,,miris has been found2 it remains 8on the shelf10 one of the rare cases where a technological gamble by ,,nasa did not pay off4 ,illustration3 ,lyle ,broadfoot1 ultraviolet spectrometer ,principal ,investigator ,ultraviolet ,spectrometer ,a second spectroscopic instrument on the ,voyager scan platform examines short-wave1 or ultraviolet1 radiation4 ,the ,principal ,investigator for the ultraviolet spectrometer 7,,uvs7 is ,a4 ,lyle ,broadfoot of ,kitt ,peak ,national ,observatory in ,tucson1 ,arizona4 ,broadfoot's expertise lies in instrumentation for atmospheric research1 and he was previously the ,principal ,investigator for a similar ultraviolet instrument on the ,mariner #aj mission to ,venus and ,mercury4 ,associated with ,broadfoot in the ,voyager investigation are fourteen other scientists from the ,united ,states1 ,canada1 and ,france4 ,in the upper atmosphere of a planet1 the lighter gases tend to diffuse1 rising above their heavier neighbors1 and unusual chemical reactions take place as the action of sunlight breaks some chemical bonds and stimulates the formation of others4 ,the study of these tenuous regions of planetary atmospheres is called aeronomy4 ,many of the exotic chemical processes that occur can best be studied by examining radiation of very short wavelength1 and it is to these problems that much of the work of the ,,uvs team is directed4 ,the observations are also sensitive to special processes in the ,jovian atmosphere resulting from the magnetosphere4 ,at the very top of the atmosphere1 energetic charged particles interacting with the atmospheric molecules produce ultraviolet aurorae that indicate the location and nature of the bombarding electrons and ions4 ,emissions from atoms in the extended gas clouds around ,io and possibly around other satellites were also expected to be observable4 ,the ultraviolet instrument is a relatively straightforward optical spectrometer1 adapted for use in space4 ,a diffraction grating disperses the light into a spectrum1 and the other optical elements are gold-coated mirrors4 ,the field of view is rectangular1 about #j4a degrees #j4i degrees4 ,an array of sensitive electronic detectors provides simultaneous measurements in #abh wavelength channels over a wavelength range from #ej to #agj nanometers4 ,photopolarimeter ,the fourth scan-platform instrument is designed to measure the brightness and polarization of light with high precision4 ,unlike the imaging system1 however1 it can look at only a single point 7one pixel7 at a time2 thus it sacrifices spatial capability in favor of precision of measurement4 ,the prime objectives of this photopolarimeter are related to study of the clouds of ,jupiter4 ,during the development stage for this instrument1 the ,principal ,investigator was ,charles ,f4 ,lillie of the ,laboratory for ,atmospheric and ,space ,physics of the ,university of ,colorado at ,boulder4 ,later1 the ,principal ,investigator's role was assumed by ,lillie's colleague1 ,charles ,w4 ,hord1 a physicist with considerable experience in space missions1 primarily with ultraviolet instruments4 ,four other scientists are ,co-,investigators on the photopolarimeter4 ,the instrument itself is essentially a small ,cassegrain reflecting telescope of #ae-;centimeter aperture4 ,two consecutive filter wheels select the color and polarization of the light1 which is then detected and measured by a photomultiplier tube4 ,the entire photopolarimeter weighs just #b4e kilograms4 ,in spite of their apparent simplicity1 however1 the ,voyager photopolarimeters were plagued with troubles almost from the moment of launch4 ,a succession of mechanical failures on ,voyager #a led to the sticking first of the polarization wheel and then of the filter wheel4 ,even when repeated commands from ,earth managed to free the wheels1 their behavior was erratic4 ,ultimately an electronic failure also took place1 leading to the reluctant decision to shut down the ,voyager #a photopolarimeter4 ,on ,voyager #b1 similar mechanical problems greatly restricted the ability of the instrument to carry out its observations4 ,the photopolarimeter thus was unable to contribute its share to unraveling the mysteries of ,jupiter and its satellites4 ,illustration3 ,charles ,w4 ,hord1 photopolarimeter ,principal ,investigator ,illustration3 ,james ,w4 ,warwick1 planetary radio astronomy ,principal ,investigator ,planetary ,radio ,astronomy ,the final ,voyager remote sensing instrument is designed to measure radio emission from ,jupiter and ,saturn over a wide range of frequencies4 ,these emissions1 which sound like hiss or static if played through an audio receiver1 result from interactions of charged particles in the magnetospheres and ionospheres of the giant planets4 ,the planetary radio astronomy 7,,pra7 ,principal ,investigator is ,james ,w4 ,warwick1 an astronomer in the ,department of ,astro-,geophysics of the ,university of ,colorado at ,boulder4 ,eleven colleagues from the ,united ,states and ,france participate as ,co-,investigators4 ,warwick has been studying ,jupiter longer than any other ,voyager ,principal ,investigator4 ,he has monitored its radio emissions since the #aifjs1 and he played a central role in the discovery that ,io influenced these emissions4 ,jupiter emits many kinds of radio radiation1 ranging from bland thermal emission at short 7centimeter7 wavelengths1 to synchrotron emission from energetic electrons at intermediate 7decimeter7 wavelengths1 to erratic1 extremely intense bursts at long 7meter and decameter7 wavelengths4 ,the origin of these latter1 nonthermal emissions constitutes one of the major unsolved problems of the ,jovian system1 and1 more generally1 of the physics of plasmas4 ,one of the most important advantages of the ,voyager ,,pra for studying ,jovian radio emission lies in its proximity to ,jupiter and hence its ability to locate the sources of different kinds of radio bursts4 ,the ,,pra instrument consists of an antenna and a radio receiver4 ,the antenna is made up of two thin metal poles1 each #aj meters long1 extended from the spacecraft after launch at an angle of #ij degrees to each other4 ,these are electrically connected to two receivers of extremely high sensitivity and broad frequency response3 from #a4b kilohertz to #dj4e megahertz4 ,the ,,pra can operate in a number of modes1 depending on the measurements desired4 ,at its lowest level of activity1 it monitors intensity in all #aih bands and transmits the data at #bff bits per second4 ,in its highest mode1 where searches are made for variations with very short time scales1 the data rate goes up to #ajh #jjj bits per second1 essentially the same as that required by imaging4 ,in fact1 the high-rate ,,pra data actually use an imaging frame as a display form1 and occasionally throughout the mission an unfamiliar looking 8image0 was transmitted that was actually a block of ,,pra data--a portrait of electrical events in the ,jovian atmosphere and magnetosphere that only ,warwick and his colleagues could interpret4 ,illustration3 ,norman ,f4 ,ness1 magnetometer ,principal ,investigator ,illustration3 ,voyager's #ac-meter-long magnetometer boom is shown fully extended4 ,in space1 under zero gravity conditions1 the triangular epoxy glass mast spirals from its housing and provides a rigid support for two magnetometer instruments--one at the end of the boom and another at about the midpoint4 #bfj-aha ,magnetometer ,the first of the direct sensing instruments to be discussed is the magnetometer1 designed to measure the magnetic fields surrounding the spacecraft4 ,such measurements can be interpreted to yield the intrinsic fields of ,jupiter and its satellites and to characterize1 in conjunction with data from particle and plasma instruments1 the processes taking place in the magnetosphere of the planet4 ,the ,principal ,investigator for this instrument is ,norman ,f4 ,ness of the ,,nasa ,goddard ,space ,flight ,center4 ,ness is an intense1 competitive scientist with a great deal of previous experience in spacecraft magnetometers1 primarily on board ,earth satellites4 ,ness is also the only ,voyager ,principal ,investigator who has previous experience at ,jupiter2 he was ,principal ,investigator on one of the two magnetometer instruments flown on ,pioneer #aa4 ,for the ,voyager investigation1 ,ness is joined by four colleagues from ,goddard and one from ,germany4 ,the magnetometer instrument consists of two systems3 a high-field magnetometer and a low-field magnetometer4 ,each system contains two identical three-axis magnetometers that measure the intensity and direction of the magnetic field4 ,the low-field system requires isolation from magnetic fields induced by electric circuits in the spacecraft itself4 ,to achieve this isolation1 it is mounted on the largest component of ,voyager--a #ac-meter boom1 about as long as the width of a typical city house lot4 ,this boom was coiled tightly in a canister during launch2 later1 when the package was opened1 it uncurled and extended automatically4 ,by using two magnetometers1 ,ness and his colleagues are able to correct for the residual artificial magnetic fields that reach even #ac meters from the main spacecraft4 ,the dynamic range extends from a maximum field of #bj gauss down to #b #aj gauss--a factor of one billion4 ,the fields can be measured as frequently as #ag times per second4 ,the total mass1 exclusive of the #ac-meter boom1 is #e4f kilograms4 ,illustration3 ,herbert ,s4 ,bridge1 plasma particle ,principal ,investigator ,plasma ,particles ,plasma is the term given to a 8gas0 of charged particles2 the electrons and protons are separate1 yet there are equal numbers of each1 producing a zero net charge4 ,if the velocities of the electrons and protons are less than about #j4a percent of the speed of light1 they can be measured by the ,voyager plasma instrument2 if their energies are higher1 they are measured by one of the other two particle instruments--the low energy charged particle 7,,lecp7 detector or the cosmic ray detector4 ,the plasma instrument1 like the magnetometer1 was designed to provide basic data on the particles and fields environment of ,voyager4 ,the ,principal ,investigator for the plasma instrument is ,herbert ,s4 ,bridge of the ,massachusetts ,institute of ,technology in ,cambridge1 ,massachusetts4 ,an experienced space physicist1 ,bridge has flown similar instruments on many ,earth satellites and planetary probes4 ,he also holds the position of ,director of the ,laboratory for ,space ,experiments at ,,mit4 ,on the ,voyager experiment1 he is joined by one ,german and ten ,u4,s4 ,co-,investigators4 ,the objectives of the plasma investigation are directed toward study of both the interplanetary medium and the ,jovian magnetosphere4 ,at ,jupiter1 ,bridge expected to determine the plasma populations and processes in the inner and outer regions of the magnetosphere and in the plasma tail that extends beyond ,jupiter1 much as a comet tail is blown outward by the solar wind4 ,the densities and temperature of electrons were measured1 and their origins determined3 ,some originate near ,jupiter and diffuse outward through the magnetosphere2 others derive from the solar wind4 ,the plasma instrument1 with a mass of #i4i kilograms1 was designed to view in two directions3 one toward the ,earth and ,sun1 primarily to study the solar wind1 and the other sideways1 looking toward the direction plasma would flow if it were caught up in the rotating ,jovian magnetic field4 ,if it is desired to look in other directions1 the entire spacecraft must be tipped1 a maneuver that was carried out several times near ,jupiter4 ,the detectors directly sense the flow of electrons1 protons1 and alpha particles 7helium nucleii1 made up of two protons and two neutrons each74 ,analysis of the energy spectra can also yield data on positive ions of higher mass4 ,illustration3 ,frederick ,l4 ,scarf1 plasma wave ,principal ,investigator ,plasma ,waves ,the plasma wave investigation on ,voyager was a late addition to the scientific payload4 ,it was selected to broaden the capability of the mission to study a wide variety of plasma processes4 ,because of the electrically charged nature of the plasma1 it responds to energy inputs in ways that ordinary gas cannot4 ,one of these modes of response yields plasma waves1 which are oscillations in density and electric field that generally cover the audio range of frequencies4 ,measurement of such waves characterizes the density and temperature of the local plasma surrounding the spacecraft1 and it also allows remote sensing detection of distant events from the plasma waves they produce4 ,the plasma wave ,principal ,investigator is physicist ,frederick ,l4 ,scarf of the ,,trw ,defense and ,space ,systems ,group of ,redondo ,beach1 ,california--he is the only ,voyager ,principal ,investigator to come from industry4 ,scarf has been associated with many particles and fields investigations in the terrestrial magnetosphere1 although he is better known as a theorist than as an experimenter4 ,a member of the ,space ,science ,board of the ,national ,academy of ,sciences1 he is familiar in ,washington as an eloquent advocate of space physics--the study of plasma-physical processes in the space environment4 ,the plasma wave instrument shares with the planetary radio astronomy investigation a pair of #aj-meter-long antennas4 ,whereas the ,,pra uses these as electric antennas to detect radio radiation1 the plasma wave system uses them to detect directly the oscillations in the plasma near the spacecraft4 ,waves are measured over a broad frequency range1 from #aj hertz 7a bit deeper than the lowest bass note we can hear7 to #ef kilohertz 7about three times higher than the highest pitch to which the human ear responds74 ,the instrument electronics have a total mass of only #a4d kilograms4 ,low ,energy ,charged ,particles ,charged particles with energies greater than a few thousand electron volts are not easily measured by a plasma instrument such as that designed by ,herb ,bridge4 ,instead1 these faster moving particles1 with speeds up to a few percent the speed of light1 are the subject of a pair of ,voyager instruments called collectively the ,,lecp1 or low energy charged particle instrument4 ,like the other particles and fields investigation1 the ,,lecp is designed to provide basic data on plasma-physical processes in the ,jovian magnetosphere and the solar wind1 and on their interactions4 ,the ,principal ,investigator for the ,,lecp investigation is ,stamatios ,mike ,krimigis1 a ,greek-born physicist from ,johns ,hopkins ,university4 ,krimigis has participated in a number of satellite studies of the terrestrial magnetosphere1 and he now serves as ,head of ,space ,physics and ,instrumentation at the ,johns ,hopkins ,applied ,physics ,laboratory4 ,he is joined in this investigation by one ,german and five ,u4,s4 ,co-,investigators4 ,the ,,lecp instrument consists of two subsystems4 ,the first1 called the low energy magnetospheric particle analyzer1 is optimized for measurement of particles within the ,jovian magnetosphere1 with high sensitivity over a broad dynamic range4 ,measurements of electrons1 protons1 and other positive ions can be carried out1 determining the energy and composition of individual particles4 ,the total energy ranges covered are #aj kiloelectron volts 7ke,v7 to #aa million electron volts 7,me,v7 for electrons and #ae ke,v to #aej ,me,v for protons and ions4 ,illustration3 ,stamatios ,mike ,krimigis1 low energy charged particle ,principal ,investigator ,the second ,,lecp subsystem is a low energy charged particle telescope1 designed to operate where the density of charged particles is low1 such as in interplanetary space or the outer magnetosphere of ,jupiter4 ,for protons and positive ions1 the energy range is from #ej ke,v to #dj ,me,v per nucleon4 ,the energy and species resolution is again sufficient to determine the composition1 both chemical and isotopic1 of many ions encountered4 ,in order to provide directional discrimination even on a spacecraft of fixed orientation1 both ,,lecp subsystems are mounted on a moving platform that steps through eight positions in a time that can be commanded to vary from #dh seconds to #dh minutes4 ,the mass of the instrument and its platform is #f4g kilograms4 ,cosmic ,rays ,the solar system is constantly bombarded by extremely energetic charged particles4 ,these are called cosmic rays1 although they are particles1 not photons--8rays0 are only produced when the particles strike something1 such as the molecules of the ,earth's atmosphere1 and give up their energy in a flash of x-rays and gamma-rays4 ,one of the ,voyager instruments is designed to study these galactic cosmic rays1 particularly to look from beyond the orbit of ,saturn1 where the cosmic ray particles will be less affected by the solar magnetic field and solar wind than they are near ,earth4 ,the cosmic ray ,principal ,investigator is ,rochus ,e4 ,vogt of the ,california ,institute of ,technology4 ,vogt has measured cosmic rays from the ground1 from balloons1 and from spacecraft for many years4 ,during #aigg and #aigh he served as ,chief ,scientist at ,,jpl1 and then assumed the job of directing the physics1 mathematics1 and astronomy programs at ,caltech4 ,among his six ,co-,investigators is ,ed ,stone1 the ,voyager ,project ,scientist4 ,because the cosmic ray instrument was not directed principally toward measurements of the ,jovian system1 it is described only briefly4 ,like the ,,lecp1 it is designed to determine the energy and composition of individual electrons and positive ions4 ,for electrons1 the energy range is from #c to #aaj ,me,v1 and for ions from #a to #ejj ,me,v per nucleon2 the corresponding velocities are from about #aj percent to #ii percent of the speed of light4 ,for the positive ions1 composition can be determined for elements from hydrogen to iron4 ,at ,jupiter1 this system could be used to determine the nature of the rare particles accelerated to very high energies in the ,jovian magnetosphere4 ,radio ,science ,the final ,voyager science investigation is in the field of radio science4 ,no special instrument was required for this study2 rather1 ,,nasa selected members of a ,radio ,science ,team who proposed investigations that could be carried out using the already existing spacecraft telecommunication system4 ,the radio science ,team ,leader is ,von ,r4 ,eshleman of the ,center for ,radio ,astronomy at ,stanford ,university4 ,eshleman is a radar physicist who has been interpreting spacecraft radio occultation data since the first such probe was carried out when ,mariner #d passed behind ,mars in #aifd4 ,the ,deputy ,team ,leader is ,g4 ,leonard ,tyler1 a colleague of ,eshleman's at ,stanford4 ,there are five other radio team members1 four of them from ,,jpl4 ,the radio science investigations are divided into two groups4 ,the first deals with the atmosphere of ,jupiter4 ,during the ,voyager flybys1 the spacecraft passed behind the planet as seen from ,earth1 and the radio signal was dimmed by the atmosphere before it was finally extinguished4 ,during an occultation1 the propagation of the radio waves is slowed down by passage through the neutral atmosphere and is speeded up by passage through the electrically charged ionosphere4 ,because of the extreme stability of the ground-based and spacecraft radio transmitters1 it is possible to measure these shifts in the signal with high precision4 ,the shifts are proportional to electron density for the ionosphere1 and to gas density for the atmosphere4 ,from a careful study of the interactions of the transmitted beam with the ,jovian atmosphere1 ,eshleman and his colleagues can reconstruct a temperature-pressure profile of the ionosphere and the upper atmosphere of ,jupiter4 ,the same approach can be used to search for tenuous atmospheres on the satellites4 ,the second area of study is in the field of celestial mechanics4 ,the frequency stability of the communications system permits measurements of the speed of the spacecraft1 relative to ,earth1 to a precision of one part in several million4 ,by careful tracking1 gravitational perturbations on the spacecraft can be detected and used to measure the gravitational fields1 and hence the masses1 of ,jupiter and its satellites4 ,illustration3 ,rochus ,e4 ,vogt1 cosmic ray ,principal ,investigator ,illustration3 ,von ,r4 ,eshleman1 radio science ,team ,leader ,these scientific instruments and their objectives were selected many years before the first ,jupiter encounter in ,march #aigg4 ,because ,voyager was an exploratory mission1 every effort was made to fly versatile instruments that could yield valuable results no matter what the nature of the ,jovian system4 ,in addition1 the ,voyager spacecraft control system permitted the instruments to receive commands from ,earth to adjust their sensitivities and observing sequences in response to new information4 ,by the spring of #aigg1 all the instruments were completed1 ready to be installed in the ,voyager spacecraft for testing and launch4 ,illustration3 ,the first picture to capture crescent ,earth and crescent ,moon in the same frame was taken by ,voyager #a1 the second-launched spacecraft1 on ,september #ah1 #aigg1 at a distance of #ab million kilometers from ,earth4 ,on the ,earth eastern ,asia1 the western ,pacific1 and part of the ,arctic can be seen4 ,since the ,moon is much less reflective than the ,earth1 ,,jpl image processors brightened the lunar image by a factor of three to ensure that both ,earth and ,moon were visible on this print4 ,p-#aihia;,c ,,chapter #e ,,the ,,voyage ,,to ,,jupiter--,,getting ,,there ,launch ,on ,august #bj1 #aigg1 exactly two years after the launch of the ,viking spacecraft to ,mars1 the first of the ,voyagers--actually ,voyager #b--was boosted into space at #aj3bi a4m4 ,,edt1 less than five minutes after the launch window opened on the first day of the thirty-day launch period4 ,sixteen days later1 at #h3ef a4m4 ,,edt on ,labor ,day1 ,september #e1 #aigg1 ,voyager #a was hurled into space on a shorter1 faster trajectory than its twin1 zipping past the orbit of the ,moon only ten hours after launch4 ,ultimately1 ,voyager #a earned its title by overtaking ,voyager #b as both spacecraft journeyed through the asteroid belt1 to arrive at ,jupiter four months ahead of ,voyager #b4 ,the ,voyagers lifted off from ,launch ,complex #da1 ,air ,force ,eastern ,test ,range1 ,kennedy ,space ,center1 ,cape ,canaveral1 ,florida1 atop the giant ,titan ,,iii-,e_/,centaur rocket4 ,it was the last time such a launch vehicle was scheduled to be used1 as1 according to plan1 the ,space ,shuttle would take over in the #aihjs4 ,thus the launching of the two ,voyagers signified both an end and a beginning3 a once-in-a-lifetime opportunity to explore1 in only #h years' time1 perhaps fifteen major bodies of the outer solar system4 ,but long before even the first ,voyager was to make its closest approach to ,jupiter--in fact1 even before ,voyager #b was off the launch pad--there were problems to overcome4 ,in early ,august #aigg1 about three weeks before launch1 failures in the attitude and articulation control subsystem 7,,aacs7 and the flight data subsystem 7,,fds71 two of the spacecraft's three main computer subsystems1 prevented the ,,vgr#gg-b spacecraft1 originally scheduled for launch on ,august #bj1 from becoming ,voyager #b4 ,instead1 the 8spare0 spacecraft ,,vgr#gg-c was substituted1 becoming ,voyager #b upon launch ,august #bj1 and ,,vgr#gg-b1 after proper repairs1 became ,voyager #a4 ,minor problems continued right up to launch4 ,the low energy charged particle instrument failed and had to be replaced1 and as late as ,t minus five minutes there was a halt in the countdown to check on a stuck valve4 ,unlike a jinxed dress rehearsal1 which is said to 8assure0 an opening-night success1 ,voyager #b's prelaunch problems were a portent of difficulties to come4 ,the ,voyager #b launch was witnessed by thousands as the spacecraft ascended gracefully into the blue ,florida sky1 accompanied by the deep-throated rumblings of its rocket1 echoing for miles across the beaches and scrub forests of ,cape ,canaveral4 ,the ,titan-,centaur performance was nearly flawless1 and ,voyager #b quickly achieved an accurate trajectory toward ,jupiter4 ,however1 even while the engines were still firing1 the spacecraft began to experience a baffling series of problems that would absorb the attention of hundreds of persons from ,pasadena to ,washington1 ,d4,c41 for the next several weeks until they were brought under control4 ,during the first minutes of flight1 there seemed to be two difficulties with the ,,aacs4 ,the first was a problem with one of the three stabilizing gyroscopes1 but fortunately1 the gyroscope began operating normally without intervention from the ground4 ,the other problem appeared to be with one of the ,,aacs computers2 the spacecraft switched to a backup computer during the ,titan burn1 and initial data transmissions were incomplete4 ,early analysis seemed to indicate that an event during the launch itself1 rather than a faulty spacecraft computer system1 was the cause of the data loss4 ,at first1 on ,august #bc1 officials suspected that perhaps the spacecraft had been bumped by the rocket motor one hour after liftoff and again about seventeen hours later1 when telemetry signals indicated that the spacecraft had been jolted4 ,however1 by the next day1 flight engineers determined that electronic gyrations in the ,,aacs seemed to have caused the difficulty4 ,illustration3 ,the ,titan-,centaur rocket used to launch ,voyager stood as tall as a #ae-story building and weighed nearly #gjj tons4 ,here the rocket waits for launch at ,kennedy ,space ,center1 with the ,voyager spacecraft enclosed in the white protective shroud at the top4 ,p-#aidga;,a ,within an hour after launch1 ,voyager #b's science scan platform boom was to have been fully extended and locked4 ,instructions to deploy were given1 and the boom moved outward2 however1 there was no signal to indicate that the boom was actually locked in place4 ,efforts to command the boom to move into the locked position were thwarted by the spacecraft4 ,the first maneuver designed to try to lock the boom was aborted by the computer command subsystem 7,,ccs7 when the ,,aacs erroneously indicated that it was in trouble4 ,three days later another maneuver was scheduled to reprogram the faulty computer in the ,,aacs1 to align the ,sun sensors1 and to try to lock the science boom4 ,to provide a direct check of the boom position1 the scan platform was turned so that the ,,tv cameras could see the spacecraft4 ,careful measurement of these pictures verified that the boom was within degree of full deployment1 but still there was no indication that it was locked into place4 ,ultimately1 it was decided that the sensor to signal actuation of the lock was at fault1 and that the boom itself was almost certainly fully extended and operational4 ,because of the postlaunch problems of ,voyager #b1 the launch of ,voyager #a was delayed twice--from ,september #a to ,september #c and then to ,september #e--;in order to inspect ,voyager #a's science boom and to try to prevent a repetition of ,voyager #b's problems4 ,an extra spring was attached to the science boom to assure its full extension4 ,finally1 as if to make up for the troubles of the first launch1 ,voyager #a's launch was both 8flawless and accurate40 ,all launch and postlaunch events went smoothly4 ,the launch window opened at #h3ef a4m4 ,,edt1 and ,voyager took off promptly at #h3ef3ja4 ,the booms and antennas deployed and locked in the first hours after launch2 all instruments scheduled to be on were on and working well4 ,,the ,,titan_/,,centaur ,,launch ,,vehicle ,the two ,voyager spacecraft were carried into space and accelerated toward ,jupiter by the ,titan ,,iii-,e ,centaur rocket1 the largest launch vehicle in the ,,nasa arsenal after the retirement of the ,saturn rockets in #aige4 ,the ,titan and ,centaur vehicles were originally developed separately and have been used with other rocket stages for many ,,nasa launches4 ,they were first combined for the two ,viking launches to ,mars in #aige1 and this powerful four-stage launch vehicle was used again in #aigg for ,voyager4 ,the ,titan_/,centaur stands nearly #ej meters tall1 about the height of a fifteen-story building4 ,fully fueled1 it weighs nearly #gjj tons4 ,at takeoff1 the thrust of the two solid-propellant ,stage-#j motors is about #aj4g million newtons4 ,these motors1 which burn for #abb seconds1 use powdered aluminum as fuel and ammonium perchlorate as oxidizer4 ,together1 they have a mass of #ejj tons4 ,the first stage of the liquid propellant core of the ,titan rocket ignites about #aab seconds after takeoff4 ,the propellant is hydrazine as fuel and nitrogen tetroxide as oxidizer4 ,the first stage is #c meters in diameter and #bj meters tall4 ,fueled1 it has a mass of #acj tons4 ,the motor provides a thrust of #b4e million newtons for a duration of #adf seconds4 ,about #d4c minutes after takeoff the ,titan ,stage ,,ii liquid propellant motor begins to fire1 and the first stage is separated and falls back into the ,atlantic4 ,the second stage is #c meters in diameter and more than #g meters long1 with a fueled mass of #ce tons4 ,the single liquid fuel motor burns for #baj seconds with a thrust of half a million newtons4 ,during the second stage burn1 the shroud covering the ,voyager spacecraft is jettisoned4 ,the ,centaur and ,titan vehicles separate #h minutes into the flight1 and the ,centaur main engine begins its burn4 ,the ,centaur is nearly #bj meters tall and #c meters in diameter1 with a mass of #ag tons4 ,the motors have a thrust of almost #bjj #jjj newtons1 operating on the most powerful chemical fuels known3 liquid oxygen and liquid hydrogen4 ,the ,centaur burns for only #a minute and #cf seconds as it attains ,earth parking orbit2 the engine then shuts down as the vehicle begins a half-hour coasting period that carries it nearly half way around the ,earth4 ,during this time1 careful tracking of the spacecraft supplies the data needed for ,earth-based computers to calculate the proper time to leave parking orbit and start the long trip toward ,jupiter4 ,about #ej minutes after liftoff1 from a position high above the ,indian ,ocean1 the second burn of the ,centaur main engine begins4 ,six minutes of additional thrust provides enough energy to break out of ,earth's orbit4 ,the ,voyager then separates from the ,centaur for a final boost toward ,jupiter4 ,the solid rocket motor in the spacecraft propulsion module 7acting as final stage of this five-stage launch sequence7 fires for #de seconds at a thrust of #fh #jjj newtons4 ,just an hour after liftoff1 the ,voyager spacecraft is on its way1 coasting on an orbit toward ,jupiter at a speed of more than #aj kilometers per second4 ,illustration3 ,the ,voyager spacecraft is dominated by the large #c4g-meter-diameter antenna used for communication with ,earth4 ,here the spacecraft undergoes final tests before launch4 ,the science instrument scan platform is folded against the spacecraft on the right2 the three cylinders on the left are the ,,rtg power sources4 #bfj-ajh;,,bc ,illustration3 ,voyager #b was the first of the spacecraft to be launched1 on ,august #bj1 #aigg1 propelled into space in a ,titan_/,centaur rocket4 ,p-#aidej;,,ac ,the ,first ,year ,is the ,roughest ,during the autumn of #aigg ,voyager #b1 and to a lesser extent ,voyager #a1 continued to plague controllers with erratic actions4 ,thrusters fired at inappropriate times1 data modes shifted1 instrument filter and analyzer wheels became stuck1 and the various computer control systems occasionally overrode ground commands4 ,apparently1 the spacecraft hardware was working properly1 but the computers on board displayed certain traits that seemed almost humanly perverse--and perhaps a little psychotic4 ,in general1 these reactions were the result of programming too much sensitivity into the spacecraft systems1 resulting in panic over-reaction by the onboard computers to minor fluctuations in the environment4 ,ultimately1 part of the programming had to be rewritten on ,earth and then transmitted to the ,voyagers1 to calm them down so that they would ignore minor perturbations1 yet still be ready to perform automatic sequences required to protect the spacecraft from major threats4 ,meanwhile1 however1 more serious problems were developing4 ,on ,february #bc1 #aigh1 during a series of movements or slews1 ,voyager #a's scan platform slowed and stopped before completing the maneuver4 ,this failure caused a great deal of concern1 since the scan platform houses the optical instruments that are crucial to the observation of the ,jovian system--the ultraviolet spectrometer1 the ,,iris1 the photopolarimeter1 and the two ,,tv cameras4 ,at ,,jpl1 tests were run on a proof-test model--an exact copy of the ,voyager spacecraft--to try to find out why ,voyager #a's scan platform had become stuck4 ,on ,march #ag1 ,voyager #a's scan platform was tested--,,jpl engineers instructed the platform to move slowly for a short distance1 and ,voyager responded as ordered4 ,further tests were conducted on ,march #bc4 ,this time the scan platform was ordered to execute a sequence of four slews1 moving away from the part of the sky where the original failure had occurred and ending with the position that it would be most useful to leave the platform in--just in case the platform should become stuck again4 ,on ,april #d the scan platform was commanded to perform a sequence of #ch slews1 and fifty more slews were performed on ,april #e4 ,all were successful4 ,yet engineers were still hesitant to force the platform to move through the region where it had originally stuck1 and extensive discussions were held to determine if the ,jupiter observations could be carried out without risking a return to the danger area4 ,it was argued1 however1 that full mobility of the scan platform really was required1 and on ,may #ca commands were sent to maneuver the scan platform through the danger region4 ,it moved normally3 ,the scan platform was operating properly again4 ,after additional slewing tests were run in mid-,june1 the scan platform was pronounced fit for operation4 ,engineers suspected that the material caught in the platform gears must have been crushed or moved out of the way by the continued slewing1 allowing the platform to move once more4 ,illustration3 ,voyager #a was launched on ,september #e1 #aigg4 ,the launch was delayed #e days to make last-minute adjustments to avoid the postlaunch difficulties experienced by ,voyager #b4 ,p-#aidhj;,,ac ,an even more serious crisis soon endangered the ,voyager #b spacecraft4 ,in late ,november #aigg1 the ,s-band radio receiver began losing amplifier power in its high-gain mode1 so the solid-state amplifier was switched to its low-power position4 ,no further problems were noted until ,april #e1 #aigh1 when ,voyager #b's primary radio receiver suddenly failed1 and shocked engineers discovered that the backup receiver was also faulty4 ,the trouble was detected after ,voyager's computer command subsystem directed the spacecraft to switch from the primary radio receiver to the backup receiver4 ,this command was issued as part of a special protection sequence3 ,if the primary radio receiver receives no commands from ,earth for seven days1 the backup receiver is switched on instead2 if the secondary receiver in turn receives no instructions over a twelve-hour period1 the system reverts to the main receiver4 ,when1 on ,april #e1 ,voyager #b's radio reception was switched from the primary to the secondary receiver1 flight engineers found that they were unable to communicate with the spacecraft--the secondary receiver's tracking loop capacitor was malfunctioning4 ,that meant that the secondary receiver could not follow a changing signal frequency sent out from ,earth4 ,the frequencies of signals transmitted from ,earth are affected by the ,doppler effect--just as the siren on a fire engine seems first to rise in pitch as the truck approaches1 then falls as the truck speeds away1 so the frequency of signals transmitted from ,earth fluctuates with the ,earth's rotation as the ,deep ,space ,network's radio antennas move toward or away from the spacecraft4 ,the engineers had to wait until the primary radio receiver was switched back on before they could communicate with the spacecraft4 ,once the primary receiver was on1 ,voyager #b began receiving instructions from ,earth1 but approximately thirty minutes later1 there was an apparent power surge in the receiver4 ,the fuses blew4 ,there was no recourse4 ,the main receiver had failed2 its loss was permanent4 ,it remained for the engineers to devise a way to communicate with the slightly deaf spacecraft4 ,illustration3 ,each ,voyager spacecraft follows a billion-kilometer path to ,jupiter4 ,except for minor thruster firings to achieve small trajectory corrections1 each ,voyager coasts from ,earth to ,jupiter1 guided by the gravitational pull of the ,sun4 ,at ,jupiter1 the powerful tug of the giant planet deflects the spacecraft and speeds them up1 imparting an extra kick to send them on their way toward ,saturn4 ,voyager #a ,voyager #b ,jupiter-,saturn-,uranus ,sun ,earth #h_/bj_/gg ,earth #i_/a_/gg ,mars #h_/bj_/gg ,jupiter #h_/bj_/gg ,jupiter #c_/e_/gi ,jupiter #g_/i_/gi ,saturn #h_/bj_/gg ,saturn #aa_/ac_/hj ,saturn #h_/bg_/ha ,uranus #h_/bj_/gg ,uranus #a_/cj_/hf ,because the switching of the radio receivers was still controlled by the special protection sequence discussed earlier1 flight engineers would have to wait for seven days--until ,april #ac--;before they could attempt communication with the spacecraft again4 ,during that week special procedures were established and rehearsed so that commands could be sent to ,voyager in the short time that the backup receiver would be on4 ,on ,thursday1 ,april #ac1 #aigh1 the seven days were up and the spacecraft should have shifted from the dead main receiver to the sick backup system4 ,there was just a twelve-hour 8window0 in which to restore communication4 ,at about #c3cj a4m4 ,,pst the ,madrid tracking station of the ,deep ,space ,network sent its first order to the spacecraft1 approximately #dgd million kilometers away4 ,almost an hour later1 word arrived from ,voyager that the command had been accepted4 7,one-way light time for a signal to travel the distance from ,earth to ,voyager at that time was almost #bg minutes47 ,elated flight controllers went ahead and transmitted nine hours of commands to the spacecraft4 ,voyager #b was successfully commanded again on ,april #ah and ,april #bf4 ,the ,april #bf commands included a course change maneuver that was executed properly on ,may #c4 ,on ,june #bc1 ,voyager #b was programmed for a backup automatic mission at ,saturn in the event that the secondary radio receiver should also fail4 ,these backup mission instructions would operate all the science experiments1 but only a minimum amount of data would be returned1 since the scan platform would only be programmed to move through three positions rather than thousands as it would in normal operation4 ,instructions for a backup minimum automatic encounter at ,jupiter were transmitted to ,voyager #b in two segments1 the second of these on ,october #ab1 #aigh4 ,with the backup instructions recorded on board the spacecraft1 ,voyager personnel felt their fears partially allayed4 ,if ,voyager #b's secondary radio receiver failed1 the spacecraft would still obtain some science data at ,jupiter and ,saturn4 ,but that would mean that there would be no mission beyond ,saturn2 our first opportunity to explore ,uranus1 its satellites1 its newly discovered ring system1 and possibly even to get a look at ,neptune1 would not come in this century4 ,another major concern affecting both ,voyager spacecraft was the proper management of hydrazine fuel reserves4 ,hydrazine is used by the thrusters on the ,voyagers for stabilization of the spacecraft and for trajectory correction maneuvers 7,,tcm74 ,each ,voyager was loaded with #aje kilograms of hydrazine budgeted for use on the long flight to ,jupiter1 ,saturn1 and beyond4 ,because of the excellent performance of the launch rockets1 both ,voyagers required less hydrazine than anticipated for their final boost into proper trajectory toward ,jupiter1 and at first it looked as though both spacecraft would have plenty of propellant to spare4 ,charles ,e4 ,kolhase1 ,manager of ,mission ,analysis and ,engineering for the ,voyager ,project1 later explained the situation3 8,voyager #a should have been launched ,september #a4 ,had it been launched on ,september #a--;and ,i'm glad it wasn't--the maneuver to correct the trajectory for a ,titan flyby would have required a change in velocity of #ajj-aaj meters per second--an enormous maneuver--and we would have had a propellant margin for going on to ,saturn of perhaps #d4e kilograms4 ,but1 by launching on the fifth of ,september we increased our margin to #bc kilograms4 ,fortunately1 for every launch date that went by1 that velocity change maneuver was shrinking at a rate of #aj meters per second per day4 ,now1 a #a meter per second change uses about a pound of hydrazine about #j4e kilogram4 ,so when we launched on the fifth of ,september1 now we suddenly had #dj pounds of hydrazine excess over what we would have had if we had launched on the first of ,september4 ,as a result1 ,voyager #a is in great shape as far as hydrazine is concerned40 ,,the ,,deep ,,space ,,network ,a vital component of the ,voyager ,mission is the communications system linking the spacecraft with controllers and scientists on ,earth4 ,the ability to communicate with spacecraft over the vast distances to the outer planets1 and particularly to return the enormous amounts of data collected by sophisticated cameras and spectrometers1 depends in large part on the transmitters and receivers of the ,deep ,space ,network 7,,dsn71 operated for ,,nasa by ,,jpl4 ,the original network of these receiving stations was established in #aieh to provide round-the-world tracking of the first ,u4,s4 satellite1 ,explorer #a4 ,by the late #aigjs1 the ,,dsn had evolved into a system of large antennas1 low-noise receivers1 and high-power transmitters at sites strategically located on three continents4 ,from these sites the data are forwarded 7often using terrestrial communications satellites7 to the mission operations center at ,,jpl4 ,the three ,,dsn stations are located in the ,mohave desert at ,goldstone1 ,california2 near ,madrid1 ,spain2 and near ,canberra1 ,australia4 ,each location is equipped with two #bf-meter steerable antennas and a single giant steerable dish #fd meters in diameter1 with approximately the collecting area of a football field4 ,in addition1 each is equipped with transmitting1 receiving1 and data handling equipment4 ,the transmitters in ,spain and ,australia have #ajj-kilowatt power1 while the #fd-meter antenna at ,goldstone has a #djj-kilowatt transmitter4 ,most commands to ,voyager are sent from ,goldstone1 but all three stations require the highest quality receivers to permit continuous recording of the data streams pouring in from the spacecraft4 ,since the mid-#aifjs1 the ,,dsn's standard frequency has been ,s-band 7#bbie megahertz74 ,voyager introduces a new1 higher frequency telemetry link at ,x-band 7#hdah megahertz74 ,the ,x-band signal can carry more information than ,s-band with similar power transmitters1 but it requires more exact antenna performance4 ,in addition1 the ,x-band signal is absorbed by terrestrial clouds and1 especially1 rain4 ,fortunately1 all three ,,dsn stations are in dry climates1 but during encounters the weather forecasts on ,earth become items of crucial concern if precious data are not to be lost by storm interference4 ,as a result of the development of larger antennas and improved electronics1 the ,,dsn command capabilities and telemetry data rates have increased dramatically over the years4 ,for example1 in #aife ,mariner #d transmitted from ,mars at a rate of only #h bits of information per second4 ,in #aifi1 ,mariners #f and #g transmitted picture data from ,mars at #af #bjj bits per second4 ,mariner #aj1 in #aigc1 achieved #aag #bjj bits per second from ,mercury4 ,voyager operates at a similar rate from ,jupiter1 about six times farther away4 ,many of these improvements in data transmission result from changes in the ,,dsn rather than in the spacecraft transmitters4 ,problems with hydrazine management developed1 however4 ,voyager #a's first trajectory correction maneuver achieved only #hj percent of the required speed change4 ,exhaust plumes from the thrusters apparently struck part of the spacecraft1 causing a #bj percent loss in velocity4 ,that being the case1 ,voyager might require more fuel than had been expected to complete the mission4 ,the extra fuel requirements did not threaten ,voyager #a itself1 since it held ample fuel to reach ,saturn2 the concern was for ,voyager #b1 where the effective loss of fuel might be enough to jeopardize the ,uranus mission4 ,illustration3 ,project members4 ,illustration3 ,project members4 ,because of the plume impingement problem on ,voyager #a1 ,voyager #b's first trajectory correction maneuver was adjusted to allow for the possibility of a #bj percent loss in thrust4 ,the ,voyager #b maneuver was successful1 but controllers felt that additional action was required to conserve fuel4 ,one way to save was by reducing requirements on control of the spacecraft orientation4 ,less control fuel would be needed if the already miniscule pressure exerted on the spacecraft by the solar wind could be reduced4 ,flight engineers at ,,jpl calculated that the pressure would be reduced if the spacecraft were tipped upside down2 however1 to accomplish this1 the spacecraft would have to be steered by a new set of guide stars4 ,by reprogramming the attitude control system it was found possible to substitute the northern star1 ,deneb1 in the constellation of ,cygnus1 for the original reference star1 ,canopus1 in the southern constellation of ,carina4 ,with this change1 as well as readjustment of ,voyager #b's trajectory near ,jupiter1 inflight consumption of hydrazine was reduced significantly4 ,in late ,august #aigh both ,voyagers were reprogrammed to ensure better science results at ,jupiter encounter2 for example1 the reprogramming would prevent imaging 7,,tv7 photographs from blurring when the tape recorder was operating4 ,by early ,november1 flight crews had begun training exercises to rehearse for the ,voyager #a flyby of ,jupiter on ,march #e1 #aigi4 ,a near encounter test was performed on ,december #ab-ad1 #aigh3 a complete runthrough of ,voyager #a's #ci-;hour near encounter period1 which would take place ,march #c-e1 #aigi4 ,participants included the flight team1 the ,deep ,space ,network tracking stations1 the scientists1 and the spacecraft itself4 ,results3 ,voyager and the ,voyager team were all ready for the encounter4 ,meanwhile1 the spacecraft were busy returning scientific data to ,earth4 ,technically1 the ,voyagers were in the cruise phase of the mission--a period that1 for ,voyager #b1 would last until ,april #bd1 #aigi1 and for ,voyager #a1 until ,january #d1 #aigi1 when each spacecraft would enter its respective observatory phase4 ,cruise ,phase ,science ,in the first few days after launch1 the spacecrafts' instruments were turned on and calibrated2 various tests for each instrument would continue to be performed throughout the cruise phase4 ,this period presented a great opportunity for the ,voyagers to study the interplanetary magnetic fields1 solar flares1 and the solar wind4 ,in addition1 ultraviolet and infrared radiation studies of the sky were performed4 ,in mid-,september #aigg the television cameras on ,voyager #a recorded a number of photographs of the ,earth and ,moon4 ,a photograph taken ,september #ah captured both crescent ,moon and crescent ,earth4 ,it was the first time the two celestial bodies had ever been photographed together4 ,in ,november both ,voyagers crossed the orbit of ,mars1 entering the asteroid belt a month later4 ,on ,december #ae1 at a distance of about #agj million kilometers from ,earth1 ,voyager #a finally speeded past its slower twin4 ,the journey through the asteroid belt was long but uneventful3 ,voyager #a emerged safely in ,september #aigh1 and ,voyager #b in ,october4 ,unlike ,pioneers #aj and #aa1 the ,voyagers carried no instruments to look at debris in the asteroid belt4 ,by ,april both spacecraft were already halfway to ,jupiter and1 about two months later1 ,voyager #a1 still approximately #bfe million kilometers from ,jupiter1 began returning photographs of the planet that showed considerable detail1 although less than could be obtained with telescopes on ,earth4 ,both the imaging 7,,tv7 and the planetary radio astronomy instruments began observing ,jupiter1 and1 by ,october #b1 #aigh1 officials announced that 8the polarization characteristics of ,jupiter's radio emissions have been defined4 ,in the high frequencies1 there is consistent right-hand circular polarization1 while in the low frequencies1 there is a consistent left-hand circular polarization4 ,this was an unexpected result40 ,in addition to scientific studies of the interplanetary medium and a first look at ,jupiter1 the plasma wave instrument1 which studies waves of charged particles over a range of frequencies that includes audio frequencies1 was able to record the sound of the spacecraft thrusters firing1 as hydrazine fuel is decomposed and ejected into space4 ,the sound was described as being 8somewhat like a #e-;gallon can being hit with a leather-wrapped mallet40 ,on ,december #aj1 #aigh1 #hc million kilometers from ,jupiter1 ,voyager #a took photographs that surpassed the best photographs ever taken from ground-based telescopes1 and scientists were anxiously awaiting the start of continuous coverage of the rapidly changing cloud forms4 ,these pictures1 together with data from several ground-based observatories1 were carefully scrutinized by a team of scientists at ,,jpl to select the final targets in the atmosphere of ,jupiter to be studied at high resolution during the flyby4 ,the observatory phase of ,voyager #a's journey to ,jupiter was about to begin4 ,illustration3 ,as ,voyager #a approached ,jupiter1 the resolution of the images steadily improved4 ,in ,october #aigh1 at a distance of about #abe million kilometers1 the image was less clear than would be obtained with an ,earth-based telescope4 ,p-#bjgij ,illustration3 ,by ,december #aj1 the spacecraft had moved to a distance of #he million kilometers1 and the resolution was about #bjjj kilometers1 comparable to the best telescopic images4 ,p-#bjhbi;,c ,illustration3 ,on ,january #i1 #aigi 7c71 at a distance of #ed million kilometers1 the image surpassed all ground-based views and approached the resolution of the ,pioneer #aj and #aa photos4 ,p-#bjibf;,c ,illustration3 ,in this1 taken ,january #bd at a distance of #dj million kilometers1 the resolution exceeded #ajjj kilometers4 ,p-#bjide;,c ,the ,observatory ,phase ,the observatory phase1 originally scheduled to start on ,december #ae1 #aigh1 eighty days before encounter1 was postponed until ,january #d1 #aigi1 to provide the flight team a holiday-season break4 ,for the next two months1 ,voyager would carry out a long-term scientific study--a 8time history0--of ,jupiter1 its satellites1 and its magnetosphere4 ,on ,january #f1 ,voyager #a began photographing ,jupiter every two hours--each time taking a series of four photographs through different color filters as part of a long-duration study of large-scale atmospheric processes1 so scientists could study the changing cloud patterns on ,jupiter4 ,even the first of these pictures showed that the atmosphere was dynamic 8with more convective structure than had previously been thought40 ,particularly striking were the changes in the planet1 especially near the ,great ,red ,spot1 that had taken place since the ,pioneer flybys in #aigc and #aigd4 ,jupiter was wearing a new face for ,voyager4 ,in mid-,january1 photos of ,jupiter were already being praised for 8showing exceptional details of the planet's multicolored bands of clouds40 ,still #dg million kilometers from the giant1 ,voyager #a had by now taken more than #ejj photographs of ,jupiter4 ,movements of cloud patterns were becoming more obvious2 feathery structures seemed painted across some of the bands that encircle the planet2 swirling features were huddled near the ,great ,red ,spot4 ,the satellites were also beginning to look more like worlds1 with a few bright spots visible on ,ganymede and dark red poles and a bright equatorial region clearly seen on ,io when it passed once each orbit across the turbulent face of ,jupiter4 ,from ,january #cj to ,february #c1 ,voyager sent back one photograph of ,jupiter every #if seconds over a #ajj-;hour period4 ,using a total of three different color filters1 the spacecraft thus produced one color picture every #d minutes1 in order to make a 8movie0 covering ten ,jovian 8days40 ,to receive these pictures1 sent back over the high-rate ,x-band transmitter1 the ,deep ,space ,network's #fd-meter antennas provided round-the-clock coverage4 ,voyager #a was ready for its far encounter phase4 ,illustration3 ,the ,galilean satellites of ,jupiter first began to show as tiny worlds1 not mere points of light1 as the ,voyager #a observatory phase began4 ,in this view taken ,january #ag1 #aigi1 at a range of #dg million kilometers1 the differing sizes and surface reflectivities 7albedos7 of ,ganymede 7right center7 and ,europa 7top right7 are clearly visible4 ,the view of ,jupiter is unusual in that the ,great ,red ,spot is not easily visible1 but can just be seen at the right edge of the planet4 ,most pictures selected for publication include the photogenic ,red ,spot4 ,p-#bjich;,c ,far ,encounter ,phase ,by early ,february ,jupiter loomed too large in the narrow-angle camera to be photographed in one piece2 #b #b three-color 7violet1 orange1 green7 sets of pictures were taken for the next two weeks2 by ,february #ba1 ,jupiter had grown too large even for that1 and #c #c sets were scheduled to begin4 ,when these sets of pictures are pasted together to form a single picture1 the result is called a mosaic4 ,illustration3 ,by ,february #a1 #aigi1 ,voyager #a was only #cj million kilometers from ,jupiter1 and the resolution of the imaging system corresponded to about #fjj kilometers at the cloud tops of the giant planet4 ,at this time1 a great deal of unexpected complexity became apparent around the ,red ,spot1 and movie sequences clearly showed the cloud motions1 including the apparent six-day rotation of the ,red ,spot4 ,p-#bjiic;,c ,illustration3 ,one of the most spectacular planetary photographs ever taken was obtained on ,february #ac as ,voyager #a continued its approach to ,jupiter4 ,by this time1 at a range of #bj million kilometers1 ,jupiter loomed too large to fit within a single narrow-angle imaging frame4 ,passing in front of the planet are the inner two ,galilean satellites4 ,io1 on the left1 already shows brightly colored patterns on its surface1 while ,europa1 on the right1 is a bland ice-covered world4 ,the scale of all of these objects is huge by terrestrial standards2 ,io and ,europa are each the size of our ,moon1 and the ,red ,spot is larger than the ,earth4 ,p-#bajhb;,c ,while the imaging experiments were in the limelight1 the other scientific instruments had also begun to concentrate on the ,jovian system4 ,the ultraviolet spectrometer had been scanning the region eight times a day2 the infrared spectrometer 7,,iris7 spent #a hours a day analyzing infrared emissions from various longitudes in ,jupiter's atmosphere2 the planetary radio astronomy and plasma wave instruments looked for radio bursts from ,jupiter and for plasma disturbances in the region2 the photopolarimeter had begun searching for the edge of ,io's sodium torus2 and a watch was begun for the bow shock--the outer boundary of the ,jovian magnetosphere4 ,on ,february #aj1 ,voyager #a crossed the orbit of ,sinope1 ,jupiter's outermost satellite4 ,yet the spacecraft even then had a long way to go--still #bc million kilometers from ,jupiter1 but closing in on the planet at nearly a million kilometers a day4 ,a week later1 targeted photographs of ,callisto began to provide coverage of the satellite all around its orbit2 similar photos of ,ganymede began on ,february #be4 ,meanwhile excitement was building4 ,as early as ,february #h and #i1 delight with the mission and anticipation of the results of the encounter in ,march were already evident4 ,garry ,e4 ,hunt1 from the ,laboratory for ,planetary ,atmospheres1 ,university ,college1 ,london1 a member of the ,imaging ,team1 discussing the appearance of ,jupiter's atmosphere as seen by ,voyager during the previous month1 said 8,it seems to be far more photogenic now than it did during the ,pioneer encounters2 ,i'm more than delighted by it--it's an incredible state of affairs4 ,there are infinitely more details than ever imagined40 ,illustration3 ,the ,voyager ,project was operated from the ,jet ,propulsion ,laboratory managed for ,,nasa by the ,california ,institute of ,technology4 ,located in the hills above ,pasadena1 ,california1 ,,jpl is the main center for ,u4,s4 exploration of the solar system4 ,,jb#agbdi;,,bc ,illustration3 ,the ,voyager ,,tv cameras do not take color pictures directly as do commercial cameras4 ,instead1 a color image is reconstructed on the ground from three separate monochromatic images1 obtained through color filters and transmitted separately to ,earth4 ,there are a number of possible filter combinations1 but the most nearly 8true0 color is obtained with originals photographed in blue 7#dhj nanometers71 green 7#efe nanometers71 and orange 7#eij nanometers7 light4 ,before these can be combined1 the individual frames must be registered1 correcting for any change in spacecraft position or pointing between exposures4 ,often1 only part of a scene is contained in all three original pictures4 ,shown here is a reconstruction of a plume on ,jupiter1 photographed on ,march #a1 #aigi4 ,the colors used to print the three separate frames can be seen clearly in the nonoverlapping areas4 ,for other pictures in this book1 the nonoverlapping partial frames are omitted4 ,p-#baaib ,the pictures from ,voyager are 8clearly spectacular10 said ,lonne ,lane1 ,assistant ,project ,scientist for ,jupiter4 8,we're getting even better results than we had anticipated4 ,we have seen new phenomena in both optical and radio emissions4 ,we have definitely seen things that are different--in at least in one case1 unanticipated--and are begging for answers we haven't got40 ,there was already1 still almost a month from encounter1 a strong feeling of accomplishment among the scientists and engineers2 they had done a difficult task and it has been successful4 ,by the last week of ,february #aigi1 the attention of thousands of individuals was focused on the activities at ,,jpl4 ,scientists had arrived from universities and laboratories around the country and from abroad1 many bringing graduate students or faculty colleagues to assist them4 ,engineers and technicians from ,,jpl contractors joined ,,nasa officials as the ,pasadena motels filled up4 ,special badges were issued and reserved parking areas set aside for the ,voyager influx4 ,twenty-four hours a day1 lights burned in the flight control rooms1 the science offices1 the computer areas1 and the photo processing labs4 ,in order to protect those with critical tasks to perform from the friendly distraction of the new arrivals1 special computer-controlled locks were placed on doors1 and security officers began to patrol the halls4 ,by the end of the month the press had begun to arrive4 ,amid accelerating excitement1 the ,voyager #a encounter was about to begin4 ,illustration3 ,the smaller-scale clouds on ,jupiter tend to be more irregular than the large ovals and plumes4 ,at the lower right1 one of the three large white ovals clearly shows internal structure1 with the swirling cloud pattern indicating counterclockwise1 or anticyclonic1 flow4 ,a smaller anticyclonic white feature near the center is surrounded by a dark1 cloud-free band where one can see to greater depths in the atmosphere4 ,this photo was taken ,march #a from a distance of #d million kilometers4 ,p-#baahc;,c ,,chapter #f ,,the ,,first ,,encounter ,the ,giant ,is ,full of ,surprises ,the ,voyager #a encounter took place at #d3db a4m4 ,,pst1 ,march #e1 #aigi4 ,about six hours before1 while the spacecraft continued to hurtle on toward ,jupiter1 overflow crowds had poured out of ,beckman ,auditorium on the campus of the ,california ,institute of ,technology4 ,there had been a symposium entitled ,jupiter and the ,mind of ,man4 ,more than one face that evening had turned to look toward the ,pasadena night1 for there1 glittering against the fabric of the sky1 was the 8star0 of the show--a planet so huge that the ,earth would be but a blemish on its ,great ,red ,spot4 ,one might have tried to imagine ,voyager #a1 large by spacecraft standards1 rapidly closing in on ,jupiter--a pesky1 investigative 8mosquito0 buzzing around the ,jovian system4 ,meanwhile1 at ,,jpl1 pictures taken by ,voyager #a were flashing back every #dh seconds1 pictures that were already revealing more and more of ,jupiter's atmosphere and would soon disclose four new worlds--the ,galilean satellites4 ,the encounter was almost at hand4 #d3db a4m4--,voyager #a had made its closest approach to ,jupiter #cg minutes earlier4 ,only now were the signals that had been sent out from the spacecraft at #d3je a4m4 reaching the ,earth4 ,but the moment of closest encounter did not have the same impact a landing would have had4 ,although the champagne would flow later for those who had worked so long and hard on this successful mission1 there was none now4 ,in the press room there were just four coffeepots working overtime to help keep the press alert4 ,in the science areas1 focus had shifted to the satellite encounters1 which would stretch over the next #bd hours2 meanwhile1 a few persons tried to catch a little sleep at their desks before the first closeups of ,io came in4 ,the instant of encounter came 444 and went 444 with no screams1 no ,new ,year's noisemakers4 ,all the excitement of the mission lay both behind 444 and ahead4 ,thursday1 ,february #bb4 ,the encounter activity was kicked off in ,washington1 ,d4,c41 by a press conference at ,,nasa ,headquarters4 ,after introductory statements by ,,nasa and ,,jpl officials1 some of the scientific results from the observatory and far encounter phases were presented4 ,both the plasma wave and the planetary radio astronomy instruments had detected very-low-frequency radio emission that generally increased in intensity whenever either the north or south magnetic pole of ,jupiter tipped toward the spacecraft4 ,the plasma wave experiment had also detected radiation that seemed to come from a region either near or beyond the orbit of ,io--perhaps even as far as the outer magnetosphere4 ,in addition1 ,frederick ,l4 ,scarf1 ,principal ,investigator for the plasma wave instrument1 released a recording of the ion sound waves created by #e to #aj kilovolt energy protons traveling upstream from ,jupiter4 ,james ,warwick1 ,principal ,investigator of the radio astronomy investigation1 enthusiastically reported the detection of a striking new low-frequency radiation from ,jupiter1 at wavelengths of tens of kilometers4 ,such radio waves cannot penetrate the ionosphere of the ,earth1 and thus had never been detected before4 ,because of their huge scale1 these bursts probably did not originate at ,jupiter itself1 but in magnetospheric regions above the planet--perhaps in association with the high-density plasma torus associated with ,io4 ,warwick commented that 8only from the magnificent perspective of the ,voyager as it approaches ,jupiter have we been able to get this complete picture40 ,illustration3 ,the ,voyager #a encounter with ,jupiter took place during a little more than #dh hours1 from the inbound to the outbound crossing of the orbit of ,callisto4 ,this figure shows the spacecraft path as it would be seen from above the north pole of ,jupiter4 ,closest approach to ,jupiter was #cej #jjj kilometers4 ,the close flybys of ,io1 ,ganymede1 and ,callisto all took place as the spacecraft was outbound4 ,voyager #a trajectory ,launch date #i_/e_/gg ,jupiter arrival date #c_/e_/gi ,satellite closest approach ,sun occultation ,earth occultation ,io ,ganymede ,periapsis ,callisto ,amalthea ,europa ,another unexpected result was announced by ,lyle ,broadfoot1 ,principal ,investigator for the ultraviolet spectrometer investigation4 ,the scientists had expected to find very weak ultraviolet emissions on the sunlit side of ,jupiter1 caused by sunlight being scattered from hydrogen and helium in ,jupiter's upper atmosphere4 8,instead we are seeing a spectacular auroral display4 ,there are two features of the emission--the auroral emission which comes from the planet and a second type of emission which appears to come from a radiating torus or shell around the planet at the orbit of ,io4 ,the spectral content of these two radiating sources is distinctly different4 ,what we find is that the auroral emission from ,jupiter's atmosphere is so strong that it completely dominates the emission spectrum even on the sunlit side of the atmosphere40 8,not since ,mariner #d carried its ,,tv camera to ,mars fifteen years ago have we been less prepared--have we been less certain of what we are about to see over the next two weeks10 said ,bradford ,smith1 ,imaging ,team ,leader4 ,he mentioned the time-lapse 8rotation movie10 in which the colorful planet spun through ten full ,jupiter days2 tiny images of the satellites passed across ,jupiter's face as though being whipped along by the rotation of the giant4 ,a week or so earlier1 when this film had been shown for the first time to the full ,imaging ,team1 it provided an occasion for good-humored rivalry between planet people and satellite people1 with jokes about the satellites getting in the way of the important studies of ,jupiter4 ,for the next few days1 the imaging focus remained on ,jupiter2 it shifted to ,io1 ,ganymede1 and ,callisto1 as each was passed in turn after closest approach to the planet4 ,tuesday1 ,february #bg4 7,range to ,jupiter1 #g4a million kilometers74 ,at a distance of #ffj million kilometers from ,earth1 within #ij ,jovian radii 7,r,j7 of ,jupiter1 ,voyager #a was prepared to begin the encounter with the planet's magnetosphere4 ,on the previous day the spacecraft had crossed the point1 at #ajj ,r,j1 at which ,pioneers #aj and #aa had found the bow shock1 the first indication of the magnetospheric boundary4 ,the start of ,voyager's plunge into the ,jovian magnetosphere was overdue1 and scientists anxiously watched the data from the particles and fields instruments1 looking for the first indication of disordered magnetic fields and altered particle densities that would mark the bow shock4 ,apparently1 higher solar wind pressure1 associated with increased solar activity since #aigd1 had compressed the magnetosphere1 but no one could predict how strong this compression might be4 ,,encounter ,,distances ,,for ,,voyager #a ,object ,range to ,center at ,closest ,best ,image ,resolution ,approach 7kilometers7 7km per line pair7 ,jupiter #cdi #jjj #h ,amalthea #dag #jjj #h ,io #ba #jjj #a ,europa #gcd #jjj #cc ,ganymede #aae #jjj #b ,callisto #abf #jjj #b ,for the first time since its discovery1 ,lyle ,broadfoot and his ,,uvs colleagues suggested a probable identification for the unexpected ultraviolet emission near the orbit of ,io4 ,the most likely candidate was sulfur atoms with two electrons removed 7,s ,,iii71 at an inferred temperature perhaps as high as #bjj #jjj ,k4 ,an additional indication of sulfur came from ,mike ,krimigis1 who reported that the low-energy charged particles instrument had detected bursts of sulfur ions streaming away from ,jupiter that had apparently escaped from the inner magnetosphere4 ,no explanations were offered1 however1 for the presence of large amounts of this element4 ,at ,,jpl1 a press room had been opened in ,von ,karman ,auditorium to accommodate the hundred or so reporters expected to arrive4 ,to keep all the interested people informed of ,voyager progress1 frequent television reports were beamed over closed-circuit ,,tv throughout ,,jpl4 ,from an in-lab television studio called the ,blue ,room1 ,,jpl scientist ,al ,hibbs1 who had played a similar role during the ,viking ,mission to ,mars1 provided hourly reports and interviewed members of the ,voyager teams4 ,as the pressure for constant commentary and instant analysis increased1 ,garry ,hunt of the ,imaging ,team was also called on to host activities in the ,blue ,room1 where his ,british accent added an additional touch of class to the operation4 ,as the encounter progressed1 the ,,jpl television reports reached a wider audience4 ,in the ,los ,angeles area1 ,,kcet ,public ,television began a nightly 8,jupiter ,watch0 program1 with ,dr4 ,hibbs as host4 ,during the encounter days1 service was extended to interested public television stations throughout the nation4 ,in this way1 tens of thousands of persons were able to experience the thrill of discovery1 seeing the closeup pictures of ,jupiter and its satellites at the same moment as the scientists at ,,jpl saw them1 and listening to the excited and frequently awestruck commentary as the first tentative interpretations were attempted4 ,unfortunately1 the commercial television networks did not make use of this opportunity1 and the greatest coverage available to most of the country was a #ij-second commentary on the nightly network news4 ,wednesday1 ,february #bh4 7,range to ,jupiter1 #e4i million kilometers74 ,at #f3cc a4m41 at a range of #hf ,r,j1 ,voyager #a finally reached ,jupiter's bow shock4 ,but by #ab3bh p4m4 the solar wind had pushed the magnetosphere back toward ,jupiter1 and ,voyager was once more outside1 back in the solar wind4 ,not until ,march #b1 at a distance of less than #de ,r,j1 would the spacecraft enter the magnetosphere for the final time4 ,at #aa a4m4 the first daily briefing to the press was given4 8,after nearly two months of atmospheric imaging and perhaps a week or two of satellite viewing1 we're happily bewildered10 said ,brad ,smith4 ,the ,jovian atmosphere is 8where our greatest state of confusion seems to exist right at the moment1 although over the next several days we may find that some of our smirking geology friends will find themselves in a similar state4 ,i think1 for the most part1 we have to say that the existing atmospheric circulation models have all been shot to hell by ,voyager4 ,although these models can still explain some of the coarse zonal flow patterns1 they fail entirely in explaining the detailed behavior that ,voyager is now revealing40 ,it was thought1 from ,pioneer results1 that ,jupiter's atmosphere showed primarily horizontal or zonal flow near the equatorial region1 but that the zonal flow pattern broke down at high latitudes4 ,but ,voyager found that 8zonal flow exists poleward as far as we can see40 ,smith also showed a time-lapse movie of ,jupiter assembled from images obtained during the month of ,january4 ,once each rotation1 approximately every ten hours1 a color picture had been taken4 ,viewed consecutively1 these frames displayed the complex cloud motions on a single hemisphere of ,jupiter1 as they would be seen from a fixed point above the equator of the planet4 ,the film revealed that clouds move around the ,great ,red ,spot in a period of about six days1 at speeds of perhaps #ajj meters per second4 ,the ,great ,red ,spot1 as well as many of the smaller spots that dot the planet1 appeared to be rotating anticyclonically4 ,anticyclonic motion is characteristic of high-pressure regions1 unlike terrestrial storms4 ,smith noted that 8,jupiter is far more complex in its atmospheric motions than we had ever imagined4 ,we are seeing a much more complicated flow of cyclonic and anticyclonic vorticity1 circulation4 ,we see currents which flow along and seem to interact with an obstacle and turn around and flow back40 ,there is a ,jovian jet stream that is 8moving along at well over #ajj meters per second4 ,several of these curious little dark features that appear to be small brown spots near ,jupiter's north temperate region have been seen to overtake one another and gobble each other up4 ,and then they occasionally spit out a piece here and there as they move along40 ,thursday1 ,march #a4 7,range to ,jupiter1 #d4h million kilometers74 ,at #e a4m41 at a distance of #ga ,r,j1 ,voyager crossed the bow shock for the third time1 catching up with the contracting magnetosphere of the planet4 ,about noon1 at #ff ,r,j1 the spacecraft finally reached the boundary of the magnetosphere1 called the magnetopause4 ,herbert ,bridge1 the plasma instrument ,principal ,investigator1 noted that the solar wind pressure as monitored by ,voyager #b1 still between the ,sun and ,jupiter1 had been for several days from two to five times greater than its level during the ,pioneer #aj and #aa encounters4 ,presumably1 this high pressure was the cause of the compressed state of the magnetosphere4 ,however1 in the previous few hours the solar pressure had dropped1 so ,bridge anticipated that the ,jovian magnetosphere might soon inflate and expand outward4 ,illustration3 ,the southern hemisphere of ,jupiter presents a tremendous diversity of atmospheric structure and motion4 ,the ,great ,red ,spot rotates counterclockwise in about six days2 above and below it high-speed jet streams flow to the right and the left1 while a complex1 dynamic cloud pattern develops in its wake4 ,this picture was taken on ,february #be1 when ,voyager #a was #i million kilometers from the planet4 ,p-#baaea;,c ,fred ,scarf intrigued the press with a tape of the sounds made by high-energy protons coming upstream from ,jupiter4 ,the plasma wave instrument had recorded the noise of the protons1 mixed with the noise of the spacecraft itself1 producing sound effects that sounded somewhat like a mixture of singing whales1 a ,nor'easter1 and the ,daytona #ejj4 ,at the press briefing1 interest in the imaging results began to shift from ,jupiter toward the satellites4 ,pictures of each of the four big ,galilean moons revealed bright and dark features as small as about #bjj kilometers across4 ,unfortunately1 this resolution is not enough to be diagnostic--the spots cannot be interpreted in terms of recognizable geological features1 such as mountains or craters4 ,today one could only speculate1 but tomorrow or the next day the answers would begin to come in4 ,deputy ,imaging ,team ,leader ,larry ,soderblom conveyed his excitement through a metaphor that would be repeated many times during the next week3 8,we're beginning a stage in this mission which represents1 ,i think1 one of the most exciting points in man's scientific exploration of the solar system--in the next few days1 we'll explore four new worlds10 seeing in a few days' time what it took us centuries to learn about other worlds in our solar system4 ,in terms of our experience with the exploration of ,mars1 8it is about #agjj ,,ad this morning1 tomorrow it will be about #ahjj1 and it will be about #aigf the year of ,viking by ,tuesday evening40 ,friday1 ,march #b4 7,range to ,jupiter1 #c4f million kilometers74 ,early in the morning1 a twelvefold increase in solar wind pressure caused another contraction of the magnetosphere1 which was behaving like a spring1 compressing in response to outside forces4 ,as the magnetopause boundary moved rapidly inward1 it crossed the spacecraft at #ei ,r,j from the planet4 ,an hour later the bow shock also flashed past1 and ,voyager was once more in the interplanetary medium4 ,by noon reinflation of the magnetosphere began again2 ,voyager crossed the bow shock for the fifth and final time at #ee ,r,j1 followed by three more magnetopause crossings1 as the magnetospheric boundary flopped in and out between #de ,r,j and #ej ,r,j4 ,illustration3 ,a few days before encounter1 the ,voyager images of the larger ,galilean satellites1 ,callisto and ,ganymede1 were beginning to show distinctive surfaces with many bright spots4 ,these two pictures were taken on ,march #e at a range of #h million kilometers2 the resolution is about #ajj kilometers4 ,although extremely tantalizing1 these images were uninterpretable1 because the spots could not be associated with any recognizable geological features1 such as mountains or craters4 ,like a naked-eye view of the ,moon1 these pictures seemed to reveal more than was actually meaningful4 ,p-#baahh;,c and ,p-#baaej;,c ,illustration3 ,ganymede ,some ,project officials began to worry about the contracted state of the magnetosphere4 ,the radiation 8hardening0 of ,voyager was carried out to protect against the energetic particle fluxes observed by ,pioneers #aj and #aa4 ,under the new conditions1 would the particles in the inner magnetosphere be more concentrated and perhaps increase the radiation levels beyond the design limits8 ,scientists asked how long the compression might last and speculated about how much energy might be pumped into the ,jovian radiation belts1 but only time could provide the answers4 ,,voyager #a ,,bow ,,shock 7,s7 ,,and ,,magnetopause 7,m7 ,,crossings ,boundary ,day ,distance 7,r,j7 ,inbound ,s #b_/bh #hf ,s #b_/bh #hb ,s #c_/ja #gb ,m #c_/ja #fg ,m #c_/jb #ei ,s #c_/jb #eh ,s #c_/jb #ef ,m #c_/jc #dg ,outbound ,m #c_/ac #aeh ,m #c_/ac #afc ,m #c_/ac #afe ,s #c_/af #aii ,s #c_/ah #bbg ,s #c_/ah #bbg ,s #c_/ai #bdj ,s #c_/bj #bef ,s #c_/bj #beh ,accurate recording of the ,x-band data being sent from ,voyager at #aae thousand bits per second required fairly clear weather at the tracking sites4 ,the three ,deep ,space ,network 7,,dsn7 stations in ,california1 ,spain1 and ,australia are located at normally dry sites4 ,however1 early in the morning1 heavy rain at the ,australian site interfered with reception of the ,voyager signal for fourteen minutes4 ,fortunately1 the loss occurred when the ,,dsn tracking of the spacecraft was being switched from ,goldstone1 ,california1 to the ,australian station4 ,mission control was able to extend ,goldstone coverage for several minutes so that only about three minutes' worth of data was lost completely4 ,a more positive announcement was made at the press conference that morning by ,donald ,shemansky of the ,ultraviolet ,spectroscopy ,team3 the discovery of a high-energy torus of doubly ionized sulfur 7,s ,,iii7 circling ,jupiter in the region of ,io's orbit4 8,we were surprised out of our chairs to see a spectacularly bright emission in the #fej-aajj angstrom region1 immediately implying that we were looking at a plasma that had to be at a temperature of about #ajj #jjj degrees40 ,the scientists estimated that the density of this torus must be at least #ejj ions per cubic centimeter1 and that the power needed to keep this plasma at such a high temperature must be in the neighborhood of #ejj billion watts4 ,as ,al ,hibbs mentioned on 8,jupiter ,watch0 that night1 #ejj billion watts of power is the total amount of installed generating capacity of the ,united ,states4 ,saturday1 ,march #c4 7,range to ,jupiter1 #b4e million kilometers74 ,early in the morning the final crossing of the magnetopause took place at #dg ,r,j1 and ,voyager finally joined the ,jovian system4 ,at the end of the day1 the rapidly moving spacecraft crossed the orbit of ,callisto1 but that satellite itself was on the far side of the planet4 ,not until the outbound crossing of its orbit on ,march #f would closeup views of ,callisto be obtained4 ,during the preceding night1 a severe summer thunderstorm in ,australia again caused a loss of data4 ,a line of storms over the tracking station blocked the high-rate1 ,x-band science data for three hours and twenty minutes4 ,attempts were made to save a part of the data by commanding the spacecraft to slow its transmission rate1 rather like speaking slowly to a partially deaf listener4 ,but by the time the craft received the signal and responded1 the storms had intensified1 and no signal could get through4 ,closeups of the ,great ,red ,spot and an extended series of observations of the glowing sodium cloud around ,io were lost4 ,a black-and-white movie assembled from photos obtained in ,january and ,february was shown at the press conference4 ,the movie was taken from observatory phase pictures photographed in blue light4 ,from these pictures1 the imaging team 8put together this so-called #8blue movie'10 said ,brad ,smith1 introducing the film4 ,the film showed changes taking place in ,jupiter's atmosphere over a period of about seventy ,jovian days4 ,near the equator there were 8bright plumes floating by--at high speed the plumes seem to wave around1 something like a flag waving in the breeze40 ,farther north1 along the edge of the north temperate zone1 one could see one of the dark ovals--8a rather fuzzy one would move up1 catch up with the one just ahead of it1 get stuck to the outside and roll around on it for a while1 then get ejected a little later40 ,dr4 ,smith also showed new closeup views of the region around the ,red ,spot1 showing not only the anticyclonic features but filamentary 8spaghettilike0 material which was rotating in a cyclonic direction1 indicating a low-pressure region4 8,the filamentary material still seems to be rather a mystery--very difficult to see the details of the motion40 ,but the photographs were already showing what seemed to be stream lines in the white ovals4 8,in appearance the white ovals seem to resemble the ,red ,spot4 ,the stream lines are at least suggestive of divergent flow1 that is1 material in each of these spots which is upwelling in these areas and then moving out tends to go around in a counterclockwise anticyclonic motion but may1 at the same time1 be slowly diverging outward40 ,illustration3 ,at a resolution of about #ajj kilometers a planetary surface just begins to reveal its personality4 ,io was photographed against the disk of ,jupiter on ,february #bf1 from a distance of about #h million kilometers4 ,such early pictures whetted the appetites of the ,voyager scientists at ,,jpl as they anxiously speculated about what they would find in closer views of the surface of this remarkable satellite4 ,p-#baahe ,b_/,w ,the large white ovals are about forty years old4 ,dr4 ,smith explained how they formed3 ,between #aici and #aidj1 8where those three white spots exist right now1 was a rather bright band similar to the north temperate zone we see on ,jupiter right now4 ,in that time period of a year or so1 three darkish spots formed4 ,a dark cloud spread out at each one of those three locations and just kept spreading longitudinally until the white material condensed between them40 ,in the end1 everything was dark except for the three ovals1 which have persisted ever since4 ,torrence ,johnson of the ,imaging ,team commented on a photo of ,io in which features resembling circular crater-type structures seemed to be visible4 8,whatever they are1 those circular features are approaching the size of the things that we would call basins if they were impact structures on other planets4 ,we don't really know whether they're impact structures4 ,they have some characteristics that look reminiscent of impact structures4 ,they could be endogenic--volcanic in origin--or internally generated in some other way40 ,johnson also showed another photograph of ,io1 this one taken against black sky1 showing a 8strikingly different face0 looking1 perhaps1 like someone's nightmare1 glaring back at the intruder from ,earth4 ,one huge feature--a 8bullseye0 or 8hoof print0 on ,io--appeared to be approximately #ajjj kilometers long4 ,no one had ever seen such a feature1 and the imaging scientists could only speculate about its significance4 ,illustration3 ,small-scale structures in the jet streams of ,jupiter's north tropical zone reveal details of atmospheric circulation4 ,the small dark oval near the right edge of the zone may offer a glimpse deep into ,jupiter's atmosphere4 ,between the regularly spaced dark ovals near the bottom of the frame are more small-scale features that are being studied for their roles in ,jovian atmospheric activity4 ,the blue-gray regions along the shear line between the equatorial zone and the north equatorial belt also appear to be windows into the deeper regions of the atmosphere4 ,this photo was taken ,february #ai by ,voyager #a from a distance of #ad million kilometers4 ,p-#baafj;,c ,a few days earlier1 someone had posted in the ,imaging ,team area a quote from a #aige review paper on the ,jovian satellites by ,david ,morrison and ,joseph ,burns4 ,the section on ,io began1 8,io is one of the most intriguing objects in the solar system40 ,this statement seemed more and more appropriate as ,voyager images improved4 ,johnson referred to this day as equivalent to the 8late #aifjs0 in our study of the ,jovian satellites4 8,we can see much more clearly than ever before1 but still not clearly enough to provide understanding of what we are seeing40 ,sunday1 ,march #d4 7,range to ,jupiter1 #a4b million kilometers74 ,at #d3cg a4m41 the near encounter phase began3 ,voyager #a was almost there6 ,in the press room1 someone taped a fortune cookie message to the ,voyager ,,tv monitor3 8,there is a prospect of a thrilling time ahead for you40 ,pulled by the powerful gravity of ,jupiter1 the spacecraft was now on a curved path through the inner ,jovian system4 ,at #b p4m4 ,,pst1 it crossed the orbit of ,ganymede1 and later in the afternoon it passed within less than #b million kilometers of ,europa1 providing ,voyager #a's closest look at this satellite4 ,during the afternoon and evening1 a number of views were obtained of ,amalthea1 the small inner satellite1 at a range of less than #ejj #jjj kilometers4 ,at #h p4m4 the orbit of ,europa was crossed1 and increasing attention was drawn to the coming encounter with ,io4 ,at about #g p4m4 a full-frame color sequence of ,io was received with a resolution of #af kilometers4 ,during the night1 as ,imaging ,team members scratched their heads trying to prepare a press release caption to interpret the peculiar structures seen1 the ,,jpl ,image ,processing ,lab rushed to prepare a color version for release the next day4 ,illustration3 ,the ,great ,red ,spot became more and more spectacular as ,voyager #a approached1 with each day revealing new and intricate detail in the clouds4 ,this view was obtained on ,march #a at a distance of #e million kilometers2 the smallest features that can be made out are about #ajj kilometers across4 ,to the west of the ,great ,red ,spot is a region of great turbulence1 and to the south is one of the three white ovals4 ,p-#baahb;,c ,illustration3 ,large brown ovals in the northern hemisphere of ,jupiter are apparently regions in which an opening in the upper1 ammonia clouds reveals darker regions below4 ,this oval1 about the same length as the diameter of the ,earth1 was at latitude #ae,n4 ,features of this sort are not rare on ,jupiter and have an average lifetime of one to two years4 ,above the feature is the pale orange north temperate belt1 bounded on the south by the high-speed north temperate current1 with winds of #abj meters per second4 ,the range to ,jupiter at the time this photograph was obtained on ,march #b was #d million kilometers1 with the smallest resolvable features being #ge kilometers across4 ,p-#baaid;,c ,illustration3 ,the best ,voyager #a photos of ,europa were obtained on ,march #d from a distance of about #b million kilometers4 ,this view of the hemisphere centered at about #cjj longitude has a resolution of about #dj kilometers4 ,most of the surface is bland and highly reflective1 being composed almost entirely of water-ice4 ,no craters resulting from meteoric impacts can be seen4 ,the most striking visual features1 which set ,europa off from the other satellites1 are the dark streaks1 as much as several thousand kilometers long1 that cross the surface4 ,just barely visible to ,voyager #a1 these streaks were dramatically apparent to ,voyager #b in its closer flyby in ,july4 ,p-#babjh;,c ,once again1 tracking station problems interrupted the smooth flow of data from the spacecraft4 ,failure at ,madrid to reestablish the correct receiver frequency after a spacecraft maneuver resulted in the loss of #ec minutes of irreplaceable data2 an additional eleven minutes were lost an hour later4 ,as the first problem was being corrected1 the tracking antenna became misaligned1 resulting in a noisy data return4 ,meanwhile1 only a few of the watchers at ,,jpl mourned the lost data1 so exciting were the other new results that kept pouring in4 8,for the highlights of this morning ,dr4 ,soderblom will come up and show some beautiful satellite pictures10 ,brad ,smith announced at the daily press briefing4 ,larry ,soderblom's excitement was hard to contain4 8,today is probably going to turn out to be one of the most memorable days in our exploration program4 ,for the planetary geologists it's truly ,christmas ,eve4 ,we see tonight the beginning of the exploration of four new worlds4 ,we're racing through time and space at an incredible rate--the rate at which we are learning things is awe-inspiring in itself40 ,callisto was still too far away to see well 8but the things we're seeing on the closer three satellites have really got us charged in anticipation40 ,ganymede--8,what can ,i say8 ,loops and swirls and incredible patches that are difficult now to hazard a guess about40 ,io--an eerie-looking red1 orange1 and yellow world--8this one we've got all figured out4 ,laughter from the press and applause4 ,it is covered with thin candy shells of anything from sulfates and sulfur and salts to all kinds of strange things40 ,the low energy charged particle instrument had discovered high-speed sulfur in the outer ,jovian magnetosphere1 ten times as far from the planet as the sulfur torus around ,io4 ,the high-speed sulfur1 which whizzed by ,voyager #a at #hjjj kilometers per second1 was first detected as the spacecraft crossed the magnetopause and entered the magnetosphere4 ,apparently this sulfur had picked up speed as it moved outward from ,io's torus1 but the mechanism for this acceleration was not known4 ,roby ,vogt reported that the cosmic ray instrument was detecting two distinct groups of atoms closer in to the planet4 ,one group was apparently of solar composition 7derived from the solar wind71 and the other showed enhanced oxygen as well as sulfur4 ,the plasma instrument had also begun to measure sulfur in the same form 7,s ,,iii7 as that detected by the ,,uvs in the ,io torus4 ,where1 the scientists asked1 could the oxygen and sulfur be coming from8 ,could ,io be the source of these atoms8 ,at one point1 as questions from the press became too detailed1 asking for 8instant analyses0 of the data1 ,dr4 ,smith was prompted to make the following statement3 8,i want to say something about what's going on right now in the imaging area because we get a lot of pointed questions4 ,we have a remarkably good system that is getting extremely good photographs4 ,not only are the cameras working well1 but ,jupiter and the satellites are cooperating by showing a lot of truly remarkable detail4 ,and it may sound unprofessional1 but a lot of the people up in the ,imaging ,team area are just standing around with their mouths hanging open watching the pictures come in1 and you don't like to tear yourself away to go and start looking at numbers on a printout4 ,we will do that1 but in the meantime we're just caught up in the excitement of what's going on40 ,while the press briefing was underway1 between #aa3ch and #aa3ej a4m41 a special experiment was being tried on board the ,voyager4 ,as the spacecraft passed through the plane of the equator of ,jupiter1 it aimed its narrow-angle camera to a point in space halfway between the cloud tops and the orbit of ,amalthea and took a single1 eleven-minute exposure4 ,the purpose3 to search for a possible faint ring around ,jupiter1 which1 if present1 could best be seen by sighting along the plane of the equator4 ,the image appeared briefly on the ,,tv monitors1 clearly showing something--a strange band of light--streaking across the center of the frame4 ,up in the imaging area1 analyses began at once to determine if the strange band were really the sought-for ring1 but it would not be until ,march #g that the identification would be confirmed and the discovery announced4 ,illustration3 ,the 8bullseye0 of ,io was first photographed at a range of #b4h million kilometers on ,march #c1 and this color version was released on ,march #d4 ,at the time1 ,imaging ,team scientists wrote that 8,the large heart-shaped feature with a dark spot near its center could be ,io's equivalent of an impact basin such as ,mare ,orientale on the ,moon4 ,its outer dimensions are about #hjj by #ajjj kilometers4 ,subsequent high-resolution coverage should reveal whether the small dark spots are impact craters1 or perhaps something more exotic such as volcanoes4 ,the reddish color of ,io has been attributed to sulfur in the salts which are believed by some to make up the surface of ,io40 ,it would be another week before this feature1 later named 8,pele0 for the ,hawaiian volcano goddess1 would be recognized as an active1 erupting volcano4 ,p-#baahg;,c ,monday1 ,march #e4 7,encounter ,day4 ,minimum range to ,jupiter1 #ghj #jjj kilometers2 speed of spacecraft1 almost #ajj #jjj kilometers per hour74 ,many celebrities1 including the ,governor of ,california1 spent the night at ,,jpl to witness the historic occasion4 ,in ,washington1 ,d4,c41 a special ,,tv monitor was set up in the ,white ,house for the ,president and his family4 ,as late ,sunday night eased into the early morning of encounter1 closeup images of ,jupiter1 looking more like abstract art than like planetary science1 flashed across the ,,tv screens1 and verbal images far less wild than the visuals from ,jupiter were heard from commentators and from members of the press4 8,are you sure ,van ,gogh didn't paint that80 8,that's not ,jupiter2 it looks like a closeup of a salad40 8,they're not showing us ,jupiter1 that's some medical school anatomy slide40 ,shortly before closest approach to ,jupiter1 ,voyager began its intensive observations of ,io4 ,much of this information1 taken while the ,australian station was tracking the spacecraft1 was recorded on ,voyager's onboard tape-recorder for playback later that day4 ,but even before the results of that imaging were known1 ,larry ,soderblom was calling ,io 8one of the most spectacular bodies in the solar system40 ,as more and more vivid photos of ,io appeared on the monitors1 members of the ,imaging ,team in the ,blue ,room buzzed with excitement4 8,this is incredible40 8,the element of surprise is coming up in every one of these frames40 8,i knew it would be wild from what we saw on approach but to anticipate anything like this would have required some sort of heavenly perspective4 ,i think this is incredible40 ,at #g3ce a4m4 ,voyager was scheduled to pass through the flux tube of ,io1 the region in which tremendous electric currents were calculated to be flowing back and forth between the satellite and ,jupiter4 ,norm ,ness suggested1 after examining magnetometer data1 that ,voyager skirted the edge of the flux tube1 and that the current in the tube was about one million amps4 ,as the flux tube results were received1 champagne bottles began to pop in the particles and fields science offices1 in celebration of the successful passage through the inner magnetosphere4 ,meanwhile1 at #g3dg a4m41 closest approach to ,io occurred1 at a range of only #bb #jjj kilometers4 ,voyager was #be #jjj times closer to this satellite than were the watchers on ,earth4 ,at #h3ad a4m41 while still within #cj #jjj kilometers of ,io1 the spacecraft passed out of sight behind the edge of ,jupiter4 ,all scientific data for the next two hours and six minutes were stored on the onboard tape recorder for later transmission to ,earth4 ,meanwhile1 the radio communication signal was used to probe the atmosphere of ,jupiter1 yielding a profile of electron density in the ionosphere and of the gas pressure and temperature in the upper atmosphere4 ,while out of sight from ,earth1 at #i3jg a4m41 ,voyager plunged into the shadow of ,jupiter4 ,as the ,sun set on the spacecraft1 the ultraviolet instrument used the absorption of sunlight to determine the composition and temperature of the upper atmosphere4 ,in the darkness1 the infrared ,,iris measured the night-side temperatures of the planet1 and long-exposure images were taken to search for aurora1 lightning1 and fireballs in the ,jovian atmosphere4 ,at #aj3bj a4m41 ,voyager reappeared from behind ,jupiter and radio contact was restored2 at #aa3bd a4m41 it emerged from shadow into sunlight1 speeding on toward encounter with ,ganymede4 ,at #h a4m4 a special press conference was held to mark the successful ,jupiter flyby4 ,noel ,hinners1 ,associate ,administrator for ,space ,science and the highest ranking ,,nasa official present1 congratulated all those who had made the ,voyager ,mission a success4 ,the encounter was the 8culmination of a fantastic amount of dedication and effort4 ,the result is a spectacular feat of technology and a beginning of a new era of science for the solar system4 ,just watching the data come in has been fantastic4 ,i had a fear that things on the satellites were going to look like the lunar highlands4 ,nature wins again4 ,if we're going to see exploration of this nature occurring in the #aihjs and #aiijs we must continue to expound the results of what we're finding here1 the role of exploration in the history of our country1 what it means to us as a vigorous national society40 ,as time passed1 it became apparent that ,voyager #a had been affected by ,jupiter's radiation environment4 ,the basic timing--the main clock on the spacecraft--had slowed down4 ,first it slowed by #f4c seconds1 but by ,march #f it was found to have slowed a total of eight seconds4 ,in addition1 the two central computers apparently got out of synchronization both with themselves and with the flight data subsystem4 ,on ,march #f it was reported that the spacecraft cameras were shuttering one frametime 7#dh seconds7 early2 this was partly offset by the eight-second spacecraft 8masterclock0 slow-down resulting in images 7according to our clocks7 being photographed forty seconds early4 ,this timing error resulted in the camera taking some pictures while the scan platform was moving1 causing some blurred images4 ,a number of the highest resolution images of ,io and ,ganymede were seriously degraded by this malfunction4 ,illustration3 ,when the first color close-up of ,io was released1 ,imaging ,team ,leader ,brad ,smith said that he had seen 8better looking pizzas02 hence this view1 taken ,march #d at a range of about #hfj #jjj kilometers1 became known as 8the pizza picture40 ,the circular feature in the center 7the piece of pepperoni7 was later revealed to be the active volcano ,prometheus1 but at the time of its release this lovely but bizarre picture baffled scientists and press alike4 ,p-#badeg;,c ,at the regular #aa a4m4 press briefing1 ,brad ,smith glowed4 8,we're all recovering from what ,i would call the most exciting1 the most fascinating1 what may ultimately prove to be the most scientifically rewarding mission in the unmanned space program4 ,the ,io pictures this morning were truly spectacular and the atmosphere up in the imaging area was punctuated by whoops of joy or amazement or both40 ,the new color photo of ,io taken the night before was released1 showing strange surface features in tones of yellow1 orange1 and white4 ,the image defied description2 the ,imaging ,team used terms like 8grotesque10 8diseased10 8gross10 8bizarre40 ,smith introduced the picture with the comment1 8,io looks better than a lot of pizzas ,i've seen40 ,larry ,soderblom added1 8,well1 you may recall that we told you yesterday that when we flew by we'd figure all this out4 ,i hope you didn't believe it40 ,one thing was certain3 ,there were no impact craters on ,io4 ,unless the satellites of ,jupiter had somehow been shielded from the meteoric impacts that cratered objects such as the ,moon1 ,mars1 and ,mercury1 the absence of craters must indicate the presence of erosion or of internal processes that destroy or cover up craters4 ,io did not look like a dead planet4 ,imaging ,team member ,hal ,masursky1 looking at the 8pizza0 picture1 estimated that the surface of ,io must be no more than #ajj million years old--that is1 some agent must have erased impact craters during the last #ajj million years4 ,this interpretation depended on how often cratering impacts occur on ,io4 ,no one could be sure that there had been any interplanetary debris in the ,jovian system to impact the surfaces of the satellites4 ,perhaps none of them would be cratered4 ,the forthcoming flybys of ,ganymede and ,callisto would soon provide this information4 ,illustration3 ,the giant volcanic feature ,pele1 about #ajjj kilometers across1 mystified ,voyager scientists when this picture of ,io was taken on ,march #e from a distance of about #djj #jjj kilometers4 ,the brilliant colors1 the strange shapes of the surface deposits1 and the absence of impact craters all testified that ,io was unlike any world previously encountered in the exploration of the solar system4 ,p-#babbf;,c ,illustration3 ,as ,voyager #a approached ,io1 the images of the surface became more and more spectacular4 ,on the morning of ,march #e1 at a range of #acj #jjj kilometers1 this picture was taken centered near longitude #cbj and latitude #aj,s4 ,the width of the picture is about #ajjj kilometers 7the distance from the ,mexican border to the northern edge of ,california74 ,there are no impact craters1 signifying a geologically young surface1 and the dark center with radiating red flows indicates recent volcanic activity of some sort4 ,p-#babgg;,c ,the close flyby of ,ganymede took place at #f3ec p4m41 at a range of #aae #jjj kilometers4 ,during the preceding four hours1 photos revealed a surface covered with impact craters4 ,watching these photos and supplying commentary for the television listeners1 ,david ,morrison remarked1 8,while ,i'm delighted to see craters1 it's just the opposite of what ,i would have expected4 ,i was telling everyone a few days ago that ,i thought ,io would have plenty of craters and that ,ganymede1 because of the ice surface1 simply would not be able to hold large craters over geological time4 ,so this is fascinating and this is confusing--both what has happened on ,io to erase the craters and why ,ganymede's surface is strong enough to preserve them40 ,just before closest approach1 at #f3ce p4m41 the ultraviolet instrument watched as a bright star passed behind ,ganymede and reemerged ten minutes later4 ,no dimming that could be attributed to an atmosphere was seen2 when the data were analyzed later1 scientists set an upper limit for any gas on this satellite at one-billionth of the atmospheric pressure at the surface of the ,earth4 ,as encounter day drew to a close1 celebrations took place all over ,,jpl4 ,for many1 however1 the excitement was tempered by exhaustion4 ,after #dh hours of intense activity1 sleep was imperative for some4 ,but the close approach to ,callisto was still to come1 as was an examination of the data already received4 ,illustration3 ,after its close flyby of ,io1 ,voyager #a headed for ,ganymede1 the largest of the ,galilean satellites4 ,this global view of ,ganymede1 taken on ,march #d at a range of #b4f million kilometers1 shows features as small as #ej kilometers across4 ,at the time1 ,voyager scientists speculated that the numerous white spots were impact craters1 surrounded in some cases by icy ejecta blankets splashed onto the surrounding surface4 ,however1 many narrow white streaks1 especially those in the lower left quadrant1 promised new and exciting geological features on this satellite4 ,p-#babjg;,c ,tuesday1 ,march #f4 ,voyager was now receding from ,jupiter1 accelerated on a new trajectory--one that would speed it on toward its ,november #aihj encounter with ,saturn4 ,but first came the encounter with ,callisto1 with closest approach at #i3ej a4m41 at a range of #abf #jjj kilometers4 ,the satellite was littered with craters and there appeared one huge 8bullseye0 pattern that might have been the result of an impact4 ,as the photos of ,callisto came in1 ,garry ,hunt described the scene in the imaging area3 8,the activity around the monitors now is quite incredible with people caught breathless by the pictures coming in4 ,every satellite we've seen has been a different world40 ,asked how he felt about the images of ,jupiter's atmosphere1 he replied1 8,i'm absolutely delighted with what ,i've seen1 and ,i'm delighted ,voyager #b is not far behind since ,i'm convinced that we'll see yet another face of ,jupiter by then4 ,the weather will have changed by ,july40 ,at the morning press briefing1 the wealth of new data began to be revealed4 8,if ,jupiter had ever posed for ,monet1 it would probably have turned out like this10 said ,brad ,smith as he introduced some enhanced 7exaggerated7 color images that had been specially processed to show more detail in the ,great ,red ,spot region4 ,indeed1 these photographs did look like paintings with ,jupiter displaying some of its best abstract art4 ,illustration3 ,as ,voyager #a approached ,ganymede on ,march #e1 many strange new surface features became visible4 ,in this frame1 taken from a distance of #bej #jjj kilometers and showing features as small as #e kilometers across1 three distinct types of terrain are seen3 polygons of old dark surface1 extensive areas of lighter1 younger material1 and brilliant white ejecta patterns 7probably water-ice7 around fresh craters4 ,the bright rays in the upper part of the picture are #cjj-ejj kilometers long2 at the bottom are several craters with only faint1 muted ejecta patterns4 ,p-#babfb;,c ,illustration3 ,voyager #a found that the surface of ,ganymede was geologically very complex4 ,this frame1 taken ,march #e from a range of #bej #jjj kilometers1 shows a region about #ajjj kilometers across centered near longitude #j and latitude #bj,s4 ,the surface displays many craters1 some 7probably the younger ones7 with bright ray systems4 ,bright grooved bands traverse the surface in various directions4 ,one of these bands1 running in a north-south direction in the lower left of the picture1 is offset along a white line that may represent a fault4 ,ganymede is the only ,galilean satellite to show indications of such lateral offsets in the crust4 ,p-#babff ,the first picture of ,amalthea was shown1 revealing an elongated1 dark1 reddish object about #bfe kilometers long4 ,smith reported3 8,there is actually some structure that one can see--a crater--a couple of bright features for which we have no interpretation and some other evidence of cratering4 ,it doesn't look like much1 but after all1 ,amalthea has never been seen as anything more than a point of light from the ,earth1 and1 in fact1 there are very few people that have even seen it as a point of light4 ,i doubt that more than one astronomer out of a hundred has actually seen ,amalthea40 ,larry ,soderblom introduced new photographs of the satellites4 ,io still showed no craters1 even at high resolution4 ,the craters that 8should0 have been on ,io were on ,ganymede and ,callisto4 ,ganymede not only had craters1 it had fault lines as well4 8,there is transverse motion along these faults4 ,things get offset1 apparently1 for hundreds of kilometers4 ,so it's the first time we've seen major kinds of transverse motion on the surface of another planet40 ,ganymede1 in effect1 had shown evidence of having its own icy version of the ,san ,andreas ,fault4 ,illustration3 ,amalthea was thought to be the innermost satellite of ,jupiter until ,voyager #b discovered tiny ,adrastea 7#aigi;,j#a74 ,the ,voyager #a camera revealed ,amalthea as an irregular dark reddish object with dimensions of #bgj #afj kilometers4 ,these three images have resolutions of 7b7 #be kilometers2 7c7 #ac kilometers2 7d7 #h kilometers4 #bfj-ejc ,illustration3 ,observation geometry for ,amalthea ,illustration3 7b7 ,illustration3 7c7 ,illustration3 7d7 ,although ,callisto was heavily covered with craters up to #bjj kilometers in diameter1 ,dr4 ,soderblom commented on the absence of larger impact basins4 ,perhaps there was one in the huge bullseye feature2 however1 it was not a 8standard0 looking basin like those on the ,moon4 ,callisto was 8extremely smooth and free of any relief4 ,the structure the bullseye1 if impact caused1 shows no relief2 the limb does not show any relief2 maybe it's possible that ,callisto cannot support relief40 ,by the next day1 the geologists on the ,imaging ,team were becoming more convinced that the ,callisto 8bullseye0 was the frozen remnant of an enormous impact into ,callisto's surface4 ,since ,callisto is composed in large part of water and has an icy crust1 the team speculated that any raised features created by the impact would eventually 8slide0 back into the surface1 8and the ripple marks from the shock wave caused by the impact were frozen0 into the surface4 ,illustration3 ,callisto was the last of the ,galilean satellites to be studied by ,voyager #a4 ,in this photo1 taken ,march #e from a distance of #a4b million kilometers1 with a resolution of about #be kilometers1 the extensive cratering of the surface began to be apparent4 ,near the upper left edge is the large impact basin ,valhalla2 the numerous light spots are craters #ajj kilometers or more in diameter4 ,this is the same side of ,callisto that was photographed at higher resolution during the ,voyager #a flyby of the satellite a day later4 ,p-#babhd;,c ,lyle ,broadfoot announced that the ultraviolet spectrometer had detected very strong auroral emission in ,jupiter's north and south polar regions4 ,the aurora seemed to be caused primarily by the excitation of molecular hydrogen1 although some atomic hydrogen was also detected4 ,auroral emissions from helium atoms were not detected4 ,illustration3 ,the largest ancient impact basin on ,callisto is called ,valhalla4 ,the central light area is about #fjj kilometers across4 ,surrounding it is a set of concentric low ridges1 looking like frozen ripple marks1 extending about #aejj kilometers from the center4 ,this picture was taken by ,voyager #a on ,march #f at a range of #cej #jjj kilometers4 ,p-#babhg;,c ,the ,,iris infrared measurements required more computer processing than other ,voyager data1 and therefore they were not available until a day or two after the observations were made4 ,however1 ,rudy ,hanel already had two new results to report4 ,first1 the ,great ,red ,spot was about #c ,c cooler than its surroundings1 and this cooling extended many kilometers above the clouds1 into the thin upper atmosphere4 ,second1 the thermal emission from ,io was peculiar1 with an unexpected shape to the spectrum4 ,tentatively1 ,hanel suggested there might be some hot spots on the surface of the satellite4 ,jupiter has been full of surprises1 but the excitement was far from over4 ,major discoveries were yet to be made4 ,illustration3 ,voyager #a discovered the rings of ,jupiter on ,march #d in a single eleven-minute exposure with the narrow-angle camera4 ,spacecraft motion during the time exposure streaked the picture1 as can be seen from the hairpin-like images of the stars4 7,the star field was unusually rich1 since it happened to include the ,beehive star cluster in ,cancer47 ,the ring image itself is a multiple exposure1 with six separate images side by side4 ,the ring does not extend out of the right side of the picture1 indicating that this image captured the outer edge of the ring1 about halfway between the cloud tops and the orbit of ,amalthea4 ,p-#babeh ,illustration3 ,camera with fields of view ,wednesday1 ,march #g4 ,at the press briefing1 ,brad ,smith made a spectacular announcement3 8,this morning ,i would like to add yet another important discovery to be claimed by this outstanding mission--that of a thin flat ring of particles surrounding ,jupiter4 ,thus ,jupiter now joins ,saturn and ,uranus to become the third planet of our solar system known to possess a planetary ring system and leaves ,neptune as the only member of the group of giant planets without a known ring4 ,the discovery of the ring was unexpected in that the current theory which treats long-term stability of planetary rings would not predict the existence of such a ring around ,jupiter4 ,the single ,voyager camera image which recorded the ring was planned by the ,voyager ,imaging ,team several years ago1 not really with any great expectation of a positive result1 but more for the purpose of providing a degree of completeness to ,voyager's survey of the entire ,jupiter system4 ,the observation1 as planned1 involved looking off to the right of the limb of ,jupiter in the planet's equatorial plane at the exact moment that the spacecraft would be crossing the equatorial plane4 ,the image actually was taken at #af hours and #eb minutes before encounter from a distance of about #a4b million kilometers4 ,exposure time was #aa4b minutes4 ,we weren't certain of the exact moment that we would cross the equatorial plane1 so we planned to open our shutter and leave it open as we went through40 ,illustration3 ,since the ,voyager #a ring photograph was taken exactly edge-on1 it was not possible to determine the width of the ring4 ,in this artist's conception1 the ring is drawn as if it were ribbon-like1 with very little width1 quite unlike the broad flat rings of ,saturn4 ,voyager #b later showed this to have been a lucky guess4 ,p-#babei ,the image showed six exposures of the ring1 together with streaked trails of background stars4 ,smith reported that the thickness of the ring was less than #cj kilometers1 and that it extended to a point #abh #jjj kilometers from the center of ,jupiter1 or #eg #jjj kilometers above the clouds4 ,in response to this discovery1 scientists were eagerly planning to alter the ,voyager #b encounter sequence to try to obtain additional information on the ring4 ,the newspapers carried stories of the ring discovery on ,thursday1 ,march #h4 ,one story was seen by two ,university of ,hawaii astronomers1 ,eric ,becklin and ,gareth ,wynn-,williams1 who were observing at ,mauna ,kea ,observatory1 at an altitude of #dbjj meters on the ,big ,island of ,hawaii4 ,within two days they had succeeded in detecting the rings by their reflected sunlight1 at an infrared wavelength of #b4b micrometers1 providing a rapid confirmation of the ,voyager discovery4 ,as the plasma measurements from the ,io flyby were analyzed1 an additional clue to the origin of sulfur and oxygen was revealed4 ,herb ,bridge reported the detection of sulfur dioxide 7,,so71 the simplest molecule composed of these two atoms4 ,the high-resolution tape-recorded pictures of ,io baffled imaging scientists4 ,a number of features looked like volcanic flows2 together with the absence of impact craters1 these features indicated a geologically active planet4 ,a central point of discussion was the recent theoretical work on ,io by ,stanton ,peale of the ,university of ,california and two ,,nasa scientists1 ,pat ,cassen and ,ray ,reynolds4 ,these authors had just published a paper in the ,march #b issue of ,science showing how tidal heating from ,jupiter's gravity could melt the interior of ,io4 ,they wrote that 8widespread and recurrent surface volcanism might occur40 ,it began to look as if the prediction had been correct4 ,thursday1 ,march #h4 ,the last ,voyager #a press briefing was held4 ,each speaker was allotted only a few minutes1 prompting ,larry ,soderblom to preface his remarks by trying to explain how difficult it was to describe four new planets--the ,galilean satellites--in the time allowed4 8,torrence ,johnson was sitting with me last night1 puzzling4 ,he said1 #8,you know1 ,larry1 it's sort of like imagining we'd flown into the solar system the day before yesterday1 and said1 8,there's a thing we'll call ,mercury1 and there's the ,moon1 and there's ,earth1 and there's ,mars4 ,now let's explain them in ten minutes04'0 ,there was ,callisto1 with the highest density of craters of any ,galilean satellite--the oldest of the ,galilean surfaces--featuring a huge 8bullseye0 that is 8the largest single contiguous feature seen so far in the solar system40 ,there was ,ganymede1 cratered1 but also overrun with fault lines that looked1 according to one person1 like 8tire tracks in the desert10 showing a surface that 8had laterally slid--faulted and sheared and sheared again--twisted and torn apart40 ,there was ,io1 the most bizarre--the one that scientists had thought would be most lunarlike--showing a surface that had apparently been 8cooked and steamed and fumed out leaving deposits all over the surface much like you might see around a fumarole at ,yellowstone ,national ,park4 ,the fact that these things exhibit such youth makes it likely that the planet is still volcanically active40 ,there was ,europa1 with huge linear features unlike those of the other three ,galileans--,europa the mystery satellite1 waiting for the probing eyes of ,voyager #b to survey it in early ,july4 ,ed ,stone summed it all up3 8,i think we have had almost a decade's worth of discovery in this two-week period1 and ,i think that all of the people who have been talking to you feel the same saturation of new information which has occurred4 ,and in fact1 we will probably be studying it in great detail for at least five years40 ,over the next day or so the press packed up and went home4 ,the ,,tv monitors showed the spacecraft's parting glance at a crescent ,jupiter1 only hinting at the vastness of space ,voyager #a would travel until its encounter with ,saturn in the autumn of #aihj4 ,maybe now there would be a relative calm that would allow the scientists to begin analyzing that 8decade's worth of data40 ,but things were not to be calm just yet4 ,fire and ,brimstone ,at about #e a4m4 on ,march #h1 ,voyager had taken a historic picture4 ,looking back at a crescent ,io from a distance of #d4e million kilometers1 the camera had been used to obtain a long-exposure view for the spacecraft navigation team--one that showed the satellite against the field of background stars4 ,during the day1 ,linda ,morabito1 an optical navigation engineer1 began to work with this picture on her computer-controlled image display4 ,she noted what appeared to be a crescent cloud1 extending beyond the edge of ,io4 ,but ,io has no atmosphere1 so a cloud rising hundreds of kilometers above the surface did not seem to make sense4 ,the next day1 working with her colleagues1 ,morabito eliminated all possibilities for the new feature on ,io except the obvious--a cloud4 ,if it were a cloud1 it must be the result of an ongoing volcanic eruption of incredible violence4 ,the picture was shown to members of the ,imaging ,team1 who agreed with the identification4 ,but it was ,friday and ,brad ,smith and ,larry ,soderblom1 along with most of the other team members1 had left for the weekend to try to get some rest4 ,the picture would have to wait two more days4 ,,highlights ,,of ,,the ,,voyager #a ,,scientific ,,findings#b ,atmosphere ,uniform wind speeds for cloud features with widely different size scales1 suggesting that mass motion and not wave motion is being observed4 ,rapid formation and spreading of bright cloud material1 perhaps the result of disturbances that trigger convective activity4 ,a pattern of east-west winds in the polar regions1 previously thought to have been dominated by convective upwelling and downwelling4 ,anticyclonic motion of material associated with the ,great ,red ,spot1 with a rotational period of about six days4 ,interactions of smaller spots with the ,great ,red ,spot and with each other4 ,auroral emissions in the polar regions1 both in the ultraviolet 7which were not present during the #aigc ,pioneer encounter7 and in the visible4 ,cloud-top lightning bolts1 similar to terrestrial superbolts1 and meteoritic fireballs4 ,a temperature inversion layer in the stratosphere and a temperature of #afj ,k at the level at which the atmospheric pressure is #j4ja bar4 ,very strong ultraviolet emission from the disk1 indicating a thermospheric temperature of more than #ajjj ,k4 ,a hot 7#aajj ,k7 upper ionosphere on the dayside that was not observed by ,pioneer #aj1 suggesting there may be large temporal or spatial changes4 ,an atmospheric composition with volume fraction of helium of #j4aa #j4jc4 ,a substantially colder atmosphere above the ,great ,red ,spot than in the surrounding regions4 ,satellites and ,rings ,at least eight currently active volcanoes on ,io1 probably the result of tidal heating of the interior of the satellite1 with plumes extending up to #bej kilometers above the surface4 ,a large hot spot on ,io near the volcano ,loki that is about #aej ,k warmer than the surrounding surface4 ,numerous intersecting1 linear features on ,europa1 possibly due to crustal cracking4 ,two distinct types of terrain1 cratered and grooved1 on ,ganymede1 suggesting that the entire ice-rich crust was once under tension4 ,an ancient1 heavily cratered crust on ,callisto1 with vestigial rings of enormous impact basins since erased by flow of the ice-laden crust4 ,the elliptical shape of ,amalthea 7#bgj #afj kilometers74 ,a faint ring of material about ,jupiter1 with an outer edge of #abh #jjj kilometers from the center of the planet4 ,magnetosphere ,an electrical current of more than a million amperes flowing in the magnetic flux tube linking ,jupiter and ,io4 ,very strong ultraviolet emissions from ionized sulfur and oxygen in the ,io plasma torus1 indicating a hot 7hundred thousand degree7 plasma that evidently was not present at the time of ,pioneer #aj encounter4 ,plasma electron densities exceeding #dejj per cubic centimeter in some regions of the ,io plasma torus4 ,a cold1 corotating plasma inside #f ,r,j with ions of sulfur1 oxygen1 and sulfur dioxide4 ,high-energy trapped particles inside #f ,r,j with significantly enhanced abundances of oxygen1 sodium1 and sulfur4 ,hot plasma near the magnetopause predominantly composed of protons1 oxygen1 and sulfur4 ,jovian radio emission at kilometer wavelengths1 which may be generated by plasma oscillations in the ,io plasma torus4 ,corotating plasma flows unexpectedly far from ,jupiter in the dayside outer magnetosphere4 ,evidence suggesting a transition from closed magnetic field lines to a ,jovian magnetotail at about #be ,r,j from ,jupiter4 ,whistler emission interpreted as lightning whistlers from the ,jovian atmosphere4 #a;,adapted from a paper by ,e4 ,c4 ,stone and ,a4 ,l4 ,lane in the ,voyager #a ,thirty-,day ,report4 #b;,adapted from a summary prepared by ,e4 ,c4 ,stone and ,a4 ,l4 ,lane for the ,voyager #a ,thirty-,day ,report4 ,illustration3 ,the dramatic discovery of active volcanoes on ,io was made by ,linda ,morabito and her colleagues from this navigation picture1 taken ,march #h at a range from ,io of #d4e million kilometers4 ,on the bright edge1 the immense plume of volcanic ash from ,pele 7,p7 rises nearly #cjj kilometers above the surface4 ,at the terminator1 the border between day and night on ,io1 a second smaller cloud from the volcano ,loki 7,p7 catches the sunlight4 ,these two eruptions--captured on this single discovery photograph--are much larger than the largest terrestrial volcanic eruption known4 ,p-#bacjf ,b_/,w ,illustration3 ,once the existence of giant volcanic eruptions on ,io was recognized1 a reexamination of the ,voyager #a encounter pictures revealed many more plumes4 ,these two views of ,prometheus 7,p7 were found by ,joseph ,veverka and ,robert ,strom on ,march #ab when they reproduced earlier pictures4 ,illustration3 ,the plume is silhouetted against the black space1 although it is also possible to see dark 8feet0 where the falling material reaches the surface4 ,p-#babie ,illustration3 ,the complex jets of material are clearly seen as dark streaks against the light background of the surface of ,io4 ,the plume itself rose more than #ajj kilometers above ,io's surface4 ,p-#babid ,meanwhile1 new information about ,jupiter was released to the public4 ,a long-exposure 7three minutes and twelve seconds7 image of the dark side of the planet1 taken with the wide-angle camera while in the shadow of the planet1 caught ,jupiter showing off some ,jovian 8fireworks40 ,a long1 broad1 white streak across the picture was a visible aurora1 the largest aurora ever seen--almost #bi #jjj kilometers long4 ,in addition1 nineteen smaller bright splotches1 looking insignificant by comparison1 were in reality 8superbolts0 of lightning4 ,since huge electrical discharges such as lightning can1 under the right circumstances1 power chemical reactions that form complex organic molecules1 the discovery of lightning on ,jupiter could have profound implications4 ,was 8lightning-inspired0 organic synthesis going on in ,jupiter's atmosphere8 ,no one knew4 ,returning to ,,jpl on ,sunday night1 ,brad ,smith got his first look at the ,morabito picture of the volcanic cloud4 ,early ,monday morning1 other ,imaging ,team scientists saw it4 ,as soon as the ,,jpl computers were operating1 ,joseph ,veverka and ,robert ,strom began working with the two interactive ,,tv terminals to look for evidence in other pictures of ongoing eruptions4 ,faint clouds or plumes would not show up in normally processed pictures1 but could be brought out easily with the computer-controlled displays4 ,by midmorning1 several additional volcanic plumes had been found4 ,illustration3 ,linda ,morabito shows the discovery photo of the volcanic eruptions on ,io4 ,p-#bagah ,illustration3 ,the dark side of ,jupiter revealed many surprising phenomena to ,voyager #a4 ,a wide-angle view1 taken on ,march #e1 led to the discovery of a double auroral arc at north-polar latitudes and numerous flashes of lightning illuminating the clouds during this #c-minute1 #ab-second exposure1 taken at a range of half a million kilometers4 #bfj-dfj ,meanwhile1 on ,march #aa1 ,john ,pearl of the ,,iris team had independently drawn the conclusion that volcanic activity must be taking place on ,io4 ,he and ,rudy ,hanel found evidence of strongly enhanced thermal emission from parts of the satellite4 ,the most prominent was a source nearly #bjj ,c hotter than its surroundings4 ,on ,march #ab1 ,pearl brought his new results to the ,imaging ,team1 and sure enough1 the hot spot was located near one of the volcanic plumes6 ,a month later1 continuing analysis of ,,iris spectra yielded identification of sulfur dioxide 7,,so7 gas over this same erupting volcano4 ,at last a source had been located for the enigmatic sulfur and oxygen ions in the magnetosphere4 ,the volcanoes provided a thread with which to weave together the disparate data on ,io4 ,a few months earlier there had been a report of a sudden brightening of ,io in the infrared2 now it seemed plausible that thermal emission from an eruption was the source4 ,the ,voyager ultraviolet experimenters had been worrying over the source of the intense sulfur emissions they had seen and had been disturbed by the changes in the gas clouds around ,io since the ,pioneer #aj and #aa flybys2 now a variable source for these gas clouds was identified4 ,in addition1 the craterfree surface and bizarre features seen in the ,voyager images could be recognized as the product of violent explosive eruptions on ,io4 ,it appeared that ,peale1 ,cassen1 and ,reynolds had found1 in their theoretical calculations1 the key to the most geologically active body ever encountered in the solar system4 ,news of the discovery was released to the press on ,monday1 ,march #ab4 ,during the next few days1 a total of eight gigantic eruptions were located in the ,voyager pictures of ,io4 ,within a few weeks1 scientists all over the world were thinking with renewed energy about this incredible satellite4 ,with four new planet-sized satellites now photographed1 there was a sudden requirement for maps and for names to be assigned to the newly discovered features4 ,the maps were produced from ,voyager images at the ,astrogeology ,branch of the ,u4,s4 ,geological ,survey at ,flagstaff1 ,arizona4 ,the names1 proposed by a group of scientists headed by ,voyager ,imaging ,team members ,tobias ,owen and ,hal ,masursky1 were given official approval by the ,international ,astronomical ,union in ,august4 ,for a time1 a dual nomenclature persisted for the erupting volcanoes on ,io4 ,the eruption plumes were given numbers1 ,p1 ,p1 etc41 while the volcanic features were given names taken from the mythology of fire and volcano legends4 ,thus the 8hoof print0 of ,io was called ,pele1 for the ,hawaiian volcano goddess1 and the #bhj-kilometer-high plume associated with it was called ,p4 ,by the time of the ,voyager #b encounter1 scientists had prepared maps on which to plot their new discoveries4 ,while the ,voyager scientists fanned out across the world to share their findings with colleagues1 attention at ,,jpl turned to ,voyager #b4 ,in response to the discoveries of the first encounter1 changes were required in the sequencing of scientific observations for ,july4 ,voyager #b1 still troubled by a faulty receiver1 might require more coddling from the spacecraft team than had its sister spacecraft1 now safely on the way to ,saturn4 ,illustration3 ,the particles of the rings of ,jupiter are stronger reflectors of red light than of blue1 as can be seen in this view1 assembled from two images taken in orange and violet light4 ,since the images were registered on the rings1 not the planet1 the bright bands of colors along the edge of ,jupiter are an artifact of misalignment4 ,p-#baggi ,,chapter #g ,,the ,,second ,,encounter3 ,,more ,,surprises ,,from ,,the 8,,land0 ,,of ,,the ,,giant ,approaching ,jupiter ,at the beginning of ,july1 the dry summer heat had returned to ,pasadena1 and so had the press4 ,the scientists had come days or weeks earlier to look at data being transmitted from the second ,voyager as it approached ,jupiter and its satellites4 ,the mood at ,,jpl seemed quieter than it had been in ,march for ,voyager #a1 although the press room would once again be deluged with observers on the day of encounter4 ,this would be our second good1 close look at the ,jovian system1 but it was to be no summer rerun4 ,voyager #b would have a different view of each world1 and1 in addition1 both ,io and ,jupiter had undergone changes1 as though to ensure that no one would become bored and fall asleep in front of a ,,tv monitor4 ,in a sense1 this encounter was to be another first look at ,jupiter and its satellites1 with a view of each object that was quite different from what had been seen before4 ,changes in ,jupiter's cloud formations became noticeable long before ,july4 ,after a gap of six weeks following the first flyby1 ,voyager #b's observatory phase began on ,april #bd1 #aigi1 seventy-six days before its ,july #i encounter with ,jupiter4 ,during this time1 the spacecraft's ultraviolet and fields and particles instruments studied the ,jovian system and its interaction with the solar wind4 ,between ,april #bd and ,may #bg1 ,voyager #b's imaging system concentrated on the motions in ,jupiter's atmosphere1 creating another approach time-lapse 8movie40 ,from ,may #bg to #bi photographs were taken in a more rapid sequence1 showing the planet during five #aj-;hour rotations4 ,from these studies it was apparent that ,garry ,hunt's prediction had been right--the weather had changed by ,july4 ,a month before the encounter ,,jpl's ,voyager ,bulletin--,mission ,status ,report announced that 8,jupiter is sporting quite a different face than it did just four months ago4 ,the bright #8tongue' extending upward from the ,red ,spot is interacting with a thin1 bright cloud above it that has traveled twice around ,jupiter in four months40 ,the turbulent region west of the ,great ,red ,spot had begun to break up and separate from the ,red ,spot4 ,the white ovals south of the ,red ,spot had drifted to the east 7about #j4ce degrees a day71 while the ,red ,spot itself had drifted west 7about #j4bf degrees a day74 ,the white zone seen just south of the ,red ,spot by ,voyager #a had become very narrow--like a thin white line just barely outlining the bottom of the spot4 ,the ,red ,spot had also changed3 ,it had become a more uniform orange-red1 perhaps reverting to the color seen by ,pioneers #aj and #aa4 ,the brown spots that had been seen in the north temperate region at the same longitude as the ,red ,spot were now on the other side of the planet4 ,a dark brown spot not present during the ,voyager #a flyby had developed along the northern edge of the brown equatorial region on the ,red ,spot side of the planet4 ,some of the white markings that seemed to have protruded into the equatorial region at the time of the first flyby were missing in the ,voyager #b photographs4 ,as ,voyager #b entered the far encounter period on ,may #bi1 all instruments on the spacecraft 7except for the photopolarimeter7 seemed to be in good shape for encounter4 ,as was the case with ,voyager #a1 the polarization wheel on ,voyager #b's photopolarimeter was stuck1 so the instrument was able to obtain only color photometry measurements4 ,illustration3 ,in early ,june1 as ,voyager #b carried out its observatory phase1 additional changes in ,jupiter's face began to be apparent4 ,these two images1 taken from a distance of #bd million kilometers1 have a resolution of about #ejj kilometers4 ,illustration3 ,the ,great ,red ,spot and the white oval south of it are seen to be followed on the west by regions of chaotic and turbulent clouds4 ,this is not the same white oval that was near the ,red ,spot in ,march2 the differential rotation of the planet carried a different oval close to the ,red ,spot during the intervening three months4 ,p-#bagac;,c ,illustration3 ,io is visible to the right of the planet1 and the shadow of ,ganymede falls on the colored clouds of ,jupiter's equatorial belt4 ,p-#bagad;,c ,illustration3 ,the ,voyager #b trajectory was complementary to that of ,voyager #a4 ,this time1 the satellites were encountered before ,jupiter1 revealing their other hemispheres4 ,as shown in this drawing1 the spacecraft flew by first ,callisto1 then ,ganymede1 then ,europa4 ,the ten-hour ,io volcano watch took place immediately after closest approach to ,jupiter4 #bfj-ecc;,a ,voyager #b trajectory ,view normal to ,jupiter equator ,sun occultation ,earth occultation ,launch date #h_/bj_/gg ,jupiter arrival date #g_/i_/gi ,periapsis ,satellite closest approach ,amalthea ,europa ,io ,ganymede ,callisto ,illustration3 ,these two faces of ,jupiter were photographed by ,voyager #b on ,may #i at a distance of #df million kilometers from the planet4 ,voyager scientists began to detect significant changes in the cloud patterns since the ,voyager #a encounter two months earlier4 #bfj-ejg ,illustration3 ,jupiter4 ,illustration3 ,jupiter4 ,illustration3 ,the weather is changing over one of the northern hemisphere brown ovals in this picture taken ,july #f4 ,the brown ovals are regions in which breaks in the upper layer of ammonia clouds reveal darker clouds below4 ,a high1 white cloud is seen moving over the darker cloud1 providing an indication of the structure of the cloud layers4 ,thin white clouds are also seen within the dark cloud4 ,at right1 blue areas1 free of high clouds1 are seen4 ,p-#bagec;,c ,although ,voyager #b's radio receiver still could not track a ,doppler-shifted radio signal from ,earth 7the problem is that it 8hears a monotone10 explained ,deputy ,project ,manager ,esker ,k4 ,davis71 the ,deep ,space ,network engineers had learned to work with the spacecraft1 determining what frequency the spacecraft would listen to at any particular time4 ,they had discovered that some of the 8housekeeping0 telemetry signals from the receiver were sensitive to the match between the incoming frequency and the receiver frequency4 ,by monitoring these signals1 they could detect a frequency drift in time to correct the transmission1 thus keeping the system in tune in spite of slow changes in the receiver4 ,the system was slow and demanding but effective2 all the necessary command sequences were successfully loaded into the computer1 and communications during the encounter were entirely successful4 ,the timing offset experienced by ,voyager #a as a result of ,jupiter's intense radiation environment was not expected to be a problem on ,voyager #b for two reasons3 ,even at closest approach1 ,voyager #b would still be more than twice as far from ,jupiter as ,voyager #a had been1 and the ,voyager #b computer was programmed to resynchronize the spacecraft's timing systems automatically every hour4 ,in this way1 even if the radiation environment proved to be much higher than anticipated1 the image smear that might occur from a timing offset would be prevented4 ,illustration3 ,complex activity in the southern hemisphere of ,jupiter continued during the ,voyager #b encounter1 although changes had occurred in the region of the ,great ,red ,spot4 ,a white oval1 different from the one observed in a similar position at the time of the ,voyager #a encounter1 was situated south of the ,red ,spot4 ,the region of white clouds extended from east of the ,red ,spot and around its northern boundary1 preventing small cloud vortices from circling the feature4 ,the disturbed region west of the ,red ,spot had also changed since the equivalent ,voyager #a image4 ,the picture was taken on ,july #c from a distance of #f million kilometers4 ,p-#bagdb;,c ,as a result of the discoveries made by ,voyager #a1 the project scientists decided to modify some of ,voyager #b's preplanned sequences4 ,as early as ,april #a1 the painstaking job of constructing new computer commands began4 ,a ten-hour ,io ,volcano ,watch was added to the spacecraft's program1 taking advantage of the fact that shortly after closest approach to ,jupiter1 the spacecraft would remain within about #a million kilometers of ,io for a long period1 keeping nearly the same face in view4 ,provisions were also made to take extensive ultraviolet measurements of the emission from the glowing torus surrounding ,jupiter near the orbit of ,io4 ,further studies would be made of the dark side of ,jupiter to search for lightning and auroral activity1 and there was also the hope that the plasma wave instrument would be able to detect lightning whistlers 7radio signals created by lightning bolts7 as the ,voyager #a instrument had done4 ,a high priority was given to observations of the newly discovered ring1 which had not been in the original ,voyager #b sequence at all4 ,the spacecraft would cross the ring plane twice1 photographing the ring during both the inbound and the outbound passages4 ,as was originally planned1 ,voyager #b would make its closest approaches to ,callisto1 ,ganymede1 ,europa1 and ,amalthea before encounter with ,jupiter4 ,because of the difference in the trajectories of the two spacecraft1 ,voyager #b would see the faces of ,callisto and ,ganymede not seen by ,voyager #a4 ,the most important difference1 however1 was that the second ,voyager would fly much closer to ,europa than ,voyager #a did1 giving scientists their first good look at the mysterious streaks scratched on the surface of that bright golden world4 ,voyager #b would also have a closer flyby of ,ganymede1 giving us a second chance to examine its strange 8snowmobile tracks40 ,the major loss1 of course1 was ,io1 which would be seen from ,voyager #b only at distances of a million kilometers or more4 ,illustration3 ,io appeared in front of ,jupiter as seen by ,voyager on ,june #be1 at a range of #ab million kilometers4 ,at a resolution of about #bjj kilometers1 the bright and dark spots on the satellite are just beginning to be resolved1 but it was not possible to determine if any eruptions were still in progress4 ,p-#bagai;,c ,the ,encounter ,wednesday1 ,july #d4 7,range to ,jupiter1 #e4c million kilometers2 range to ,earth1 #iba million kilometers74 ,while most of the nation celebrated ,independence ,day with picnics1 sports events1 and fireworks1 the scientists and engineers at ,,jpl were working around the clock4 ,voyager #b had already entered ,jupiter's territory1 crossing the bow shock for the first time on ,july #b at a distance of #ii ,r,j from ,jupiter1 indicating that the magnetosphere had expanded in the interval between the two encounters4 ,at about noon on ,july #c1 the spacecraft encountered the magnetopause1 but on ,july #d1 the data from the particles and fields instruments were ambiguous4 ,apparently the magnetosphere was pulsating in response to changing pressures1 and the spacecraft was playing tag with the rapidly shifting boundaries of the bow shock and the magnetopause4 ,as the low energy charged particle instrument began to measure particles coming from inside the ,jovian magnetosphere1 it became apparent that some important changes had taken place since ,voyager #a's encounter4 ,from the composition of the particles1 it appeared that they were largely of solar origin1 unlike the heavy concentrations of ions of sulfur and oxygen seen by ,voyager #a4 ,scientists began to speculate that the ,io volcanoes1 which presumably eject sulfur and oxygen into the magnetosphere1 might have declined in activity4 ,in the evening1 the first images of ,io at a resolution high enough to allow the volcanic plumes to be seen would be beamed back to ,earth4 ,,encounter ,,distances ,,for ,,voyager #b ,object ,range to ,center at ,closest ,best ,image ,resolution ,approach 7kilometers7 7km per line pair7 ,jupiter #gbb #jjj #ae ,amalthea #eeh #jjj #aj ,io #a #acj #jjj #bj ,europa #bjf #jjj #d ,ganymede #fb #jjj #a ,callisto #bae #jjj #d ,illustration3 ,voyager scientists anxiously awaited the first views of ,io that would show whether the volcanic eruptions seen in ,march were still active4 ,this picture was taken on ,july #d1 at a range of #d4g million kilometers1 about the same as that of the volcano discovery picture on ,march #h4 ,one large plume is clearly visible1 rising nearly #bjj kilometers above the surface4 ,at the time of release of this picture on ,july #f1 the scientists wrote1 8,the volcano apparently has been erupting since it was observed by ,voyager #a in ,march4 ,this suggests that the volcanoes on ,io probably are in continuous eruption40 ,p-#bagch;,b_/,w ,thursday1 ,july #e4 7,range to ,jupiter1 #d4d million kilometers74 ,the press room at ,von ,karman ,auditorium opened and the members of the press1 most of them veterans of the first encounter1 arrived at ,,jpl4 ,meanwhile1 the spacecraft continued to measure fluctuations in the magnetospheric boundary4 ,by noon1 ,,jpl had reported at least eleven crossings of the bow shock as the solar wind flirted with ,jupiter's magnetosphere4 ,apparently the solar wind was much more variable in ,july than it had been in ,march4 ,at times the bow shock seemed to be thicker than that experienced by ,voyager #a2 one ,voyager #b crossing took ten minutes1 whereas the longest ,voyager #a crossing was only one minute long4 ,even though the processes affecting the magnetosphere seemed more complex1 the magnetosphere was less compressed2 when ,voyager #b actually entered the magnetosphere at a distance of #fb ,r,j1 it was much farther from ,jupiter than ,voyager #a had been at its final crossing 7#dg ,r,j74 ,photos obtained the day before from over #d million kilometers showed that at least one of ,io's volcanoes was still active4 ,a total of eight ongoing eruptions had been seen by ,voyager #a1 and scientists were anxious to see how many of these were still erupting four months later4 ,,voyager #b ,,bow ,,shock 7,s7 ,,and ,,magnetopause 7,m7 ,,crossings ,boundary ,day ,distance 7,r,j7 ,inbound ,s #g_/jb #ii 7multiple7 ,s #g_/jb #ig ,s #g_/jc #hg ,m #g_/jd #gb 7multiple7 ,m #g_/je #ga ,s #g_/je #fi ,s #g_/je #fg ,m #g_/je #fb ,outbound ,m #g_/bc #afi ,m #g_/bc #agc ,m #g_/bd #agd ,m #g_/bd #age ,m #g_/bd #agf ,m #g_/bd #agg ,m #g_/be #ahd ,m #g_/be #ahe ,m #g_/bg #bac ,m #g_/ca #bec ,m #h_/ja #beh ,m #h_/ja #bfb 7multiple7 ,m #h_/jc #bgi 7multiple7 ,s #h_/jc #bhc 7multiple7 ,illustration3 ,although ,voyager #b did not come as close to ,io as had ,voyager #a1 some changes in the surface during the four months between encounters were so large that they could still be easily seen4 ,these two pictures of the region of the volcano ,pele were taken in early ,march and early ,july1 respectively4 ,the most dramatic change was the filling in of the indentation in the ejecta ring1 turning the hoofprint into a symmetric oval4 ,the oval is about #ajjj by #gjj kilometers in outermost dimension1 and the area that changed amounts to more than #aj #jjj square kilometers4 #bfj-fhg;,,ac ,illustration3 ,volcano ,pele on ,io4 ,illustration3 ,volcano ,pele on ,io4 ,while attention at ,,jpl focused on the unfolding drama of the ,jupiter encounter1 many members of the world's press seemed more interested in the fate of ,skylab1 which was nearing its death plunge into the ,earth's atmosphere4 ,launched in #aigc1 ,skylab had been one of ,,nasa's more successful projects4 ,three crews of astronauts had visited it1 carrying out intensive studies of the ,sun and breaking one record after another for the duration of manned space flight4 ,since the departure of the final group of three astronauts in #aigd1 ,skylab had been sinking gradually lower as a result of friction with the extreme upper atmosphere of ,earth4 ,during the past year1 higher temperatures in the atmosphere had increased this drag1 and now the end was near4 ,with a strange fascination1 the world watched the end of this old spacecraft1 almost seeming to forget the spectacular new results being transmitted from ,jupiter4 ,to the frustration of the ,voyager team and the press 8camped out0 in ,von ,karman ,auditorium for the second encounter1 the exaggerated stories of a possible ,skylab disaster took precedence over ,voyager news4 ,ultimately1 ,skylab fell over the ,indian ,ocean and ,australia on ,wednesday1 ,july #aa1 just as the major findings of ,voyager #b were being released4 ,friday1 ,july #f4 7,range to ,jupiter1 #c4e million kilometers74 ,with the first satellite encounter still two days away1 ,voyager #b continued to make a variety of measurements of ,jupiter and all the ,galilean satellites4 ,as the distance to ,io decreased1 it was possible to see detailed surface features as well as to look for the volcanic plumes at the edge of the disk1 silhouetted against black space4 ,by the end of the day1 the ,great ,red ,spot loomed so large that six imaging frames 7a #b #c mosaic7 were required to encompass it and its immediate surroundings4 ,at the first formal press conference of the ,voyager #b encounter1 ,project ,scientist ,ed ,stone reviewed the progress of the mission4 ,because the ailing spacecraft receiver was working so well1 ,ray ,heacock1 ,voyager ,project ,manager1 announced that the major trajectory correction maneuver at ,jupiter had been rescheduled to take place only two hours after closest approach4 ,since the geometry was especially favorable at this time1 the #gf-minute rocket burn could put the spacecraft on its planned route to ,saturn with a minimum expenditure of fuel1 thereby preserving the option of sending the spacecraft on to ,uranus4 ,the ,io torus was under observation1 both directly by the ultraviolet spectrometer1 and indirectly by the charged particle instruments4 ,the ,,lecp instrument had begun to pick up sulfur ions1 but at lower energies and lower concentrations than those recorded during the first encounter4 ,in the ultraviolet1 glows could be seen both from the torus and from aurorae in the polar regions of ,jupiter4 ,photographs of ,io showed that the heart-shaped feature surrounding ,pele 7,p71 ,io's largest volcano1 had changed shape4 ,the indentation of the heart had disappeared1 making the heart into an oval4 ,apparently a new deposit of volcanic ejecta had blanketed the surface1 altering its color4 ,perhaps an earlier obstruction in the volcanic vent1 or the shape of the vent itself1 had caused the area surrounding ,pele to look heart-shaped4 ,in any case1 whatever had caused the indentation was now gone4 ,at the same time1 new photos failed to show a plume above ,pele1 and there was speculation that changes in this eruption might be related to the altered population of charged particles in the magnetosphere4 ,illustration3 ,the ,voyager #b pictures of ,callisto looked remarkably similar to those obtained of the other side of the satellite by ,voyager #a4 ,seen from a distance of #b4c million kilometers1 the large craters 7#ajj kilometers or more across7 appear as light spots4 ,no new major impact features such as ,valhalla1 discovered by ,voyager #a1 are visible on the hemisphere seen by ,voyager #b4 ,p-#bagdj;,c ,saturday1 ,july #g4 7,range to ,jupiter1 #b4f million kilometers74 ,as the spacecraft rapidly closed on ,callisto1 better and better photographs were taken of the previously unseen hemisphere4 ,as with the ,voyager #a observations1 however1 the main impression was one of heavy cratering1 unrelieved by other geologic structures4 ,meanwhile1 the coverage of ,io had improved as the satellite rotated to the point at which a census of the volcanic eruptions seen in the first encounter began to emerge4 ,at the #aa3jj a4m4 press conference1 ,larry ,soderblom announced that four of the volcanoes discovered by ,voyager #a had been looked at again by ,voyager #b1 and three of them--,prometheus 7,p71 ,loki 7,p71 and ,marduk 7,p7--were still active4 ,however1 there was no trace of volcanic activity coming from ,pele1 the source of the largest plume seen by ,voyager #a4 ,p was either greatly subdued or had turned off completely4 ,dr4 ,soderblom also announced that ,voyager #b images had detected another giant ring structure on ,callisto1 bringing the total to three1 and there were probably more4 ,this particular ring feature was estimated to be about #aejj kilometers across4 ,it was also noted that although ,callisto generally seemed to be saturated with shoulder-to-shoulder craters1 the crater density near the ring structures seemed to be lower4 ,saturday was a fairly quiet time for the scientists but not for the spacecraft or the spacecraft team4 8,we blocked out about #g hours10 explained ,michael ,devirian1 ,ground ,data ,systems ,development1 ,integration1 and ,test ,director1 8in which we could send it a set of commands and re-send it if necessary to make sure all close-encounter commands were received by ,voyager #b until all the commands got through4 ,the whole thing went perfectly the first time40 ,so everything was 8go0 for close encounter4 ,the near encounter phase began at #f3cf p4m4 ,,pdt4 ,illustration3 ,a new face of ,ganymede was revealed to ,voyager #b4 ,this image was taken ,july #g from a distance of #a4b million kilometers and clearly shows the large dark area ,regio ,galileo1 as well as much of the lighter grooved terrain discovered by ,voyager #a4 ,the bright spots are impact craters4 ,this image also shows what appear to be polar caps1 extending down to about latitude #de in both the northern and southern hemispheres4 #bfj-fgj ,sunday1 ,july #h4 7,range to ,jupiter1 #a4e million kilometers74 ,at #b3cj a4m4 the first long-exposure sequence of ring pictures was taken1 and at #c3jj a4m4 the intensive period of the ,callisto encounter began4 ,eighty high-resolution images were obtained of the satellite1 centered around closest approach 7#bae #jjj kilometers7 at #f3ac a4m4 ,incoming photos showed some features that looked like double-walled craters1 but no more giant ring structures were seen4 ,it appeared that there was an asymmetry in the distribution of large impact features over ,callisto's surface4 8,callisto may turn out to be the most heavily cratered body in the solar system10 ,torrence ,johnson remarked4 ,garry ,hunt was to add later on1 8,there's just not room for another crater on that body--it's totally full40 ,at the press conference1 ,brad ,smith confirmed the earlier finding that ,io's volcano ,pele was quite dead--at least for now4 ,although ,p had not yet been looked at1 all other volcanoes discovered by ,voyager #a were still active1 but no new plumes had been found4 ,however1 new ultraviolet images of ,p 7,loki7 suggested that the eruption had increased in size4 7,in a later report1 the imaging team announced that ,p had increased in height to #age kilometers and had changed to a two-column plume47 ,the new photographs of ,jupiter's ring showed it to be quite narrow and ribbonlike1 ,dr4 ,smith announced4 ,the artist's drawing 7released during the ,voyager #a encounter71 intended to show the outer edge of the ring1 turned out to be a good representation of the actual ring1 ,dr4 ,smith said4 ,there seemed to be less high-speed sulfur and oxygen inside ,jupiter's magnetosphere than there had been during the ,voyager #a encounter1 ,george ,gloeckler announced4 ,voyager #b's low energy charged particle instrument was finding substantial amounts of carbon1 silicon1 magnesium1 and other elements of solar origin1 but the ,io-associated elements were almost depleted4 ,the ultraviolet instrument had found as much glowing sulfur in ,io's torus as before1 but less of it seemed to be raised to energies high enough to leave the torus and be detected elsewhere in the magnetosphere4 ,there were other indications of ,jupiter's changing weather4 ,in a ,voyager report ,sunday evening1 ,garry ,hunt remarked1 8,one very exciting observation came the other day which caused major excitement down in the imaging area4 ,we actually saw a white cloud starting to intrude across a dark barge large brownish oval-shaped feature in ,jupiter's northern hemisphere4 ,atmospheric scientists get very excited by that because this is showing us how the colors layer themselves up--that white cloud is clearly above the dark brown4 ,we're desperately trying to understand the relationship of colors on ,jupiter40 ,illustration3 ,the first close-up views of ,europa were both exciting and perplexing to ,voyager scientists4 ,the best ,voyager #a resolution had been only about #cj kilometers1 but the ,voyager #b trajectory permitted a much closer flyby4 ,these pictures1 taken on ,july #i at a range of #bdj #jjj kilometers1 have a resolution of about #e kilometers4 ,the bright icy crust of ,europa is covered with a spectacular series of dark streaks1 giving the satellite a cracked appearance4 ,in a few cases1 narrower light lines run down the centers of the dark streaks1 which are typically a few tens of kilometers in width4 ,very few1 if any1 impact craters are visible on ,europa4 ,p-#bagfj;,c and ,p-#bagfd;,c ,illustration3 ,europa ,monday1 ,july #i4 7,range to ,jupiter at encounter1 #gbb #jjj kilometers74 ,encounter day6 ,and not just one encounter1 but a whole sequence3 ,ganymede1 ,europa1 ,amalthea1 ,jupiter1 and ,io4 ,by early ,sunday evening1 a wealth of new data on ,ganymede was pouring in4 ,not only was this a side of the satellite not seen before1 but ,voyager #b would pass closer to ,ganymede than had ,voyager #a4 ,encounter took place at #a3jf a4m41 at a range of #fb #jjj kilometers4 ,between #i3jj p4m4 ,sunday night and #a3cj a4m4 ,monday morning a total of #bag photos1 plus infrared and ultraviolet spectra1 were scheduled4 ,sixty-nine photos were sent back in real time2 others were recorded for playback later4 ,as the ,ganymede pictures appeared on the ,,tv screens1 they revealed a world of tremendous variety4 ,some regions were heavily cratered3 8,ganymede looks like ,mercury or the highlands of the moon10 one ,voyager scientist remarked4 ,other parts of the surface1 however1 showed very different features3 long1 parallel mountain ridges that looked like grooves made with a giant's rake2 narrow1 segmented lines2 white ejecta blankets from impacts that looked like a dazzling1 snow-covered landscape4 ,some of the pictures suggested cracking and slipping of ,ganymede's crust1 while others showed what appeared to be remnants of ancient terrain unaffected by subsequent intense geologic activity4 ,many of the highest resolution frames were not seen at this time2 they were recorded on the spacecraft for playback later4 ,starting at about #h a4m41 ,earth began receiving the first closeup views of ,europa4 ,europa 8could be the most exciting satellite in the whole ,jovian system10 said ,larry ,soderblom1 8because it's sort of the transition body between the solid silicate body1 ,io1 and the ice balls1 ,ganymede and ,callisto40 ,the icy crust looked as though it 8had been ruptured all over--as though it was in pieces--just as though it had been broken in place and left there40 ,at #aa3dc a4m41 closest approach took place at a range of #bjf #jjj kilometers4 ,by this time the scientists were dazzled by what they had seen2 some were calling ,europa the most bizarre of all the ,galilean satellites4 ,in the ,imaging ,team viewing area1 ,david ,morrison compared ,europa's surface to 8a cracked egg10 and ,gene ,shoemaker said1 8,it looks like sea ice to me40 ,when someone commented that the canal-like streaks were reminiscent of ,mars1 ,torrence ,johnson replied1 8,it looks like some pictures of ,mars ,i've seen1 but only on the walls of ,lowell ,observatory40 ,another quipped1 8,where is ,percival ,lowell1 now that we need him60 ,there were to be two press conferences3 one to present spacecraft and scientific results and one to celebrate the second successful flyby and to talk of new goals--,saturn1 and perhaps ,uranus4 ,at the first conference1 ,ed ,stone began by discussing the radiation ,voyager was experiencing4 ,one of ,jupiter's surprises was that the radiation environment was greater than had been anticipated1 and this caused problems with the radio receiver4 ,the receiver frequency was shifting 8more rapidly than we had anticipated10 said ,ray ,heacock1 8and we have not been able to keep an uplink continuously with the spacecraft40 ,the solution was to keep sending up commands at different frequencies until a frequency the spacecraft would accept was found4 ,just how bad were the radiation levels8 ,ed ,stone commented1 8,the penetrating radiation at a given distance is more intense now at this distance than it was when ,voyager #a flew by40 ,from a preliminary analysis it seemed that1 overall1 ,voyager #b would still be subjected to lower radiation levels than ,voyager #a had been1 but to higher levels than had been expected4 ,in addition1 ,voyager #b's radio receiver was much more sensitive4 ,the higher than expected radiation intensity also led ,voyager scientists to have the ultraviolet spectrometer shut off1 since that instrument was also quite sensitive to the radiation4 ,the fourth member of the ,galilean satellites had finally been seen1 and ,larry ,soderblom happily introduced ,europa4 8,well1 some few months ago1 before the ,voyager #a encounter1 we thought we had some idea of what planets were like--at least the planets in the inner solar system3 ,mars1 ,mercury1 the ,moon1 the ,earth4 ,and we've discovered many times over in the last couple of months how narrow our vision really was4 ,included in the ,jovian collection of satellites are the oldest 7,callisto71 the youngest 7,io71 the darkest 7,amalthea71 the brightest 7,europa71 the reddest 7,amalthea and ,io71 the whitest 7,europa71 the most active 7,io71 and the least active 7,callisto74 ,today we found the flattest 7,europa740 ,in spite of the appearance of a cracked or broken surface1 ,europa showed no topography at all4 ,toward the sunset line1 where the low angle of illumination should reveal even low relief1 8the surface disappears--as if it were the surface of a billiard ball40 ,it seemed clear that ,europa has much less relief than the other two icy satellites1 ,ganymede and ,callisto4 ,but why can't ,europa's surface support relief8 ,perhaps ,europa has a thick ice mantle--on the order of #ajj kilometers4 ,if ,europa is affected by tidal heating1 then such an ice mantle might be 8sort of soft and slushy0 rather than rigid as are the crusts of ,ganymede and ,callisto4 8,the fact that the surface of ,europa cannot support relief of any substantial amount suggests that the surface must be soft40 ,but1 ,dr4 ,soderblom added1 there does not appear to be much lateral motion or rotation causing the surface markings--they don't seem to be offset--rather1 8it's as if ,europa had been cracked1 broken1 by some process which crushed it like an eggshell and just left the pieces sitting there4 ,expansion and contraction of ice and water are a good way to crunch up the surface40 ,illustration3 ,regio ,galileo is the largest remnant of the ancient1 heavily cratered crust of ,ganymede4 ,this ,voyager #b color reconstruction was made from pictures taken at a range of #caj #jjj kilometers2 the scene is about #acjj kilometers across4 ,numerous craters1 many with central peaks1 are visible4 ,the large bright circular features have little relief and are probably the remnants of old large craters that have been annealed by the flow of icy near-surface material4 ,the closely spaced1 arcuate linear features are analogous to features on ,callisto1 such as the 8ripple0 marks surrounding the ancient impact feature ,valhalla4 ,p-#bagfa;,c ,illustration3 ,at high resolution1 the grooved terrain on ,ganymede shows a wonderful complexity4 ,surface features as small as #a kilometer across can be seen in this mosaic of ,voyager #b images taken ,july #i4 ,the grooves are basically long1 parallel mountain ridges1 #aj to #ae kilometers from crest to crest--about the same scale as the ,appalachian mountains in the ,eastern ,united ,states4 ,the numerous impact craters superposed on the mountain ridges indicate that they are old--probably formed several billion years ago4 #bfj-fcg ,the other side of ,ganymede presented quite a different face from the one ,voyager #a had seen4 ,here were the dark ancient cratered terrains1 the shoulder-to-shoulder craters reminiscent of ,callisto1 and there was a huge circular feature on ,ganymede looking like the remnant of a ,callisto-style ringed basin1 preserved in the ancient1 dark terrain4 ,the very large dark feature revealed by ,voyager in the northern hemisphere which bears these impact scars was later named 8,regio ,galileo10 for the discoverer of the ,galilean satellites4 ,it was seen in the low-resolution ,pioneer #aj picture of ,ganymede taken in #aigc1 but its nature was not understood4 ,it is so large it has even been glimpsed on occasions of exceptionally stable 8seeing0 with ground-based telescopes4 #c3bi p4m4 ,,pdt--,jupiter ,encounter6 ,in the press room half a dozen cameras clicked in unison as the universal clock declared the ,voyager #b had made its closest approach to ,jupiter--#fej #jjj kilometers from the cloud tops1 zipping by at about #gc #jjj kilometers per hour--neither as close nor as fast as ,voyager #a4 ,by the time of the special press conference at #d3cj p4m41 everyone at ,,jpl was in a party mood4 ,thomas ,a4 ,mutch1 who had replaced ,noel ,hinners as ,,nasa ,associate ,administrator for ,space ,science1 ,robert ,parks1 and ,rodney ,mills were the speakers4 ,the ,jovian system is a place of 8incredible beauty and mystery4 ,jupiter has been a nice place to go by1 but we wouldn't want to stop there--we're going on to ,saturn10 ,rod ,mills explained1 and ,bob ,parks agreed4 ,tim ,mutch had a different perspective4 8,although we have just heard ,jupiter somewhat downgraded in favor of ,saturn1 nonetheless what we have been witnessing1 first in ,march and now1 in ,july1 is a truly revolutionary journey of exploration4 ,we have gone beyond the familiar part of the solar system to objects that are so exotic that their very existence1 at least as far as ,i'm concerned1 was something ,i'd accepted intellectually1 but didn't really accept in an immediate sense4 ,we're starting out in our own space program on a new stage of space exploration--on our own long journeys beyond the solar system to distant lands4 ,we never like to think1 or rather1 it's statistically unlikely1 that we're at a turning point in history4 ,but if you look back at history books1 such events are clearly read into the record4 ,and ,i submit to you that when the history books are written a hundred years from now1 two hundred years from now1 the historians are going to cite this particular period of exploration as a turning point in our cultural1 our scientific1 our intellectual development40 ,although everyone was already celebrating another successful mission1 the encounter was far from over4 ,data continued to come in2 there was still the ten-hour ,io ,volcano ,watch1 which had begun at #d3ca p4m42 there were more observations of ,jupiter1 including scheduled ring observations and dark side searches for aurorae and lightning bolts4 ,there was a lot of work and excitement yet to come4 ,jupiter had another surprise in store for ,voyager #b4 ,illustration3 ,during the #aj-;hour ,io volcano watch on ,july #i1 the spacecraft kept nearly the same face of ,io in view4 ,most of the surface was turned away from the ,sun1 however1 and only a thin crescent could be seen1 shrinking as the observations continued4 ,these four frames were all photographed with identical exposures from a range of about #a million kilometers4 ,these images show ,amirani 7,p7 and ,maui 7,p7 on the west edge1 brightening as the ,sun illuminates them more nearly from behind4 #bfj-fgg ,illustration3 ,io volcanoes4 ,illustration3 ,masubi 7,p7 is faintly visible in the crescent 7above and below74 ,illustration3 ,io volcanoes4 ,illustration3 ,loki 7,p7 rises #bej kilometers above the surface1 catching the morning sunlight on the east edge of ,io4 ,tuesday1 ,july #aj4 7,range to ,jupiter1 #a4d million kilometers74 ,the ,io watch continued through the night4 ,as time passed1 the satellite rotated in the same direction as the motion of the spacecraft1 keeping nearly the same side in view4 ,because of this1 a few volcanoes could be closely watched1 but most would be missed entirely4 ,during the sequence1 the illuminated crescent steadily shrank1 until at the end1 volcanic plumes could be seen on both edges1 one illuminated by the setting ,sun1 the other shining in the dawn light4 ,at the #aa a4m4 press conference1 ,esker ,davis announced that engineers had lost contact with the spacecraft radio receiver ,monday evening 7probably due to ,jupiter's radiation7 and had to 8chase it around most of the night10 sending commands at various frequencies until they locked on to the frequency the spacecraft would accept4 ,the major trajectory correction maneuver1 begun at about the same time contact with the receiver was lost1 was successful4 ,the #gf-minute thruster firing1 done at periapsis instead of two weeks after encounter1 enabled the spacecraft to get a bigger 8boost0 from ,jupiter than was originally planned1 amounting to a fuel saving of about #aj kilograms of hydrazine1 enough to preserve the option of going on from ,saturn to ,uranus4 ,andrew ,ingersoll discussed some results of the analysis of the ,jovian atmosphere4 8,at first1 ,voyager seemed to do nothing but emphasize the chaos1 not the order40 ,but1 with the help of ground-based observations1 ,reta ,beebe found that there is a 8regular alternation of eastward and westward jets0 underlying the seemingly chaotic visible features4 8,the turbulence we see in the visible clouds seems to be a minor side show1 or a process without much energy or mass compared to the very great energy and mass that might be moving around in the deep atmosphere40 8,we're continuing to operate in our panic mode to try to get pictures to the press10 ,brad ,smith said as he introduced new photographs of the satellites4 ,in earlier photos1 ,ganymede had seemed to have two different kinds of terrain--an ancient1 cratered1 ,callisto-like surface1 and the stranger1 grooved terrain--terrains that might be representative of two very different types of major episodes in ,ganymede's history4 ,the most recent images showed a much more confused picture1 with several additional types of surface geology4 ,at the daily project science briefing1 another interpretation was being discussed4 ,lyle ,broadfoot reported that new measurements of the position of the ultraviolet aurora demonstrated that it was caused by charged particles from the ,io torus1 not from the outer parts of the magnetosphere4 ,apparently these plasma particles arise in the volcanic eruptions1 are trapped for a time in the torus1 and then fall into the polar regions of ,jupiter1 where they excite auroral emissions4 ,a terrestrial aurora1 in contrast1 is caused by particles that originate in the solar wind4 ,jim ,sullivan of the plasma investigation estimated that about two tons of material each second are fed from ,io into the plasma torus4 ,this plasma1 driven by the rotation of the ,jovian magnetic field1 appears to be able to supply the million-million watts of power radiated in the ultraviolet4 ,by #e3jj p4m4 the excitement had died down2 many of the scientists had parties to attend that evening1 and some members of the press were planning parties of their own4 ,the schedule of spacecraft activities also seemed to have slowed4 ,there were dark-side observations planned to search for lightning and aurorae4 ,there would be a few more ring pictures--not too much to see on the monitors that night 444 or so many people thought4 ,but a few people were waiting around1 perhaps to catch a glimpse of lightning or auroral activity1 or to wait for another look at ,jupiter's faint ring4 ,between #e3eb and #f3af p4m41 six long-exposure1 wide-angle photographs of the dark side of ,jupiter had been scheduled to search for aurorae and lightning4 ,the spacecraft was #a #dej #jjj kilometers from ,jupiter and about two degrees below the equatorial plane4 ,shortly after #f p4m41 the first of these ring photographs appeared on the ,,tv monitors with unexpected brilliance4 ,taken in orange and violet light1 the images showed the outline of ,jupiter and1 protruding from it1 two narrow lines--one reaching all the way to ,jupiter's limb1 the other broken off1 apparently hidden by the shadow of the giant planet4 ,seen from the new perspective of the shadow of ,jupiter1 the tenuous rings were remarkably clear4 ,a sudden renewal of excitement surged through the devotees remaining in the press room4 ,about #f3ae1 ,brad ,smith came down to join the press to watch the remainder of this series of pictures come in4 8,hey ,brad1 are you going to burn out the camera with the ring80 someone joked4 8,well1 the rings do forward scatter nicely1 don't they80 ,dr4 ,smith replied4 ,as the wide-angle pictures were followed by narrow-angle views1 more and more detail became apparent4 ,for the first time1 a definite width for the ring could be seen1 and there was even a hint of additional material inside the main ring4 ,all in all1 ,voyager had provided one more splendid series of pictures before it took off for ,saturn4 ,illustration3 ,from a vantage point #b4e degrees above the ring plane1 ,voyager #b was able for the first time to determine the width of ,jupiter's ring4 ,this picture shows that the ring is ribbon-like and only a few thousand kilometers wide1 quite unlike the broad rings of ,saturn4 ,p-#bageg;,b_/,w ,wednesday1 ,july #aa4 ,,jpl ,public ,information ,officer ,frank ,bristow opened the #aj a4m4 press conference with an announcement3 8,we'll have the report from the ,imaging ,team including the tremendous pictures that we received here last night of the ,jupiter ring that excited the entire team40 ,brad ,smith showed the ring pictures4 8,as many of you who were here last night know1 we got some rather nice pictures of the ring of ,jupiter4 ,it's as though ,voyager #b was fearful that we might be becoming just a little bit apathetic after this series of marvelous discoveries and felt that it had to dazzle us one more time before it left for ,saturn4 ,the rings appear very much brighter than we had expected them to be40 ,the outer ring is about #fejj kilometers wide4 ,there is material inside the ring4 ,there is a rather sharp outer boundary and a somewhat diffuse inner region4 8,and it is now our belief that the material in the ring goes all the way down to the surface of ,jupiter40 ,there is a very narrow relatively bright outer ring and an extremely faint inner ring that goes all the way down to ,jupiter's cloud tops4 ,larry ,soderblom summarized the satellite data3 ,with respect to the ,galilean satellites1 8,we're in a relatively high state of ignorance40 ,the ,io ,volcano ,watch images seemed to indicate that plume ,p was now the highest volcano on ,io1 since ,p seemed to have become quiet4 ,io may be the easiest ,galilean satellite to try to understand1 because we can actually see the geological processes that are shaping the planet4 ,io's 8twin10 ,europa1 seems to be where 8our highest state of ignorance0 lies4 8,the faint bright streaks which show some relief are evidently different from the diffuse dark bands which don't seem to show topography1 but the similarity of these forms that both the light and dark markings are of planetary scale suggests that they must be related40 ,ed ,stone speculated about the other two ,galilean satellites4 ,ganymede and ,callisto are essentially identical in size1 mass1 and probably composition4 ,by examining them1 we can perhaps learn what happens when bodies with very similar chemistry have different 8life histories0 and different surface properties 7there are indications that ,ganymede's crust may not have been as rigid as ,callisto's74 ,going further1 he added that ,callisto and ,mercury1 the least dense and the most dense1 respectively1 of the terrestrial-style planets1 although totally different in composition and density1 seem to have similar surfaces and similar histories4 ,what would have happened to ,mercury if it had been made of ice1 water1 and rock as ,callisto is8 ,would it have evolved as ,callisto did8 ,illustration3 ,one of the most spectacular of the ,voyager #b images was obtained from inside the shadow of ,jupiter4 ,looking back toward the planet and the rings with its wide-angle camera1 the spacecraft took these photos on ,july #aj from a distance of #a4e million kilometers4 ,the ribbon-like nature of the rings is clearly shown4 ,the planet is outlined by sunlight scattered from a haze layer high in the atmosphere4 ,on each side1 the arms of the ring curving back toward the spacecraft are cut off by the planet's shadow as they approach the brightly outlined disk4 ,p-#baggd;,b_/,w ,illustration3 ,the rings of ,jupiter proved to be unexpectedly bright when seen with the ,sun nearly behind them4 ,strong forward scattering of sunlight is characteristic of small particles4 ,these two views were obtained by ,voyager #b on ,july #aj from a perspective inside the shadow of ,jupiter4 ,the distance of the spacecraft from the rings was about #a4e million kilometers4 ,although the resolution has been degraded by camera motion during the time exposures1 these images reveal that the rings have some radial structure4 #bfj-faj;,b_/,w and #bfj-fgd ,illustration3 ,rings of ,jupiter4 ,,highlights ,,of ,,the ,,voyager #b ,,scientific ,,findings#c ,atmosphere ,the main atmospheric jet streams were present during both ,voyager encounters1 with some changes in velocity4 ,the ,great ,red ,spot1 the white ovals1 and the smaller white spots at #da,s1 appear to be meteorologically similar4 ,the formation of a structure east of the ,great ,red ,spot created a barrier to the flow of small spots which earlier were circulating about the ,great ,red ,spot4 ,the ethane to acetylene abundance ratio in the upper atmosphere appears to be larger in the polar regions than at lower latitudes and appears to be #a4g times higher on ,voyager #b than on ,voyager #a4 ,an ultraviolet map of ,jupiter shows the distribution of absorbing haze4 ,the polar regions are surprisingly dark1 suggesting that the absorbing material must be at high altitudes4 ,equatorial ultraviolet emissions indicate planet-wide precipitation of charged particles into the atmosphere from the magnetosphere4 ,the high-latitude ultraviolet auroral activity is due to charged particles that originate in the ,io torus4 ,satellites and ,ring ,system ,the ring consists of a bright1 narrow segment surrounded by a broader1 dimmer segment1 with a total width of about #ehjj kilometers4 ,the interior of the ring is filled with much fainter material that may extend down to the top of the atmosphere4 ,images of ,amalthea in silhouette against ,jupiter indicate that the satellite may be faceted or diamond shaped4 ,volcanic activity on ,io changed somewhat1 with six of the plumes observed by ,voyager #a still erupting4 ,the largest ,voyager #a plume 7,pele7 had ceased1 while the dimensions of another plume 7,loki7 had increased by #ej percent4 ,several large-scale changes in ,io's appearance had occurred1 consistent with surface deposition rates calculated for the large eruptions4 ,europa is remarkably smooth with very few craters4 ,the surface consists primarily of uniformly bright terrain crossed by linear markings and very low ridges4 ,there are four basic terrain types on ,ganymede1 including younger1 smooth terrain and a rugged impact basin first observed by ,voyager #b4 ,callisto's entire surface is densely cratered and is likely to be several billion years old4 ,equatorial surface temperatures on the ,galilean satellites range from #hj ,k 7night7 to #aee ,k 7the subsolar point on ,callisto74 ,magnetosphere ,the outer region of the magnetosphere contains a hot plasma consisting primarily of hydrogen1 oxygen1 and sulfur ions4 ,the hot plasma generally flows in the corotation direction out to the boundary of the magnetosphere4 ,beyond about #afj ,r,j1 the hot plasma streams nearly antisunward4 ,outbound the spacecraft experienced multiple magnetopause crossings between #bjd ,r,j and #bae ,r,j4 ,the abundance of oxygen and sulfur relative to helium at high energy increases with decreasing distance from ,jupiter4 ,measurements of high energy oxygen suggest that these nuclei are diffusing inward toward ,jupiter4 ,the ultraviolet emission from the ,io plasma torus was twice as bright as four months earlier and the temperature had decreased by #cj percent to #fj #jjj ,k4 ,the low-frequency 7kilometric7 radio emissions from ,jupiter have a strong latitude dependence and often contain narrowband emissions that drift to lower or higher frequencies with time4 ,a complex magnetospheric interaction with ,ganymede was observed in the magnetic field1 plasma1 and energetic particles up to about #bjj #jjj kilometers from the satellite4 #c;,adapted from a summary prepared by ,e4 ,c4 ,stone and ,a4 ,l4 ,lane for the ,voyager #b ,thirty-,day ,report4 ,illustration3 ,a new inner satellite of ,jupiter1 provisionally designated #aigi;,j#a1 was discovered by ,david ,jewitt and ,ed ,danielson of ,caltech in these ,voyager #b ring photographs4 ,illustration3 ,in a #ae-second exposure with the wide-angle camera1 the edge-on ring shows as a faint line1 and the satellite is the dot indicated by the arrow4 #bfj-hjg ,illustration3 ,in a narrow angle #if-second exposure1 the motion of the satellite can be seen4 ,again1 the faint band is the ring1 blurred by camera motion1 and the arrow indicates the streak due to the satellite4 ,a star streak is located above and to the left of the satellite2 note that the length and angle of the two trails are different1 owing to satellite motion4 ,p-#bbagb ,a ,new ,satellite ,one of the most fascinating discoveries of ,voyager #b was not recognized at first4 ,graduate student ,david ,jewitt of the ,california ,institute of ,technology1 working with ,imaging ,team member ,ed ,danielson1 began a detailed analysis of all the ring photos in late summer4 ,in early ,october he determined that a short streak on a photo taken ,july #h1 previously presumed to be an image of a star trailed by the time exposure1 did not correspond to any known star position4 ,perhaps this was a new satellite6 ,additional sleuthing turned up a second image of the same part of the ring that also showed the anomalous object1 together with trails due to known stars4 ,the differing angles and lengths of the trails of the object and the stars confirmed that this was indeed a #adth satellite of ,jupiter4 ,following the guidelines of the ,international ,astronomical ,union1 it was designated #aigi;,j#a1 pending later assignment of a mythological name4 ,the proposed name is ,adrastea1 a nymph who nursed the infant ,zeus in ,greek legend4 ,the newly discovered satellite orbits ,jupiter at a distance of #eh #jjj kilometers above the equatorial cloud tops1 placing it just at the outer edge of the ring and much closer to the planet than is ,amalthea1 previously thought to be the innermost satellite4 ,it travels at #cj kilometers per second 7nearly #gj #jjj miles per hour71 circling ,jupiter in just seven hours and eight minutes4 ,from its brightness1 scientists guessed that it might be #cj-dj kilometers in diameter4 ,the proximity of ,adrastea to the ring suggests a relationship between the two4 ,when the discovery was announced to the press in mid-,october1 it was speculated that the ring material might originate on the satellite1 perhaps eroded away by the energetic charged particles in the inner ,jovian magnetosphere4 ,once again1 ,voyager had added to our perspective on planetary processes1 suggesting that undiscovered but similar small satellites might also be associated with the rings of ,saturn and ,uranus4 ,voyager #b had certainly added a few years' of data of its own to ,voyager #a's 8ten years' worth of data40 ,it had given a different view of the ,jovian system1 helping to solve some of the mystery surrounding ,jupiter and its satellites1 and creating new mysteries4 ,as ,voyager #b sped out away from ,jupiter1 riding along the giant planet's huge magnetotail1 attention turned to ,saturn3 ,what would ,pioneer #aa1 the ,pathfinder1 discover in ,september #aigi8 ,what would the ,voyagers learn in ,november #aihj and ,august #aiha8 ,would all go well8 ,would ,voyager #b fly on to ,uranus8 ,there was also a yearning to examine more closely1 with the ,galileo ,project1 what had been unknown for so long1 yet had become so familiar in only a few months' time--the little dark1 red 8potato0 ,amalthea1 the volcano-covered world ,io1 the mysterious 8cracked billiard ball0 ,europa1 cratered and groovy ,ganymede1 ancient ,callisto1 and the king of the planets itself1 a colorful1 banded world of stable climate and ever-changing weather patterns4 ,illustration3 ,a fifteenth satellite of ,jupiter was discovered in the spring of #aihj by ,steven ,synnott of ,,jpl4 ,it was first seen on this ,voyager #a image taken ,march #e1 #aigi1 in which the #ge-kilometer-diameter satellite shows as a dark oval against the planet4 ,also visible is the shadow of the satellite1 designated #aigi;,j#b4 ,this satellite orbits between ,io and ,amalthea with a period of #af hours and #aa minutes4 ,p-#bbehj;,b_/,w ,illustration3 ,the ,jupiter seen by the ,voyager cameras is a cloud-belted world of rapid jet streams and complex cloud forms4 ,prominent in this ,voyager #a image1 taken ,february #e at a range of #bh4d million kilometers1 is the alternating structure of light zones and dark belts1 and the ,great ,red ,spot and numerous smaller spots4 ,also easily visible are the two inner ,galilean satellites1 ,io and ,europa4 ,the resolution in this picture is #ejj kilometers1 about five times better than can be obtained from ,earth-based telescopes4 ,callisto can be faintly seen at the lower left4 ,p-#bajhc;,c ,,chapter #h ,,jupiter--,,king ,,of ,,the ,,planets ,a ,star ,that ,failed ,more massive than all the other planets combined1 ,jupiter dominates the planetary system4 ,the giant revealed by ,voyager is a gas planet of great complexity2 its atmosphere is in constant motion1 driven by heat escaping from a glowing interior as well as by sunlight absorbed from above4 ,energetic atomic particles stream around it1 caught in a magnetic field that reaches out nearly #aj million kilometers into the surrounding space1 embracing the seven inner satellites4 ,from its deep interior through its seething clouds out to its pulsating magnetosphere1 ,jupiter is a place where forces of incredible energy contend4 ,at its birth1 ,jupiter shone like a star4 ,the energy released by infalling material from the solar nebula heated its interior1 and the larger it grew the hotter it became4 ,theorists calculate that when the nebular material was finally exhausted1 ,jupiter had a diameter more than ten times its present one1 a central temperature of about #ej #jjj ,k1 and a luminosity about one percent as great as that of the ,sun today4 ,at this early stage1 ,jupiter rivaled the ,sun4 ,had it been perhaps #gj times more massive than it was1 it would have continued to contract and increase in temperature1 until self-sustaining nuclear reactions could ignite in its interior4 ,if this had happened1 the ,sun would have been a double star1 and the ,earth and the other planets might not have formed4 ,however1 ,jupiter did not make it as a star2 after a brief flash of glory1 it began to cool4 ,at first ,jupiter continued to collapse4 ,within the first ten million years of its life1 the planet was reduced to nearly its present size1 with only a few percent additional shrinkage during the past #d4e billion years4 ,the luminosity also dropped as internal heat was carried to the surface by convection and radiated away to space4 ,after a million years ,jupiter emitted only one-hundred thousandth as much radiation as the ,sun1 and today its luminosity is only one-ten billionth of the ,sun's4 ,jupiter's internal energy1 although small by stellar standards1 has important effects on the planet4 ,about #aj watts of power1 comparable to that received by ,jupiter from the ,sun1 reach the surface from the still-luminous interior4 ,the central temperature is still thought to be about #cj #jjj ,k1 sufficient to maintain the interior in a molten state4 ,scientists generally agree that ,jupiter is an entirely fluid planet1 with no solid core whatever4 ,composition and ,atmospheric ,structure ,because of its great mass1 ,jupiter has been undiscriminating in its composition4 ,all gases and solids available in the early solar nebula were attracted and held by its powerful gravity4 ,thus it is expected that ,jupiter has the same basic composition as the ,sun1 with both bodies preserving a sample of the original cosmic material from which the solar system formed4 ,illustration3 ,jupiter is a gas giant1 composed of the same elements as the ,sun and stars--primarily hydrogen and helium4 ,its internal structure is dominated by the properties of hydrogen1 its most abundant constituent and by the high temperatures in the deep interior that remain from its luminous youth4 ,most of the interior is liquid3 metallic hydrogen at great depths and high pressures1 and normal hydrogen nearer the surface4 ,in the upper few thousand kilometers1 the hydrogen is a gas4 ,the primary known or suspected cloud layers are1 from the top down1 thin hydrocarbon 8smog02 ammonia2 ammonium hydrosulfide2 water-ice1 and liquid water4 #bfj-hbh ,cloud top-aerosols ,ammonia crystals ,ammonium hydrosulfide clouds ,ice crystal clouds ,water droplets ,trace compounds ,fluid molecular hydrogen ,transition ,zone ,fluid metallic hydrogen ,possible core ,the primary constituents of ,jupiter have long been suspected to be hydrogen and helium1 the two simplest and lightest atoms4 ,however1 it has proved impossible to derive accurate measurements of the abundance of these two elements from astronomical observations4 ,on the basis of a rather simple infrared measurement1 ,pioneer investigators found ,he_/,h #j4ad #j4jh4 ,on ,voyager1 ,,iris was able to obtain much improved infrared spectra1 yielding an initial value of ,he_/,h #j4aa #j4c4 ,voyager scientists expect that further analysis will reduce the uncertainty to about #j4ja4 ,the ratio of #j4aa is in excellent agreement with the solar value of about #j4ab1 supporting the idea that ,jupiter and the ,sun have similar elemental compositions4 ,astronomers have known for a long time that1 in addition to hydrogen and helium1 the compounds methane 7,,ch7 and ammonia 7,,nh7 are present in the visible atmosphere of ,jupiter4 ,in the #aigjs1 additional spectra in the infrared resulted in the discovery of water 7,h,o71 ethane 7,c,h71 germane 7,ge,h71 acetylene 7,c,h71 phosphine 7,,ph71 carbon monoxide 7,,co71 hydrogen cyanide 7,,hcn71 and carbon dioxide 7,,co74 ,all these are trace constituents1 with two of them1 ethane and acetylene1 apparently formed at high altitudes by the action of sunlight on methane4 ,a total of approximately #ajj #jjj infrared spectra1 many of small regions on the disk1 were obtained by ,,iris4 ,these spectra generally show hydrogen1 helium1 methane1 ammonia1 phosphine1 ethane1 and acetylene4 ,in addition1 excellent spectra were obtained in 8hot spots10 regions in which breaks in the upper clouds permit radiation from deeper layers to escape4 7,the hot spots generally correspond to dark brown regions on photographs of the planet47 ,,iris measured temperatures in the hot spots up to -#ac ,c but no higher2 apparently this temperature corresponds to the top of a deeper cloud deck4 ,spectral features indicative of the presence of water vapor and germane were clearly seen in the hot spots4 ,further analysis of the ,,iris spectra will be required to derive the abundances of the gases detected4 ,however1 even the preliminary data showed how variable ,jupiter can be1 especially in its upper atmosphere4 ,the two hydrocarbons1 ethane and acetylene1 vary in relative abundance with latitude2 there is less acetylene near the poles4 ,in addition to this planetwide trend1 smaller variations were seen from place to place and between the observations in ,march and ,july4 ,all the variations will eventually provide information on the processes of formation1 transportation1 and destruction of hydrocarbons in the upper atmosphere4 ,,elements ,,detected ,,in ,,the ,,jovian ,,magnetosphere ,element ,atomic ,instruments ,number ,hydrogen 7,h7 #a ,,uvs1 ,plasma1 ,,lecp1 ,,crs ,helium 7,he7 #b ,plasma1 ,,lecp ,carbon 7,c7 #f ,,lecp ,nitrogen 7,n7 #g ,,crs ,oxygen 7,o7 #h ,,uvs1 ,plasma1 ,,lecp1 ,,crs ,neon 7,ne7 #aj ,,crs ,sodium 7,na7 #aa ,,lecp1 ,,crs ,magnesium 7,mg7 #ab ,,crs ,silicon 7,si7 #ad ,,crs ,sulfur 7,s7 #af ,,uvs1 ,plasma1 ,,lecp1 ,,crs ,iron 7,fe7 #bf ,,crs ,voyager did not make any direct measurements of the chemical composition of the clouds1 but theorists generally agree that the uppermost clouds are ammonia cirrus1 and that layers of ammonium hydrosulfide 7,,nh,,sh7 and water exist at deeper levels4 ,all these clouds are formed in the troposphere1 the layer of the atmosphere in which convection takes place4 ,the top of the ammonia cloud deck is thought to have a pressure of about #a atmosphere and a temperature of about -#aac ,c4 ,ammonia cirrus is white1 yet ,jupiter's clouds display a spectacular range of colors4 ,voyager did not determine the nature of the coloring agents2 they may be minor constituents--trace impurities in a sea of white clouds4 ,perhaps organic polymers1 formed from atmospheric chemicals such as methane and ammonia that have reacted with lightning1 are responsible for the oranges and yellows4 ,the color of the ,red ,spot could be caused by red phosphorus 7,p74 ,according to this theory1 phosphine 7,,ph7 from deep in ,jupiter's atmosphere is brought to high altitudes by the upwelling of the ,great ,red ,spot4 ,ultraviolet light1 penetrating the upper reaches of the ,red ,spot1 splits the phosphine molecules1 and1 through a series of chemical reactions1 converts the phosphine into pure phosphorus4 ,however1 this theory fails to explain the existence of the smaller red spots on ,jupiter2 these spots are not at such high altitudes as the ,great ,red ,spot 7which is the highest and coldest of ,jupiter's visible clouds71 so it is unlikely that ultraviolet light could react with any phosphine in these areas to produce red phosphorus4 ,illustration3 ,although the ,voyager spacecraft never flew over the poles of ,jupiter1 it is possible to reconstruct from several images the ,view that would be seen from directly above or below the planet4 ,note the absence of a strong banded structure near both poles4 ,the regular spacing of cloud features is obvious4 ,in the ,southern hemisphere1 the three white ovals are #ij degrees apart in longitude1 but a fourth oval at the other quadrant is missing4 ,the irregular black areas at each pole are places for which no ,voyager data exist4 ,the resolution of the original pictures from which these polar projections were made was about #fjj kilometers4 ,illustration3 ,north pole4 ,p-#bafch;,c ,illustration3 ,south pole4 ,p-#bafci;,c ,various forms of elemental sulfur might be responsible for the riot of color we see on ,jupiter4 ,sulfur forms polymers 7,s1 ,s1 ,s1 ,s17 that are yellow1 red1 and brown1 but no sulfur in any form has been detected on ,jupiter4 8,we never promised you we were going to identify the colors on ,jupiter with this mission10 one of the atmospheric scientists remarked1 8but we will have a probe that is going into the atmosphere in the mid-#aihjs--,galileo40 ,perhaps the mystery of the ,jovian clouds will have to wait till then4 ,temperature maps of ,jupiter were obtained by ,,iris in radiation arising at different levels above the clouds4 ,maps show temperatures at pressures of #j4h atmosphere near the clouds1 and #j4b atmosphere near the top of the troposphere4 ,in addition to the low temperatures over the bright zones and the higher temperatures over dark belts1 there is a great deal of smaller scale structure4 ,it is interesting that a cold area corresponding to the ,great ,red ,spot is clearly visible even near the top of the troposphere1 indicating that this feature disturbs the atmosphere to very high altitudes4 ,the structure of the atmosphere of ,jupiter above the troposphere was investigated through the radio occultation experiment as well as by ,,iris4 ,the level in which the minimum temperature of about -#agc ,c occurs has a pressure of #j4a atmosphere4 ,above this point lies the stratosphere1 in which temperatures increase with altitude as a result of sunlight absorbed by the gas or by aerosol particles resembling smog4 ,at #gj kilometers above the ammonia clouds1 the temperature is about -#aac ,c4 ,above this level1 the temperature stays approximately constant1 although at extreme altitudes the temperature again rises in the ionosphere4 ,illustration3 ,if one could 8unwrap0 ,jupiter like a map1 views such as these would be obtained4 ,the comparison between the pictures shows the relative motions of features in ,jupiter's atmosphere4 ,it can be seen1 for example1 that the ,great ,red ,spot moved westward and the white ovals eastward during the time between the acquisition of these pictures4 ,regular plume patterns are equidistant around the northern edge of the equator1 while a train of small spots moved eastward at approximately latitude #hj ,s4 ,in addition to these relative motions1 significant changes are evident in the recirculating flow east of the ,great ,red ,spot1 in the disturbed region west of the ,great ,red ,spot1 and as seen in the brightening of material spreading into the equatorial region from the more southerly latitudes4 ,p-#bagga;,c ,illustration3 ,the planet as it appeared about ,march #a4 ,illustration3 ,as it was in early ,july4 ,weather on ,jupiter ,the ,voyager pictures reveal a planet of complex atmospheric motions4 ,spots chase after each other1 meet1 whirl around1 mingle1 and then split up again2 filamentary structures curl into spirals that open outward2 feathery cloud systems reach out toward neighboring regions2 cumulus clouds that look like ostrich plumes may brighten suddenly as they float toward the east2 spots stream around the ,red ,spot or get caught up in its vortical motion--all in an incredible interplay of color1 texture1 and eastward and westward flows4 ,such changes can be noticed in the space of only a few ,jovian days4 ,illustration3 ,differing characteristics of ,jupiter's meteorology are apparent in high-resolution images1 such as this one taken by ,voyager #a on ,march #b at a range of #d million kilometers4 ,the well-defined pale orange line running from southwest to northeast 7north is at the top7 marks the high-speed north temperate current with wind speeds of about #abj meters per second4 ,toward the top of the picture1 a weaker jet of approximately #cj meters per second is characterized by wave patterns and cloud features which have been observed to rotate in a clockwise manner at these latitudes of about #ce,n4 ,these clouds have been observed to have lifetimes of one to two years4 ,p-#baaic;,c ,on a broader time scale1 greater changes on the face of ,jupiter can be seen4 ,features drift around the planet2 even the large white ovals and the ,great ,red ,spot slide along in their respective latitudes4 ,belts or zones intrude upon each other1 resulting in one of the banded structures splitting up or seeming to squeeze together and eventually disappear4 ,small structures form1 then die4 ,the largest spots may slowly shrink in size1 and the ,red ,spot itself changes its size and color4 ,the ,jupiter of ,pioneers #aj and #aa was quite unlike the planet seen by ,voyager #a4 ,at the time of the ,pioneer exploration1 the ,great ,red ,spot1 embedded in a huge white zone1 was more uniformly colored1 and pale brown bands circled the northern hemisphere4 ,in the intervening years1 the south temperate latitudes have changed completely1 developing the complex turbulent clouds seen around the ,red ,spot by ,voyager #a4 ,yet1 even between the two ,voyagers1 ,jupiter appeared to be undergoing a dynamic 8facelift40 ,at a quick glance1 ,voyager #b photographs showed the visage that had been familiar since early in #aigi1 but a closer look showed that it is not quite the same4 ,the white band below the ,great ,red ,spot1 fairly broad during the first flyby1 had become a thin white ribbon where it rims the southern edge of the ,spot4 ,the turbulence to the west of the ,red ,spot had stretched out and become 8blander0 than it was before4 ,small rotating clouds seemed to be forming out of the waves in this region4 ,the cloud structure that had been east of the ,red ,spot during the ,voyager #a flyby spread out1 covering the northern boundary and preventing small clouds from circling the huge red oval4 ,the ,red ,spot itself also changed4 ,its northern boundary seemed--at least visually--to be more set off from the clouds that surround it1 and the feature appeared to be more uniform in color1 perhaps reverting back to the personality it had in ,pioneer days4 ,illustration3 ,jupiter's cloud patterns changed significantly in the few months between the two ,voyager flybys4 ,most of the changes are the result of differential rotation1 in which the prevailing winds at different latitudes shift long-lived features with respect to those north or south4 ,thus1 for example1 the three large white ovals shifted nearly #ij degrees in longitude1 relative to the ,great ,red ,spot1 between ,march and ,july4 ,p-#baeii ,illustration3 ,cloud patterns from ,voyager #b4 ,the most obvious features in the atmosphere of ,jupiter1 after the banded belts and zones1 are the ,great ,red ,spot and the three white ovals4 ,these have often been described as 8storms0 in ,jupiter's atmosphere4 ,the ovals are about the size of the ,moon1 and the ,red ,spot is larger than the ,earth4 ,voyager has revealed that in many respects the white ovals1 which formed in #aici1 resemble their ancient red relative4 ,all four spots are southern hemispheric anticyclonic features that exhibit counterclockwise motion2 hence they are meteorologically similar4 ,other smaller bright elliptical and circular spots also exhibit anticyclonic motion1 rotating clockwise in the northern hemisphere and counterclockwise in the southern hemisphere4 ,in general1 these features are circled by filamentary rings that are darker than the spots they surround4 ,hints of interior spiral structure can be seen in some of these spots4 ,all the elliptical features in the southern hemisphere lie to the south of the strong westward-blowing jet streams4 ,the spots tend to become rounder the closer they are to the poles4 ,along the northern edge of the equator are a number of cloud plumes1 which appear to be regularly spaced all around the planet4 ,some of the plumes have been observed to brighten rapidly1 which may be an indication of convective activity2 indeed1 some of the plume structures seem to resemble the convective storms that form in the ,earth's tropics4 ,the plumes travel eastward at speeds ranging from about #ajj to #aej meters per second1 but they do not move as a unit4 ,illustration3 ,the ,great ,red ,spot of ,jupiter is a magnificent sight1 whether viewed in normal or exaggerated color4 ,these pictures were taken by ,voyager #a at a range of about #a million kilometers2 the area shown is about #be #jjj kilometers1 with features visible on the originals that are as small as #cj kilometers across4 ,the ,red ,spot is partly obscured on the north by a thin layer of overlying ammonia cirrus cloud4 ,south of the ,red ,spot is one of the three white ovals1 which are also anticyclonic vortices in the atmosphere4 ,illustration3 ,this frame is in natural color4 ,p-#badcj;,c ,illustration3 ,the red and blue have been greatly exaggerated in this frame to bring out fine detail in the cloud structure4 ,p-#badca;,c ,the most visible cloud interactions take place in the region of the ,great ,red ,spot4 ,material within the ,red ,spot rotates about once every six days4 ,infrared measurements show that the ,red ,spot is a region of atmospheric upwelling1 which extends to very high altitudes2 however1 the divergent flow suggested by this upwelling seems to be very small--one bright feature was observed to circle the ,red ,spot for sixty days without appreciably changing its distance from the spot's center4 ,during the ,voyager #a flyby1 spots were seen to move toward the ,red ,spot from the east1 flow along its northern border1 then either flow on to the west past the ,red ,spot or into the outer regions of its vortex4 ,a spot caught on the outer edge of the ,red ,spot flow might break in two as it reached the eastern edge of the spot1 with one piece remaining in the vortex and the other moving off to the east4 ,alternatively1 a spot floating toward the ,red ,spot from the east might be pushed northward to join the eastward current flowing north of the giant red oval4 ,by the time of the second ,voyager encounter1 a ribbon of white clouds curled around the northern border of the ,red ,spot1 blocking the motion of small spots that might otherwise have been caught up in the vortex4 ,spots approaching the ,red ,spot from the east just turned around and headed back in the direction from which they had come4 ,illustration3 ,voyager #b captured the ,red ,spot region four months after ,voyager #a1 when some changes had taken place in the cloud circulation pattern around it4 ,this is a mosaic of ,voyager #b frames1 taken on ,july #f4 ,the white oval to the south is not the same one that was present in a similar location during the ,voyager #a flyby1 because of differential rotation at the two latitudes4 #bfj-fjf ,in the northern hemisphere1 small brown anticyclonic features speed around the planet1 often colliding with one another4 ,on collision1 the spots may combine and roll around together for a while4 ,ultimately1 part of the mass of the combined spots is ejected as a streamer1 and the remaining material continues on its eastward path4 ,order out of ,chaos ,despite all the turbulence in ,jupiter's atmosphere--this ever-changing chaotic mixture of cyclonic and anticyclonic flows1 of ovals and filaments1 of reds1 browns1 and whites--a pattern may be emerging3 ,there is an underlying order to the seemingly random mixing of patterns we see in the ,jovian atmosphere4 ,first1 the changing weather patterns are in some sense cyclic4 ,the fact that ,jupiter may be reverting to the appearance it presented at the time of ,pioneers #aj and #aa is not surprising4 ,from ,earth-based studies1 astronomers have found that the face of ,jupiter often goes through a major change every few years4 ,the transition is very rapid1 but the planet maintains its new 8look0 for some time--until the next major transition4 ,at the time of the ,pioneer flybys1 8,the ,red ,spot was prominent1 but it was surrounded by intense cloud4 ,there was no visible structure at all in the south tropical zone--it was totally bland4 ,you couldn't see the turbulent area to the west10 explained ,garry ,hunt4 8,and1 ,i believe that the buildup of cloud that we're seeing to the east of the ,red ,spot is the beginning of the transition that will produce the ,pioneer look40 ,there is even more order underlying ,jupiter's changing atmosphere4 ,this order is revealed in part in the alternating belts and zones4 ,it is believed that the cloud-covered zones are regions of rising air1 and the belts are regions of descending air4 ,the internal energy of ,jupiter provides the power to maintain this pattern of slow vertical circulation4 ,in addition1 there are horizontal or zonal flows that are much more regular than the changing cloud patterns4 ,illustration3 ,the three large white ovals are the longest-lived features in ,jupiter's atmosphere1 after the ,great ,red ,spot4 ,like the ,red ,spot1 they are anticyclonic1 or high-pressure1 regions4 ,during ,voyager #b encounter1 one of the ovals was just south of the ,red ,spot4 ,this picture shows the other two ovals as they looked in early ,july4 ,the clouds show very similar internal structures4 ,to the east of each of them1 recirculating currents are clearly seen4 ,p-#baged;,c ,illustration3 ,in this frame1 a similar structure is seen to the west of the cloud4 ,illustration3 ,a sequence of pictures of ,jupiter1 taken once per rotation 7about ten hours71 can be used to construct a time-lapse movie of the circulation of the ,jovian atmosphere4 ,these frames are from the ,voyager #a 8,blue ,movie0 of the ,great ,red ,spot region4 ,every odd ,jovian rotation is shown1 so the #bd frames correspond to #dh ,jupiter days1 or about #bj ,earth days4 ,white spots can be seen entering the ,red ,spot from the upper right and being carried around by its six-day rotation until they are ejected toward the lower right4 ,above the ,red ,spot1 the flow is toward the right2 below it1 toward the left4 ,the rotation of the ,spot is counterclockwise1 or anticyclonic4 #bfj-ddi ,illustration3 ,at different latitudes1 the strong prevailing zonal winds produce different apparent rotation rates4 ,plotted here are the horizontal velocities measured from a pair of ,voyager images taken one rotation 7about ten hours7 apart4 ,also shown for comparison are older ground-based measurements obtained from careful timings of the apparent rotation rate at different latitudes4 ,the excellent agreement of the two plots indicates the stability of the zonal winds4 ,also1 the wind pattern shows much greater symmetry between northern and southern hemispheres than do the more superficial cloud patterns4 8,at first1 ,voyager seemed to do nothing but emphasize the chaos1 not the order in ,jupiter's atmosphere10 ,andy ,ingersoll stated4 8,there are turbulent regions in which individual little spots seem to change every ,jovian rotation4 ,and the whole texture in certain turbulent regions is unrecognizable in one earth day40 ,with the ,voyager spacecraft1 more detail could be seen than ever before2 in addition1 changes could be observed on a small timescale1 as they happened4 8,it became much more of a mystery how large-scale order could exist in the face of all this small-scale chaos4 ,but ,i think we are beginning to see the order underneath4 ,what we are looking at when we observe ,jupiter are minute cloud particles representing only a small fraction of the mass of the atmosphere40 ,the large-scale order the scientists had found was a regular alternation of eastward and westward jets4 8,if we take all the measurements from ,earth-based observations over the last #ge years1 we find that every current that has ever been seen from the ,earth over #ge years is visible in one ten-hour rotation4 ,they're all there--they were just invisible40 ,the ever-changing appearance was dancing above a regular1 almost stationary pattern of alternating flows1 which may come from deep within ,jupiter's atmosphere4 ,why this alternating pattern persists remains a mystery4 ,even if the underlying pattern can be thought of as a sort of ,jovian climate1 it still does not explain the mechanics of 8the minor sideshow0--the changing weather patterns4 ,analysis of the ,voyager pictures is sure to keep planetary meteorologists busy for many years to come4 ,lights in the ,night ,sky ,toward the end of the first encounter period1 ,voyager #a flew behind ,jupiter1 and the spacecraft's wide-angle camera scanned the northern hemisphere on the nightside of the planet1 searching for aurorae and lightning bolts4 ,the most impressive darkside feature found was a tremendous aurora in the north polar region4 ,but this was not the first time ,jovian aurorae had been detected4 ,very-high-energy auroral emissions resulting from ultraviolet glows of atomic and molecular hydrogen had been detected prior to encounter on the bright side of ,jupiter by the ultraviolet spectrometer4 ,the ultraviolet observations indicate that atmospheric temperatures in the auroral regions are at least #ajjj ,k4 ,in both the visible and the ultraviolet spectra1 the aurorae are confined to the polar regions and result from charged magnetospheric particles striking the upper atmosphere4 ,the ultraviolet aurorae are created when high-energy particles from the ,io plasma torus spiral in toward ,jupiter on magnetic field lines4 ,several meteor trails were also evident in the darkside pictures of ,jupiter's atmosphere4 ,traveling at roughly #fj kilometers per second as they entered1 these fireballs brightened quickly and seemed to survive for about #ajjj kilometers before they died4 ,clusters of lightning bolts--indicative of electrical storms--were also discovered on ,jupiter's nightside4 ,this particular phenomenon does not seem to depend on latitude4 ,the ,voyager #a photograph that captured the huge ,jovian aurora also caught the electrical discharges of #ai superbolts of lightning1 and ,voyager #b photographs located eight additional flashes4 ,radio emission 7whistlers7 from lightning discharges were also detected by the ,voyager radio astronomy receivers and the plasma wave instrument4 ,illustration3 ,the night side of ,jupiter is not dull4 ,a large aurora 7northern light7 arcs across the northern horizon1 while farther south about twenty large bolts of lightning illuminate electrical storms in the clouds4 ,similar pictures also revealed fireballs1 or large meteors1 burning up in the atmosphere of ,jupiter4 ,p-#babhc;,b_/,w ,magnetic ,field ,deep in the interior of ,jupiter1 the pressures are so great that hydrogen becomes an electrical conductor1 like a metal4 ,currents driven by the rapid rotation of the planet are thought to flow in this metallic core4 ,the result is a magnetic field that penetrates the space around ,jupiter4 ,direct measurements of the ,jovian magnetic field were first made by the ,pioneers1 and ,voyager results generally confirm the initial findings4 ,the strength of the ,jovian field is about #djjj times greater than that of the ,earth4 ,the dipolar axis is not at the center of ,jupiter1 but offset by about #aj #jjj kilometers and tipped by #aa degrees from the axis of rotation4 ,each time the planet spins1 the field wobbles up and down1 carrying with it the trapped plasma of the radiation belts4 ,the ,voyager particles and fields instruments concentrated not on the planetary magnetic field but on the processes taking place in the magnetosphere4 ,illustration3 ,a probe of the ,jovian atmosphere is obtained each time a spacecraft passes behind the planet as seen from the ,earth4 ,passage through the ionosphere and atmosphere alters the phase of the radio telemetry signal1 and subsequent computer analysis allows members of the ,radio ,science ,team to reconstruct the profile of the atmosphere4 ,shown here is the atmospheric temperature as a function of pressure as derived from the ,voyager #a ,x-band occultation data1 corresponding to a point at latitude #ab,s1 longitude #fc4 ,the two curves represent extreme interpretations of the same data2 the best fit lies somewhere between4 ,accuracy is high at greater depths but poor at levels above a pressure of about #j4jc bar4 ,clearly shown is the temperature minimum near #j4a bar and the steady increase of temperature with depth as the radio beam probed toward the cloud level near #a4j bar4 ,illustration3 ,at a wavelength near #e micrometers1 the primary gases in the atmosphere of ,jupiter are particularly transparent1 and the infrared radiation from the planet comes from relatively great depths4 ,at these depths1 it is possible to see evidence of gases such as water vapor that condense at higher altitudes where the temperatures are lower4 ,this ,,iris spectrum in the #e-micrometer spectral region shows features identified with water 7,h,o71 germane 7,ge,h71 and deuterated methane 7,,ch,d71 as well as the more easily detected ammonia 7,,nh74 ,wavelength 7m7 ,h,o ,,ch,d ,ge,h ,,nh ,voyager #a ,voyager #b ,wave number 7cm7 ,voyager #a brightness temperature 7,k7 ,voyager #b brightness temperature 7,k7 ,illustration3 ,the structure of the ,jovian atmosphere can be derived from infrared spectra as well as from the radio occultation data4 ,this profile of temperature as a function of pressure covers the same range of altitudes as does the preceding figure and is in good agreement4 ,both profiles locate a temperature minimum of about #aaj ,k near a pressure of #j4a bar 7#ajjj mb #a bar #a atm74 ,the ,,iris data also show variation of structure with position1 including a cooler minimum temperature 7about #ajj ,k7 over the ,great ,red ,spot4 ,illustration3 ,the structure of the atmosphere can be inferred from ,,iris spectra at many locations over the disk of ,jupiter4 ,scientists are beginning to assemble this vast amount of information into maps that show the temperatures at a given pressure4 ,the temperature contours are labeled in degrees ,kelvin4 ,the banded structure1 with higher temperatures near the dark equatorial belt1 is most clearly evident at the lower altitude4 ,surprisingly1 the cool region associated with the ,great ,red ,spot 7latitude #bc,s7 is more apparent at high altitude4 ,illustration3 ,the observed temperature at a depth near the cloud tops 7#j4h bar74 ,illustration3 ,the observed temperature at an altitude about #cj kilometers higher 7#j4ae bar74 ,the ,magnetosphere ,giant ,jupiter has an enormous realm--from the size of its satellite system to its tremendous aurorae and superbolts of lightning1 to the huge planet-sized cloud features that surround its atmosphere4 ,the most gargantuan ,jovian feature is its magnetosphere1 which envelopes the satellites and constantly changes in size1 pumping in and out at the whim of the solar wind4 ,the ,pioneer and ,voyager spacecraft provided four cuts through this dynamic region1 showing that its borders in the upwind solar direction lie between #ej ,r,j and #ajj ,r,j from ,jupiter4 ,downwind1 away from the ,sun1 the magnetosphere extends much farther2 some scientists postulate that a magnetotail may reach as far as the orbit of ,saturn4 ,charged particles in the magnetosphere are subject to powerful forces4 ,tightly embedded in ,jupiter1 the magnetic field spins with a ten-hour period as the planet rotates4 ,the particles are caught in the spinning field and accelerated to high speeds4 ,the result is a co-rotating plasma in the magnetic equator of ,jupiter1 extending outward to at least #bj ,r,j4 ,beyond this distance1 the flow breaks up and the magnetosphere is more unstable4 ,within the co-rotation region1 the spinning plasma sets up a powerful electric current girdling the planet4 ,charged particles can be accelerated in the magnetosphere to high energies1 corresponding to speeds tens of thousands of kilometers per second4 ,some of these particle streams escape from the inner parts of the magnetosphere and can penetrate the magnetopause and be ejected from the ,jovian system4 ,on ,voyager #a1 the low energy charged particle instrument began detecting these streams of 8hot0 plasma on #bb ,january1 when ,voyager #a was still #fjj ,r,j 7almost #ej million kilometers7 from the planet4 ,voyager #b first detected ,jovian particles at an even greater distance1 #hjj ,r,j4 ,hydrogen and helium ions 7protons and alpha particles7 dominate the magnetosphere at great distances from ,jupiter1 but increasing amounts of sulfur and oxygen appeared as the spacecraft crossed the magnetopause4 ,the heavier ions presumably originate from ,io4 ,in the inner magnetosphere1 the ,galilean satellites have a powerful influence on the populations of fast-moving particles4 ,during the ,voyager #a encounter1 the primary effect was observed at ,io1 where the satellite apparently sweeps up energetic electrons4 ,in the million-volt energy range1 these particles are depleted near ,io1 with peaks observed both inside and outside the satellite's orbit4 ,voyager #b passed close to ,ganymede1 and here also major effects were seen1 with the satellite apparently absorbing electrons4 ,in the wake created by the motion of the co-rotating plasma past ,ganymede1 the particle populations showed large and complex variations4 ,illustration3 ,the magnetosphere of ,jupiter can be 8seen0 from ,earth by its emissions at radio wavelengths4 ,the recent development of imaging radio telescopes in ,great ,britain1 ,the ,netherlands1 and the ,united ,states allows frequent mapping of the large-scale features of the innermost magnetosphere1 inside the orbit of ,io4 ,these six images showing one rotation of ,jupiter were obtained near the time of the ,voyager #a flyby by ,imke de ,pater at the ,westerbork ,radio ,observatory near ,leiden4 ,the brightest emission is shown by dark red or black4 ,the size of the planet is indicated by a dotted white circle4 ,the tilt of the magnetosphere relative to the rotation axis of the planet can be seen by the wobble of the magnetosphere as ,jupiter rotates4 ,just inside the ,jovian magnetosphere is the 8hot spot0 of the solar system3 a #cjj-djj million degree plasma detected by ,voyager #a while it was still about #e million kilometers from ,jupiter4 ,t4 ,p4 ,armstrong commented1 8,even the interior of the ,sun is estimated to be less than #bj million degrees40 ,s4 ,m4 ,krimigis added that the temperature of this plasma is 8the highest yet measured anywhere in the solar system40 ,fortunately for ,voyager1 this region of incredibly hot plasma is also one of the solar system's best vacuums4 ,the spacecraft was in little danger because the bow shock protects this region from the solar wind1 and most of the particles in ,jupiter's magnetosphere are held in much closer to the planet4 ,illustration3 ,each ,voyager passed through the boundaries of the magnetosphere--the bow shock 7,,bs7 and the magnetopause 7,,mp7--on both the inbound and the outbound legs of its passage through the ,jovian system4 ,in this diagram1 the heavy solid line represents the spacecraft trajectory1 as seen looking down from the north4 ,also shown are the positions of the bow shock in ,march and of the magnetosphere in both ,march and ,july4 ,illustration3 ,several regions of plasma 7charged particles7 make up the ,jovian magnetosphere4 ,these sketches1 based on ,voyager data1 show the magnetosphere as viewed from above 7a7 and as seen from the ,jovian equatorial plane 7b74 ,most of the plasma co-rotates with the planet and is confined near the magnetic equator1 where it forms a broad plasma sheet about #ajj ,r,j across4 ,the very hot plasma in the outer magnetosphere discovered by ,voyager is thought to play an important role in establishing the size of the ,jovian magnetosphere4 ,although the density is low1 only about one charged particle per hundred cubic centimeters1 this plasma actually carries a great deal of energy because of the high speed of the particles4 ,it is this plasma pressure1 rather than the magnetic field pressure1 that appears to hold off the pressure of the solar wind4 ,however1 the balance between hot plasma inside the magnetopause and the solar wind outside is not very stable4 ,the ,voyager experimenters suggest that a small change in solar wind pressure can cause the boundary to become suddenly unstable4 ,a large quantity of the hot plasma can then be lost1 producing the bursts seen at large distances and permitting a sudden collapse of the outer magnetosphere4 ,continued injection of hot plasma from within would then reinflate the magnetosphere1 which would expand like a balloon until another instability developed4 ,processes of this sort may be the cause of the rapidly varying magnetospheric boundaries observed by both ,voyager spacecraft4 ,illustration3 ,the rings of ,jupiter are best seen when looking nearly in the direction of the ,sun1 since the small particles that comprise them are good forward scatterers of sunlight4 ,this mosaic is of ,voyager #b images 7two wide angle and four narrow angle7 obtained from a perspective behind the planet and inside the shadow of ,jupiter4 ,the spacecraft was #b degrees below the equator of ,jupiter and #a4e million kilometers from the rings4 ,the shadow of the planet can be seen to obscure the near segment of the ring near the edge of the planet4 ,the brightest region of the ring is about #a4h ,r,j from the center of ,jupiter4 #bfj-fgh;,b ,rings of ,jupiter ,one of the spectacular discoveries of ,voyager was the existence of a ring system of small particles circling ,jupiter4 ,saturn and ,uranus were known to have rings1 but none had been seen before at ,jupiter4 ,as revealed by the ,voyager cameras1 the rings extend outward from the upper atmosphere to a distance of #ec #jjj kilometers above the cloud tops1 #a4h ,r,j from the center of the planet4 ,the main rings1 however1 are much narrower1 spanning from #dg #jjj to #ec #jjj kilometers above ,jupiter4 ,there are two main rings1 a #ejjj-kilometer-wide segment1 and a brighter1 outer #hjj-kilometer segment4 ,the thickness of the rings is unknown1 except that it is certainly less than #cj kilometers1 and probably under #a kilometer4 ,illustration3 ,structure within the ring can be seen in the best ,voyager #b images1 taken about #bg hours after closest approach to ,jupiter4 ,this enlarged portion of a wide-angle picture taken with a clear filter shows a bright core about #hjj kilometers across with a dimmer region a few thousand kilometers across on the inside1 and a narrow dim region on the outside4 #bfj-fgd ,the rings of ,jupiter are quite tenuous1 which explains why they are invisible from ,earth although they have been detected from ,earth since ,voyager4 ,seen face on1 the brightest part of the ring blocks less than one part in ten thousand of the light passing through1 making it essentially transparent4 ,in fact1 the ring does not even offer much resistance to a spacecraft2 ,pioneer #aa traversed the ring in #aigd with no obvious ill consequences4 ,apparently the individual particles that make up the ring are widely dispersed4 ,they can be seen only when the rings are viewed nearly edge-on1 or toward the ,sun1 where they show up well in forward scattered light4 ,it is this extra brilliance when backlit that created the excellent photos taken by ,voyager #b from inside the shadow of ,jupiter4 ,the individual ring particles are probably dark1 rocky fragments that are very small--essentially dust grains4 ,they move around ,jupiter in individual orbits1 circling the planet in #e - #g hours4 ,scientists postulate that such orbits are not stable and that the particles fall slowly in toward ,jupiter4 ,apparently the rings are constantly renewed from some source1 which may be the satellite ,adrastea 7,j#ad71 discovered by ,voyager #b4 ,there has also been speculation that ,adrastea may influence the ring structure by sweeping particles out of the ring4 ,at present the rings of ,jupiter remain mysterious4 ,they are clearly very different from the rings of ,saturn and ,uranus1 and reaching an understanding of their origins and dynamics presents many challenges to planetary scientists4 ,illustration3 ,the four ,galilean satellites of ,jupiter are planet-like worlds1 revealed by ,voyager to be as diverse and fascinating as the terrestrial planets ,mercury1 ,venus1 ,earth1 and ,mars4 ,in this ,voyager #a composite1 all four are shown in their correct relative size1 as they would appear from a distance of about #a million kilometers4 ,relative color and reflectivity are also approximately preserved1 although it is not possible to show on a single print the full range of brightness from the dark rocky surface of ,callisto to the brilliant white of ,europa or orange of ,io4 #bfj-dii;,c ,illustration3 ,io 7longitude #adj74 ,illustration3 ,europa 7longitude #cjj74 ,illustration3 ,callisto 7longitude #cej74 ,illustration3 ,ganymede 7longitude #cbj74 ,,chapter #i ,,four ,,new ,,worlds ,jupiter's ,satellite ,system ,in a sense1 the ,voyager ,mission revealed a new planetary system4 ,astronomers had long been fascinated by the large ,galilean satellites of ,jupiter1 but they had only looked from afar1 watching the dancing points of light in their telescopes1 and1 occasionally1 as the atmosphere steadied1 seeing these points resolve themselves into tiny disks before dissolving again in the turbulence of the terrestrial atmosphere4 ,much had been learned from telescopic studies1 but not until the ,voyager flights had we truly seen the ,galilean satellites4 ,the historic hours as ,voyager #a cruised past each satellite on ,march #e and #f1 #aigi1 fundamentally altered our perspective4 ,four new worlds were revealed1 as diverse and fascinating as the more familiar terrestrial planets4 ,although not yet household words1 the names ,io1 ,europa1 ,ganymede and ,callisto have now been added to ,mercury1 ,venus1 ,moon1 and ,mars in the lexicon of important 8,earth-sized0 bodies in the solar system4 ,jupiter has fifteen known satellites1 counting the two new satellites discovered by ,voyager4 ,these moons vary greatly in size1 composition1 and orbit4 ,the four outermost satellites1 ,sinope1 ,pasiphae1 ,carme1 and ,ananke1 circle the planet in retrograde orbits of high inclination2 their distances from ,jupiter vary between #bj and #bd million kilometers 7#bij ,r,j to #ccc ,r,j74 ,these small bodies1 none more than #ej kilometers in diameter1 require nearly two years for each orbit of ,jupiter4 ,it is possible that they are captured asteroids1 but so little is known about them that astronomers cannot tell if their surface properties resemble those of asteroids1 or if these four satellites are even similar to each other4 ,the next group of ,jovian satellites consists of four small difficult-to-observe objects4 ,these are ,lysithea1 ,elara1 ,himalia1 and ,leda1 the latter discovered by ,charles ,kowal of ,hale ,observatories in #aigd4 ,they have similar orbits1 varying in distance from ,jupiter between #aa and #ab million kilometers 7about #afj ,r,j74 ,like the outer group1 these satellites have orbits of high inclination2 unlike the outer group1 they move in the proper1 prograde direction around ,jupiter4 ,the largest1 ,himalia 7#agj kilometers in diameter7 and ,elara 7#hj kilometers diameter71 are known to be very dark1 rocky objects1 and it seems probable that the others are similar4 ,it is unlikely that the census of the outer groups of irregular satellites is complete1 and new satellites less than #aj kilometers in diameter will probably be discovered4 ,the ,jovian system is dominated1 of course1 by the large ,galilean satellites1 which vary in size from just smaller than the ,moon 7,europa7 to nearly as large as ,mars 7,ganymede74 ,these satellites are in regular1 nearly circular orbits in the same plane as the equator of ,jupiter1 and all four lie within the inner magnetosphere of ,jupiter1 where they interact strongly with energetic charged particles and plasma4 ,most of this chapter will be devoted to a discussion of these fascinating worlds4 ,we now know of three additional small satellites inside the orbit of ,io1 orbiting close to ,jupiter4 ,the first1 ,amalthea1 was discovered in #ahib2 it orbits ,jupiter in just twelve hours at a distance of #aha #jjj kilometers 7#b4ee ,r,j74 ,a smaller object1 ,adrastea 7officially #aigi;,j#a for the first new satellite of ,jupiter discovered in #aigi71 is much closer1 at #acd #jjj kilometers 7#a4gf ,r,j74 ,as described in ,chapter #g1 it skirts the outer edge of the ring1 circling ,jupiter in just over seven hours4 ,the inner satellite moves faster than ,jupiter's rotation2 seen from the planet1 it would rise in the west and set in the east4 ,both ,amalthea and ,adrastea are buried deep within the inner magnetosphere where they are continually bombarded by energetic electrons1 protons1 and ions4 ,depletion of the ,jovian radiation belt particles was observed at the orbits of both satellites by ,pioneer #aa1 which went much closer to ,jupiter than the ,voyagers1 testifying to the intensity of the interaction between these objects and their surroundings4 ,illustration3 ,callisto was revealed by the ,voyager cameras to be a heavily cratered and hence geologically inactive world4 ,this mosaic of ,voyager #a images1 obtained on ,march #f from a distance of about #djj #jjj kilometers1 shows surface detail as small as #aj kilometers across4 ,the prominent old impact feature ,valhalla has a central bright spot about #fjj kilometers across1 probably representing the original impact basin4 ,the concentric bright rings extend outward about #aejj kilometers from the impact center4 #bfj-dej ,,satellites ,,of ,,jupiter ,distance ,from ,jupiter ,name #aj ,jupiter ,period ,year of kilometers ,radii 7days7 ,discovery ,adrastea ,j#ad #acd #a4gf #j4cj #aigi ,amalthea ,j#e #aha #b4ee #j4di #ahib #aigi;,j#b ,j#ae #bbb #c4aa #j4fg #aihj ,io ,j#a #dbb #e4ie #a4gg #afaj ,europa ,j#b #fga #i4dg #c4ee #afaj ,ganymede ,j#c #ajgj #ae4aj #g4ae #afaj ,callisto ,j#d #ahhj #bf4fj #af4gj #afaj ,leda ,j#ac #aa #aaj #aef #bdj #aigd ,himalia ,j#f #aa #dgj #afa #bea #aijd ,lysithea ,j#aj #aa #gaj #afd #bfj #aich ,elara ,j#g #aa #gdj #afe #bfj #aijd ,ananke ,j#ab #bj #gjj #bia #fag #aiea ,carme ,j#aa #bb #cej #cad #fib #aich ,pasiphae ,j#h #bc #cjj #cbg #gce #aijh ,sinope ,j#i #bc #gjj #ccc #geh #aiad ,illustration3 ,the state of the interiors of the ,galilean satellites can be judged from their sizes and densities4 ,these cross-sectional views represent the best guess following the ,voyager flybys as to the composition and structure of the objects4 ,io1 with a density equal to that of the ,moon and a long history of volcanic activity1 is a dry1 rocky object4 ,europa is less dense1 and it probably has a global ocean of ice as much as #ajj kilometers thick over a rocky interior4 ,ganymede and ,callisto both have densities near #b grams per cubic centimeter1 suggesting a composition about half water and half rock4 ,there is probably a rocky core surrounded by an icy mantle4 ,io ,active volcanoes ,sulfur and frozen ,,so ,molten silicate interior ,europa ,global fracture patterns ,ice crust ,rocky interior ,ganymede ,fresh craters expose ice ,young grooved terrain with intricate fracture ,old1 dark cratered areas ,ice crust ,water or ice mantle ,callisto ,large basins reduced by ice flow ,fresh craters expose ice ,ice_/rock crust ,water or ice mantle ,moon ,mercury ,long after the flybys of ,jupiter1 continued analysis of ,voyager images revealed another new satellite1 ,jupiter's fifteenth4 ,initially designated #aigi;,j#b1 the unexpected new satellite orbits the planet at #c4ag ,r,j1 between ,io and ,amalthea4 ,stephen ,synnott of the ,,jpl ,optical ,navigation ,team discovered the satellite on pictures taken during the ,voyager #a events on ,march #e1 #aigi1 while searching for additional images of satellite #aigi;,j#a4 ,it is about #ge kilometers in diameter1 but nothing else is known about its physical properties4 ,together1 the #ae satellites circling giant ,jupiter form a mini-solar system4 ,perhaps the outer1 irregular satellites were captured or resulted from the catastrophic collisions of one or more larger satellites with passing asteroids4 ,the inner seven satellites constitute a coherent system1 almost certainly formed together with ,jupiter and sharing a common #d4e-;billion-year history4 ,they are fascinating as individual worlds1 and also as brothers and sisters1 and the study of their interrelationships undoubtedly will provide insights into the general problems of planetary formation and evolution4 ,,sizes ,,and ,,densities ,,of ,,the ,,galilean ,,satellites ,name ,diameter ,density 7grams per 7kilometers7 cubic centimeter7 ,io #cfdj #c4e ,europa #cacj #c4j ,ganymede #ebgj #a4i ,callisto #dhdj #a4h ,callisto ,callisto is the least active geologically of the ,galilean satellites4 ,basically a dead world1 it bears the scars of innumerable meteoric impacts1 with virtually no sign of major internal activity4 ,callisto is a world of craters1 and to understand it we must explore the role that cratering plays in molding planetary surfaces4 ,the space between the planets is filled with debris1 ranging from the larger asteroids1 hundreds of kilometers in size1 down to microscopic grains of dust4 ,inevitably1 each planet collides with some of these fragments4 ,the smaller particles do little damage2 in the case of a planet with an atmosphere1 like ,earth1 they burn up as meteors before reaching the surface1 whereas on an airless planet1 they erode the surface by sandblasting the exposed rock4 ,the larger impacts are another matter1 and the craters they produce can be the dominant features on the surface of a planet4 ,illustration3 ,voyagers #a and #b photographed most of the surface of ,callisto at resolutions of a few kilometers or better4 ,shown here is a preliminary shaded relief map4 ,additional measurements will improve the accuracy of the coordinate system4 #bfj-fgb ,illustration3 ,callisto is a world of craters1 as is well shown in this ,voyager #b photomosaic taken from a distance of #djj #jjj kilometers4 ,craters about #ajj kilometers in diameter cover the surface uniformly4 ,many have bright rims1 perhaps composed of exposed water-ice4 ,there are very few craters larger than #aej kilometers in diameter1 however1 indicating that the scars of very large impacts do not survive on the surface of ,callisto4 ,p-#bagdf;,b_/,w ,illustration3 ,the concentric rings surrounding ,valhalla are perhaps the most distinctive geological feature on ,callisto4 ,this ,voyager #a close-up shows a segment of the ridged terrain4 ,the presence of superposed impact craters shows that the rings formed early in ,callisto's history2 however1 the density of craters is less here than on other parts of the satellite1 where the surface is older4 ,p-#bbaid ,we who live on ,earth tend not to realize the importance of cratering1 for the simple reason that our planet has very few craters1 and these are frequently of volcanic rather than meteoric origin4 ,why are we so favored8 ,is there an invisible shield to protect us from the cosmic shooting gallery8 ,clearly not2 the ,earth has experienced just as many cratering impacts as has the ,moon or other planets4 ,the difference is not that craters are formed less often1 but that the great geological activity of ,earth--erosion1 volcanism1 mountain building1 continental drift1 etc4--erases craters as fast as they are formed4 ,on the average1 a #aj-kilometer-wide crater is formed on ,earth about once every million years1 but all those older than a few million years have been eroded away1 filled in1 or crushed beyond recognition by crustal motion4 ,if a planet lacks great internal geologic forces1 large craters can survive almost indefinitely4 ,such is the case for the ,moon4 ,most of the volcanism and other activity on the ,moon ceased #c billion years ago1 as the dating of lunar samples obtained by the ,apollo astronauts showed4 ,since that time1 the lunar surface has been passively accumulating impact scars4 ,the longer any particular surface area has been exposed1 the more densely packed are the craters4 ,thus crater density is the first thing a planetary geologist looks for in photos of a new world4 ,craters are the touchstone of this field1 revealing the degree of internal activity and allowing the determination of the relative ages of different surface units4 ,on ,callisto the density of craters is very high4 ,in some places they are packed as closely as one can imagine1 particularly for craters several tens of kilometers in diameter4 ,although no one knows the exact rate of formation of impact craters on the ,jovian satellites1 geologists on the ,voyager ,imaging ,team estimate that it would require several billion years to accumulate the number of craters found on ,callisto4 ,they therefore conclude that ,callisto has been geologically inactive almost since the time of its formation4 ,although superficially similar to the heavily cratered surfaces of the ,moon and ,mercury1 ,callisto is far from identical to these rocky worlds4 ,one of the most obvious differences is a lack of craters larger than about #aej kilometers on ,callisto1 together with a tendency for large craters to have much shallower depths4 ,apparently the ice-rock composition of ,callisto alters the ability of the crust to retain large craters4 ,geologists speculate that the ice flows over many millions of years1 filling in crater floors and gradually obliterating the largest craters4 ,there is also a conspicuous absence of mountains on ,callisto1 again suggestive of a weak1 icy crust4 ,illustration3 ,the ,,iris instrument measured the temperature of spots on the surface of ,callisto as each ,voyager sped past4 ,the measurements shown here were all made at equatorial latitudes 7between -#aj and #be74 ,shown are very low predawn temperatures 7-#aij ,c7 followed by an increase to a noon-time maximum of about -#abj ,c1 and then a drop again as the ,sun sets4 #bfj-gce ,the most prominent features in the ,voyager pictures are the ghost remnants of what must have been immense impact basins4 ,the largest of these1 the 8bullseye0 of the ,voyager #a images1 has been named ,valhalla1 for the home of the ,norse gods4 ,these ghost basins have lost nearly all their vertical relief4 ,what remains is a central1 light-colored zone 7probably the location of the original crater71 surrounded by numerous concentric rings of subdued1 bright ridges4 ,such features had never been seen before on any planet1 and they appear to be the characteristic geologic feature of an ice-rock planet4 ,little is known about the composition of ,callisto's surface1 the material from which sunlight is reflected4 ,it appears to be primarily dark rock or soil1 but it lacks diagnostic spectral features1 except for one infrared band due to water molecules bound in the soil4 ,the many lighter spots and arcs that outline craters in the high-resolution pictures may be regions in which the ice is showing through1 but these cover only a very small fraction of the exposed surface4 7,it should be noted that1 although ,callisto is the darkest of the ,galilean satellites1 the term 8dark0 is relative1 for even ,callisto is brighter than ,earth's moon47 ,the daytime surface temperature of ,callisto1 observed both from the ground and by ,voyager1 is about -#aah ,c4 ,the ,voyager infrared interferometer spectrometer also determined the minimum temperature1 reached just before dawn1 of -#aic ,c4 ,no atmosphere is expected at these cold temperatures1 and none was seen4 ,analysis of ,voyager images provided an improved diameter for ,callisto of #dhdj kilometers1 yielding an average density of #a4h grams per cubic centimeter4 ,as noted previously1 it is this low density that leads to the conclusion that ice or water is an important component of the interior of ,callisto4 ,the ice has never been detected directly1 but the peculiar nature of the craters seen by ,voyager adds strong circumstantial support to this conclusion4 ,callisto1 with its heavy cratering1 is the most familiar-looking of the ,galilean satellites2 if all of them had turned out to be as geologically dead as ,callisto1 planetary geologists would certainly have been disappointed4 ,however1 each satellite1 progressing in toward ,jupiter1 presents increasing evidence of internal activity4 ,ganymede ,the largest of the ,galilean satellites 7#ebgj kilometers in diameter71 ,ganymede was expected to be similar to ,callisto in many ways4 ,both have low densities 7for ,ganymede1 #a4i grams per cubic centimeter71 indicating a bulk composition of about half rocky materials and half water4 ,in addition1 their diameters differ by only eight percent1 and both are far enough from ,jupiter to escape the severe pounding ,io receives from magnetospheric charged particles4 ,thus it was with great interest that ,voyager scientists looked at the differences that emerged between these two satellites4 ,illustration3 ,shaded relief map of ,ganymede4 #bfj-fgc ,the surface of ,ganymede as revealed by the ,voyager cameras is one of great diversity1 indicating differing periods of geologic activity4 ,at one extreme there are numerous dark areas that resemble the surface of ,callisto in both albedo 7reflectivity7 and crater density4 ,the largest of these1 ,regio ,galileo1 stretches from the equator to latitude #de and is #djjj kilometers across1 nearly as large as the continental ,united ,states4 ,this ancient terrain even preserves the remnants of a ,callisto-type impact basin in the form of a system of parallel1 curving1 subdued ridges about #aj kilometers wide1 #ajj meters high1 and spaced about #ej kilometers apart4 ,the central part of this ghost basin is missing1 however2 it was presumably destroyed by subsequent geologic activity4 ,other regions of the surface of ,ganymede are clearly the product of intense internal geologic activity4 ,generally1 these regions are of higher albedo and consist of many straight parallel lines of mountains and valleys4 ,the ,voyager geologists call these the grooved terrain because of their appearance from a great distance4 ,typically1 these mountain ridges are #aj-ae kilometers across and about #ajjj meters high1 similar in scale to some sections of the ,appalachian ,mountains in the ,eastern ,united ,states4 ,no higher relief exists1 presumably for the same reason it is absent on ,callisto4 ,in many places the grooved terrain forms between areas of the older1 darker surface1 giving the appearance of mountains extruded between separating plates of ancient crust4 ,in other areas the relationships are much more complex1 with curved systems of grooves and ridges overlying each other1 displaying intricate crosscutting relationships4 ,apparently ,ganymede has experienced a series of mountain-building events4 ,the grooved terrains show a substantial range in age1 as indicated by the crater densities4 ,the oldest have nearly the same density as the ancient1 dark plains1 suggesting that formation of the grooved terrain began early1 perhaps #d billion years ago4 ,the youngest grooved terrain has only about one-tenth as many craters1 but this is still as many as are seen in the #c4e-;billion-year-old lunar plains4 ,the ,voyager geologists believe that even in these areas geologic activity ceased billions of years ago4 ,illustration3 ,the hemisphere of ,ganymede that faces away from the ,sun displays a great variety of terrain4 ,in this ,voyager #b mosaic1 photographed at a range of #cjj #jjj kilometers1 the ancient dark area of ,regio ,galileo lies at the upper left4 ,below it1 the lighter grooved terrain forms bands of varying width1 separating older surface units4 ,on the right edge1 a prominent crater ray system is probably caused by water-ice splashed out in a relatively recent impact4 #bfj-fga ,illustration3 ,the sinuous nature of some of the narrower ,ganymede groove systems can be seen in this oblique view1 obtained on ,march #e by ,voyager #a4 ,the area shown is about the size of ,california1 with features visible as small as #e kilometers across4 ,the ridges appear to be the result of deformation of the crust of ,ganymede4 ,p-#babce ,other types of surfaces are seen on ,ganymede4 ,some regions are lightly cratered and smooth1 with no indication of mountain building4 ,in one place1 there is a rough mountainous area that looks more like the jumbled lunar mountains than the long ridges and valleys of the rest of ,ganymede4 ,many of the larger craters are distinguished by brilliant white halos and rays that suggest that impacts may have splashed large quantities of water or ice over the surface4 ,many of the geologic features seen on ,ganymede appear to have been caused by breaking1 faulting1 or spreading of the crust4 ,in a few cases1 there even seem to be indications of transverse1 or sideways1 motion along faults4 ,this evidence is extremely exciting to geologists1 since similar crustal motion on ,earth is associated with the drift of continental plates1 drawn by convection currents deep in the mantle4 ,such activity has never been seen before on another planet4 ,astronomers on ,earth had known since #aiga that about half the surface of ,ganymede was covered with exposed water ice and about half with darker rock4 ,an examination of the albedo variations in the ,voyager pictures suggests that the ice is exposed near large craters and1 to a lesser extent1 in the grooved terrain1 but no direct measurements were made by ,voyager of the composition of different parts of the surface4 ,the presence of ice on the surface suggested to many astronomers that ,ganymede might have a very tenuous atmosphere of water vapor or oxygen1 which might be released by the breakdown of water vapor by sunlight4 ,during the ,voyager #a flyby1 a sensitive test for an atmosphere was made by the ultraviolet instrument from observations of the star ,kappa ,centauri as it was occulted by ,ganymede4 ,no dimming of the starlight was seen1 yielding an upper limit for the surface pressure of the gases oxygen1 water vapor1 or carbon dioxide of #aj bar1 or one hundred-billionth the atmospheric pressure at ,earth4 ,the differences between the geologic histories of ,ganymede and ,callisto are surprisingly large4 ,no one knows the reason4 ,perhaps only a small increase in internal temperature is necessary to initiate geologic activity in an icy planet1 and for some reason ,ganymede crossed this threshold for a part of its history1 whereas ,callisto did not4 ,illustration3 ,the complex patterns of the grooved terrain on ,ganymede are apparent in high-resolution images4 ,this picture1 taken by ,voyager #a on ,march #e1 has a resolution of about #c kilometers and shows a region about the size of the state of ,pennsylvania4 ,the mountain ridges are spaced about #aj to #ae kilometers apart and rise about #ajjj meters1 similar to many of the mountains of ,pennsylvania4 ,the transections of different mountain systems indicate that they formed at different times4 ,a degraded crater near the left center of the picture is crossed by ridges1 indicating that it predated the period of crustal deformation and mountain building4 ,p-#babgi ,illustration3 ,ray systems of exposed water-ice are visible in this high-resolution mosaic of ,ganymede1 obtained by ,voyager #b on ,july #i at a range of about #ajj #jjj kilometers4 ,the rough mountainous terrain at lower right is the outer portion of a large fresh impact basin that postdates most of the other terrain4 ,at the bottom1 portions of grooved terrain transect other portions1 indicating an age sequence4 ,the dark patches of heavily cratered terrain 7right center7 are probably ancient icy material formed prior to the grooved terrain4 ,the large rayed crater at upper center is about #aej kilometers in diameter4 ,p-#baggj;,b_/,w ,illustration3 ,the many variants of smooth and grooved terrain on ,ganymede suggest a complex geologic history for this satellite4 ,four high-resolution views by ,voyager #b are grouped together4 #bfj-fgh;,a ,illustration3 ,a band of low mountain ridges has apparently been cut and offset by a fault4 ,illustration3 ,multiple sets of mountain ridges transect at nearly right angles4 ,some impact craters were formed before1 and some after1 the grooved terrain4 ,illustration3 ,many short parallel ridges butt into each other1 making a crazy-quilt pattern4 ,illustration3 ,in the center of this frame is an unusual smooth area1 perhaps the result of flooding of the surface by material that filled in the grooves4 ,europa ,as one proceeds inward through the ,galilean satellites1 these worlds become less and less familiar to the planetary geologist4 ,this was an unexpected effect4 ,callisto and ,ganymede were expected to have unusual properties as a result of their large percentage of ice4 ,the densities of ,europa and ,io are more normal for the smaller1 terrestrial-type planets1 and before ,voyager many scientists expected these two satellites might look much like the ,moon1 which they resemble in size4 ,europa1 with a diameter of #cacj kilometers1 is about #ae percent smaller than the ,moon4 ,its density is #c4j grams per cubic centimeter1 indicating a basically rocky composition4 ,however1 cosmic mixtures of rocky and metallic materials are often a bit denser than this1 leaving room for a substantial component of ice or water4 ,calculations indicate that if all the ice were at the surface1 it might form a crust up to #ajj kilometers thick4 ,illustration3 ,shaded relief map of ,europa4 #bfj-fei ,telescopic observations of ,europa demonstrated long before ,voyager that this satellite is almost completely covered with ice4 ,it is a white1 highly reflecting body1 looking1 from a great distance1 like a giant snowball4 ,in the early ,voyager pictures1 ,europa always showed a bland1 white disk1 in striking contrast to the spottiness of ,ganymede or the brilliant colors of ,io4 ,voyager #a never got closer to ,europa than #gcd #jjj kilometers1 and at that distance it remained a nearly featureless planet1 with no obvious impact craters or other familiar geologic structures4 ,what did show in the ,voyager #a pictures1 however1 were numerous thin1 straight dark lines crisscrossing the surface1 some extending up to #cjjj kilometers in length4 ,to the members of the ,imaging ,team1 these features were 8strongly suggestive of global-scale tectonic processes1 induced either externally 7as by tidal despinning7 or internally 7as by convection740 ,it was with the greatest interest that the ,voyager #b images1 taken from about four times closer1 were anticipated4 ,the spectacular pictures obtained of the satellite in ,july were perhaps more confusing than clarifying4 ,europa is entirely covered with dark streaks that vary in width from several kilometers to approximately #gj kilometers and in length from several hundred to several thousand kilometers4 ,most streaks are straight1 but others are curved or irregular4 ,the streaks lie on otherwise smooth1 bright terrain1 featureless except for numerous random dark spots1 most less than #aj kilometers in diameter4 ,voyager #b photos showed1 in addition to the smooth terrain with its dark streaks1 regions of darker1 mottled terrain4 ,this mottled terrain appears rough on a small scale1 and it may contain small craters just on the limit of resolution 7about #d kilometers74 ,only three definite impact craters have been identified1 each about #bj kilometers across4 ,this small number of craters suggests either that the surface is relatively young or that craters are not preserved for long in the icy crust4 ,although the dark streaks give ,europa a cracked appearance1 the streaks themselves are not obviously cracks4 ,they are not depressed below their surroundings2 in fact1 they have no topographic structure whatever4 ,europa is extraordinarily smooth1 and the dark streaks look rather like marks made with a felt-tipped pen on a white billiard ball4 ,the streaks are not even very dark2 the contrast with adjacent smooth terrain is only about #aj percent4 ,one of the most remarkable geologic phenomena discovered by ,voyager is the light streaks that appear on ,europa4 ,these are smaller than the dark streaks1 only about #aj kilometers in width1 but much more uniform4 ,seen at low ,sun angle1 they also show vertical relief of less than a few hundred meters4 ,these light ridges are seen best at low ,sun and tend to be invisible at higher illumination angles4 ,the most amazing thing about the light ridges is their form4 ,instead of being straight1 they form scallops or cusps1 with smooth curves that repeat regularly on a scale of #ajj to a few hundred kilometers4 ,in some of the low-,sun-angle pictures1 the surface of ,europa seems to be covered with a beautiful network of these regular curving lines4 ,the impression is so bizarre that one tends not to believe the reality of what is seen4 ,nothing remotely like it has ever been seen on any other planet4 ,at present the geology of ,europa remains beyond our understanding4 ,presumably there is a thick ice crust1 perhaps floating on a liquid water ocean4 ,presumably there is sufficient heat coming from the interior to have produced cracking or motion in the ice crust1 and the light and dark streaks preserve a pattern in some way related to this internal activity4 ,however1 the actual mechanisms for producing the observed features so lightly traced on this smooth white world remain for scientists to decipher4 ,illustration3 ,europa looks like a cracked egg in this computer mosaic of the best ,voyager #b images4 ,in this presentation1 the variation of surface brightness due to the angle of the ,sun has been removed by computer processing1 so that surface features can be seen equally well at all places4 ,the many broad dark streaks show up well1 but this presentation does not bring out the much fainter and more enigmatic light streaks4 ,these pictures were taken from a distance of about #bej #jjj kilometers and show features as small as #e kilometers across4 #bfj-fhf ,io ,the most spectacular of the ,galilean satellites is ,io4 ,even in low-resolution images1 its brilliant colors of red1 orange1 yellow1 and white set it apart from any other planet4 ,the dramatic scale of its volcanic activity confirms that ,io is in a class by itself as the most geologically active planetary body in the solar system4 ,the diameter of ,io is #cfdj kilometers1 and its density is #c4e grams per cubic centimeter4 ,both values are nearly identical to those of our ,moon4 ,were it not for ,io's proximity to ,jupiter1 it would probably be a dead1 rocky world much like ,earth's satellite4 ,careful examination of all the ,voyager images of ,io1 some of which have resolutions as good as #a kilometer1 has failed to reveal a single impact crater4 ,yet the flux of crater-producing impacts at ,io must be even greater than for the other ,galilean satellites1 because of the focusing effect of ,jupiter's gravity4 ,the absence of craters alone would indicate that ,io has an extremely young and dynamic planetary surface1 even without the observation of active volcanoes4 ,calculations indicate that craters on ,io must be filled in or otherwise obliterated at a rate corresponding to the deposition of at least #ajj meters per million years1 and quite probably a factor of ten greater1 or #a meter every thousand years4 ,illustration3 ,one of the most remarkable of all the ,voyager discoveries was the arcuate white ridges on ,europa4 ,visible only at very low ,sun angle1 these curved bright streaks are #e to #aj kilometers wide and rise at most a few hundred meters above the surface4 ,their graceful scalloped pattern is unique to this planet and has defied explanation4 ,also visible in this view1 taken by ,voyager #b on ,july #i at a range of #bbe #jjj kilometers1 are dark bands1 more diffuse than the light ridges1 typically #bj to #dj kilometers wide and hundreds to thousands of kilometers long4 ,p-#bagff ,illustration3 ,shaded relief map of ,io as it appeared in early ,march #aigi4 #bfj-fcd;,,bc ,illustration3 ,the great erupting volcanoes on ,io produce distinctive surface markings4 ,these are three views of ,prometheus 7,p74 ,the bright ring on the surface rims the areas of fallout from the plume1 and it is probably an area in which sulfur dioxide frost is being deposited on the surface4 #bfj-dea ,illustration3 ,prometheus4 ,illustration3 ,prometheus4 ,illustration3 ,prometheus4 ,illustration3 ,these views are of ,loki 7,p7 as seen by ,voyager #a4 #bfj-dea ,illustration3 ,the asymmetric structure of the plume can be seen4 ,illustration3 ,loki4 ,illustration3 ,an ultraviolet image has been used to produce a false-color composite2 the large ultraviolet halo above the visible-light plume may be due to scattering from sulfur dioxide gas rather than solid particles4 ,illustration3 ,perhaps the most spectacular of all the ,voyager photos of ,io is this mosaic obtained by ,voyager #a on ,march #e at a range of #djj #jjj kilometers4 ,a great variety of color and albedo is seen on the surface1 now thought to be the result of surface deposits of various forms of sulfur and sulfur dioxide4 ,the two great volcanoes ,pele and ,loki 7upper left7 are prominent4 #bfj-dfd ,in place of impact craters1 the surface of ,io has a great many volcanic centers1 which generally take the form of black spots a few tens of kilometers across4 ,in a few cases1 high-resolution pictures show the characteristic shapes associated with volcanic calderas on ,earth and ,mars1 and1 if the other volcanic centers are similar1 about #e percent of the entire surface of ,io is occupied by calderas4 ,these are extremely black1 reflecting less than #e percent of the sunlight2 often they are surrounded by irregular1 diffuse halos nearly as black as the central spot4 ,the calderas seem more like the ,valles caldera in ,new ,mexico1 which is associated with vents that produced large quantities of ash1 than with those of ,hawaiian-type shield volcanic mountains4 ,there is evidence in many of the ,voyager photos of extensive surface flows on ,io4 ,these originate in dark volcanic centers and either spread to fan shapes1 typically #ajj kilometers across1 or else snake out in long1 twisting tentacles4 ,some of the flows are lighter than the background and some are darker4 ,most are red or orange in color1 often outlined by fringes of contrasting albedo4 ,the equatorial regions of ,io are quite flat1 with no vertical relief greater than about #a kilometer high2 indeed1 many of the volcanic centers do not appear to correspond to mountains or domes at all4 ,there are1 however1 a number of long1 curvilinear cliffs or scarps and narrow1 straight-walled valleys a few hundred meters deep4 ,these appear to be places in which the crust has broken under tension1 somewhat similar to terrestrial faults and the valleys called graben4 ,a few rugged mountains of uncertain origin are visible in low ,sun elevation pictures4 ,near the poles of ,io the terrain is more irregular4 ,there are few volcanic centers1 but more mountains1 some with heights of several kilometers4 ,in addition1 there are regions that appear to be made of stacked layers of material4 ,these so-called layered terrains are revealed when erosion cuts into them1 exposing the layers along the cliff or scarp4 ,the largest such plateau or mesa has an area of about #ajj #jjj square kilometers4 ,the scarps sometimes intersect each other1 suggesting a complex history of deposition1 faulting1 and erosion4 ,voyager geologists believe that these scarps may be areas in which the release of liquid sulfur or sulfur dioxide has undercut cliffs1 analogous to internal sapping by groundwater at similar scarps on ,earth4 ,perhaps the most distinctive surface features on ,io are the circular or oval albedo markings that surround the great volcanoes4 ,the first of these to be seen was the #cjj-kilometer-wide white donut of ,prometheus1 on the equator at longitude #aej4 ,much more spectacular is the hoofprint of ,pele1 about #gjj by #ajjj kilometers4 ,these symmetric rings mark the locations of the kinds of eruptions that generate large fountains or plumes1 and may be produced by condensible sulfur or sulfur dioxide raining down from the volcanic fountain4 ,at least one new ring appeared during the four months between the ,voyager encounters1 centered at longitude #ccj1 latitude #bj1 but by the time ,voyager #b photographed this area1 no plume remained active4 ,during the ,voyager #a flyby1 temperature scans of the surface of ,io were made with the infrared interferometer spectrometer 7,,iris74 ,a number of localized warm regions were found1 the most dramatic being just south of the volcano ,loki4 ,here the images showed a strange1 ,u-shaped black feature about #bjj kilometers across4 ,the ,,iris team interpreted its data to indicate a temperature for the black feature of #ag ,c 7or room temperature71 in contrast to the surrounding surface at -#adf ,c4 ,perhaps the dark feature was some sort of lava lake1 either of molten rock or molten sulfur4 ,the melting point of sulfur is #aab ,c4 ,if there were a scum of solidifying sulfur on top of the 8lake10 this interpretation might well be the correct one4 ,the brilliant reds and yellows of the surface of ,io immediately suggest the presence of sulfur4 ,when heated to different temperatures and suddenly cooled1 sulfur can assume many colors1 ranging from black through various shades of red to its normal light-yellow appearance4 ,even before ,voyager1 laboratory studies had shown that sulfur matches the overall properties of the spectrum of ,io1 including the low albedo in the ultraviolet and the high reflectivity throughout the infrared4 ,contemporary with the ,voyager flybys1 additional telescopic observations and laboratory studies by ,fraser ,fanale at ,,jpl and ,dale ,cruikshank at the ,university of ,hawaii identified another component on ,io1 sulfur dioxide4 ,sulfur dioxide is an acrid gas released from terrestrial volcanoes1 where it combines with water in the ,earth's atmosphere to produce sulfuric acid4 ,at the temperature of the surface of ,io1 sulfur dioxide is a white solid4 ,researchers guessed that the extensive bright white areas in the ,voyager pictures of ,io might be covered with sulfur dioxide frost or snow4 ,the presence of this material on ,io was confirmed when the infrared ,,iris instrument obtained a spectrum of sulfur dioxide gas over the erupting volcano ,loki during the ,voyager #a encounter4 ,illustration3 ,close-ups of ,io reveal a wide variety of volcanic phenomena4 ,this ,voyager #a view of an equatorial region near longitude #cjj shows several large surface flows that originate in volcanic craters or calderas4 ,at the right edge is a light flow about #bej kilometers long4 ,another dark1 lobate flow with bright edges is just left of center1 with an exceedingly dark caldera to its left4 #bfj-dfh;,a ,the discovery of the ongoing eruptions on ,io1 made shortly after the ,voyager #a flyby1 did much to clarify the confused evidence pouring in concerning the apparent youth of ,io's surface4 ,here1 under the very eyes of ,voyager1 eruptions were taking place on a scale that dwarfed anything ever seen before4 ,the discovery picture alone1 taken from a distance of #d million kilometers1 showed two eruptions 7,pele and ,loki71 each of which was much larger than the most violent volcanic eruption ever recorded on ,earth4 ,voyager #a found eight giant eruptions1 with fountains or plumes rising to heights of between #gj and #bhj kilometers4 ,to reach these altitudes1 the material must have been ejected from the vents at speeds of between #cjj and #ajjj meters per second1 several times greater than the highest ejection velocities from terrestrial volcanoes4 ,although widely spaced in longitude1 these volcanoes were concentrated toward the equator2 seven of the eight were at latitudes between #cj and -#cj1 and the eighth at -#dd4 ,when ,voyager #b arrived four months later1 it was able to reobserve seven of the eight volcanoes4 7,to be identified reliably1 the volcanic plumes must be silhouetted against dark space at the edge of the disk47 ,six of these were still erupting2 one1 ,pele1 the largest plume seen by ,voyager #a1 had ceased activity4 ,the plume associated with ,loki had also changed markedly1 increasing in height from #ajj to #baj kilometers in visible light4 7,all the plumes appear larger when viewed in the ultraviolet47 ,loki had developed a more complex structure2 in ,march it appeared to originate near the south end of a #bej-kilometer-long dark feature1 but in ,july there was a double plume1 with activity at both ends of the dark feature4 ,illustration3 ,differences in surface elevation can clearly be seen in a few of the ,io close-ups from ,voyager #a4 ,this remarkable picture is of the center of the great volcano ,pele1 at latitude #ae,s and longitude #bbd4 ,a low mountain with flow features can be seen4 ,in the background1 there are several large irregular depressions with flat floors that appear to be the result of collapse4 ,the diffuse dark features in the center are probably the ejecta plumes being erupted from the ,pele vent4 ,p-#babbj;,b_/,w ,illustration3 ,at the highest resolution obtained by the ,voyager cameras1 ,io revealed some landscapes that looked familiar to terrestrial geologists4 ,this picture1 taken by ,voyager #a at a range of only #ca #jjj kilometers1 shows a region about the size of the state of ,maryland at a resolution of #cjj meters4 ,clearly seen is a volcano not too different from some of those on the ,earth or ,mars4 ,at the center is an irregular composite crater or caldera about #ej kilometers in diameter with dark flows radiating from its rim4 ,the style of volcanism illustrated here is quite different from the explosive plumes or fountains with their associated rings of bright material deposited on the surface4 ,this volcano is located at about longitude #ccj1 latitude #gj,s4 #bfj-ejb ,illustration3 ,in addition to its giant volcanic plumes or fountains1 ,io possesses other indications of current volcanic activity4 ,one of these takes the form of intermittent blue-white patches that may be caused by gas venting from the interior4 ,in this pair of photographs1 the same region of the surface is shown about six hours apart4 ,on the right1 there is an arcuate bright gas cloud2 on the left the same region is black4 ,it is believed that the venting gas is sulfur dioxide1 and that the condensation of this gas produces fine particles of 8snow0 that look blue4 #bfj-ejh ,illustration3 ,some of the most dramatic changes in the surface of ,io between ,march and ,july took place in the vicinity of the volcano ,loki1 at longitude #caj and latitude #ae,n4 ,on the left is a ,voyager #a view2 on the right1 one from ,voyager #b4 ,the 8lava lake0 associated with ,loki has become less distinct1 apparently as a result of deposits that fell on the northern part of the dark ,u-shaped feature4 ,perhaps the surface had also cooled between these photos4 ,in the upper left center1 a new dark volcanic caldera with bright spots near it and a large1 faint bright ring had appeared by ,july1 although it was not active at the time ,voyager #b flew by4 #bfj-fhg;,,ac ,no new eruptions were seen by ,voyager #b2 between them1 the two spacecraft effectively surveyed the whole surface of ,io for plumes down to #dj kilometers height4 ,interestingly1 the smallest plume seen was #gj kilometers high2 there appeared to be a real absence of smaller eruptions4 ,from the number and size of the observed eruptions1 it is possible to calculate the resurfacing rate for ,io due to these plumes4 ,the result is that each plume is erupting about #aj #jjj tons of material per second1 or more than #ajj billion tons per year4 ,this quantity corresponds to about #aj meters of deposition over the whole surface in a million years4 ,when additional note is taken of surface flows1 the deposition rate could easily be ten times higher1 or #ajj meters per million years1 in agreement with the rate estimated from the absence of impact craters4 ,energy for the ,io ,volcanoes ,clearly1 something extraordinary is happening to ,io to generate the observed level of volcanism4 ,the primary heat source for the interiors of the terrestrial planets is the decay of the long-lived radioactive elements thorium and uranium4 ,but ,io would have to be supplied with a hundred times its quota of these elements to explain the observed activity4 ,a way out of this difficulty was provided by a theoretical investigation carried out by ,stanton ,peale of the ,university of ,california at ,santa ,barbara and ,pat ,cassen and ,ray ,reynolds at the ,,nasa ,ames ,research ,laboratory4 ,working in the months before the first ,voyager flyby1 they calculated that the tidal effects of ,jupiter on ,io could generate large-scale heating of the satellite4 ,io is about the same distance from ,jupiter as the ,moon from ,earth1 but the much greater mass of ,jupiter raises enormous tides in its satellite4 ,these tides distort its shape1 but no other effect would be present if ,io remained at a constant distance from ,jupiter4 ,what ,peale1 ,cassen1 and ,reynolds realized was that the distance of ,io from ,jupiter varies as the result of small gravitational perturbations from the other ,galilean satellites4 ,therefore the tidal distortions also vary1 in effect squeezing and unsqueezing ,io each orbit4 ,such flexing pumps energy into the interior of ,io in the form of heat2 theorists calculated that the heat supplied could be as high as #aj watts4 ,they predicted1 in a paper published just three days before the ,voyager flyby of ,io1 that 8widespread and recurrent surface volcanism might occur10 and that 8consequences of a largely molten interior may be evident in pictures of ,io's surface returned by ,voyager40 ,illustration3 ,voyager #b obtained beautiful views of the volcanic eruptions during its ten-hour ,io volcano watch on ,july #i4 ,on the edge of the crescent image are the volcanoes ,amirani 7,p7 and below it ,maui 7,p71 each sending up fountains about #ajj kilometers above the surface4 ,the blue color is probably the result of sunlight scattered by tiny particles of sulfur dioxide snow condensing in the erupting plume4 ,p-#baghj ,,volcanic ,,eruptions ,,on ,,io ,plume ,name ,location ,height ,during ,activity ,during ,number 7latitude_/longitude7 ,voyager #a ,voyager #b ,flyby ,flyby 7kilometers7 #a ,pele -#bj_/bee #bhj ceased #b ,loki #bj_/cjj #ajj increased #c ,prometheus -#e_/aee #gj increased #d ,volund #bj_/age #ie no data #e ,amirani #be_/abj #hj similar #f ,maui #bj_/abj #hj similar #g ,marduk -#be_/baj #abj similar #h ,masubi -#dj_/ #ej #gj similar ,illustration3 ,between the two encounters1 the volcanic eruption at ,loki 7,p7 changed character4 ,the single plume emanating from the western end of an apparent dark fissure seen in ,march was joined by a second fountain of similar size about #ajj kilometers to the east4 ,the plume also increased in height1 from about #abj kilometers in the ,voyager #a image 7left7 to #age kilometers in the ,voyager #b image 7right74 #bfj-ffb;,a and ,b ,illustration3 ,the detailed structure near the volcano ,loki is like nothing seen elsewhere on ,io4 #bfj-fdb;,b ,illustration3 ,when this ,voyager #a picture was taken1 the main eruptive activity 7,p7 came from the lower left of the dark linear feature 7perhaps a rift7 in the center4 ,below it is the 8lava lake10 a ,u-shaped dark area about #bjj kilometers across4 ,in this specially processed image1 detail can be seen in the dark surface of this feature1 possibly due to 8icebergs0 of solid sulfur in a liquid sulfur lake4 ,illustration3 7,bottom7 ,the ,,iris on ,voyager #a found this 8lava lake0 to be the hottest region on ,io1 with a temperature about #aej ,c higher than that of the surrounding area4 ,illustration3 ,one model for the structure of ,io indicates that an ocean of liquid sulfur with a solid sulfur crust covers most of the satellite4 ,heat escapes from the interior in the form of lava1 which erupts beneath the sulfur ocean4 ,secondary eruptions in the sulfur ocean heat liquid sulfur dioxide1 which is mixed with solid sulfur in the crust4 ,the rapid expansion of sulfur dioxide gas then produces the great eruptive plumes1 which consist of a mixture of solid sulfur1 sulfur dioxide gas1 and sulfur dioxide snow4 ,solid sulfur solid ,,so ,solid sulfur liquid ,,so ,molten sulfur ,solid silicate ,the ,voyager observations appear to confirm the theoretical calculations4 ,the tidal heat source has presumably been acting since ,io was formed more than #d billion years ago4 ,with a totally molten interior and continuing large-scale volcanism1 ,io has had an opportunity to thoroughly sort out its composition4 ,in the process it would have lost all the volatile gases such as water and carbon dioxide1 explaining why ,io now has no appreciable atmosphere in spite of the outpouring of material from the interior4 ,in addition1 most of the sulfur from the interior could have risen to the surface1 where it would be constantly recycled through volcanic activity4 ,the presence of large amounts of sulfur on the surface may help explain the extraordinary nature of the ,io volcanoes4 ,one model considered for the satellite postulates that it is covered by a sea of liquid sulfur several kilometers deep1 with a crust of solid sulfur and1 below the surface1 liquid sulfur dioxide4 ,calculations by ,sue ,kieffer of the ,u4,s4 ,geological ,survey and others indicate that the expansion of the sulfur dioxide in such a model can explain the observed eruption velocities of up to a kilometer per second4 ,the volcanic plumes on ,io appear to be made primarily of sulfur and sulfur dioxide4 ,both are molten as they emerge from the vent1 but they quickly cool as the plume rises #ajj or more kilometers into the near vacuum of space4 ,unlike terrestrial volcanoes1 there is almost no gas in the plumes4 ,it requires about half an hour for the fine particles of solidified sulfur and sulfur dioxide snow to fall back to the surface1 where they form the colorful rings that mark the major eruptive sites4 ,almost all the roughly #ajj #jjj tons of material erupted each second by the ,io volcanoes snows back to the surface4 ,but apparently a part--perhaps #aj tons per second--escapes from ,io and is captured by the ,jovian magnetosphere4 ,another part contributes to an ionosphere--a tenuous atmosphere of electrons and ions--that surrounds ,io4 ,the injection of several tons of particles each second into the magnetosphere has dramatic consequences that can be seen even from ,earth4 ,illustration3 ,direct evidence of an atmosphere on ,io was obtained during the ,voyager #a flyby by the ,,iris4 ,in the region near the volcano ,loki and its associated 8lava lake10 infrared spectra clearly showed the signature of sulfur dioxide gas4 ,it is not known whether this gas was a temporary feature associated with the eruption of ,loki or if it might be present on ,io more generally4 ,other evidence1 however1 points to the sulfur dioxide atmosphere as a transient feature4 ,a small amount of the sulfur dioxide escapes and is broken apart by sunlight to provide the oxygen and sulfur ions observed in the ,io torus4 ,sulfur dioxide gas cloud ,plume -#bce ,f ,hot surface areas 7#de ,f7 ,the ,io ,torus ,surrounding ,jupiter at the distance of ,io is a donut-shaped volume1 or torus1 of plasma that originates at the satellite4 ,at first1 the atoms escaping from ,io expand outward as a gas1 but soon they are stripped of electrons and become electrically charged4 ,some of these gases1 such as sulfur dioxide1 apparently originate in the large volcanic eruptions2 other1 such as the sodium cloud being studied with ,earth-based telescopes1 result from sputtering of surface materials by energetic particles in the magnetosphere4 ,after they are ionized by the loss of one or more electrons1 the atoms are caught by the spinning magnetic field of ,jupiter and become a part of what is called a co-rotating plasma1 spinning at #gd kilometers per second with the same ten-hour period as ,jupiter itself4 ,the ,io torus was easily detected on ,voyager by the ultraviolet spectrometer1 even from a distance of #aej million kilometers4 ,the strongest ultraviolet radiation comes from twice-ionized sulfur 7atoms that have lost two electrons7 7,s ,,iii71 emitting a wavelength of #fi nanometers or about one-eighth the wavelength of visible light4 ,the spectrometer also detected glows from atoms of triply ionized sulfur 7,s ,,iv7 and twice-ionized oxygen 7,o ,,iii74 ,illustration3 ,at the time of the ,voyager #a encounter1 the most abundant heavy ions in the ,jovian magnetosphere were sulfur and oxygen4 ,multiply ionized sulfur and oxygen both emit strongly in the ultraviolet1 where they could be observed by the ultraviolet spectrometer4 ,this spectrum of the ,io torus registers the tremendous amount of ultraviolet energy 7about a million million watts7 being radiated4 ,to emit so strongly1 the temperatures in the torus must be near #ajj #jjj ,k4 ,illustration3 ,direct measurement of the heavy ions associated with the ,io torus were made by the ,voyager #a ,,lecp instrument4 ,here the amounts of various elements are shown for two cases3 the ,jovian inner magnetosphere 7solid line7 and a typical solar event4 ,both the ,jovian and the solar particles have been scaled to show similar amounts of oxygen1 but the solar particles are also rich in carbon and iron1 whereas ,jupiter has a great deal of sulfur4 ,evidence such as this demonstrates that the sulfur does not come from the ,sun2 rather1 the sulfur and most of the oxygen appear to be the product of the ,io sulfur dioxide volcanic eruptions4 ,scans across the torus showed that it had a thickness of #a4j ,r,j and was centered at a distance of #e4i ,r,j from ,jupiter4 ,the torus is centered on the magnetic1 rather than the rotational1 equator of the planet4 ,to produce the intense glow observed1 the electron temperatures in the torus must be #ajj #jjj ,k1 with an electron density of about #ajjj per cubic centimeter4 ,the brightness in the ultraviolet corresponds to a radiated power of more than a million million 7#aj7 watts4 ,this enormous amount of energy must be continuously supplied by the magnetosphere4 ,the ultraviolet emissions from the ,io torus seen by ,voyager were dramatically different from those seen in #aigc and #aigd with the simpler ultraviolet instrument on board ,pioneers #aj and #aa4 ,these changes correspond to more than a factor of #aj in brightness4 ,as noted by the ,voyager ,team1 8,because of the remarkable differences we conclude that the ,jupiter-,io environment has changed significantly since ,december #aigc4 ,the observed differences are so spectacularly large that this conclusion does not depend on a detailed comparison of the two instruments1 their calibrations1 or the observing geometry40 ,the reason for this change1 or the degree to which it reflects a large-scale variation in the volcanic activity of ,io1 is one of the major questions arising from the ,voyager mission4 ,the ,io torus can also be observed from the ground4 ,in #aigf1 spectra showed the glow of singly ionized sulfur 7,s ,,ii71 and in #aigi singly ionized oxygen 7,o ,,ii7 was detected4 ,the observations of ,s ,,ii and ,o ,,ii are particularly interesting because they provide a measure of the density and temperature of the plasma4 ,in ,april #aigi1 between the two ,voyager flybys1 ,carl ,pilcher of the ,university of ,hawaii succeeded in obtaining a telescopic image of the torus in the light of ,s ,,ii4 ,he measured nearly the same ring diameter 7#e4c-e4g ,r,j7 as had ,voyager2 interestingly1 both agree that the sulfur torus is centered slightly inside the orbit of ,io 7#f4j ,r,j74 ,illustration3 ,the emission of light from sulfur ions in the ,io torus is so strong it can be measured from the ,earth4 ,in these pictures1 ,university of ,hawaii astronomer ,carl ,pilcher photographed the torus in the light of ionized sulfur on the night of ,april #i1 #aigi4 ,as the planet rotates1 the torus is seen first partly opened1 then edge-on1 and again opened in the opposite direction4 ,the dark band on the right of each image is due to light from ,jupiter scattered in the telescope1 as shown in the bottom picture1 which contains the scattered light only4 ,direct measurements of the torus were made from ,voyager #a as the spacecraft passed twice through this region1 once inbound and once outbound4 ,the low energy charged particle instrument and the cosmic ray instrument both determined that the composition of the ions in the ,io torus was primarily sulfur and oxygen4 ,ionized sodium was also observed4 ,for several years1 ground-based telescope observations had revealed a cloud of neutral sodium around ,io2 the ,voyager instruments picked up these atoms after they had each lost an electron and become trapped in the magnetosphere4 ,these instruments also derived the electron and ion density 7about #ajjj per cubic centimeter7 and confirmed that the ions were co-rotating with the inner magnetosphere4 ,another ,io-associated phenomenon searched for by ,voyager was the ,io flux tube4 ,as a conductor moving through the ,jovian magnetic field1 ,io generates an electric current1 estimated to have a strength of about #aj million 7#aj7 amperes and a power of the order of a million million 7#aj7 watts4 ,the region of space through which this current flows from the satellite to ,jupiter is called the flux tube4 ,voyager #a was targeted to fly through the ,io flux tube4 ,this was an important decision1 since this option precluded the possibility of obtaining occultations by either ,io or ,ganymede4 ,the event was to take place on ,march #e1 just after closest approach to ,io4 ,the effects of the flux tube were clearly observed by the magnetometer1 the ,,lecp instrument1 and other particle and field instruments2 however1 subsequent analysis indicated that the spacecraft had not penetrated the region of maximum current flow2 it probably missed the center of the flux tube between #ejjj and #aj #jjj kilometers4 ,the flux tube is not the only connection between ,io and ,jupiter4 ,radio emissions from the atmosphere are triggered by ,io's orbital position1 and the aurorae that illuminate ,jupiter's polar regions are the result of charged particles falling into the planet from the ,io torus4 ,other charged particles can occasionally escape outward and be detected as far away as ,earth4 ,io is unquestionably a remarkable world4 ,the only planetary body known to be geologically more active than the ,earth1 it provides many extreme examples to test the theories of geoscientists4 ,its intimate interconnections with the ,jovian magnetosphere and the planet itself provide a unifying theme to the complex processes taking place in the inner parts of the ,jovian system4 ,illustration3 ,the ,galileo ,probe will make a fiery entry into the ,jovian atmosphere1 carrying a payload of scientific instruments for the first direct sampling of the atmosphere of a giant planet4 ,shown here is the moment1 at a pressure level of about #j4a bar1 when the parachute is deployed and the still-glowing heatshield drops free from the ,probe4 ,,chapter #aj ,,return ,,to ,,jupiter ,a ,successor to ,voyager ,the spectacular discoveries of the ,voyagers did not exhaust our interest in the ,jovian system4 ,both the giant planet and its system of satellites will almost certainly play a central role in any future program of solar system exploration and research4 ,thus1 even as the two ,voyager spacecraft directed their attention further outward toward ,saturn1 ,,nasa had begun development of the next ,jupiter mission1 named ,galileo4 ,galileo is an ambitious1 multiple-vehicle planetary mission4 ,it has two major interlocking elements3 a probe to be placed in the atmosphere of ,jupiter and an orbiter to explore ,jupiter1 its satellites1 and its magnetosphere4 ,by using individual satellite flybys to alter its orbit1 the ,galileo spacecraft can carry out a satellite 8tour0 consisting of flybys of the ,galilean satellites at different geometries and a deep penetration into the magnetosphere in the unexplored region of space behind ,jupiter4 ,both an atmospheric probe for the planet and a long-lived orbiter to study the satellites and magnetosphere are logical successors to ,voyager4 ,in #aigd1 three years before ,voyager launch1 the ,space ,science ,board of the ,national ,academy of ,sciences was already emphasizing the scientific advantages of both of these approaches4 ,in suggesting goals for #aige-aihe1 the ,board wrote1 8,we recommend that a significant effort in the ,,nasa planetary program over the next decade be devoted toward the outer solar system4 ,jupiter is the primary object of outer solar system exploration40 ,looking at specific mission goals1 the ,board recommended that 8the primary objectives in the exploration of ,jupiter and its satellites for the period #aige-aihe in order of importance are 7#a7 determination of the chemical composition and physical state of its atmosphere1 7#b7 the chemical composition and physical state of the satellites1 and 7#c7 the topology and behavior of the magnetic field and the energetic particle fluxes4 ,in order to carry out this program1 it will be necessary to utilize orbiting spacecraft and probe-delivering spacecraft40 ,in the same period ,,nasa carried out studies of two possible orbiter and probe missions4 ,working through the ,ames ,research ,center1 a scientific panel chaired by ,james ,van ,allen explored the adaptation of the ,pioneer #aj and #aa spinning spacecraft to carry a probe to ,jupiter and to carry out an orbiter mission emphasizing magnetospheric studies4 ,william ,b4 ,hubbard of the ,university of ,arizona chaired a ,,jpl-based panel investigating the use of a ,mariner-class fully stabilized spacecraft similar to ,voyager to carry out a satellite-oriented orbiter mission4 ,in #aigf these concepts were combined in a study1 again chaired by ,dr4 ,van ,allen1 of a ,voyager-type orbiter with probe-carrying capability4 ,this mission concept was given the name ,,jop1 for ,jupiter ,orbiter ,probe1 and lead responsibility was assigned by ,,nasa to ,,jpl1 with ,ames carrying out the design of the probe4 ,in #aigg1 as ,voyager activity was building toward autumn launch1 a struggle was underway in ,washington to obtain approval for the new ,jupiter orbiter and probe mission4 ,budgeting authority was requested in the ,president's ,fiscal ,year #aigh budget1 but only after extensive testimony and several ,congressional votes was the mission approved4 ,the official new start for ,,jop1 soon to be renamed ,galileo1 was set for ,july #a1 #aigg1 and the scientific investigators and their instruments were selected in ,august4 ,at ,,jpl1 many members of the ,voyager ,team made a smooth transition to the ,galileo ,project4 ,much of the knowledge that had gone into the design of the ,voyager spacecraft and its subsystems was now incorporated into ,galileo4 ,similarly1 at ,ames the knowledge gained from the design of the ,pioneer ,venus probes1 which were launched to ,venus in #aigh1 a year after ,voyager launch1 was applied to design of a ,jupiter probe4 ,among the individuals who brought their ,voyager experience to ,galileo were ,john ,casani1 who left the position of ,voyager ,project ,manager to become ,galileo ,project ,manager1 and ,torrence ,johnson of the ,voyager ,imaging ,team1 who became ,galileo ,project ,scientist4 ,the ,scientific ,capability of ,galileo ,the investigations of ,jupiter and its system planned for the ,galileo ,project represented substantial advances over those carried out by ,voyager4 ,in part1 this was the result of new spacecraft capabilities1 particularly the atmospheric entry probe4 ,it also represented increasing sophistication in scientific instrumentation over the seven-year interval between the selection of the payloads for the two missions4 ,the main emphasis in the study of ,jupiter itself is on direct measurements with the ,probe4 ,for the first time it will be possible to examine directly the atmosphere of a giant planet4 ,by measuring the temperature and pressure as it descends through the clouds1 the ,probe can determine the structure of the atmosphere with much higher precision than could ever be obtained from remote observations4 ,the structure1 in turn1 provides information on dynamics--the circulation and heat balance of the ,jovian atmosphere4 ,in addition1 the ,probe can make direct measurements of the composition of the gases1 with sensitivity in some cases to quantities as low as a few parts per billion4 ,in addition to the elemental abundance1 the amount of different isotopes can also be measured4 ,direct studies of the clouds of ,jupiter can be made from the ,galileo ,probe4 ,with a device called a nephelometer 7literally1 cloud-meter71 the sizes and compositions of individual aerosol particles will be determined4 ,an infrared instrument will determine the temperatures of the cloud layers and measure the amounts of sunlight deposited in different regions of the atmosphere4 ,another instrument will search for lightning2 it has the ability to detect both the flash of light and the radio static generated by each bolt4 ,additional studies of the atmosphere1 similar to those of ,voyager1 can be carried out from the ,galileo ,orbiter4 ,television pictures1 ultraviolet and infrared spectra1 and measurements of the polarization of reflected light will all be obtained with the same scan platform instruments that are used to study the surfaces of the satellites4 ,a full battery of fields and particles instruments is planned for the ,galileo ,orbiter4 ,many of these are direct descendants of ,voyager instruments4 ,in general1 their capabilities have been improved1 particularly their ability to determine the composition of charged particles4 ,there is a steady progression from ,pioneer to ,voyager to ,galileo3 ,the early measurements were concentrated on particle energies1 but more sophisticated instruments yield the composition of the ions and the details of their motion4 ,many of the advances expected from ,galileo in magnetospheric studies result from the ,orbiter's ability to explore many parts of the environment of ,jupiter4 ,the ,pioneer and ,voyager spacecraft made single cuts through the magnetosphere1 and often it was difficult to distinguish temporal from spatial effects4 ,galileo will repeatedly swing around ,jupiter1 sampling conditions at many distances from the planet over a time span of two years or more4 ,in addition1 it is planned to adjust the orbit of ,galileo to swing out into the magnetotail1 the turbulent region of the magnetosphere that stretches 8downwind0 from ,jupiter for hundreds of ,jupiter radii4 ,no flybys can reach the magnetotail2 an orbiting spacecraft is required4 ,the ,galilean satellites naturally will be a primary focus of ,galileo science1 particularly after ,voyager4 ,it is planned to have as many as a dozen individual encounters1 most of them at much closer range than the ,voyager flybys4 ,to take advantage of these opportunities1 the ,galileo scan platform will carry two new remote sensing systems4 ,illustration3 ,the ,orbiter section of the ,galileo spacecraft will carry both remote sensing and direct measuring instruments for the study of ,jupiter1 its satellites1 and its magnetosphere4 ,several remote sensing instruments--an imagery system1 a near infrared mapping spectrometer1 an ultraviolet spectrometer1 and a photopolarimeter_/radiometer--will be mounted on a scan platform4 ,the particles and fields instruments will be on a spinning section of the spacecraft4 ,the ,orbiter is expected to operate for at least two years around ,jupiter1 providing one close flyby of ,io and several each of ,europa1 ,ganymede1 and ,callisto4 ,p-#bjggb ,instead of the vidicon television camera on ,voyager1 ,galileo imaging will be done with a new solid-state detector called a charged coupled device 7,,ccd74 ,the ,,ccd has a wider spectral response and greater photometric accuracy4 ,in addition1 its increased sensitivity permits shorter exposures1 so that even on very close flybys the pictures will not be blurred by spacecraft motion4 ,substantial coverage at a resolution of #ajj meters should be possible1 compared to ,voyager's best resolution of #a kilometer for ,io and #d kilometers for ,europa4 ,illustration3 ,galileo will be launched by the ,space ,shuttle1 the central element of the new ,,nasa ,space ,transportation ,system4 ,together with its upper stage launch rocket1 ,galileo will be placed into ,earth's orbit in the large ,shuttle bay1 about #bj meters long and #e meters in diameter4 ,after releasing ,galileo1 the ,shuttle will be piloted back to a landing at ,cape ,canaveral1 to be used again for many future flights4 #e-gh-bceii ,illustration3 ,the ,space ,shuttle and the ,inertial ,upper ,stage of ,galileo as they will appear in ,earth orbit4 ,the upper stage has been released from the ,shuttle bay and is being prepared for launch to ,jupiter4 ,to determine the composition of satellite surface materials1 ,galileo will also carry a near-infrared mapping spectrometer 7,,nims74 ,this instrument will obtain measurements over the visible and infrared spectra of areas as small as #aj kilometers across4 ,with ,,nims1 it should be possible to investigate the composition of individual features as small as the volcanic calderas on ,io or the ejecta blankets of ,ganymede's craters4 ,galileo ,mission ,design ,the ,galileo ,orbiter and ,probe are to be launched with ,,nasa's new ,space ,shuttle and ,inertial ,upper ,stage4 ,to carry the maximum possible payload to ,jupiter1 a close flyby of ,mars is planned en route4 ,the gravitational field of ,mars will give a boost to ,galileo1 just as that of ,jupiter was used by ,voyager to swing on to ,saturn4 ,the exact launch date and trajectory for ,galileo have not yet been specified1 but if all goes well1 the ,orbiter spacecraft will approach ,jupiter from the dawn side of the planet sometime in the mid-#aihjs4 ,it will not be moving as fast as ,voyager1 since it must be placed into orbit around ,jupiter rather than flashing past on its way to the outer solar system4 ,on its initial trajectory1 ,galileo will probably come within #e ,r,j of ,jupiter1 slightly closer than ,voyager #a4 ,at this time it will fire its rocket engines 7supplied by the ,federal ,republic of ,germany in a cooperative program with ,,nasa7 to shed excess speed and let itself be captured by ,jupiter's gravity4 ,the first pass will also be the time for a close flyby of ,io4 ,illustration3 ,the most critical period of the ,galileo flight will be the ,probe entry at ,jupiter4 ,the ,probe must strike the atmosphere at precisely the correct angle and speed to be slowed down without being destroyed4 ,at a pressure level of about #j4a bar the rapid deceleration period ends and the heat shield is released4 ,a parachute is deployed to slow the descent further1 and the ,probe then has a period of nearly an hour to study the atmosphere and clouds of ,jupiter4 ,the ,probe mission ends when its batteries run down or when it is crushed by the pressure of the ,jovian atmosphere near the #bj-;bar level1 whichever comes first4 ,,sl#gh-ede7c7 ,,galileo ,,probe ,,science ,,investigations ,probe ,scientist3 ,l4 ,collin1 ,,nasa ,ames ,investigation ,principal ,investigator ,primary ,objectives ,atmospheric structure ,a4 ,seiff1 ,,nasa ,ames ,measure temperature1 density1 pressure1 and molecular weight to determine the structure of ,jupiter's atmosphere4 ,neutral mass ,h4 ,b4 ,neimann1 ,,nasa ,measure the spectrometer ,goddard composition of the gases in ,jupiter's atmosphere and the variations at different levels in the atmosphere4 ,helium abundance ,u4 von ,zahn1 ,bonn ,u4 ,measure with high interferometer 7,germany7 accuracy the ratio of hydrogen to helium in ,jupiter's atmosphere4 ,nephelometer ,b4 ,ragent1 ,,nasa ,ames ,determine the sizes of cloud particles and the location of cloud layers in ,jupiter's atmosphere4 ,net flux radiometer ,r4 ,w4 ,boese1 ,,nasa ,ames ,measure energy being radiated from ,jupiter and the ,sun1 at different levels in ,jupiter's atmosphere4 ,lightning and radio ,l4 ,j4 ,lanzerotti1 ,measure lightning emission ,bell ,labs flashes in ,jupiter's atmosphere1 from the light and radio transmissions from those flashes4 ,energetic particles ,h4 ,m4 ,fischer1 ,u4 ,measure energetic ,kiel 7,germany7 electrons and protons in the inner regions of the ,jovian radiation belts and determine their spatial distributions4 ,because of the intense radiation environment1 the ,galileo ,orbiter will not be able to spend much time in the inner magnetosphere1 near the orbit of ,io4 ,to do so would risk damage to the spacecraft electronics and a premature end to the mission4 ,additional thruster firing during the first orbit can be used to raise the periapse to #aj ,r,j or greater4 ,no more close passes by ,io will be possible1 but studies of this satellite can be made on each subsequent orbit with imaging resolutions of about #aj kilometers1 sufficient to see details of the volcanic eruptions and monitor volcano-associated changes in the surface4 ,at each subsequent orbit1 ,galileo will be programmed for a close flyby of one of the other satellites4 ,several passes each of ,callisto1 ,ganymede1 and ,europa should be possible4 ,the satellite tour does not need to be fully planned in advance2 by adjusting the spacecraft trajectory with small bursts of the thruster motors1 navigation engineers can modify the orbit to permit adaptation to scientific needs4 ,as the ,orbiter mission progresses1 the spacecraft will also sample many parts of the magnetosphere1 including one long excursion1 at least #aej ,r,j1 into the magnetotail4 ,the total duration of the ,orbiter mission is planned to be at least #bj months4 ,additions to the basic mission are possible if the spacecraft remains healthy and fuel reserves are adequate4 ,in contrast1 the ,galileo ,probe mission lasts only a few hours4 ,as the ,probe approaches the atmosphere of ,jupiter at the awesome speed of #bf kilometers per second1 it will be traversing a region of space never before explored4 ,an energetic particle detector will investigate the innermost magnetosphere before the entry begins4 ,then1 within a period of just a few minutes1 friction with the upper atmosphere must dissipate the ,probe energy until it is falling gently in the ,jovian air4 ,jupiter1 being the largest planet1 presents the most challenging atmospheric entry mission ever undertaken by ,,nasa4 ,the design of the ,galileo ,probe calls for a massive heat shield to protect the instruments during the high-speed entry phase4 ,after the ,probe has slowed to subsonic velocities1 a parachute will be deployed1 and the heat shield1 having done its job1 will be dropped free4 ,,galileo ,,orbiter ,,science ,,investigations ,project ,scientist3 ,t4 ,v4 ,johnson1 ,,jpl ,investigation ,principal ,investigator ,primary ,objectives ,solid state imaging ,m4 ,j4 ,s4 ,belton1 ,kitt ,provide images of ,peak ,observatory ,jupiter's 7,team ,leader7 atmosphere and its satellites2 study atmospheric structure and dynamics on ,jupiter2 investigate the composition and geology of the satellite surfaces2 study the active volcanic processes on ,io4 ,ultraviolet spectrometer ,c4 ,w4 ,hord1 ,u4 ,study composition and ,colorado structure of the upper atmospheres of ,jupiter and its satellites4 ,near-infrared mapping ,r4 ,w4 ,carlson1 ,,jpl ,provide spectral spectrometer 7,,nims7 images and reflected sunlight spectra of ,jupiter's satellites1 indicating the composition of their surfaces2 measure reflected sunlight and thermal emission from ,jupiter's atmosphere to study composition1 cloud structure1 and temperature profiles2 monitor hot spots on ,io4 ,photopolarimeter_/radiometer ,j4 ,e4 ,hansen1 ,,nasa ,measure temperature ,goddard profiles and energy balance of ,jupiter's atmosphere2 measure ,jupiter's cloud characteristics and composition4 ,magnetometer ,m4 ,g4 ,kivelson1 ,,uc ,measure magnetic ,los ,angeles fields and the ways they change near ,jupiter and its satellites2 measure variations caused by the satellites interacting with ,jupiter's field4 ,plasma particles ,l4 ,a4 ,frank1 ,u4 ,iowa ,provide information on low-energy particles and clouds of ionized gas in the magnetosphere4 ,energetic particles ,d4 ,j4 ,williams1 ,,noaa ,measure composition1 ,space ,environment distribution1 and ,lab energy spectra of high-energy particles trapped in ,jupiter's magnetosphere4 ,plasma waves ,d4 ,a4 ,gurnett1 ,u4 ,iowa ,investigate waves generated inside ,jupiter's magnetosphere and waves radiated by possible lightning discharges in the atmosphere4 ,dust detection ,e4 ,grn1 ,determine size1 ,max-,planck-,institut speed1 and charge 7,germany7 of small particles such as micrometeorites near ,jupiter and its satellites4 ,celestial mechanics ,j4 ,d4 ,anderson1 ,,jpl ,use the tracking data 7,team ,leader7 to measure the gravity fields of ,jupiter and its satellites2 search for gravity waves propagating through interstellar space4 ,radio propagation ,h4 ,t4 ,howard1 ,use radio signals ,stanford ,u4 7,team from the ,orbiter ,leader7 and ,probe to study the structure of the atmospheres and ionospheres of ,jupiter and its satellites4 ,interdisciplinary ,f4 ,p4 ,fanale 7,,jpl71 ,p4 ,j4 ,gierasch 7,cornell ,scientists3 ,u471 ,d4 ,m4 ,hunten 7,u4 ,arizona71 ,h4 ,masursky 7,u4,s4 ,geological ,survey71 ,m4 ,b4 ,mc,elroy 7,harvard ,u471 ,d4 ,morrison 7,u4 ,hawaii71 ,g4 ,s4 ,orton 7,,jpl71 ,t4 ,owen 7,,su ,new ,york71 ,j4 ,b4 ,pollack 7,,nasa ,ames71 ,c4 ,t4 ,russell 7,,uc ,los ,angeles71 ,c4 ,sagan 7,cornell ,u471 ,f4 ,l4 ,scarf 7,,trw71 ,g4 ,schubert 7,,uc ,los ,angeles71 ,c4 ,p4 ,sonett 7,u4 ,arizona71 ,j4 ,a4 ,van ,allen 7,u4 ,iowa74 ,the ,probe will spend nearly an hour descending from a pressure level of about #j4a bar1 where the heat shield is jettisoned1 to a depth of #aj-bj bars4 ,during this time it will make most of its scientific measurements1 relaying them back to ,earth via the ,probe carrier4 ,designers expect the ,probe to sink through regions of ammonia clouds1 ammonium hydrosulfide clouds1 and ice and water clouds during this hour4 ,by the time it has descended below the water clouds1 the increasing pressure will exceed the strength of some ,probe components4 ,engineers expect the ,probe to have completed its mission1 exhausted its battery power1 and been crushed by the atmospheric pressure before the #bj-;bar level is reached4 ,lifeless1 it will then sink on into the thick1 hot lower atmosphere of ,jupiter4 ,illustration3 ,during its two years in orbit1 the ,galileo ,orbiter will carry out many investigations of the planet1 the ,galilean satellites1 and the ,jovian magnetosphere4 ,repeated close flybys of the satellites are used to modify and shape the orbit to provide additional flybys at an optimum viewing geometry4 ,initially1 the orbit is a long loop that extends in the general direction of the sunset side of ,jupiter4 ,the orbit is then contracted1 and the encounters with the satellites rotate it behind the planet for a long excursion into the magnetotail late in the tour4 ,beyond ,galileo ,after ,galileo1 the future cannot be predicted4 ,perhaps there will no longer be a program of planetary exploration4 ,but if humanity still has the vision to seek a future in the stars1 there will surely be other ,jupiter missions4 ,perhaps the next mission will concentrate on ,jupiter itself4 ,probes could be built to withstand pressures as high as several hundred bars1 feeling their way deep into the murky depths of the planet4 ,or a hot-air balloon could be deployed from a probe to carry instruments for long-term studies of the atmosphere4 ,a number of proposals have also been made for additional satellite missions1 including orbiters or landers for ,ganymede and ,callisto4 ,or perhaps it will be desirable to land a vehicle on one of the satellites and collect a sample and return it to ,earth for laboratory analysis4 ,whatever the future holds1 it is clear that the ,pioneer and ,voyager missions blazed the path to ,jupiter and beyond4 ,the little ,pioneers proved that it could be done1 and the ,voyagers expanded their vision1 exploring and discovering new worlds more remarkable and exciting than anyone could have imagined4 ,,appendix ,a ,,pictorial ,,maps ,,of ,,the ,,galilean ,,satellites ,these maps were prepared for the ,voyager ,imaging ,team by the ,u4,s4 ,geological ,survey in cooperation with the ,jet ,propulsion ,laboratory1 ,california ,institute of ,technology and the ,national ,aeronautics and ,space ,administration4 ,copies are available from ,branch of ,distribution1 ,u4,s4 ,geological ,survey1 #abjj ,south ,eads ,street1 ,arlington1 ,,va #bbbjb1 and ,branch of ,distribution1 ,u4,s4 ,geological ,survey1 ,box #bebhf1 ,federal ,center1 ,denver1 ,,co #hjbbe4 ,preliminary ,pictorial ,map of ,callisto ,atlas of ,callisto #a3be1jjj1jjj ,topographic ,series ,jc #be,m #b,,rmn1 #aigi ,i-#abci ,this map was compiled from ,voyager #a and #b pictures of ,callisto4 ,placement of features is based on predicted spacecraft trajectory data and is highly approximate4 ,the linkage between ,voyager #a and ,voyager #b pictures is particularly tenuous4 ,placement errors as large as #aj are probably common throughout the map1 and a few may be even larger4 ,feature names were approved by the ,international ,astronomical ,union in #aigi4 ,airbrush representation is by ,p4 ,m4 ,bridges4 ,jc #be,m #b,,rmn3 ,abbreviation for 7,jupiter7 ,callisto1 #a3be1jjj1jjj series1 second edition1 shaded relief map1 ,r1 with surface markings1 ,m1 and feature names1 ,n4 ,illustration3 ,north polar region2 ,south polar region ,scale #a3ac #ihj #jjj at #ef latitude ,polar stereographic projection ,illustration3 ,callisto ,scale #a3be #jjj #jjj at #j latitude ,mercator projection ,preliminary ,pictorial ,map of ,ganymede ,atlas of ,ganymede #a3be1jjj1jjj ,topographic ,series ,jg #be,m #b,,rmn1 #aigi ,i-#abdb ,this map was compiled from ,voyager #a and #b pictures of ,ganymede4 ,placement of features is based on predicted spacecraft trajectory data and is highly approximate4 ,the linkage between ,voyager #a and ,voyager #b pictures is particularly tenuous4 ,placement errors as large as #aj are probably common throughout the map1 and a few may be even larger4 ,a large unresolved discrepancy exists in the area bounded by the -#de and -#ee parallels between #abj and #ahj longitude4 ,relative placement of features is distorted in that area4 ,feature names were approved by the ,international ,astronomical ,union in #aigi4 ,airbrush representation is by ,j4 ,l4 ,inge4 ,jg #be,m #b,,rmn3 ,abbreviation for 7,jupiter7 ,ganymede1 #a3be1jjj1jjj series1 second edition1 shaded relief map1 ,r1 with surface markings1 ,m1 and feature names1 ,n4 ,illustration3 ,north polar region2 ,south polar region ,scale #a3ac #ihj #jjj at #ef latitude ,polar stereographic projection ,illustration3 ,ganymede ,scale #a3be #jjj #jjj at #j latitude ,mercator projection ,preliminary ,pictorial ,map of ,europa ,atlas of ,europa #a3be1jjj1jjj ,topographic ,series ,je #be,m #b,,rmn1 #aigi ,i-#abda ,this map was compiled from ,voyager #a and #b pictures of ,europa4 ,placement of features is based on predicted spacecraft trajectory data and is highly approximate4 ,feature names were approved by the ,international ,astronomical ,union in #aigi4 ,airbrush representation is by ,j4 ,l4 ,inge4 ,je #be,m #b,,rmn3 ,abbreviation for 7,jupiter7 ,europa1 #a3be1jjj1jjj series1 second edition1 shaded relief map1 ,r1 with surface markings1 ,m1 and feature names1 ,n4 ,illustration3 ,north polar region2 ,south polar region ,scale #a3ac #ihj #jjj at #ef latitude ,polar stereographic projection ,illustration3 ,europa ,scale #a3be #jjj #jjj at #j latitude ,mercator projection ,preliminary ,pictorial ,map of ,io ,atlas of ,io #a3be1jjj1jjj ,topographic ,series ,ji #be,m #b,,rmn1 #aigi ,i-#abdj ,this map was compiled from ,voyager #a and #b pictures of ,io4 ,placement of features is based on preliminary control information provided by ,m4 ,e4 ,davies of the ,rand ,corporation1 ,santa ,monica1 ,california1 and is probably accurate within #ej to #ajj km4 ,feature names were approved by the ,international ,astronomical ,union in #aigi4 ,airbrush representation is by ,p4 ,m4 ,bridges4 ,ji #be,m #b,,rmn3 ,abbreviation for 7,jupiter7 ,io1 #a3be1jjj1jjj series1 second edition1 shaded relief map1 ,r1 with surface markings1 ,m1 and feature names1 ,n4 ,illustration3 ,north polar region2 ,south polar region ,scale #a3ac #ihj #jjj at #ef latitude ,polar stereographic projection ,illustration3 ,io ,scale #a3be #jjj #jjj at #j latitude ,mercator projection ,,appendix ,b ,,voyager ,,science ,,teams ,imaging ,science ,bradford ,a4 ,smith1 ,university of ,arizona1 ,team ,leader ,geoffrey ,a4 ,briggs1 ,,nasa ,headquarters ,a4 ,f4 ,cook1 ,smithsonian ,institution ,g4 ,e4 ,danielson1 ,jr41 ,california ,institute of ,technology ,merton ,davies1 ,rand ,corp4 ,g4 ,e4 ,hunt1 ,university ,college1 ,london ,tobias ,owen1 ,state ,university of ,new ,york ,carl ,sagan1 ,cornell ,university ,lawrence ,soderblom1 ,u4,s4 ,geological ,survey ,v4 ,e4 ,suomi1 ,university of ,wisconsin ,harold ,masursky1 ,u4,s4 ,geological ,survey ,radio ,science ,von ,r4 ,eshleman1 ,stanford ,university1 ,team ,leader ,j4 ,d4 ,anderson1 ,jet ,propulsion ,laboratory ,t4 ,a4 ,croft1 ,stanford ,research ,institute ,gunnar ,lindal1 ,jet ,propulsion ,laboratory ,g4 ,s4 ,levy1 ,jet ,propulsion ,laboratory ,g4 ,l4 ,tyler1 ,stanford ,university ,g4 ,e4 ,wood1 ,jet ,propulsion ,laboratory ,plasma ,wave ,frederick ,l4 ,scarf1 ,,trw ,systems1 ,principal ,investigator ,d4 ,a4 ,gurnett1 ,university of ,iowa ,infrared ,spectroscopy and ,radiometry ,rudolph ,a4 ,hanel1 ,goddard ,space ,flight ,center1 ,principal ,investigator ,b4 ,j4 ,conrath1 ,goddard ,space ,flight ,center ,p4 ,gierasch1 ,cornell ,university ,v4 ,kunde1 ,goddard ,space ,flight ,center ,p4 ,d4 ,lowman1 ,goddard ,space ,flight ,center ,w4 ,maguire1 ,goddard ,space ,flight ,center ,j4 ,pearl1 ,goddard ,space ,flight ,center ,j4 ,pirraglia1 ,goddard ,space ,flight ,center ,r4 ,samuelson1 ,goddard ,space ,flight ,center ,cyril ,ponnamperuma1 ,university of ,maryland ,d4 ,gautier1 ,meudon1 ,france ,s4 ,kuman1 ,university of ,southern ,california ,ultraviolet ,spectroscopy ,a4 ,lyle ,broadfoot1 ,kitt ,peak ,national ,observatory1 ,principal ,investigator ,j4 ,l4 ,bertaux1 ,service d',aeronomie du ,,cnrs1 ,france ,j4 ,blamont1 ,service d',aeronomie du ,,cnrs1 ,france ,t4 ,m4 ,donahue1 ,university of ,michigan ,r4 ,m4 ,goody1 ,harvard ,university ,a4 ,dalgarno1 ,harvard ,college ,observatory ,michael ,b4 ,mc,elroy1 ,harvard ,university ,j4 ,c4 ,mc,connell1 ,york ,university1 ,canada ,h4 ,w4 ,moos1 ,johns ,hopkins ,university ,m4 ,j4 ,s4 ,belton1 ,kitt ,peak ,national ,observatory ,d4 ,f4 ,strobel1 ,naval ,research ,laboratory ,sushil ,atreya1 ,university of ,michigan ,william ,r4 ,sandel1 ,university of ,southern ,california ,donald ,shemanski1 ,university of ,southern ,california ,photopolarimetry ,charles ,w4 ,hord1 ,university of ,colorado1 ,acting ,principal ,investigator ,d4 ,l4 ,coffeen1 ,goddard ,institute for ,space ,studies ,j4 ,e4 ,hansen1 ,goddard ,institute for ,space ,studies ,k4 ,pang1 ,science ,applications ,inc4 ,planetary ,radio ,astronomy ,james ,w4 ,warwick1 ,university of ,colorado1 ,principal ,investigator ,anthony ,riddle1 ,radiophysics1 ,inc4 ,jeffrey ,pearce1 ,radiophysics1 ,inc4 ,j4 ,k4 ,alexander1 ,goddard ,space ,flight ,center ,a4 ,boischot1 ,observatoire de ,paris1 ,france ,w4 ,e4 ,brown1 ,jet ,propulsion ,laboratory ,t4 ,d4 ,carr1 ,university of ,florida ,samuel ,gulkis1 ,jet ,propulsion ,laboratory ,f4 ,t4 ,haddock1 ,university of ,michigan ,c4 ,c4 ,harvey1 ,observatoire de ,paris1 ,france ,y4 ,le,blanc1 ,observatoire de ,paris1 ,france ,r4 ,g4 ,peltzer1 ,university of ,colorado ,r4 ,j4 ,phillips1 ,jet ,propulsion ,laboratory ,d4 ,h4 ,staelin1 ,massachusetts ,institute of ,technology ,magnetic ,fields ,norman ,f4 ,ness1 ,goddard ,space ,flight ,center1 ,principal ,investigator ,mario ,h4 ,acuna1 ,goddard ,space ,flight ,center ,k4 ,w4 ,behannon1 ,goddard ,space ,flight ,center ,l4 ,f4 ,burlaga1 ,goddard ,space ,flight ,center ,r4 ,p4 ,lepping1 ,goddard ,space ,flight ,center ,f4 ,m4 ,neubauer1 ,technische ,universitat1 ,f4,r4,g4 ,plasma ,science ,herbert ,s4 ,bridge1 ,massachusetts ,institute of ,technology1 ,principal ,investigator ,j4 ,w4 ,belcher1 ,massachusetts ,institute of ,technology ,j4 ,h4 ,binsack1 ,massachusetts ,institute of ,technology ,a4 ,j4 ,lazarus1 ,massachusetts ,institute of ,technology ,s4 ,olbert1 ,massachusetts ,institute of ,technology ,v4 ,m4 ,vasyliunas1 ,max ,planck ,institute1 ,f4,r4,g4 ,l4 ,f4 ,burlaga1 ,goddard ,space ,flight ,center ,r4 ,e4 ,hartle1 ,goddard ,space ,flight ,center ,k4 ,w4 ,ogilvie1 ,goddard ,space ,flight ,center ,g4 ,l4 ,siscoe1 ,university of ,california1 ,los ,angeles ,a4 ,j4 ,hundhausen1 ,high ,altitude ,observatory ,low-,energy ,charged ,particles ,s4 ,m4 ,krimigis1 ,johns ,hopkins ,university1 ,principal ,investigator ,t4 ,p4 ,armstrong1 ,university of ,kansas ,w4 ,i4 ,axford1 ,max ,planck ,institute1 ,f4,r4,g4 ,c4 ,o4 ,bostrom1 ,johns ,hopkins ,university ,c4 ,y4 ,fan1 ,university of ,arizona ,g4 ,gloeckler1 ,university of ,maryland ,l4 ,j4 ,lanzerotti1 ,bell ,telephone ,laboratories ,cosmic ,ray ,r4 ,e4 ,vogt1 ,california ,institute of ,technology1 ,principal ,investigator ,j4 ,r4 ,jokipii1 ,university of ,arizona ,e4 ,c4 ,stone1 ,california ,institute of ,technology ,f4 ,b4 ,mc,donald1 ,goddard ,space ,flight ,center ,b4 ,j4 ,teegarden1 ,goddard ,space ,flight ,center ,james ,h4 ,trainor1 ,goddard ,space ,flight ,center ,w4 ,r4 ,webber1 ,university of ,new ,hampshire ,,appendix ,c ,,voyager ,,management ,,team ,,nasa ,office of ,space ,science ,thomas ,a4 ,mutch1 ,associate ,administrator for ,space ,science ,andrew ,j4 ,stofan1 ,deputy ,associate ,administrator ,adrienne ,f4 ,timothy1 ,assistant ,associate ,administrator ,angelo ,guastaferro1 ,director1 ,planetary ,division ,rodney ,a4 ,mills1 ,program ,manager ,milton ,a4 ,mitz1 ,program ,scientist ,walter ,jakobowski1 ,viking and ,flight ,support ,manager ,,nasa ,office of ,space ,tracking and ,data ,systems ,william ,schneider1 ,associate ,administrator of ,space ,tracking and ,data ,systems ,acquisition ,charles ,a4 ,taylor1 ,director1 ,network ,operations and ,communication ,programs ,frederick ,b4 ,bryant1 ,director1 ,network ,system ,development ,programs ,maurice ,e4 ,binkley1 ,director1 ,,dsn ,systems ,,nasa ,office of ,space ,transportation ,systems ,john ,f4 ,yardley1 ,associate ,administrator for ,space ,transportation ,systems ,joseph ,b4 ,mahon1 ,director1 ,expendable ,launch ,vehicles ,joseph ,e4 ,mc,golrick1 ,chief1 ,small and ,medium ,launch ,vehicles ,b4 ,c4 ,lam1 ,titan ,,iii ,manager ,jet ,propulsion ,laboratory1 ,pasadena1 ,california ,bruce ,c4 ,murray1 ,laboratory ,director ,gen4 ,charles ,h4 ,terhune1 ,jr41 ,deputy ,laboratory ,director ,robert ,j4 ,parks1 ,assistant ,laboratory ,director for ,flight ,projects ,raymond ,l4 ,heacock1 ,project ,manager ,esker ,k4 ,davis1 ,deputy ,project ,manager ,peter ,t4 ,lyman1 ,deputy ,project ,manager ,richard ,p4 ,laeser1 ,mission ,director ,george ,p4 ,textor1 ,deputy ,mission ,director ,charles ,e4 ,kohlhase1 ,mission ,planning ,office ,manager ,james ,e4 ,long1 ,science ,directorate ,manager ,charles ,h4 ,stembridge1 ,deputy ,arthur ,l4 ,lane1 ,assistant ,project ,scientist for ,jupiter ,francis ,m4 ,sturms1 ,sequence ,design and ,integration ,directorate ,manager ,robert ,k4 ,wilson1 ,deputy ,michael ,j4 ,sander1 ,development1 ,integration and ,test ,directorate ,manager ,robert ,g4 ,polansky1 ,deputy ,michael ,w4 ,devirian1 ,space ,flight ,operations ,directorate ,manager ,raymond ,j4 ,amorose1 ,deputy ,marvin ,r4 ,traxler1 ,tracking and ,data ,system ,manager ,kurt ,heftman1 ,mission ,control and ,computing ,center ,manager ,california ,institute of ,technology1 ,pasadena1 ,california ,edward ,c4 ,stone1 ,project ,scientist ,,additional ,,reading ,,technical ,jupiter1 ,t4 ,gehrels1 ,ed41 ,u4 of ,arizona ,press1 ,tucson1 #abed pages 7#aigf74 ,space ,science ,reviews1 special ,voyager instrumentation issues1 ,vol4 #ba1 ,no4 #b1 pgs4 #ge-bcb 7,november #aigg72 ,vol4 #ba1 ,no4 #c1 pgs4 #bcd-cgf 7,december #aigg74 8,melting of ,io by ,tidal ,dissipation10 by ,s4 ,j4 ,peale1 ,p4 ,cassen1 and ,r4 ,t4 ,reynolds1 ,science1 ,vol4 #bjc1 pgs4 #hib-hid 7#b ,march #aigi74 ,science1 special ,voyager #a issue1 ,vol4 #bjd1 pgs4 #ide-ajjh 7#a ,june #aigi74 ,nature1 special ,voyager #a issue1 ,vol4 #bhj1 pgs4 #gbe-hjf 7#cj ,august #aigi74 8,jupiter's ,ring10 by ,t4 ,owen et al41 ,nature1 ,vol4 #gha1 pgs4 #ddb-ddf 7#aa ,october #aigi74 ,science1 special ,voyager #b issue1 ,vol4 #bjf1 pgs4 #ibe-iif 7#bc ,november #aigi74 ,geophysical ,research ,letters1 special ,voyager issue1 ,vol4 #g1 pgs4 #a-fh 7,january #aihj74 ,,nontechnical 8,the ,solar ,system10 special issue of ,scientific ,american1 ,vol4 #bbc1 ,no4 #c 7,september #aige74 8,the ,galilean ,satellites of ,jupiter10 by ,d4 ,p4 ,cruikshank and ,d4 ,morrison1 ,scientific ,american1 ,vol4 #bcd1 ,no4 #e1 pgs4 #ajh-aaf 7,may #aigf74 ,pioneer ,odyssey1 by ,r4 ,o4 ,fimmel1 ,w4 ,swindell1 and ,e4 ,burgess1 ,,nasa ,,sp-#cif1 #bag pages 7#aigg74 ,murmurs of the ,earth3 ,the ,voyager ,interstellar ,record1 by ,carl ,sagan et al41 ,random ,house1 ,new ,york1 #aigh4 8,jupiter and ,family10 by ,j4 ,eberhart1 ,science ,news1 ,vol4 #aae1 pgs4 #afd-agc 7#ag ,march #aigi74 8,the ,far-,out ,worlds of ,voyager10 by ,j4 ,k4 ,beatty1 ,sky and ,telescope1 ,vol4 #eg1 pgs4 #dbc-dbg 7,may #aigi7 and pgs4 #eaf-ebj 7,june #aigi74 8,return to ,jupiter and ,co410 by ,j4 ,eberhart1 ,science ,news1 ,vol4 #aaf1 pgs4 #ai-ba 7#ad ,july #aigi74 8,voyage to the ,giant ,planet10 by ,c4 ,sutton1 ,new ,scientist1 ,vol4 #hc1 pgs4 #bag-bbj 7#ai ,july #aigi74 8,voyager ,views ,jupiter's ,dazzling ,realm10 by ,r4 ,gore1 ,national ,geographic1 ,vol4 #aeg1 ,no4 #a1 pgs4 #b-bi 7,january #aihj74 8,the ,galilean ,moons of ,jupiter10 by ,l4 ,a4 ,soderblom1 ,scientific ,american1 ,vol4 #bdb1 ,no4 #a1 pgs4 #hh-ajj 7,january #aihj74 8,the ,great ,red ,spot10 by ,d4 ,schwartzenburg1 ,astronomy1 ,vol4 #h1 pgs4 #f-ac 7,july #aihj74 8,four ,new ,worlds10 by ,d4 ,morrison1 ,astronomy1 ,vol4 #h 7,september #aihj74 ,illustration3 ,national ,aeronautics and ,space ,administration ,transcriber's ,notes --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,silently corrected a few palpable typos4 --,moved captions nearer the relevant images2 tweaked image references within captions accordingly4 --,in the text versions only1 text in italics is delimited by underscores4