A Publication .", THEPLANET4 SOCIETY o o o. 0-e--e-

Board of Directors CARL SAGAN BRUCE MURRAY VOYAGER: OPENING THE SOLAR SYSTEM President Vice President Director. Laboratory for Planetary Professor of Planetary Studies, Cornell University Science. California Institute of Technology BY CARL SAGAN LOU IS FRIEDMAN Executive Director HENRY TANNER California Institute THOMAS O. PAINE of Technology , n this issue of The Planetary Report we spect and admiration. They are real American Former Administrator. NASA Chairman, National JOSEPH RYAN celebrate the epic journey of the Voyager 1 heroes. Commission on Space O'Me/veny & Myers and Voyager 2 spacecraft and look forward Meanwhile, at almost a million miles a day, Board of Advisors with keen anticipation to the Voyager 2 en~ the two spacecraft are sweeping past the plan­ DIANE ACKERMAN JOHN M. LOGSDON counter with the Neptune system on August etary part of the solar system. Their instru­ poet and author Director. Space Policy Institute George Washington University 25, 1989. Launched in August and September ments may survive long enough to detect their ISAAC ASIMOV Buthor HANS MARK 1977, the two spacecraft were programmed passage through the heliopause, the charged­ Chancellor. RICHARD BERENDZEN University of Texas System only to explore the Jupiter and Saturn systems, particle and magnetic-field boundary between President, American University JAMES MICHENER but they have far exceeded that original mis­ the solar system and interstellar space. Inex­ JACQUES BLAMONT author sion specification. Voyager l 's trajectory to orably, they will enter interstellar space and Scientific Consultant, Centre National d'Eludes Spatiales, MARVIN MINSKY Titan precluded its visit to any other worlds. wander forever in the dark between the stars. France Donner Professor of Science, MassachuseUs Institute But Voyager 2, after gravity assists by Jupiter Because interstellar space is such a benign and RAY BRADBURY of Technology poet and author and Saturn, became in 1986 the first artifact of placid medium, the rate of erosion of the Voy­ PHILIP MORRISON ARTHUR C. CLARKE Institute Professor. Massachusetts the human species to reach the Uranus system, ager spacecraft will be very slow. Even a bil­ author Institute of Technology and this August will play the same role in the lion years from now, the two spacecraft will be CORNELIS DE JAGER PAUL NEWMAN Professor of Space Research, actor Neptune system. very much as they are today (although, of The-Astronomicallnstitute at Utrecht, The Netherlands BERNARD M. OLIVER The spacecraft have provided our first de­ course, inoperative). If there are interstellar Chief, SET! Program, FRANK DRAKE NASA/Ames Research Center tailed, close-up information about dozens of spacefaring civilizations, it is possible that Professor of Astronomy and Astrophysics, University of SALLY RIDE new worlds- some of them previously known sometime in the next billion years one or both California at Santa Cruz former astronaut only as fuzzy disks in the eyepiece of ground­ of the spacecraft will be intercepted and exam­ LEE A. DUBRIDGE ROALD Z. SAGDEEV based telescopes, some merely as points of ined. To prepare for such a contingency, each former presidential Institute for Space Research, science advisor Academy of Sciences light, and many entirely undiscovered before of the spacecraft has, affixed to its side, a of/he USSR JOHN GARDNER Voyager approached. One of these moons, Mi­ golden phonograph record (and instructions founder. Common Cause HARRISON H. SC HMI TT former US Senator. NM randa of Uranus, was discovered not only in for use), containing greetings from our civi­ THEODORE M. HESBURGH and former astronaut President Emeritus, my lifetime but by my thesis adviser, Gerard lization, as well as a variety of information University of Notre Dame LEWIS THOMAS Chancel/or. Memorial Sloan Kuiper. How astonished he would have been about our science, our technology, our music SHI RLEY M. HUFSTEDLER Kettering Cancer Center educator and jurist at its stunning, twisted terrain as radioed back (90 minutes of "Earth's Greatest Hits"), our JAMES VAN ALLEN GARRY E. HUNT Professor of Physics, by Voyager 2. Another moon he discovered, evolution and ourselves. Both in their ex­ space scientist, United Kingdom University of Iowa Nereid of Neptune, will be revealed to us for ploratory mission and in the messages that the first time later this summer. Among the they carry, these spacecraft are for the ages. many discoveries, definite or probable, made Neptune is the final port of call on Voy­ t~ee~~~~~6ff~:O~ T~~S~a~~~r-~l~rl.~I~~~~ ~~t~~n~S A::~~e~t Pasadena. CA 91106, (818) 793-5100. It is available to members of The by Voyager are the repeated formation and de­ ager's Grand Tour. There are no more worlds Planetary Society. Annual dues are $20 in the US, $25 in Canada, and $30 outside the US or Canada. struction of moons and rings, the volcanism of on its itinerary. Before Voyager 2 passes the Editor, CHARLENE M. ANDERSON; 10, the configuration of outer-planet magneto­ planetary frontier into interstellar space, it has Technical Editor, JAMES D. BURKE; Guest Technical Editor, ELLIS D. MINER: spheres, the rich organic chemistry in the outer the opportunity to take (as I very much hope it Assistant Editor, DONNA STEVENS: solar system--especially Titan's- and the will) one last picture---over its shoulder, of the Copy Editor, KARL STULL; Art Director. BARBARA SMITH possible existence of oceans on Titan and inner solar system. The planets will appear as Viewpoints expressed in columns ~ editorials are those of the authors and Europa. a sparse sprinkling of points of light. One of g~ ;g! i ~~:~a~ ~ge~~eTh~t Wa~~~~ ~~?:ty~ a netary Society, its officers The spacecraft have opened most of the them, a tiny blue dot set against the spangle of In Canada, Second Class Mail RegislratlOll Number 9567 solar system- both in extent and in mass- to the Milky Way, will be the Earth. From the the human species. They represent a triumph distance of Neptune, it will seem no more than of American technology, admired even by a faint star. I believe that this picture could COVER: The blue planet Neptune and its those who have deep misgivings about other have a profound influence on how we view tantalizing moon Triton are the last sched­ policies of our nation. They provide an exam­ ourselves, as powerful as the images taken by uled destinations on Voyager 2'_s Grand-Tour ple of what contemporary human technology, the Apollo astronauts of our lovely, finite and of the outer solar system. Jupiter, Saturn and Uranus have all given up secrets to this freed to pursue peaceful exploratory objec­ fragile planetary home. robotic emissary from Earth, and the space­ tives, is capable. The data are made freely craft will now probe the secrets of its last available, much of it in real time, to all the cit­ Carl Sagan o/Cornell University, President planet. After swinging past Triton and izens of our planet. Those who built and oper­ o/The Planetary Society, is Distinguished examining its atmosphere, Voyager 2 will begin a quest to find the edge of our solar ated Voyager--especially the engineering staff Visiting Scientist at the Jet Propulsion Labo­ system, and then will travel on forever at the Jet Propulsion Laboratory, who time and ratory and a member 0/ the Voyager imaging through the realm of the stars. again devised brilliant solutions to unexpected team. He also chaired the NASA committee Painting: Paul Hudson, reprinted with permission, problems uncovered when the spacecraft, in that designed and/abricated the Voyager © National Geographic Society effect, radioed home for help--deserve our re- Interstellar Record_ NEWS l BRIEFS

Senator Albert Gore Jr. (D-TN) charges that Bush administration of­ In the last issue of The Planetary Report, we promised that Academician Roald Z. Sagdeev, ficials "censored" a scientist's Scientific Director of the Phobos mission, would comment on the failure of both of the mis­ planned testimony on global warm­ sion's spacecraft. Time constraints prevented Sagdeev from completing his statement, but he ing to downplay the problem. Dr. suggested we reprint instead selections from this article by K. Gringauz, which originally James E. Hansen, director of appeared in Pravda. The translation is by Colleen B. London of the Space Physics Research NASA's Goddard Institute for Space Laboratory of the University of Michigan. Studies, told The New York Times that the Office of Management and he only new type of spacecraft developed by the industry in recent years for planetary Budget edited his testimony to soft­ T research-the Phobos-was launched in the middle of 1988. (The Vega spacecraft are a en the conclusions and make the modification of the earlier Venera series.) From the point of view of carrying scientific ex­ prospects of climatic change appear periments, it has a number of shortcomings: It is difficult to put scientific equipment onto it more uncertain. in the necessary manner, the amount of transmitted information is not enough, and the scien­ "It distresses me that they put tific payload is too small in relation to its overall weight. words in my mouth; they even put it These shortcomings are connected with an unjust and inappropriate relationship between in the first person," Hansen told the the Academy of Sciences and the aerospace industry. The chief designer of spacecraft, Aca­ newspaper, adding that he had tried demician S. P. Korolov, was well-aquainted with all the members of the academy's institutes to "negotiate" with the budget office who put experiments on the spacecraft of his OKB (Special Designing Bureau), and, at the over the wording but "they refused time of the design of the spacecraft, he was in constant communication with them. His to change." coworkers' thinking was in accord with the physicists', because they were solving the same -from the Associated Press problems. The same style was maintained by G. N. Babakin, to whom Korolov transferred from his bureau the designs of planetary spacecraft. Many American adults suffer from In the years of stagnation [when Brezhnev was Party Secretary], everything gradually "cosmic illiteracy" according to a changed. Now the Institute of Space Research finds itself in the position of a junior and (al­ nationwide survey taken by re­ most powerless) partner. The heads of the industry determine on their own the configuration searchers from the Massachusetts of a spacecraft, the characteristics of its service system, the weight and energy consumption Institute of Technology and North­ of the scientific equipment, and then they come to the Institute of Space Research with the ern Illinois University. Out of the proposal: Take it or leave it; that's all there is or will be. One gets the impression that the 1,111 adults polled, only 55 percent fewer the experiments the scientists come up with, the happier the heads of the industry are. knew that the Sun is a star, only 37 . . . We should note that the loss of the first Phobos spacecraft . . . is connected with hu­ percent believe that the Sun will man incompetence and irresponsibility and not with any technical malfunction. The Acade­ eventually bum out and only 24 per­ my of Sciences has no choice but to make peace with this situation, since it doesn't have cent were aware that the universe is any means of control of the industry which builds the spacecraft. . . . In this case, the pro­ expanding. ducer dictates what is made. If these funds were given to the Academy of Sciences, and if the When asked if the Sun is "a plan­ construction offices of the Soviet Space Agency were financially motivated to produce what et, a star or something else," 25 per­ the customer wanted, the technical quality of Soviet spacecraft and their ground testing cent said the Sun is a planet, 15 per­ would, without a doubt, greatly improve. cent said it was "something else" and The design and implementation of the Energia and Buran systems entailed the surmount­ 5 percent had no idea whatsoever. ing of enormous difficulties, and they are the outstanding pride of engineering accomplish­ -from the Los Angeles Times ment. But they are only an engineering feat. To lump together these expenditures with those of basic scientific research is totally without justification. And to answer the question as to "The editors of Popular Science be­ why we needed to have this system at the present time is certainly not easy. lieve that if spending is to stay at or At the international forum in Moscow on the occasion of the thirtieth anniversary of the near current levels, we should build launch of the first Sputnik, there was announced an impressive program for Soviet research and expand our programs of moni­ on Mars (for the next decade, using unmanned spacecraft). However, its accomplishment is toring Earth from space and sending starting with astonishing sluggishness. Thus, for example, the development of the scientific scientific probes throughout the so­ equipment for the Mars '94 spacecraft (which is supposed to be launched in 1994, with the lar system and beyond. same design as that of the Phobos spacecraft) has not yet even begun. This in spite of the "But we also believe that it would fact that in the rest of the world they are already intensively working on the development of be in the best interest of the country equipment designated for launches in 1995 or 1996. The period necessary for design and de­ and the world if we were to launch a velopment of scientific equipment to be carried on board spacecraft is normally five to six 30-year program leading to the years. The delay of work on the Mars '94 project, which will result in a decline in the quali­ manned exploration of Mars in co­ ty of the research, clearly appears to be of very little concern for the industry and equally as operation with the Soviet Union and little for the Presidium of the Academy of Sciences. other governments. A space station All of this goes to show that the continuation of basic space research, initiated with such is a necessary intermediate step. brilliance in our country, is currently arousing a deep concern, and the plans for building Such a program would give a sense spacecraft for scientific research and manned spaceships need to be fundamentally readjusted. of national purpose and a scientific In the USA the budget and the programs for space research are publicly examined and bonanza, and it would improve the voted on by the Congress. It would be wonderful if such a system were introduced by the chances of world peace." new Supreme Soviet of the USSR which will be elected this year. -from an editorial by C. P. Gilmore -K. Gringauz, Chief Scientist, Space Research Institute, Academy of Sciences of the USSR in Popular Science 3 VOYAGER TOGETHER

As originallyenvi­ sioned, the Grand Tour of the outer planets would have begun with a launch to Jupiter, Saturn and Pluto in the mid- 1970s, followed by a launch to Jupiter, Uranus and Neptune in 1979. Political and budgetary problems forced planners to scale back to one launch to Jupiter and Saturn (Voyager 1) and another to Jupiter and Saturn with possible flybys -If everything went exactly right~f Uranus and Neptune (Voyager 2). Painting: JPUNASA

oyager 2 is about to stage another Gravity Assists Franc;ois Tisserand published a simple planetary spectacular by flying The gravity-assist technique used by mathematical method for assessing V through the neptunian system in Voyager trajectory designers to pass comet trajectories before and after a late August, transforming that distant their spacecraft from planet to planet close planetary encounter. The creative and dark region from obscure to famil- had its intellectual roots in the venera­ leap that made Voyager possible-from iar. Since the flamboyant flybys of ble study of comets. By the 19th centu­ Tisserand's criterion to the engineering Jupiter in 1979, we have come to ry astronomers realized that a comet utilization of gravity assist for a space­ expect a reliable flow of scientific could, on its looping orbit across half craft-was accomplished by a series of information from the Voyager space­ the solar system, pass close to a planet, trajectory analysts working on the craft. But the path from the 1960s pro­ and that the comet's orbit would be sig­ Mariner and Apollo programs. A tip of posal of a "Grand Tour" to the mature nificantly altered as a result, especially the cap is due also to science fiction project of the last ten years had many if the planet were massive Jupiter. The writer Lester del Rey, who forecast the twists. Techniques and technologies change of trajectory could be so great technique in 1939. had to be developed. Barriers of pro­ that an unwary astronomer might fail to The idea behind a gravity assist is gram definition and funding had to be recognize the comet as one and the simple. Pass the spacecraft behind a scaled. And the early phase of flight same object when observed before and planet (or natural satellite) and the operations can only be described as after a close jovian encounter. slight gravitational tug of the vehicle 4 "scary." In 1889 the French astronomer will subtract from the planet a minute by William I. McLaughlin

amount of energy, which, by the law of prove valuable in planning the conservation of energy, is then added to more ambitious Voyager explo­ the spacecraft. The effects of this rations. The Pioneers gave warn­ exchange, while slight for the planet, ings of the intense flux of high­ can be dramatic for the much less mas­ energy trapped particles in the sive spacecraft. jovian magnetosphere, and they A second effect from gravity assist is alleviated fears over the dangers a bending of the spacecraft's trajectory that might lurk in the asteroid by the gravitational pull of the planet. belt. Thus, by careful selection of an "aim The TOPS project addressed point" near the assisting planet, the some key issues that had to be spacecraft can gain energy and change resolved before voyaging into the course. Either of these maneuvers outer solar system. The genera­ would otherwise require expending tion of electricity for spacecraft precious propellant. subsystems was one such prob­ The gravity-assist technique was lem. Conversion of sunlight to employed during some of the Apollo electrical power, which had been flights when the spent third-stage of the employed on flights through the Saturn V launch vehicle was directed to inner solar system, was not feasi­ an aim point near the Moon and slung ble for missions to Jupiter and into an orbit about the Sun. The first beyond. The "Thermo-electric" planetary application of this method in the TOPS acronym referred to occurred in 1974 when Mariner 10 was production of electrical power directed toward Mercury by means of from the heat generated by an encounter with Venus. The Voyager radioactive decay; this technolo­ concept of planet-hopping by means of gy was the precursor of the successive gravity assists required care­ radioisotope thermoelectric gen­ ful planning and just the right condi­ erators (RTGs) that now keep tions in the solar system-there is a both Voyagers and the Pioneers shopworn joke that the administrations supplied with power. of John Adams and Thomas Jefferson Returning images from deep missed an opportunity for the Grand space required development of a Tour when the outer planets were last new telecommunications system favorably aligned for successive gravi­ of considerable channel capacity. tyassists. A high-frequency (X-band) sys­ tem was devised so that all of the Technology Development information in a picture taken by In the late 1960s, as the Apollo lunar­ the spacecraft's camera could be landing program was approaching cul­ returned to Earth in just a few mination, NASA looked to the outer minutes of transmission time planets and initiated two projects: the even at the range of Saturn. The Pioneer missions to Jupiter and Saturn, X-band system allows the energy at the Ames Research Center, and the pattern radiated by the distant Thermo-electric Outer Planets Space­ spacecraft to be focused toward craft (TOPS) project, at the Jet Propul­ Earth rather than spread over a sion Laboratory. larger region of space. The two Pioneer spacecraft, Traditionally, interplanetary launched in 1972 and 1973, are still spacecraft had been navigated by functioning and returning scientific interpreting signals from their data from the outer solar system. In radio systems. To carry out its addition to exploring Jupiter (Pioneer complex mission plan, Voyager 10 in 1973 and Pioneer 11 in 1974) and needed to "see for itself," and so Saturn (Pioneer 11 in 1979), these through "optical navigation" the durable spacecraft lived up to their spacecraft's camera measures name by sampling the space environ­ planetary and satellite images ment and reporting data that would (continued on page 7) ORIGINAL DESIGN I PLASMA WAVE DETECTOR

The Voyager spacecraft evolved from an early design done at the Jet Propulsion Laboratory called the "Thermo-electric Outer Planets Spacecraft" or TOPS (right). The spacecraft that finally flew (below) is now a familiar friend to the millions of people who have followed its odyssey through our solar system. RTG's Drawings: JPUNASA

LOW-GAIN ANTENNA

DEPLOYABLE HIGH-GAIN ANTENNA 4.3-m (14 ft.) DIAMETER

FINAL DESIGN

D

J

A High-gain antenna-To communicate with Earth and to measure struc­ H Photopolarimeter-To record the scattering of light by an \ ture of atmospheres and rings as radio beam passes through them. The atmosphere or surface. Also measures intensity of starlight passing antenna is 3.65 meters (12 feet) in diameter. through rings to determine structure and amount of material present. B Cosmic Ray Detector-To intercept cosmic rays, atomic nuclei and I Low-Energy Charged Particles Detector-To measure composi­ electrons thought to be the most energetic particles in nature. tion and energy spectrum of low-energy charged particles trapped in C Plasma Detector-To characterize properties of plasmas (hot, ionized planetary magnetospheres. Also measures the distribution and variation gases affected by magnetic fields). Also determines properties of the solar of galactic cosmic rays. wind. J Planetary Radio Astronomy and Plasma Wave Antennas-To detect radio emissions from charged particles in planetary magneto­ D' Imaging System, Wide-Angle Camera-Camera and refracting tele­ spheres, and from lightning in atmospheres. Also measures plasma scope to provide wide views of planetary objects. waves in magnetospheres and detects interactions between magneto­ E Imaging System, Narrow-Angle Camera-Telephoto camera and spheres and solar wind . Can "hear" collisions of small particles with the reflecting telescope for close-up views of planetary objects. spacecraft. F Ultraviolet Spectrometer-To study the chemical composition, temper­ K Radioisotope Thermoelectric Generators-Power plant for ature and structure of atmospheres, and to study ultraviolet light from stars. spacecraft, producing electrical energy through conversion of heat from G Infrared Spectrometer and Radiometer-To measure the tempera­ radioactive decay of plutonium 238. tures of planets and satellites. Can determine molecular composition of L Magnetometer-To characterize magnetic fields, structure of mag­ atmospheres and measure solar radiation reflected from a body. netospheres and magnetospheric interactions with moons. (continued from page 5) Uranus and Neptune. That option was against a stellar background. Optical not exercised with Voyager 1. Scientific navigation is important because it interest in Titan, Saturn's largest satel­ establishes the location of the vehicle lite, dictated that Voyager 1 be sent on a relative to the targets (rather than rela­ trajectory that precluded the needed tive to Earth only). Voyager was the gravity assist for a Uranus flyby. But first US mission that absolutely Voyager 2 threaded all four pearls­ required optical navigation to meet its Jupiter, Saturn, Uranus and Neptune scientific objectives, but as early as -on the string of its flight path. It is 1969 and 1971 the Mariner missions to not dynamically possible to arrange a Mars tried it out in preparation for gravity assist at Neptune that would exploration of the outer planets. The send Voyager 2 to Pluto. Viking missions to the Red Planet, launched in 1975, perfected optical Just Getting to Jupiter navigation for Voyager. Today the Voyager flight team func­ tions with a quiet confidence derived Five Planets, Five Spacecraft from 12 years of successful operations. Human curiosity is a powerful force Success didn't come easy in the begin­ that has led us from Stone Age caves to ning. the brink of interstellar travel. Curiosity Voyager 2 lifted off from Kennedy about the outer solar system prompted Space Center on August 20, 1977 atop a 1969 study by the Space Science a Titan 3 -EICentaur launch vehicle. Board of the National Academy of Sci­ The launch of Voyager 1 followed on ences. The study recommended that the September 5 (numerical order in the United States undertake an investiga­ mission was restored when Voyager 1 tion of the outer planets, which would arrived at Jupiter four months ahead of be in a favorable alignment in the late Voyager 2). On September 5, 1970s. A 1971 Academy study echoed Voyager 2 was the troublesome twin. 1977, a Titan 3 the sentiment and developed the theme. Even before launch, failures in its rocket carrying a The plan for the Grand Tour was to onboard computer subsystems had to Centaur upper stage launched launch four spacecraft to visit a total of be rectified. The problems continued Voyager 1 from five planets-that is, two spacecraft after liftoff as the attitude-control sub­ Kennedy Space and one planet more than was eventual­ system behaved in an unexpected fash­ Center toward Jupiter and ly accomplished. The launches were to ion. Newspaper headlines spoke Saturn. have been broken into two sets, with anthropomorphically of a "Mutiny in two spacecraft being sent to Jupiter, Space." Then there was concern that Saturn and Pluto in 1976 and 1977 and the boom holding the platform on two sent to Jupiter, Uranus and Nep­ which the remote-sensing instruments tune in 1979. (An earlier version of the were mounted, including the camera, plan featured five spacecraft to explore was not fully deployed. the outer planets.) One by one these problems were Budget pressures began to build sorted out by the mission flight team, within NASA, causing the agency to and Voyager 2 seemed to have emerged even today. search for ways to decrease the scope from its chrysalis of troubles. But in the Within a short time the flight team of the mission and reduce its cost. A spring of 1978 the most fearful blow had installed a Backup Mission Load compromise was reached that proposed fell: The primary radio receiver failed. (BML), a sequence of commands that a prime mission to Jupiter and Saturn, The spacecraft switched to its backup provides some insurance against further to be accomplished by launching two receiver, which was found to be severe­ problems with the radio. Even if the spacecraft in 1977. A third spacecraft ly flawed (due to a faulty tracking-loop command link is lost, BML will ask would be launched in 1979 to visit capacitor). In effect the range of fre­ Voyager 2 to execute a minimal pro­ Jupiter and Uranus. quencies on which Voyager 2 could gram of observations at Neptune and The "Mariner-Jupiter-Saturn" pro­ receive instructions was greatly send the results automatically back to ject, "MJS77" for short, was approved reduced; to make matters worse, this Earth. by Congress in 1972 and became the narrowed receiving bandwidth kept The Grand Tour draws to a close. Voyager Project that we know today, shifting with even small changes in the Will this greatest of space explorations with the name change taking place temperature of the spacecraft. All com­ seem as magnificent to future genera­ early in 1977, before launch. The 1979 munication with Voyager 2 could have tions as it does to us , who watch in mission, "MJU79," fell by the wayside been lost, leaving it to drift helplessly wonder? Bet on it. despite strenuous efforts by the late Jim through the solar system. Long and others at JPL. The engineering teams rapidly put William l. McLaughlin was manager Mission designers for MJS77 were together a new system by which they of the Flight Engineering Office for well aware that at Saturn, the presump­ could fit commands into Voyager 2 ' s the Voyager/Uranus mission and is tive end point of the mission, it would elusive range of reception. Communi­ currently manager of the Mission Pro - still be possible to resuscitate the Grand cating with Voyager 2 is a skill that the file and Sequencing Section at the Jet Tour and send the spacecraft on to flight team must continue to practice Propulsion LaboratOlY· 7 rrER: FIRST STOP ON

upiter, with its dazzling satellite system, was the first target in the J epic journey of the Voyager space­ craft. Jupiter is the giant of the planets, with more mass than all of the other eight planets combined. Its powerful gravity has clung to the primordial gas­ es hydrogen and helium, giving Jupiter a composition similar to that of the Sun and stars. The chemistry of Jupiter is dominat­ ed by hydrogen and its compounds, such as water, ammonia and methane. Swirling ammonia clouds float high in its extensive atmosphere. Its interior is primarily liquid hydrogen at a high temperature, with no solid surface. Although it is immense relative to the other planets, Jupiter's mass doesn't compare with that of a star. In order to sustain a star's internal nuclear reac­ tions, the planet would require nearly 70 times more mass. However, Jupiter does radiate substantial heat in the in­ frared part of the spectrum. Its interior is still hot as a result of gravitational energy released when the planet formed out of the solar nebula some 4.5 billion years ago. Even before Voyager, we knew the basics of Jupiter's size and composi­ tion. Further, radio astronomers had mapped Jupiter's huge magnetosphere of charged particles, trapped in a mag­ netic field stronger than that of any oth­ er planet. Close-up observations of magnetospheric and atmospheric phe­ nomena were prime objectives of the Voyager flybys, but we also wanted a good look at the giant planet's array of moons. Jupiter is at the center of its own miniature "solar system." Among the 13 satellites that we knew about before Voyager, the four known as the Galilean satellites are larger than our Moon. The largest, Ganymede, is near­ ly as big as Mars. From telescopic mea­ surements, astronomers had determined that the outer two Galilean satellites -Callisto and Ganymede-are com­ posed in large part of water ice, while the smaller inner satellites-Io and Eu­ ropa-are primarily silicate in compo­ sition, like the terrestrial planets. Little more was known about these worlds until the revelations of the Voyagers. r ----

VOYAGER'S GRAND TOUR by David Morrison

Closing In Voyager 1 flew a long curving trajecto­ ry that took it past the planet on March 5 , 1979 just 270,000 kilometers (170,000 miles) above the cloud tops. Its path through the satellite system Right: The erupting sulfur volcanoes of Jupiter's moon 10 were among the was calculated to permit close exami­ most astonishing discoveries of the nation of Callisto, Ganymede and 10 Voyager mission. Caught between the during the preceding days. At a range gravitational pulls of sister satellite of just 22,000 kilometers (14,000 Europa and the immense planet, 10 is heated by tides that drive the most ac­ miles), the 10 flyby was close enough tive volcanism yet seen in our solar , for imaging objects as small as one system. The plume seen here is erupt­ kilometer (0.6 mile) over much of the ing from the volcano Loki. satellite surface. Below: Years after the Voyager space­ Following a similar path, Voyager 2 craft passed through the jovian sys­ tem, researchers on Earth are con­ reached Jupiter on July 9, after encoun­ tinuing to examine the data and glean ters with Callisto, Ganymede and Eu­ more knowledge about these strange ropa. The best Voyager 2 satellite flyby worlds. This map of Ganymede, the largest moon yet visited in our solar took in Ganymede at a range of 62,000 system, reveals varied terrains. Gany­ kilometers (39,000 miles). The path of mede displays dark, heavily cratered each spacecraft was exactly adjusted so regions and strange, grooved features that are unique to this world. that the powerful gravity of Jupiter would accelerate them on to­ ward Saturn, the next target. For several months before encounter, Voyager 1 pho­ tographed the complex cloud motions in Jupiter's atmo­ sphere to generate a "movie" for analysis of atmospheric circulation. Then, beginning about fifteen days before the flyby, the full battery of Voyager instruments began their measurements of the planet, its satellites and the space through which the craft was passing. Each day the precision and resolution of the planetary measurements improved, generating a wealth of new data for the Voyager scientists to inter­ pret. The usually deliberate pace of scientific investiga­ tion accelerated, with new theories being created and ei­ ther reinforced or disproved within a matter of days, or sometimes hours. On February 28, 1979 Voyager 1 reached the boundary of Now it began to study the magnetic distance of about 3 million kilometers Jupiter 's magnetosphere, and the en­ field of the jovian system. At first, (2 million miles), or 47 Jupiter radii, counter period began. Previously, the rapidly changing pressure from the so­ that Voyager 1 finally crossed the mag­ craft had been in the interplanetary lar wind caused the highly elastic mag­ netospheric boundary for good. Two medium, measuring properties of the netosphere to swell and contract several days later the spacecraft reached the or­ solar wind streaming out from the Sun. times, and it was not until March 3, at a bit of Callisto, the outermost of the four 9 Galilean satellites. And from there on, Right: Callisto, the outermost of discovery piled upon discovery at a the Galilean satellites (10, Europa, dizzying pace. Ganymede and Callisto, all dis­ covered by Galileoj, is probably the most heavily cratered body A Ring and 3 New Moons \ in our solar system. As this en­ As it swept through the jovian system, hanced-contrast image shows, the spacecraft's instruments described its face is "saturated" with craters; crater is piled upon the complexities of the magnetosphere. crater, so that eil£h crater Because it is closely coupled to formed by an impact destroys Jupiter's magnetic field, the inner parts another crater. Image: JPUNASA of the magnetosphere rotate with the Far right: The smallest moon of same lO-hour period as the planet, ac­ Jupiter is possibly the most in­ triguing. Long dark cracks mar celerating the trapped charged-particles its smooth, icy surface. Few of to tremendous speeds. Radiation from the impact craters that cover charged particles striking the spacecraft most solar system satellites are visible on Europa. Some as yet increased steadily until the moment of unknown process is keeping this closest approach to Jupiter, near the or­ moon's face smooth. Some sci­ bit of 10, the inner Galilean satellite. entists have speculated that an 10 is coupled to Jupiter by a loop of ocean of liquid water may lie under Europa's thin ice crust, intense electrical current called the 10 and when a meteorite or comet flux tube. Voyager 1 steered right strikes the surface, traces of its through this flux tube, measuring the impact are erased by water ooz­ strength of the current. ing into the resulting crater. At about the same time, the space­ Map: US Geological Survey, Flagstaff craft's cameras succeeded in pho­ tographing what turned out to be a ring of dust circling Jupiter. The search for such a ring had been considered a long shot before the encounter, but there it was, confined within the orbits of two small satellites, newly discovered by Ganymede's surface preserves the fur and sulfur compounds condense in the Voyager cameras. The tiny ring par­ record of a complex history of melting, the cold of space and fall back to blan­ ticles, acquiring an electrical charge mountain formation, and slow evolution ket the surface, painting it a variety of from the magnetosphere, spread above of the thickness and strength of its crust. colors from white through shades of or­ and below the planet's equator; thus Ju­ Europa, observed from afar, re­ ange to red. piter's ring is thicker than the flat rings mained an enigma to Voyager 1. It Voyager 1 also recorded heat radia­ of Saturn. A third small satellite was al­ would be studied in better detail four tion from the volcanoes. The strongest so discovered near the planet, bringing months later by Voyager 2. of the 10 "hot spots" was a sort of lava the total of jovian satellites to 16. lake called Loki, about 200 kilometers The two largest satellites of Ju­ Live Volcanoes (125 miles) across, with the "lava" piter-Ganymede and Callisto-being The most spectacular result of the probably composed of liquid sulfur. composed of about half water ice, rep­ Voyager 1 Jupiter flyby was the discov­ Little 10, no larger than Earth's Moon, resented a class of objects never before ery of intense volcanic activity on 10, a turned out to have the most active geol­ viewed at close range. Scientists ex­ satellite distorted and heated by the ef­ ogy and the highest level of volcanism pected that the geology of such ice­ fects of tides raised by Jupiter. A few of any body in the solar system. and-rock objects of planetary size weeks before the encounter, three as­ would be different from the more fa­ tronomers had calculated that 10 might Atmosphere and Local Color miliar rocky worlds, and they were not be tidally heated and had predicted that Although much attention was focused disappointed. its surface would show evidence of vol­ on discoveries concerning the satellites, Callisto, although lacking evidence canism. Sure enough, the spacecraft the magnetosphere and the ring of of internal geological activity, displays cameras photographed a strange, color­ Jupiter, the planet itself proved to be of a multitude of impact craters, some of ful topography, lacking in impact exceptional interest. Its colorful atmo­ them quite different from any impact craters but showing many signs of past sphere is in constant motion, displaying features seen in the inner solar system. volcanic eruptions. a wide variety of storms, jet streams The simple fact that Callisto's surface The big surprise came a few days af­ and other meteorological phenomena. is saturated with craters-that is, the to­ ter the encounter, when members of the Some features are reminiscent of at­ tal number of craters cannot increase, Voyager navigation team found a faint­ mospheric circulation on Earth, others because any new impact would have to ly illuminated arc above the satellite in are unique to this giant planet, whose destroy an existing crater-demon­ one of their long-exposure pictures. atmosphere is driven by escaping inter­ strates that early in its history the outer This unexpected feature turned out to nal heat as well as absorbed sunlight. solar system experienced a high rate of be one of eight active eruptions cap­ The largest and best known feature is impacts, perhaps analogous to the late tured in the photographic record, each the Great Red Spot; the Voyager cam­ heavy bombardment of the inner plan­ fountaining sulfur and sulfur dioxide to eras and spectrometers showed that this ets as recorded in the heavily cratered a height of tens to hundreds of kilome­ storm, unlike terrestrial storms, is a cir­ 10 highlands of the Moon. ters above the surface. The ejected sul- culating region of high atmospheric pressure, rather than a low pressure re­ Voyager 2'5 Turn yond the Earth where extensive liquid gion. Only four months separated Voyager 2 water can be found. Here was another The sheer amount of detail was sur­ from Voyager 1, and before the results satellite with its own special geologic prising, as high-resolution images of the of the first flyby could be fully pro­ history and a visage different from any atmosphere revealed a kaleidoscope of cessed, the observatory phase for the planet or moon previously investigated. swirling colors, with relatively little second spacecraft was under way. On The four month interval between en­ mixing between adjacent features of July 2, 1979 Voyager 2 entered the jo­ counters proved ideal for investigating different hues. Although we learned vian magnetosphere at a distance from changes in the magnetosphere and cir­ much about the atmosphere's composi­ the planet of more than 5 million kilo­ culation of the atmosphere, and it al­ tion and dynamics, its chemistry retains meters (3 million miles). The magne­ lowed sufficient time for us to adjust a fundamental mystery: We don't yet tosphere had expanded between en­ Voyager 2's program after Voyager 1's understand the chemical actions that counters. observations. Voyager 2's photographs impart such bright colors to the swirling By July 6 long-range photos of 10 of the jovian ring were among the high­ jet streams anq storms of this planet. confirmed that most of the active vol­ lights of the July 9 encounter. Perhaps they are compounds of sulfur, canoes discovered by Voyager 1 were The two Jupiter encounters are right­ perhaps organic chemicals produced by still erupting. On July 8 Voyager 2 ly celebrated as splendid technological the action of ultraviolet light on the up­ passed Callisto, and early the next day and scientific accomplishments. Fol­ per atmosphere. No one knows. it obtained the best photos yet of lowing Voyager 2's Jupiter encounter, Infrared readings of Jupiter revealed Ganymede. While much of the surface NASA Associate Administrator for that the second most abundant gas after of Ganymede appears to be old and Space Science Thomas A. Mutch said, hydrogen is helium, and thus the plan­ heavily cratered terrain, as on Callisto, "We're starting a new stage of space et's bulk composition is the same as younger mountains cover about half of exploration, and when history books that of the Sun-75 percent hydrogen, this moon. These may have been pro­ are written a hundred or two hundred 25 percent helium. Analysis of the tem­ duced by alternating cycles of expan­ years from now, historians will cite this perature and structure of the atmo­ sion and compression of the crust, re­ period of exploration as a turning point sphere indicates that the high clouds are sulting from the cooling that followed in our cultural, our scientific, our intel­ composed of frozen crystals of ammo­ the formation of the satellite. lectual development." The success of nia, and below we think there is a layer Europa, poorly imaged by Voyager the Saturn and Uranus encounters fully of ammonium hydrosulfide clouds. Fur­ 1, proved as extraordinary as its com­ justified this optimistic assessment of ther down, there may be clouds of water panion moons, with a smooth icy the Voyager program. And now, Nep­ ice and water droplets like those on surface crossed by hundreds of light tune beckons. Earth. These unseen regions will be and dark lines that look like cracks in measured directly by the Galileo probe, sea ice. Possibly this icy crust covers a David Morrison is Chief of the Space scheduled for launch in October of this global ocean of water. If so, Europa is Science Division, NASA Ames Re- year. the only place in the solar system be- search Center. 11 -- RN: JEWEL OF THE SOLAR

duced and distributed. Much current research on the evolution of the solar system derives directly from efforts to understand the beautiful and surprising patterns seen in Saturn's rings. We were dazzled by the filigree struc­ ture of the rings of Saturn. The "record groove" radial structure of the rings was revealed to the Voyager cameras from as far as 10 million kilometers (6 million miles). Two photopolarimeter experi­ ments picked up finer-scale structure, resolving features down to several hun­ dred meters. In these experiments the instruments rapidly sampled the fluctu­ ating brightness of the star delta Scorpii as the spacecraft's motion caused the star to trace a path through the shadow of Saturn on the rings. In addition to being an aesthetic bonanza, the extent and variety of all this ring structure was a scientific surprise. Saturn's rings are made of icy parti­ cles in high-speed orbit around the planet. While orbiting Saturn at 3 kilo­ meters per second (75,000 miles per hour), the ring particles jostle each other ever so gently-their typical col­ lision velocity is millimeters per sec­ ond. The large number of particles and their gentle collisions give the rings The "Rt,lJrn';Rn~" system is un­ he Voyager Saturn encounters in strength. There was a mood of festival many of the characteristics of a fluid, questionably November 1980 and August 1981 in the air. A similar feeling pervaded the notably viscosity, which is a prime con­ the most beau­ can leave little doubt as to the Voyager 2 encounter nine months later. tributor to ring structure. (Viscosity, the tiful in our T solar system. poverty of our collective scientific Discoveries came at a fast and furi­ tendency of a fluid to resist flowing, is JPL image pro­ imagination and the absolute necessity ous clip during both Saturn encounters. a manifestation of molecular friction, cessors creat­ for exploration. Naturally, we had our Much of the "instant science" that we the result of a large number of particles edthismon­ tage from im­ theories and our consensus wisdom. all did in those hectic weeks has (mer­ and their collisions.) We had thought, ages taken by Based on these, we planned a variety of cifully) been forgotten, replaced by incorrectly, that viscosity would pre­ both Voyager observation sequences months in slower but steadier strides in the years vent fine structure from forming. spacecraft. The satellites advance and eagerly awaited the that followed. In this article, there is Careful analysis of the light scattered seen here are results. We knew we would get answers only enough space for some of the by the rings, done in the years since (clockwise to some of our questions, and we knew highlights dealing with the rings, the encounter, has revealed that ring parti­ from top right) there would be some surprises. But we cle surfaces are more "compacted" in Titan, Iapetus, icy satellites and Titan. Tethys, Mlmas, had no clue as to the wealth of phe­ regions where collisions are more vig­ Enceladus, nomena that would be revealed when Grooved Rings, Not Smooth orous. The collisions pack the particle Dione and the Voyagers entered the Saturn system. Scientists have long thought the Saturn surfaces, much as one makes a snow­ Rhea. Image: JPUNASA Arriving at JPL (Jet Propulsion Lab­ system to be rich in clues about the ori­ ball, but not so violently as to fragment oratory) late in the evening a week gin of the solar system, which evolved the particles and produce clouds of dust before the Voyager 1 encounter, I found from a pancake-shaped "protoplanetary (as we had thought during encounter). the entire lab complex ablaze with nebula" of gas and particles orbiting the The degree of jostling determines the lights and alive with throngs of people. forming Sun. Disks of particles, such as effectiveness of viscosity in creating Colleagues from across the country had the rings of Saturn, provide an excel­ and removing fine structure in the rings. gathered to get a little closer look at the lent laboratory for studying how the The cause of the irregular filigree new data and toss around ideas, and the disk of dust and gas that ultimately structure, still unresolved, probably 12 international press corps was out in became our planetary system was pro- involves subtle and counter-intuitive SYSTEM by Jeffrey N. Cuzzi

ways in which viscosity can work in where particle orbit orbiting particle disks. periods were simple ra­ tios of the satellite orbit Bombarded Rings and "Spokes" period). Nevertheless, it We have uncovered clues that small took nearly two years meteoroids-perhaps comet fragments for us to recognize oth­ -hit the rings at a very high velocity, er similar patterns as typically 30 kilometers (19 miles) per spiral "bending waves," second. At these velocities, an impact or warps in the ring would be like the explosion of a hun­ plane. dred times the meteoroid's mass of Over the eight or so TNT. Such energetic impacts may help years since the encoun­ explain one of the most mysterious dis­ ters, the spiral wave coveries made by Voyager-the patterns have been "spokes" flickering across the Bring. studied in great detail, These huge features appear nearly and we understand instantaneously (in only a few minutes) their theoretical basis yet cover tens of thousands of kilome­ well enough now to ters, and spread into wedgelike patterns use them to determine that last for several hours before fading the mass in the regions through which away. The spokes are probably generat­ they travel. Using these and other data, ed by microscopic grains liberated from we have determined the mass of the the surfaces of ring particles. entire ring system to be comparable to Exactly how this process works is that of the saturnian moon Mimas, still not understood. It may begin with which has a diameter of 400 kilometers a large cloud of charged gas, produced (250 miles). by a meteoroid impact, that moves across the rings, causing tiny grains to Young Rings fly off the surface of their parent parti­ A fundamental change in perspective is cle. It certainly involves some interac­ emerging concerning the evolutionary tions with the planet's magnetic field as time scales of ring-moon systems. We well as the still-hypothetical impact realized quite early on that angular "trigger." We think that it takes a mete­ momentum (for an orbiting object, the oroid with a radius between 10 cen­ product of its mass times its velocity timeters and a meter (4 to 40 inches) to times its distance from Saturn) is trans­ produce a spoke. Of course, meteoroids ferred via spiral density waves from the this large are not common; only the rings to the moons at such a substantial vast extent of the rings-their area is rate that it is difficult to explain how ten thousand times that of Earth-pro­ either the rings or the moons could long vides a screen large enough for watch­ endure in their current configuration. ing this movie. This was the first note in a now persis­ tent refrain of indications that, through­ Warps in the Ring Plane out the solar system, rings may be Top: At Saturn, Voyager cameras revealed that the In addition to viscosity and meteoroid much younger than their planets. bright, broad, apparently smooth bands seen in bombardment, gravity plays an impor­ Another clue came indirectly from telescopes were actually composed of thousands of tant role in shaping the structure of the investigations of why there are so few thin "ringlets." This false-color image focuses on the boundary between two major differentiated sections rings, notably in the nearly regular, electrons in Saturn's ionosphere (an called the B ring (yellow) and C ring (blue). wavelike features detected in the earli­ upper atmospheric region characterized Middle: Another surprising discovery of the Voyager est Voyager images and in the stellar by an abundance of atoms with an elec­ mission was the phenomenon of "spokes" in Saturn's and radio occultation experiments. trical charge) as compared to the iono­ rings, huge wedge-shaped features perhaps triggered In this case the data confirmed our sphere of Jupiter. A flow of water mol­ by meteroid impact. pre-Voyager theoretical prediction that ecules into Saturn's ionosphere from Bottom: The Voyagers revealed that the F ring outside Saturn's main ring system is kinked, with individual spiral density waves, such as delineate the apparent "atmosphere" enveloping strands seemingly crossing each other. Gravitational the arms of a spiral galaxy, would form the rings could account for this deple­ interaction with small satellites traveling on either side at locations susceptible to the gravity of tion, since water molecules are efficient of this ring may cause this strange structure. Images: JPUNASA a maSSive moon (that is, locations (continued on page 14) 13 scavengers of electrons. However, the amount of water required to sustain this transfer over the age of the solar system would be comparable to the ring mass itself. A loss of mass on that scale cre­ ates a problem for survival of the rings. Moreover, solving the ionosphere problem this way poses yet another puzzle. The only mechanism that could generate this much vapor seems to be micrometeoroid bombardment. Mete­ oritic impurities in the ring material, as measured by their reflection and emis­ sion of visible light and radio waves, Left: Saturn's outermost large satellite, Iapetus, is a most peculiar world. The are a small fraction of the amount of hemisphere that leads in its orbit around the planet is many times darker than its material that would have hit the rings trailing hemisphere. The source of this dark-probably organic-material is a mystery. Some scientists believe that it fell onto the moon from other dark objects; over the age of the solar system, if the others think that it erupted from beneath Iapetus' surface. infall rate is as large as we suspect. Middle: In this map of Tethys compiled from Voyager images, the giant impact crater This set of arguments thus also points Odysseus, about 400 kilometers (250 miles) across, dominates one face of the moon. to youthful rings. Similar arguments On the opposite side Ithaca Chasma takes precedence. This feature was possibly apply to the rings of Jupiter and formed by the same impact that created Odysseus. Uranus. Recent formation of planetary Far Right: Perhaps no other satellite visited by Voyager aroused as much scientific interest as Titan. Voyager 1 found a rich array of organic molecules; these molecules rings, as we will see in a later section, coalesce in aerosol particles, forming haze layers (blue in this false-color image) and bears on the possible co-evolution of red, organic clouds. Images: JPUNASA rings and moons.

Old Moons niscent of glacial flows. Its smoothed­ martian moon Phobos is the only other Impact craters are the dominant land­ over appearance means that the surface object in the solar system to display a form on the saturnian satellites-scars of Enceladus must be extremely recent crater so large compared with its own left over, primarily, from the early years ---certainly less than a billion years old size; just a slightly larger impact would of the solar system when the last few and maybe as young as "yesterday." have split Mimas into fragments. planetesimals, bodies up to 200 kilo­ Of all the satellites in the solar sys­ Such may have been the fate of ham­ meters in size, were swept up on the tem, Enceladus has the brightest sur­ burger-shaped Hyperion, which tum­ hardening surfaces of newly formed face. Because it remains unsullied by bles erratically in its orbit, unlike all moons and planets. (The incessant hail the continual infall of impurity-laden other close-in satellites, which con­ of tiny meteoroids now scouring the meteorites, we must again conclude stantly present the same face to the par­ rings and moons is a remnant of this that this bright, nearly pure icy surface ent planet. Hyperion's tumbling, influ­ primordial torrent.) Many subtle infer­ is quite young. In another observation enced by massive Titan, is an example en-ces can be read in the cratering of a short-lived phenomenon, we have of "chaotic" motion, being determinis­ record, but controversies remain. found that Enceladus is surrounded by tic but unpredictable (see The Planetary The bombardment history on the sur­ a hazy belt (the E ring) of microscopic Report, May/June 1989, for more on faces of Saturn's icy Mimas, Tethys, particles that are rapidly destroyed by the study of chaos in the solar system). Rhea, Dione and Iapetus is manifestly magnetospheric and meteoritic erosion. We believe that Saturn's small inner different from that left on Jupiter's large Taken together, the evidence sug­ satellites, with radii of 100 to 300 kilo­ moons, which have many more large gests extremely recent resurfacing of meters (60 to 200 miles), have been craters. The large craters on Jupiter's Enceladus by some sort of ice flows. destroyed and reaccreted several times moons were probably created by plan­ However, Enceladus has too few radio­ over, probably 3 to 4 billion years ago, etesimals coming from remote reaches active elements to heat itself substan­ when the last planetesimals were still of the early solar system, while the tially, and it is too small to hold heat for being swept up. smaller craters on Saturn's moons may very long. We would not expect Ence­ be a record of debris created within the ladus to have a molten interior even if, Ringmoons Saturn system-possibly by the partial like most other satellites, it turns out to As hinted previously, we are beginning or complete destruction of other moons. have gone through a molten state at the to realize that rings and moons are not Whatever their cause, the craters time of its formation. Thus, the recent isolated phenomena but blend smoothly blanketing Mimas, Tethys, Dione and source of energy for the observed resur­ into one another. Voyager imaged Rhea are too plentiful to be recent; they facing remains a mystery. moonlets in and around the rings of probably record events occurring 3 to 4 Jupiter and Uranus and just outside the billion years ago. Reassembled Moons rings of Saturn. By analysis of distur­ The concept of moons being destroyed bances in the nearby ring material, we Enceladus the Exception occasionally during the history of the can infer the presence of at least one The surface of the small moon Ence­ solar system has achieved new other saturnian moon, about 10 kilome­ ladus, on the other hand, tells of more respectability since the Saturn encoun­ ters (6 miles) in radius orbiting within recent events. Close-up Voyager images ters. Mimas, innermost of the satellites the Encke gap in the A ring. An show Ence1adus to be nearly devoid of known before Voyager, was found to entourage of tiny objects may surround craters over much of its surface; sport a huge crater, spanning nearly the F ring, sporadically "tweaking" the 14 instead, there are wavy patterns remi- one-third of the moon's diameter. The orbits of its particles into their stranded, some places in the dense atmosphere. The Voyager infrared spectrometer detected a pharmacopeia of organic molecules in Titan's atmosphere-from

simple hydrocarbons like ethane (C2H6),

acetylene (C 2H2) and propane (C3HS) to more complex compounds containing nitrogen, such as hydrogen cyanide (HCN), cyanoacetylene (HC3N) and cyanogen (C 2N2). Organic chemists have long known that the chemical pathway that leads from simple atoms like hydrogen, nitrogen and carbon to the complex molecules from which our cells and genetic material are made gets started with these very compounds. This pathway would be rapidly short-circuit­ kinked appearance. meteoroid destruction of ringmoons. ed in the hydrogen-rich atmospheres of Objects this small are susceptible to Further insight along these lines is the gas planets, but in Titan's nitrogen­ destruction by meteoroids. The risk of clearly one of the main goals of the methane atmosphere, no one knows such destruction increases with proxim­ Neptune encounter. how far the path may lead. ity to a giant planet, because its strong Major advances in our understanding gravitational field accelerates approach­ Titan: Recipe for Life? of this giant organics factory have con­ ing meteoroids, multiplying their force Most scientists believe that life is a tinued in the years since encounter. In of impact. The great destructive power product of a long chemical chain that 1983, we realized that the ethane pro­ of meteoroids that get close to the giant has its beginning in oceans or atmo­ duced in Titan's atmosphere must be planets may help explain why, around spheres abundant in nonliving organic liquid at Titan's surface temperature all the gas giants visited so far by the material ("organic" in chemistry means and pressure. Furthermore, the amount Voyagers, we find more numerous but "containing carbon"). Complex organic produced over the age of the solar sys­ smaller satellites and, as we look nearer molecules are not unique to Earth; tem is large enough that much of the the planet, ring systems. It appears that amino acids have been found even in surface of Titan could be covered with satellites unlucky enough to orbit close primitive meteorites. In a classic labo­ an ocean of liquid hydrocarbons hun­ to the planet are much more likely to be ratory experiment, Stanley Miller and dreds of meters to tens of kilometers dealt a shattering blow. Harold Urey showed that an atmo­ deep. Such an ocean, containing the Before Voyager, we assumed that sphere containing only simple gases floating, sunken or dissolved residues Saturn's rings were as old as Saturn, like methane (CH4), ammonia (NH3) of more complex carbon-nitrogen­ but the apparently youthful features and water (H20), and energized by hydrogen compounds, would be a play­ seen in all known planetary ring sys­ electrical discharge, can yield complex ground of organic chemistry like none tems now point toward recent creation organic material such as amino acids other in our solar system. of rings-possibly by destruction of and other building blocks of life. moonlets. Gigantic Titan presents just such an A Chance to Go Again To produce Saturn's main rings, a environment in its atmosphere and on We have discovered dynamism not moon the size of Mimas would have to its surface. only in the turbulent atmospheres of the be pulverized, an event of low proba­ Titan's surface is completely gas giants but in their ring-moon sys­ bility since it would require a colliding obscured by a thick haze of organic tems as well. The Saturn system is projectile of several kilometers' radius. aerosol particles, produced in its nitro­ especially well endowed with dynamic However, the uranian rings and Sat­ gen-methane atmosphere by an alche­ phenomena, and our understanding urn's stranded F ring each contain only my of ultraviolet light, magnetospheric continues to increase steadily almost a the mass of a moonlet with a radius of protons and electrons, and cosmic rays. decade after the initial encounter. 1 kilometer (0.6 mile). Before Voyager, the depth of its atmo­ Yet fundamental questions remain It may be that many rings, like us, sphere was unknown; one of the most that have no answer in the Voyager data are temporary residents of the solar sys­ suspenseful periods of the Voyager 1 set. NASA, in cooperation with the tem. A planetary ring that we see today Saturn encounter came as the space­ European Space Agency, has recently may be only one of a succession of fea­ craft flew behind Titan with its radio proposed to initiate the Cassini mission tures that spring into existence and then transmissions slicing through the thick to Saturn, consisting of a Saturn orbiter vanish many times over during the life­ atmosphere. It was touch and go and a Titan atmospheric probe. If time of the Sun. whether any surface would be detected Cassini and its sister mission CRAF It is surely no accident that the ring before the beam faded into the noise, (Comet Rendezvous Asteroid Flyby) are arcs around Neptune lie in the same but detected it was-more than 350 approved by Congress this year, we and region of transition from rings to kilometers (220 miles) below the top­ future generations will continue to moons that we have seen around the most layer of haze. receive the rewards and insights of plan­ other gas giants. Most likely Neptune's We found a surface pressure 60 per­ etary exploration into the next century. ring arcs will be found to mingle with cent greater than on Earth. Subtle their own population of "ringmoons." inflections in the intensity of the weak­ leff Cuzzi, Research Scientist at The arcs may be nothing more than ening radio beam suggest that there NASA's Ames Research Center, is a transient clumps of debris created by may be thick rain clouds of methane at member of the Voyager imaging team. 15 us: BENEATH THAT BLAND

oyager 2's third destination on its Grand Tour was the Sun's V seventh planet, Uranus. On Jan­ uary 24, 1986, the spacecraft flew with­ in 82,000 kilometers (51,000 miles) of the planet. This was Uranus' fIrst visi­ tor from Earth. As at Jupiter and Saturn, we discov­ ered much about the planet, its magnet­ ic and radiation fields, its rings and its satellites. Uranus' almost featureless face betrayed few secrets to the space­ craft's cameras, and the thin, extremely dark rings were difficult to image. To penetrate the planet's mysteries re­ quired the best efforts of all team mem­ bers working on Voyager 2's instru­ ments. The radio science investigation in particular became a major contribu­ tor to our knowledge of the uranian system. The radio team used the regular communications link with Earth in sev­ eral ingenious ways. The extreme sta­ bility of the radio's frequency permits Voyager navigators to measure the spacecraft's speed to within millimeters per second. By measuring variation in speed resulting from tiny tugs from the The face of Uranus was the least detailed on any world visited by Voys planet and its satellites, they could features In images of the planet, scientists were forced to resort to hej measure the gravitational fields and so false-color techniques. In the image to the right, researchers have ave derive the masses of the bodies. treme tilt and to show that faint atmospheriC features correspond to tt. However, the radio experiment, using the spacecraft's communication As the spacecraft passed behind the atmosphere and taught us much about the atmosphere and clouds of planet (and was occulted, or hidden, as seen from Earth), the radio signal passed through Uranus' ionosphere and decipher. Voyager 2's cameras followed Voyager 2 's instruments carefully atmosphere. Just as light passing several cloud features long enough to measured the planet's temperature and through water is slowed and bent, so derive wind speeds. Seven of the eight the amount of sunlight it absorbs. Anal­ the radio signal changed, enabling sci­ appeared in the mid-latitudes, moving ysis showed that little or no internal entists to discern structure in the iono­ in the same direction as the planet ro­ heat escapes from Uranus, whereas its sphere and atmosphere, and construct a tates, between 50 and 150 meters per larger cousins Jupiter and Saturn still temperature and pressure profile of this second (110 to 330 miles per hour). The glow with heat left over froin their for­ strange world. eighth cloud, closer to the south polar mation. Scientists on the infrared inter­ Voyager 2's trajectory was carefully region, moved even faster, near 190 me­ ferometer spectrometer team concluded chosen so that the radio beam would al­ ters per second (420 miles per hour). that less than 13 percent-and perhaps so pass through the tenuous ring sys­ The radio science results showed that none--of Uranus' heat is generated in tem. Changes in the signal's amplitude the atmosphere at the equator rotates the interior. Since escaping heat drives and phase told scientists much about 110 meters per second slower than the atmospheric turbulence, this lack of the amount of matter in the rings and planet rotates; winds reverse direction heat may partly explain the planet's the size of their component particles. as they approach the equator. This wind bland appearance. It's also possible that profile indicates that Uranus' atmo­ extensive layering within Uranus' deep Atmosphere spheric gases rise near the poles and atmosphere acts as insulation, inhibit­ After the kaleidoscopic storms of sink near the equator. (This is in con­ ing the release of heat. If so, the interior Jupiter and the butterscotch banding of trast with Earth, a very different planet, may have cooled very little since its Saturn, the blue-green disk of Uranus where air rises near the equator and formation. seemed rather bland. Uranus' atmo­ then sinks at mid-latitudes, causing the The bland blue face of Uranus seen 16 spheric patterns proved very difficult to jet stream.) in Voyager 2 images of the planet rep- generates this "electroglow" is still a by Richard A. Simpson topic of lively debate. and Ellis D. Miner ExrERIOR Magnetic Field For 37 days, beginning 5 days before Voyager 2 reached Uranus, the plane­ tary radio astronomy investigation monitored radio signals generated by surface is sometimes considered to be interactions between charged particles the level of the atmosphere where the and the magnetic field. By tracking pressure equals one bar (a unit equal to these signals, we learned that the planet the pressure of Earth's atmosphere at rotates on its axis every 17.24 hours. sea level). On Uranus, the distance be­ The magnetometer investigation veri­ tween one-bar readings at the equator fied this period. It also found that the gives a radius of about 25,559 kilome­ the magnetic field is tilted 58.5 degrees ters (15,882 miles). Using data from from the rotation axis, and offset from the spacecraft and from Earth-based the center of the planet by nearly 8,000 observations, the radio scientists ex­ kilometers (5,000 miles). trapolated that the radius from Uranus' This highly tilted and offset magnet­ center to its poles is about 24,973 kilo­ ic field is the strangest among planets meters (15,517 miles). Thus Uranus, yet visited by spacecraft. Uranus' rota­ like Earth, is slightly squashed at the tion, tilt and interaction with the solar poles (because of rotation). wind combine to produce a corkscrew­ The radio team identified a minimum like structure trailing out behind the temperature of about -220 degrees Cel­ planet. The solar wind, a stream of sius (-360 degrees Fahrenheit) at the charged particles thrown off by the 100 millibar level of the atmosphere. Sun, blows most planets' magneto­ The temperature increases with depth spheres into a teardrop shape. to about -172 degrees Celsius at the 2.3 The offset from the planet's center bar depth and increases with altitude to causes magnetic-field strengths at about -160 degrees at 0.5 millibar. Uranus' one-bar "surface" to vary by a The radio science and infrared in­ factor of ten between the north and struments determined that the atmo­ south magnetic poles. Trapped within sphere above the obscuring blue clouds this wobbling magnetic field is an in­ is about 85 percent hydrogen with the tense radiation field, which consists al­ rest mostly helium. The other gas giant most exclusively of electrons and pro­ ger. To discern any atmospheric planets, as well as the Sun, are similar tons. The source of these particles is 'oic contrast-enhancement and mixtures of these two most abundant probably the hydrogen exosphere seen rlaid a grid to sh9w Uranus' ex- elements in the universe. by the ultraviolet investigation. 'e planet's rotation about its axis. s beam, was able to penetrate the The radio scientists saw a dramatic Uranus. Image: JPUNASA change in the radio signal at the 1.2 bar Rings level of the atmosphere, representing a In 1977, James Elliot of MIT (then at methane cloud layer some two to four Cornell) and his colleagues discovered resents only the topmost layer of kilometers thick. These clouds are the the uranian rings when they noticed clouds. The atmospheric depths were products of the temperature and pres­ that the light from a star blinked several hidden from view. The radio occulta­ sure conditions at the 1.2 bar level in times just before the planet occulted it. tion experiment, which sliced through Uranus' atmosphere, just as water va­ Similar Earth-based occultation obser­ the atmosphere as Voyager 2 passed be­ por in Earth's atmosphere condenses vations disclosed most of the rings' di­ hind the planet, covered a narrow re­ into clouds at certain levels. Below the mensions before Voyager 2 's closest gion near the equator and penetrated cloud layer, methane may represent as approach, but we still had much to about 250 kilometers (150 miles) into much as 2.3 percent of the uranian at­ learn about these narrow and perhaps the atmosphere. Even at this depth, we mosphere; above the clouds the con­ temporary structures. were still seeing only the outermost centration is far less. With its photopolarimeter, imaging, ,skin of Uranus. At greater altitudes, the ultraviolet ultraviolet and radio science instru­ To determine Uranus' equatorial ra­ spectrometer measured a very extended ments, Voyager 2 refined the telescopic ,dius, scientists watched as Voyager 2 layer of hydrogen with temperatures up measurements and made the first de­ disappeared behind the planet and reap­ to 525 degrees Celsius (980 degrees tailed images of the rings. It also dis­ peared on the other side. Then they Fahrenheit). The radio science team also covered two new rings. measured the distance between the two found evidence of an extended iono­ Since colors are a clue to the compo­ points. Determining these two points, sphere (an atmospheric layer character­ sitions and histories of solar system ob­ however, required an arbitrary choice ized by charged particles) that may be jects, it was very important to examine since Uranus, a gas planet, does not of­ related to this "exosphere" of hydrogen. them in visible light. As the spacecraft fer an obvious edge from which to The hydrogen interacts with sunlight so passed through the planet's shadow, the measure. On gas giants, which have that the illuminated atmosphere glows Voyager 2 cameras, glanced back at the very tiny solid cores, if any at all, the with ultraviolet light. The process that rings to see them in the "forward-scat- 17 each of the nine rings known before 14 kilometers per second (30,000 miles encounter, it created profiles of their per hour), while the gravitational ef­ optical depth (this is a measure of the fects of the planet and satellites tugged light-intercepting capacity of a cloud of on the spacecraft. The effects let us use material, and so gives an indication of tracking data, combined with imaging the properties of the particles). Uranus' data and eight years of telescopic ob­ rings are extremely narrow, typically servations from Earth, to determine the I less than 10 kilometers across, whereas masses of Uranus and its largest Saturn's system is thousands of kilome­ moons. The satellites seem to be made ters in width. The spacecraft was never­ primarily of water ice and rock. The '\ theless able to pick out rich structural densest, Titania, is about 69 percent detail. Several instruments picked up more dense than water; the lightest, Mi­ J tenuous companions to the eta and randa, is 25 percent more dense than I delta rings. The imaging and photopo­ water. Earth is 450 percent more dense larimeter systems reported additional than water. rings that did not show up in the radio Detailed images of the five largest science data, most probably because uranian satellites-Oberon, Titania, the particles there are much smaller Umbriel, Ariel and Miranda-revealed than the radio wavelength. a far greater diversity than scientists The combined results indicate that, had anticipated. Their grooved and unlike the saturnian and jovian rings, cracked surfaces suggest a higher level the uranian system contains surprising­ of tectonic activity than we would ex­ ly few particles smaller than fist-sized. pect for such small, icy bodies. Tecton­ We would expect such small particles ics is the movement in planetary crusts to be created as icy ring particles col­ that is found on larger bodies-such as lide, perhaps to re-accrete later into Earth- with some sort of internal heat larger particles. This is what we would to drive the motions. Many forces, expect in a system that had reached col­ some perhaps tidal, some impact, may lisional equilibrium, which seems to be have reworked the surfaces of the ura­ what we saw at Jupiter and Saturn. nian satellites. So what's happening to the dust at Titania, Ariel and Miranda are criss­ Uranus? The bloated hydrogen exo­ crossed by surface fractures. Ice ap­ sphere may be sweeping the rings clean pears to have flowed across Ariel's sur­ of dust, while the rings erode before face. Miranda possesses three baffling our eyes. Voyager 2 may be telling us "coronae" that may be results of partial that planetary rings are relatively tran­ melting of the interior during its youth. sient phenomena. As satellites mature, the materials that At only 480 kilometers (300 miles) in diameter, Miranda is the smallest The plasma wave investigation comprise them can, under suitable con­ of Uranus' five major satellites, but recorded the effects of micron-sized ditions, differentiate and settle into what it lacks in size it makes up in particles striking the spacecraft as it denser, rocky cores with lighter, icy strangeness. The face of this little moon displayed the most varied crossed the ring plane of Uranus. The surfaces. If Miranda's coronae, each of terrain Voyager 2 has yet seen in its team estimated the cloud of fine parti­ which is nearly rectangular in shape reconnaissance of the outer solar cles to be about 3,500 kilometers and contains a series of concentric system, with fault valleys, parallel (2,200 miles) thick with a maximum ridges, impact craters and strange structures, are the result of ice flowing ovoid features unique to Miranda. concentration of 1.6 particles in each up to the surface, the process of differ­ The top image is a photomosaic of 1,000 cubic meters. Such a low-density entiation must have been halted in mid­ Voyager images; the bottom is a cloud would be invisible to other inves­ course. Or perhaps the coronae are the map made from those images. Map: US Geological Survey, Flagslaff tigations, and the spacecraft suffered no result of processes that we don't yet un­ perceptible damage. derstand. Voyager 2 taught us much about the tered" light that reveals fine material. Satellites motion, composition and origin of the The cameras found a tenuous sheet of Voyager 2 discovered 10 new satellites, objects that make up the uranian sys­ dust spanning the entire ring system. all orbiting inside the 5 previously tem. These data illustrate both similari­ Uranus' rings are extremely dark, re­ known satellites. The imaging system ties and differences among the giant flecting only about 5 percent of the was able to resolve the disks of 7 of the planets of the outer solar system. After sunlight falling on them. Like Saturn's 15 moons; the others remain only the Neptune encounter in August, our glimmering ring system, the uranian points of light. All the new satellites are initial reconnaissance of these bodies rings are probably made up of water ice smaller and darker than the five "classi­ will be complete. The results will pro­ particles, but this usually bright materi­ cal" satellites. As with the rings, this vide substance for scientific discussion al may be contaminated by methane darkness may be due to the high-energy for decades to come.

(CH4). The energetic exosphere discov­ protons in Uranus' exosphere bombard­ ered by Voyager 2 could strip the hy­ ing the methane ice on the satellite sur­ Richard A. Simpson is an Associate drogen from the methane, leaving be­ faces, releasing the hydrogen and leav­ Member of the Radio Science Team on hind a dark carbon coating on the ring ing behind a carbon (organic) residue. the Voyager Project. Ellis D. Miner is particles. Voyager 2 moved through the urani­ Voyager 's Deputy Project Scientist at 18 As the radio beam passed through an system at a relative velocity of over 1PL. ------

VOYAGER 2 ApPROACHES NEPTUNE

by Ellis D. Miner

xcitement is building rapidly at Only four days earlier, Voyager 2 satellites with data collected by the Caltech's Jet Propulsion Labora­ will have celebrated the 12th anniver­ Goldstone tracking array. VLA pro­ Etory as Voyager 2 plummets to­ sary of its launch from Cape Canaveral. vides an additional tracking capability ward its August 25 closest approach to The spacecraft's twin, Voyager 1, was equivalent to one and a half 70-meter the solar system's eighth planet. We're launched on a faster trajectory 16 days antennas. closing the gap at the rate of 1,450,000 after Voyager 2. Both spacecraft have Changes we've made to the space­ kilometers (900,000 miles) per day. Im­ operated continuously since launch. craft to accommodate lower light levels ages from Voyager 2' s high-resolution Each is tracked for 8 to 16 hours a day at Neptune include reprogramming vidicon camera keep improving in de­ during interplanetary cruise periods and Voyager 2's computers to (1) allow tail (since late 1988 they have exceeded 24 hours a day during planetary en­ longer imaging exposures, (2) steady anything obtained from Earth). With all counter periods. the spacecraft and (3) provide new the pre-encounter tests of the spacecraft means of compensating for relative mo­ complete, the Voyager Project team at Tuning Up the Equipment tions of the spacecraft and the target JPL is rehearsed and ready. We will be Because Neptune is so much farther body. Typical exposures at Neptune handling a flood of new data during the from the Sun and Earth than Uranus, will be 15 seconds or longer (whereas 5 four-month Neptune encounter period, light levels are lower and the received seconds was typical at Uranus). The from June 5 through October 2,1989. signal strength is weaker. These condi­ new capability permits exposures of up At its closest approach to Neptune, tions have made it necessary to modify to 61 seconds, plus integer multiples of Voyager 2 will sail over the north polar the spacecraft as well as the receiving 48 seconds. Camera wobble has been regions of the planet only 29,200 kilo­ antennas on Earth. All three of NASA's slowed from 1/l0th the angular rate of meters (18,100 miles) from the planet's 64-meter antennas (at Goldstone, Cali­ the hour hand on a clock to 1/25th that center, a scant 5,000 kilometers (3,100 fornia; near Canberra,' Australia; and rate. Image motion compensation can miles) from its polar cloud tops. The near Madrid, Spain) have been enlarged now be done by nodding the spacecraft moment of closest approach will actu­ to 70-meter diameters and made more to match target motion during an expo­ ally occur on August 24 at 9 p.m. Pa­ efficient. A high-efficiency 34-meter sure without turning far enough to in­ cific Daylight Time (PDT), but the ra­ antenna has been added at Madrid so terrupt the flow of data back to Earth. dio signals from Voyager 2, traversing that each site now has one 70-meter an­ During the Uranus encounter, we com­ the 4.5 billion kilometers (2.8 billion tenna and two 34-meter aritennas. pared the challenge of Voyager 2 's pho­ miles) to tracking stations near Canber­ Non-NASA antennas will also track tography to taking a picture of a piece ra and Parkes, Australia, and Usuda, Voyager 2 during the Neptune en­ of coal against a black background at Japan, will be received at 1:06 a.m. counter. After taking part in the 1986 twilight. At Neptune, the lighting is PDT on August 25. Voyager 2 's pas­ Uranus encounter, the Parkes (Aus­ about half as good. . sage behind the planet will be moni­ tralia) Radio Telescope will again assist tored by those stations from I: 12 a.m. the Canberra stations in data collection, What to Look For through 2:01 a.m. PDT. and both Parkes and Usuda (Japan) will To appreciate how far away it really is, A short 5 hours, 14 minutes after help collect radio science data as the consider that Neptune has not complet­ "Neptune closest approach," Voyager spacecraft passes behind Neptune and ed an orbit around the Sun since its dis­ 2 will fly within 40,000 kilometers Triton. The Very Large Array (VLA) covery in 1846. Distance makes Nep­ (25,000 miles) of the center of the plan­ near Socorro, New Mexico, will collect tune a very difficult object to learn et's largest moon, Triton, and just half Voyager 2 telemetry data with its twen­ much about. We know its orbital period an hour later will take four minutes or ty-seven 25-meter antennas and com­ is 165 years, but its rate of rotation is less to slice through the moon's shadow. bine that signal via communications still uncertain. Recent studies have 19 yielded rotation periods from 17 to 18 hours, but these times are based on the motions of clouds across the disk and may not represent the rotation period of Neptune's interior. Magnetic and radiation fields un­ doubtedly surround the planet, but we don't know their strength and extent. There may also be a rudimentary ring system, possibly consisting of a few elongated and relatively isolated collec­ tions of debris, created by collisions of small satellites that we have not yet dis­ covered. These or other satellites may also prevent the ring material from spreading and dissipating, either along the ring or toward the planet. Two teams of observers in Arizona may have detected such a satellite as it briefly blocked the light of a star in May 1981. Searches for undiscovered satellites will be a prime objective of Voyager 2. Triton, Nereid and 1989Nl are the known satellites of Neptune. Triton is the only major moon in the solar sys­ Above: Neptune and its large moon Triton are the last tem to travel in a retrograde (back­ ports of call on Voyager 2' s Grand Tour. wards) orbit around its planet. Four of Painting: Michael Carroll Jupiter's tiny outer moons are retro­ Right Background: Even while Voyager 2 was still 134 grade, and Saturn's little Phoebe is ret­ million kilometers (83 million miles) away from rograde; these five moons, however, are Neptune, it was already picking out detail in the atmosphere. These five views show how the planet believed to be captured asteroids. With looked through five C!f the filters on the spacecraft's a diameter of 2,800 to 3,600 kilometers narrow-angle camera. (1,700 to 2,200 miles), Triton is much Image: JPUNASA too large to be an asteroid. It travels around Neptune in just under six days along an orbit inclined 21 degrees from Neptune's equator. Its orbit is nearly circular at an average distance of about 354,000 kilometers (220,000 miles) The diameter of 1989Nl is some­ instruments will record variations in from the center of the planet. what smaller; it occupies a prograde, brightness at different times of day (and Telescopic measurements show that circular orbit about 117,500 kilome­ thus in changing levels of sunlight). Triton has methane ice on its surface. ters (72,800) miles) from Neptune's These observations, when combined Temperatures at the surface may be near center. with infrared temperature readings, will 50 degrees kelvin (-370 degrees Fahren­ provide a measure of heat flowing from heit), close to the condensation tempera­ The Agenda of Measurements the interior of Neptune. ture of nitrogen, and some scientists Atmosphere. Images obtained before The most definitive measurements of have speculated that condensed nitro­ the encounter period reveal banded the deep atmosphere near and below gen (probably solid) may exist on the structure and discrete clouds, and thus the cloud tops will come from radio surface, perhaps forming frozen lakes. Neptune will not emulate the bland ap­ waves transmitted through the atmo­ Even without the nitrogen, Triton's pearance of Uranus. Encounter measure­ sphere by the spacecraft as it passes methane would cause a thin atmosphere ments of the atmosphere began June 5 "behind" the planet as viewed from to form; if nitrogen is present, Triton with color imaging every 3.6 hours until Earth. These data will provide a profile might have an atmosphere more sub­ mid-August, when the planet will no of the atmosphere's structure and com­ stantial than that of Mars. longer fit in a single high-resolution position. These data will help to pro­ Nereid's orbit is inclined 28 degrees frame. vide measurements of helium and to Neptune's equator and is extremely Infrared temperature and composi­ methane abundance, the variations of elongated. Nereid's distance from Nep­ tion measurements begin on August 16. temperature and pressure with depth, tune ranges during a single 359-day_or~ _ Then, during the period from seven and the vertical locations of cloud and bit from a minimum of about 1.4 mil­ days before until seven days after Nep­ haze layers. lion kilometers (870,000 miles) to a tune closest approach, Voyager 2 will Magnetic and Radiation Fields. Six maximum of 9.5 million kilometers map the planet and take higher-resolu­ of Voyager 2' s eleven scientific investi­ (5.9 million miles). Its diameter is tion images of selected atmospheric gations will assay the solar wind/plan­ thought to be between 300 and 1,100 features. During the closest 48 hours, etary magnetosphere environment, 20 kilometers (200 and 700 miles). infrared, imaging and photopolarimeter measuring charged particles, plasmas, so large that the atmosphere is likely to obscure the surface (as was the case for Saturn's Titan). Triton will be scruti­ nized by the optical instruments for an entire six-day orbital period. High-reso­ lution studies begin with a full-color, full-disk image just two hours before Neptune closest approach. For an eight-hour period, beginning two hours after Neptune closest ap­ proach, Triton will be the sole target of Voyager 2' s remote-sensing instru­ ments. Moderate-resolution color mapping will be followed by higher­ resolution, non-color mapping. After an infrared atmospheric study, Voyager 2 will perform highest-resolution imag­ ing along with ultraviolet, infrared and photopolarimeter coverage, revealing features smaller than one kilometer. This eight-hour period may also provide the best measurements of Triton's mass (and hence its density), as calculated from the moon's effect on Voyager 2's flight path. Above and Left: With its substantial atmosphere, The ultraviolet and photopolarimeter Neptune's large satellite Triton is ripe with possibili­ instruments will then aim beyond the ties. Some scientists have concluded that lakes of liq­ horizon at a star to detect any atmo­ uid nitrogen may lie on its surface. Others say such sphere-caused variations in the star's lakes, if they exist, must be frozen. We will not know exactly what this moon will look like until Voyager 2 brightness. Further investigations of flies past in August, but some artists have already vis- Triton's atmosphere by the ultraviolet ited Triton in their imaginations. Here are two artists' and radio science instruments will span versions of what this moon might look like. the period when Voyager 2 passes Top painting: MariLynn Flynn Left painting: Michael Carroll through Triton's shadow. Studies of its crescent phase and dark side will occupy the remainder of the Triton close approach period. The remainder of the Neptune en­ counter period is occupied with contin­ magnetic fields and the lower-frequen­ beams. The plasma wave instrument ued studies of Neptune's and Triton's cy radio waves generated by their inter­ will also "listen" for evidence of tiny dark sides and of Neptune's magnetic actions. Periodic radio emissions will ring particles striking the spacecraft tail. Final calibrations for most of the likely be the best indicators of Nep­ during the inbound and the outbound instruments will also be performed pri­ tune's internal rotation period, as they crossings of Neptune's equatorial plane. or to the official end of the encounter have been at Jupiter, Saturn and Uranus. Nereid. Because of the long loop of period on October 2. Data analysis and Voyager 2 is expected to travel with­ Nereid's orbit, Voyager 2 won't get publication will likely continue for in the magnetic field of Neptune from closer to that moon than 4.6 million years to come. August 24 until August 28. The pas­ kilometers (2.9 million miles). At that As one who has had the pleasure of sage of Voyager 2 over Neptune's north range, high-resolution images will being intimately involved in Voyager polar region may give us our first op­ show features as small as 90 kilometers science planning, data collection and portunity to measure directly the polar (60 miles). While we won't be able to publication of results since the 1977 magnetic and radiation fields of one of resolve individual craters, regional launches, the author here thanks mem­ the solar system's four giant gaseous brightness differences should be appar­ bers of The Planetary Society, the Unit­ planets. ent, and we can compare them with ed States Congress, the National Aero­ Ring Material. Neptune's ring arcs, earlier imaging sequences to determine nautics and Space Administration and if they exist, bear little resemblance to Nereid's rotation rate. The unusual or­ others who have been instrumental in the ring systems seen at Jupiter, Saturn bital characteristics militate against advocating, supporting and applauding and Uranus. Voyager 2 will search for synchronous rotation, in which a satel­ the Voyager 2 adventure. For centuries ring material in orbit around Neptune. lite keeps the same face toward the to come it will stand as one of human­ If any ring arcs are discovered a week planet it orbits. kind's great explorations. or more before closest approach, then Triton. A fitting climax to Voyager remote-sensing instruments will be 2' s Grand Tour of the Outer Planets Ellis Miner is Voyager's Deputy Pro­ trained on them to make high-resolution may well be the encounter with Triton. ject Scientist at 1PL. He was previous­ mosaics and measure blockage of Here is a satellite large enough to hold ly involved in the Mariner 6,7,9 and starlight, sunlight and spacecraft radio an easily detectable atmosphere but not 10 and Viking missions. 21 The Soviet Phobos mission to the planet Mars and its largest moon ended abruptly on March 27, 1989. But before the spacecraft was lost, it managed to return some fascinating and important data on the Red Planet. Some of the most spectacular results came from the • Termoskan instrument, which examined the planet in the thermal-infrared part of the spectrum. In essence, it measured heat radiated ! from the surface. These Termoskan images reveal differences in temperature between areas on the planet surface. The areas that appear dark in 'I these images have lower temperatures, but they are not necessarily dark to the eye. Conversely, light areas have higher temperatures. From information about temperature, scientists can infer other important details about the surface. For example, fine dust cools and warms more quickly than solid rock. During the day, dusty regions will be warmer, and therefore will appear brighter in the thermal-in­ frared spectral band seen by this instrument. At night, the opposite will be true: fine dust will appear darker and colder than solid rock.

~ S~ This image of the crater Schiaparelli was taken at noon. The dusty areas appear light and the coarser surface features are dark. This large crater, about 400 kilometers (250 miles) in diameter, was one of the first to be imaged by spacecraft from Earth when the Mariner 4 flew by the planet in 1965.

e~ ~ Chaotic terrain is one of the most intriguing types of surface features found on Mars. Some scientists believe that catastrophic floods may have formed such regions when water stored in subsurface aquifers was suddenly released. The chaotic terrain seen in this image connects the great canyon Valles Marineris (Mariner Valley) to Chryse Planitia (the Plain of Gold) where the Viking 1 lander now sits.

1/aLte4- ~ Valles Marineris is often called the Grand Canyon of Mars, but that name does little to convey the immensity of this feature, which stretches one­ fifth of the way around the planet. The section of the canyon seen in this Termoskan image covers about 1,000 kilometers (600 miles), or about one-quarter of the canyon's length. The area seen in the "chaotic terrain" image lies just off the upper right of this image, where the canyon turns to the north. 22 The bottom image shows the same area as seen in visible light. Image s courtesy of Margarita Naraeva, Glavkosmos VOYAGER INTERSTELL4R MISSION

by Dan F. Finnerty

hile all eyes have been focusing less and less remains to provide heat, so na pointing. When the spacecraft can on the upcoming Voyager 2 the Voyagers' "batteries" are steadily no longer track the Sun, they will drift Wflyby of Neptune, Voyager Pro­ running down. away from Earth-point, eventually tum­ ject members have been quietly consid­ Current predictions hold that the bling out of control. Analysis of the ering new challenges for the veteran RTGs can power basic spacecraft oper­ Sun-sensing equipment indicates that pair of spacecraft. Mission managers ation until about 2025. Full instrument the Voyagers will probably be able to are planning to operate these spacecraft operations should be possible through recognize the Sun until about 2030. well into the next century- by which 2000, and all fields and particles in­ None of these projections takes into time they will have left the planets of struments should be operable through account the possibility of component the solar system far behind-in the about 2015. failures. For the most part, each major Voyager Interstellar Mission (VIM). The strength of the radio signals spacecraft subsystem is backed up by a Just how long are the Voyagers likely transmitted back to Earth depends on redundant system, and the Voyagers to survive, and what are they likely to several factors. Most important is the have enough onboard autonomy to take find along the way? How will logistics distance to Earth, which of course corrective action in the event of fail­ change when Voyager is no longer a increases as the two spacecraft follow ures. However, there have already been priority mission? These are questions their separate paths into interstellar several critical equipment failures that that mission planners are seeking to space. We can build larger tracking sta­ limit the ability of the spacecraft to sur­ answer on the eve of the last Voyager tions to acquire the ever fainter signals, vIve. planetary encounter. or we can simply instruct the Voyagers Voyager 2 lost its primary command to send data back more slowly. At the receiver shortly after launch and has How Far Can Voyager Go? slowest rate of transmission for science been operating on its backup receiver Just how long the Voyager spacecraft data (about 43 bits per second), the ever since. If that should fail, it would can continue to function depends on Voyagers can likely be tracked until be unable to receive further operating many things, one of the most important 2015 by stations with 34-meter anten­ instructions. Also, the primary radio being the electrical power needed to nas, and possibly, if either spacecraft amplifier tube has worn out, leaving no operate the computers, radios and sci­ still has electrical power, beyond 2030 backup for that either. If the backup entific instruments. When the power by 70-meter tracking stations. tube fails, Voyager 2 will be unable to runs out, the spacecraft will cease func­ The Voyager spacecraft carry transmit science data, although engi­ tioning forever. hydrazine fuel for use in two kinds of neering-only telemetry could still be The strength of the signals radioed maneuvers: adjusting trajectory for transmitted. back to Earth is also critical. If the sig­ planetary flybys, and maintaining atti­ Voyager 1 has suffered a failure of nals become too faint, ground tracking tude control so that the antenna points one of its flight data system memories. stations will be unable to decode and toward Earth. In the VIM era, there will If the backup memory fails, the space­ interpret the data that the Voyagers are be no need to make further trajectory craft will no longer be able to process sending. Equally important, there must corrections; hence all remaining fuel data for transmission to Earth. It is be sufficient fuel for each spacecraft to will be available for attitude control. If impossible to assess when or whether a fire its thrusters and keep its high-gain we adopt a conservative attitude-con­ catastrophic failure would occur on antenna pointed directly at Earth. If the trol strategy-using precision antenna­ either spacecraft. Barring such prob­ antenna drifts off point, the signals will pointing only when absolutely neces­ lems, the two Voyagers should function radiate uselessly into space. sary- the hydrazine should last until until at least 2015, and perhaps as long Electrical power for the Voyager about 2025, and possibly longer. as 2025 or even 2030. spacecraft comes from radioisotope thermoelectric generators (RTGs), End of Mission Science: Which Plugs to Pull which directly convert the heat from As the Voyagers go farther from Earth, Mission planners have considered a the radioactive decay of plutonium into they also go farther from the Sun, range of options for the Voyager Inter­ electricity. As the plutonium decays, which they use as a reference for anten- stellar Mission, from a minimal "keep 23 alive" configuration to full operation of resources for the remaining experi­ Beyond the Solar System all instruments. To be scientifically use­ ments. In another set of observations-this ful, the mission should at least continue time by the cosmic ray instrument-the operation of the fields and particles The Solar Medium Voyagers have come in contact with instruments (magnetometer; cosmic ray, The Voyager spacecraft have already intriguing particles of interstellar ori­ plasma, and low-energy charged-particle returned much valuable data about the gin. The cosmic ray instrument mea­ instruments; and the planetary radio Sun. Since launch the fields and parti­ sures very high energy particles (atom­ astronomy and plasma wave receivers). cles instruments have monitored the ic nuclei traveling near the speed of These instruments, while continuing to solar wind, which blows out from the light), which can originate from super­ sample the outer solar system environ­ Sun at 400 kilometers (250 miles) per nova explosions, galactic magnetic ment, would search for the farthest reach second, and when solar flares erupt, the fields and other high-energy phenome­ of the Sun's magnetic field, and then, planetary radio astronomy receiver na. If they are electrically charged, such having crossed this "heliopause," would picks up intense radio bursts. particles are deflected by the Sun's go on to analyze the interstellar wind (a The plasma (PLS) and low-energy magnetic field. Neutral particles, how­ stream of charged particles flowing out charged-particle (LECP) instruments ever, can directly penetrate the helio­ from the stars). directly measure the density, composi­ pause. Once inside the heJiosphere, A valuable enhancement to this basic t;on, energy and flow direction of high­ flowing at relatively low speeds of 25 mission would be continued studies of speed streams of solar material injected kilometers (15 miles) per second, the active galaxies, active binary stars, and into space. The magnetometer observes neutral particles may become ionized pulsars by the ultraviolet spectrometer. associated fluctuations in the local and captured by the solar wind, which Ultraviolet wavelengths can't be interplanetary magnetic field. Taken as flings them back toward the heliopause. observed from Earth because of the a whole, Voyager observations provide At the heJiopause they are accelerated screening effects of the ozone layer, fundamental information on the physics by processes similar to those thought to and no other spacecraft currently in and dynamics of the Sun's magneto­ cause the "shock wave" signals detect­ operation possess the Voyagers' capa­ sphere, the extended region influenced ed by the plasma wave sensor. The bility at far-UV wavelengths. Informa­ by the Sun's magnetism. interstellar particles rebound at about tion obtained at these wavelengths is The Voyagers may already be detect­ one-tenth the speed of light into the particularly valuable because the most ing evidence of the heliopause. The solar system, where they are being energetic astronomical phenomena plasma wave instrument has on occa­ detected by the cosmic ray instru­ radiate most of their energy in the ultra­ sion picked up radio emissions that ments. This scenario, too, awaits con­ violet. Measurements in this portion of may be caused by the collision of the firmation from further VIM data. the electromagnetic spectrum would solar wind with the interstellar wind. Neutral interstellar particles that flow fundamentally influence models of Theorists suggest that near the colli­ into the solar system are called the such objects. sion, as the solar wind slows suddenly, "anomalous cosmic ray component" Several experiments will end during the magnetic field flowing with it because of their lack of carbon, one of VIM, the most prominent being imag­ becomes compressed, generating strong the most abundant elements in nature. ing. Proposed observations were, electric currents. These currents then Current thinking holds that this lack of unfortunately, too difficult to perform rapidly accelerate solar wind particles, carbon results from the ease with which within the limitations of the extended generating the observed radio emis­ interstellar carbon is ionized, acquiring mission, too unlikely to succeed or too sions. This "shock wave" interpretation an electrical charge, and then blocked expensive. Similarly, the infrared inter­ is still somewhat speculative; observa­ by the Sun's magnetic field from entry ferometer and spectrometer, photopo­ tions continue in an attempt to deter­ into the solar system. In 1985 Voyager larimeter, and radio science experi­ mine the source of the signals. We can instruments made the first positive ments had to be dropped. Terminating expect even more interesting results as detection of this anomalous cosmic ray these experiments was a difficult deci­ the Voyagers get closer to the component, sensing both argon and sion to make, but it freed up significant heliopause. -unex pectedl y--carbon.

oyager Project planners can guess 8571 20319 20629 23274 when the Voyager 2 spacecraft will pass certain "landmarks" such as the heliopause, which signals the edge of our Sun's sphere of magnetic influence, and the Oort Cloud of comets, which marks the Enters the Crosses the Closest Closest Closest Closest end of the solar system . Gauging heliosheath, heliopause into approach to approach to approach to approach to Voyager 2's speed and direction, the boundary the interstellar Barnard's Star, Proxima Alpha Cen- Lalande 21185. region of the medium. which may Centauri, star tauri, desti- Voyager 2 they can estimate when it might Sun's helio- have a nearest to our nation of many passes within make its closest approach to distant sphere. planetary Sun. Voyager science fiction 4.65 light years stars. As centuries pass, Voyager companion. 2 passes within imaginings. of the star, 2'5 actual itinerary will be known Voyager 2 3.21 light years Voyager 2 when the only to the spacecraft itself-and to passes within of the star, passes within spacecraft is any life-forms that happen across it. 4.03 light years when the 3.47 light years 1.15 light years of the star, This timeline represents the of the star, spacecraft is from the Sun. when the 1 .00 light years when the guesses of the Voyager program spacecraft is from the Sun. spacecraft is planning office as to the probe's 0.42 lig ht years 1.02 light years adventures among the stars. from the Sun. from the Sun. A Short Segment from the Information We saw these carbon particles-in addition to taking the most accurate . Digitally Encoded on the Voyager Interstellar Record measurements yet of interstellar hydro­ gen, helium, nitrogen, oxygen and This Voyager spacecraft was constructed by the United States of America. neon-because 1985 was near a period We are a community of 240 million human· beings among the more than of minimum solar activity, which made 4 billion who inhabit the planet Earth. We human beings are still divided it easier for interstellar particles to pen­ into nation states, but these states are rapidly becoming a single global etrate the solar magnetosphere. Cosmic civilization. rays came in mainly along a plane We cast this message into the cosmos. It is likely to survive a billion extending from the solar equator, in the years into our future, when our civilization is profoundly altered and the "neutral sheet" formed where the mag­ surface of the Earth may be vastly changed. Of the 200 million stars in the netic field lines from the Sun's north Milky Way galaxy, some-perhaps many-may have inhabited planets and south poles meet. and spacefaring civilizations. If one such civilization intercepts Voyager During the current solar cycle, which and can understand these recorded contents, here is our message: will reach its maximum in about 1992, This is a present from a small distant world, a token of our sounds, our the magnetic polarity of the Sun is science, our images, our music, our thoughts and our feelings. We are reversed from the previous cycle, which attempting to survive our time so we may live into yours. We hope some­ peaked in 1981. What will happen at the day, having solved the problems we face, to join a community of galactic neutral sheet? Scientists await Voyager civilizations. This record represents our hope and our determination, and readings of cosmic rays during the solar our good will in a vast and awesome universe. maximum with great curiosity. Will the rejuvenated magnetic field JIMMY CARTER block out these cosmic rays entirely? President of the United States of America Or will they be detected from different directions? What phenomena will the The White House other instruments measure? The instru­ June 16, 1977 ments of the spacecraft will be tuning in to a wealth of new science.

Garrison Operations Operating the Voyager spacecraft in the safer, easier and less expensive. sion will be realized only after decades VIM era will differ greatly from current To maximize telecommunications of steady observation and careful anal­ methods. The flight team will shrink capability at extreme distances, the ysis- I find it interesting, to say the from more than 200 people to about 40. flight data system will be completely least, that on my retirement date in The way commands are generated for reprogrammed. The memory space 2017, the Voyagers may still be on the the spacecraft will also change dramati­ made available by shutting down cam­ job. And while the Voyager Interstellar cally. Instead of command sequences eras and other instruments will be used Mission will not offer the same drama custom crafted for each activity, a set of to enhance data processing for the as the planetary encounters, the scien­ onboard "command blocks" will be remaining instruments. The new data tific value of exploring the outer limits stored and reused as often as needed. modes have been designed to work of the solar system and interstellar Use of command blocks, which can be well with 34-meter tracking stations, space will redouble the triumph of activated by simple ground commands, since the 70-meter stations will be Voyager. will reduce the complexity of operating taken up by the Magellan, Galileo, the spacecraft, though it will also limit Mars Observer and other high-priority Dan F. Finnerty is Deputy Science the flexibility of observations. Overall, projects. Investigations Support Team Chief on operating the Voyagers is going to be The full value of the Voyager mis- the Voyager Projec.t.

Voyager 2 enters Voyager 2 Closest Closest Closest Closest the Oort Cloud, leaves the Oort approach to approach to approach to approach to the sphere of Cloud and so Ross 154. Sirius, brightest 440phiuchi. DM+271311. comets orbiting . exits our solar Voyager 2 star visible from Voyager 2 Voyager 2 the Sun, esti­ system. passes within Earth. Voyager passes within passes within mated to extend 5.75 light years 2 passes within 6.72 light years 6.62 light years between 20,000 of the star, 4.32 light years of the star, of the star, to 200,000 AU when the of the star, when the when the out. (An Astro­ spacecraft is when the spacecraft is spacecraft is nomical Unit 6.39 light years spacecraft is 21.88 light 47.38 light equals the mean from the Sun. 14.64 light years from years from distance from years from the Sun. the Sun. Earth to the the Sun. Sun.) ~------~------~-~~~~------~.

ject, to build a model of the universe. Needless to say, one scale model of the universe, even if it filled a great hall, could hardly show the details of anything as puny as our Milky Way galaxy, let alone the features of the solar system that are so relevant to us. So Awad adopted a scheme called "the powers of ten" that has been popularized in a couple of books and in an edu­ cational movie. Instead of building one model, Awad has built 75 by Clark R. Chapman models, each within a cube just a foot and a half per side, and each representing a linear dimension exactly 10 times that of its neighbor. The models are suspend­ ed overhead in a darkened gallery. As you walk from model to model, it's as though you had moved 10 times he universe is incredibly vast. It is also intricate­ farther away each time. The display begins, simply ly complex. We relate to our world on a "human enough, with a close-up of Carl Sagan's book Cosmos T scale," comparing things to a bread box or to a on the hood of a car. Moving to the next model, you mile. Only with difficulty do we visualize our planet as see the whole parking lot, and the book is now tiny. fu a planet, yet Earth is small enough to fit inside three more steps, you see the whole eastern coast of Jupiter's Great Red Spot. Jupiter is puny compared Florida, as if you had been launched in the shuttle a with the Sun. But the Sun is miniscule compared with couple of minutes before. In three more steps there is a a red giant, like Betelgeuse in the shoulder of Orion. beautifully intricate model of the whole planet Earth; Interstellar distances, the galactic scale, clusters of the book and the parking lot have long since been galaxies, and so on-all beyond our ability to visual­ squeezed to the vanishing point. ize. Or are they? Continuing from model to model, you then see the A few months ago, I had a chance to see a scale whole solar system, which thereafter shrinks to become model of the universe. It is hard to believe that a mu­ an insignificant comer of our galaxy. We see the local seum exhibit could so successfully show the human group of galaxies, then larger clusters of galaxies, and scale, the scale of the universe and everything else in super clusters, before [mally reaching a somewhat fan­ between. But a man named George Awad, a member ciful representation of the entire visible universe. The of The Planetary Society's New Millennium Commit­ effect is breathtaking because of the craftsmanship of tee, has built a fantastic array of models that does just the planetary models and the use of fiber optics to de­ that. It is not located in any museum or planetarium, at pict stars. and galaxies. George Awad is fascinated by the diversity of the A group of least not yet. But it is open to the public, after a fash­ schoolchil­ ion. If you are in New York City, I suggest that you ar­ universe, and he couldn't stick with just the geocentric dren visit range to see it. perspective. At places, the model branches out to show George George Awad is president of a company that makes other planets at the same scale as Earth, or other starS Awad's mode/of the architectural models-scale models of skyscrapers at the same scale as our Sun or the solar system. Alto­ universe. and developments. But his hobby is astronomy, and he gether, there are 75 separate models hung in a rectan­ turned his craftspeople loose on an awe-inspiring pro- gular array in the gallery. As a scientist, I was particu­ larly struck by the extraordinary precision of the work. Every lunar crater is shown to correct scale, all the or­ bital inclinations have been measured properly, and the integrity of what we know about the cosmos has not been sacrificed to serve artistic ends, although the ex­ hibit is exquisitely beautiful, indeed. Awad has used his own resources to build this fabu­ lous model. It is located in a mid-town building, not in a museum. Since Awad first unveiled his model a year ago, he has been in touch with various museums and planetariums around the country. There has been an ex­ pression of interest from the National Air and Space Museum, among others, but there are no firm plans for the exhibit to be shown outside New York City. Meanwhile, it is possible for you to see this three­ dimensional model of the universe through the powers of ten. George Awad is happy to arrange dates for pub­ lic viewing at 260 West 36th Street. Just telephone Awad Astronomical Models in advance at (212) 563- 5480. Although unheralded, this project is surely one of the major wonders to see in the Big Apple.

Clark R. Chapman is a member of the imaging team of the Galileo mission, scheduled for an October launch toward Venus and Jupiter. SOCIETY

field related to planetary sci­ -Andrea Schweitzer, Po­ For more information, CRAFOORD PRIZE ence. Ms. Avegno, a student mona College, "The Gravi­ write to: Unesco Coupons James Van Allen of the Uni­ of remarkable accomplish­ ty-Assist Technique and Its Programme, 7 Place de versity of Iowa, a Planetary ment, is on her way to the Application to the Trajectory Fontenoy, F-75700 Paris, Society Advisor, has won University of Notre Dame to of Voyager 2" France. Or write to us for the 1989 Crafoord Prize, study aeronautics. -Adam Showman, Stan­ the address of the issuing which is awarded each year - Louis D. Friedman, Ex­ ford University, "The Search agency nearest you.- Iva by the Royal Swedish Acad­ ecutive Director for Extraterrestrial Life" Svitek, Assistant Manager, emy of Sciences in an area Congratulations to all the Data Processing not covered by the Nobel NEW MILLENNIUM winners of fellowships and Prize (astronomy is one such AWARDS scholarships that are part of CONTEST- area). The New Millennium Com­ The Planetary Society'S 10TH ANNIVERSARY THEME Honored for three decades mittee Scholarship competi­ commitment to the future The 10th anniversary of The of research and leadership in tion has also come to a close through education. All these Planetary Society will be in magnetospheric physics, with final judging of student students, and others pursu­ 1990. While we celebrate Van Allen is best known as essays on the pros and cons ing studies that will one day our many accomplishments, the discoverer of the upper­ of establishing a lunar base guide us to the stars, deserve we will also look toward the atmospheric region called within the next 20 years. our encouragement.-LDF exciting possibilities of the the Van Allen belts. The The winners are: coming decade. Academy cited him as a pio­ -Eric Engstrom of Cham i­ SEDS As part of the celebration, neer combining "superior nade College Preparatory INTERNATIONAL MEETING we are sponsoring a special expertise both in rocketry School, West Hills, CA Students for the Exploration 10th Anniversary Theme and in relevant scientific in­ ($1 ,000) and Development of Space contest. The winner will be strumentation. " -Amy Houts of Riverview will hold their 8th Internation­ recognized in The Plane­ Van Allen, who took part High School, Sarasota, FL al Conference from August 24 tary Report and will be in Explorer 1, Mariner 2, ($500) to 28 in Pasadena, in con­ awarded a trip to a 1990 an­ and Pioneer 10 and 11 , cur­ -Edward Lux of Thomas junction with Planetfest '89. niversary event (locale to be rently serves as an interdis­ S. Wootton High School, Planetary Society Director determined) . ciplinary scientist for the Rockville, MD ($250) Thomas O. Paine will deliv­ Send your original motto Galileo mission to Jupiter. -Justin Tsai of West High er the keynote address, or phrase that best sums up His response to the award School, Torrance, CA heading up an exciting pro­ the essence of The Planetary was "one of humility, be­ ($250).--LDF gram of panel discussions Society of the future to: 10th cause my style has been to and student presentations, Anniversary Theme, c/o The work with students and ju­ COLLEGE FELLOWSHIPS coverage of the Voyager 2 Society. Entries must be in nior colleagues. One of my The Planetary Society has encounter with Neptune and by October 2.- Tim Lynch, greatest pleasures has been selected five promising un­ the Arthur C. Clarke Awards Director of Programs and to watch their evolution, de­ dergraduates to receive Banquet. Development velopment and achievement $1,000 grants in support of For registration informa­ their studies in a field related tion, write to Irwin Horo­ of leadership in this field." KEEP IN TOUCH -Charlene M. Anderson, to planetary science. These witz, SEDS Conference, Director of Publications winners earned distinction 112-58 California Institute Our mailing address: on the basis of academic of Technology, Pasadena, The Planetary Society NATIONAL achievement and a 2,500- CA 91125.-Susan Lend­ 65 N. Catalina Avenue MERIT SCHOLARSHIP word essay. roth, Manager of Events Pasadena, CA 91106 Jennifer L. Avegno of Saint -David Baker, University and Communications Call for an updated events of Texas, Austin, "The Gen­ . Mary's Dominican High calendar: eral Atmospheric Circula­ UNESCO COUPONS School, New Orleans, has (818) 793-4328 east of the tions of Earth and Venus" In countries where foreign been named as the 1989 Mississippi -James Hugunin, Stanford currency is scarce, Unesco winner of a four-year Na­ (818) 793-4294 west of the University, "A Case for Coupons generally offer the tionalMerit Scholarship Mississippi sponsored by The Planetary Manned Spaceflight" best rate of exchange and Society. -Charles Liu, Harvard may be used as money for General calls: Each year this award goes University, "Recent Work on educational purchases, in­ (818) 793-5100 to an outstanding high school the Core-Instability Model cluding membership in The Sales calls ONLY: senior planning to major in a of Giant Planet Formation" Planetary Society. (818) 793-1675 27 Scientists have said that there is of collapse. Such a collapse could take on Mars (like cliffs on Earth) common­ plenty of water under the ground on place after the ground ice melted and ly have piles of debris at their bases. Mars. What solid evidence do we flowed away. However, the debris piles associated have to support such a claim? The surface of Mars contains many with many martian cliffs have forms -Ponniah Sivanesan, Danville, Illi­ unusual craters (rampart craters) that that suggest they have undergone slow nois we have not seen anywhere else in the deformation and flow. Their appear­ solar system. These craters are sur­ ance is much like that of rock glaciers There is strong geologic evidence that rounded by lobes of material that on Earth, which are large tongues of ice water exists underneath much of the extend outward from the crater like and rock that creep slowly down martian surface. Water in this form is petals around the center of a flower. We slopes. The ice acts as a lubricant, often called "ground ice." Some think that these lobes were formed by allowing rock particles to slide past one regions on Mars (the "chaotic" and material that "flowed out" from the another, resulting in slow movement "fretted" terrain) appear as if they were crater when it was created by a crash­ like that of a fluid. This suggests that formed by the melting of underground ing comet or meteorite. Such fluid the material making up the cliff debris ice. Several large channels on Mars, behavior suggests that the crater was on Mars (which comes from the cliffs which we believe to have formed by formed in ice-rich rock or soil, which themselves) contains a significant catastrophic flooding, flow out of such would have become mud when heated amount of ice. regions. These areas are lower than by the impact. A similar process may be responsible their surroundings and have complex, Mars also contains surface features for "terrain softening" on Mars. Craters hummocky floors, indicating some kind consistent with the creep of ice. Cliffs in equatorial regions on Mars appear FACTI NOS

very "crisp" with sharp crater rims program has been supported in the Comet Halley is an oddball of the solar sys­ and steep slopes. By contrast, the past by The Planetary Society, was tem, according to a group of scientists at Ari­ appearance of the "softened" craters the next to observe 1989 FC, in early zona State University in Tempe. in nonequatorial regions can be April. Nevertheless, the asteroid's During its 1986 flyby, astronomer Susan explained by the presence of ground orbit was difficult to calculate from so Wyckoff and her tearn measured Halley's car­ ice. The ice allows the underground few observations, and 1989 FC was in bon-12/carbon-13 ratio using a telescope and material to move slowly and to flow danger of being lost forever. Because spectrometer. They found it to be about 65 to to a more subdued shape. This relax­ of interest, like yours, about when it 1, compared to the 89 to 1 ratio measured in ation slowly modifies the sharp rims will be back and whether or not it all other solar system objects. Halley's high of craters until they become rounded. might hit Earth, astronomers worked proportion of carbon-I 3 is closer to that of ' Because we see sharp, pristine craters hard during the last nights in April to interstellar dust and gas. [The carbon-I 3 atom close to the martian equator and mod­ photograph its much dimmer image. has an extra neutron in its nucleus, making it ified craters far from the equator, They were successful, so we will heavier than the commonest isotope, carbon- ground ice may exist close to the sur­ be able to find it next spring, when it 12. Both isotopes are believed to have been face in regions where temperatures comes around again. Fortunately, it made inside exploding stars. Their relative are colder. will not present a hazard on the next quantity gives a clue to the history of the The Mars Observer is scheduled pass. Closer passes are possible in interstellar medium.) The researchers say their for launch in 1992. This orbiting future decades, however, so astrono­ analysis suggests that Halley may be an alien spacecraft will carry along a gamma­ mers will be trying to improve the body, born fairly recently outside the solar ray spectrometer that can detect water orbital calculations. It is important to system and captured in a close encounter with ice to a depth of one meter (3 feet) or do so because 1989 FC measures at the Sun. so in the martian surface. This will be least 200 by 500 meters (600 by 1,600 Other comet experts are still skeptical about our first attempt to directly detect feet). According to the final chapter in the capture idea. Paul Feldman of Johns Hop­ ground ice on Mars. the new book, Cosmic Catastrophes, kins University in Baltimore says scientists -DAVID G. JANKOWSKI, Cornell which I wrote with David Morrison, need more comet measurements to determine University the impact of such an object would if Halley truly is an adopted child. have devastating consequences, possi­ -from F. Flam in Science News An asteroid crossed Earth's orbit on bly even threatening the viability of March 23rd but was not discovered civilization. That is all the more rea­ Voyager 2 images taken 90 minutes apart on ' until March 31. With all our ad­ son to strive to discover and catalogue April 3 of this year show a large dark spot and vanced technology, how did we miss the thousands of as-yet-undiscovered a dark band circling the south polar region of it? Will it return regularly like Hal­ objects like 1989 Fe. Neptune. The spot, which is 10 percent darker ley's Comet? -CLARK R. CHAPMAN, Plane­ than its surroundings, travels around the plan­ -Natalie Pope, Burbank, Illinois tary Science Institute et every 17 to 18 hours. Detection of the spot will help atmospheric scientists to determine , The close pass of asteroid 1989 FC by How do the mechanical, imaging rotation rates of Neptune's atmospheric fea­ our planet on March 23rd was news­ and propulsion systems operate tures much sooner than they had expected. worthy because no other asteroid has properly on the Voyager and Pio­ The spot's dimensions, relative to the size been seen to come as close (less than neer spacecraft considering the of Neptune, are comparable to Jupiter's Great twice the distance to the Moon). But extremely frigid conditions existing Red Spot. it was hardly a remarkable event in in the far reaches of the solar sys­ -from the Jet Propulsion Laboratory's the cosmic scheme of things. Dead tem? Voyager ,Bulletin comets and asteroid fragments are - Justin L. MacGregor, Prescott numerous and they are not easy to Valley, Arizona Alan Stem, an astronomer at the University of discover. For example, it has been Colorado, calculates that comets in theOort estimated that there are probably The systems operate properly because Cloud are likely to collide amongst them­ about 1,000 objects larger than a kilo­ the spacecraft keeps its insides warm. selves so often that their surfaces will be meter (0.6 mile) in diameter whose Surfaces facing the Sun are painted or cratered and fractured to a depth -of several orbits cross Earth's orbit. But only covered so that they absorb and reject hundred feet. This finding conflicts with the several dozen of them have been dis­ the right amount of radiant energy. genenil belief that material in the distant Oort covered so far. Patient observers Surfaces facing dark space are insu­ Cloud is undisturbed and largely unchanged using Schmidt camera telescopes con­ lated to limit their loss of the internal since the early days of solar system history. tinue to search the skies. heat generated by operating electrical Since comets that swing through the inner Henry Holt, who works in the systems. And, because the Voyagers solar system are unlikely to have a clean, search project led by Gene Shoemak­ have traveled from near the Sun to far untouched surface, astronomers searching er, discovered 1989 FC from the from the Sun, some of their electronic them for primordial solar system material are Mount Palomar Observatory about a compartments have active thermal out of luck unless they can get samples from week after its closest approach when control in the form of thermostatic well b.elow their battered surface layers. it was rapidly zooming away. That louvers that can close to conserve "About once a year some comet hits anoth­ was good fortune-it might have heat or open to radiate it away. er," notes Stem. But because the collisions been missed altogether, and nearly - JAMES D. BURKE, Jet Propul­ t~ke place at relatively low speeds, "only one was. Eleanor Helin, whose observing sion Laboratory comet in a hundred or a thousand has actually been destroyed." -from Astronomy 29 PLANETFEST '89 PREVIEW! I~- l PLANETFEST '89 1..:--__-- Enamel Pin - Blue and Special Sales .tems and Events! gold Planetfest logo on white enamel background, 1114" diagonal size. #671 THE PLANETARY VOYAGER 2 Sportshirt - SOCIETY Cloisonne 100% cotton, machine Pin - FIRST TIME washable, with the special OFFERED - gold Voyager 2 Neptune encounter lettering on black design. Available in berry, background, ~B" #562, and navy, #561, diagonal size. #670 S M L XL sizes. SET OF TWO PINS - Both the Planetfest '89 and The Planetary VOYAGER 2 Society pins in a box. FRISBEE-9" #672 across, with the Voyager 2 spacecraft gold imprint, available in dark blue, red, or green. #563

PLANETFEST '89 Sweatshirt - PLANETFEST '89 T-Shirt - FANNYPACK­ 50150 blend, machine washable, 100% cotton, machine washable, with the Planetfest '89 logo Silver with black with the Planetfest '89 logo design design. Available in white, imprint "The on front. Available in white, S M L XL sizes. #680 Planetary Society," S M L XL sizes. #681 lets you carry what GLITTER PENCILS PLANETFEST '89 Coffee Mug - Midnight blue you need in style - Available in a classic ironstone mug with 22kt gold imprint. #682 and with ease! #264,A.1t::::::l1... rainbow of colors. THE PLANETARY SOCIETY Coffee Mug - #675 Classic ironstone mug featuring the Voyager 2 spacecraft. Available in midnight blue with white GLITTER PENCILS imprint, white with royal blue imprint, and light blue INA TUBE ­ with white imprint. #265 Colorful glitter KEYRINGS - Available pencils, art eraser with the Planetfest '89 or and sharpener. The Planetary Society logo Blue, red, and purple imprinted designs. #677 and #676 with either the PLANETFEST '89 Planetfest '89, #673 DECALS - Stick to a or The Planetary notebook, folder, or Society logo, #674 anywhere! #685

Celebrate Voyager 2's Flyby of Neptune! Come to the Pasadena Center this August for Planetfest '89 You can pre-register for events over the telephone Wed., Aug. 23: Symposium #1: A craft and missions, plus a special, using a charge card. VISA, Voyager Retrospective. Panelists international space art show! include Dr. Carl Sagan, Dr. Bruce MasterCard, and American Sat., Aug. 26: " Best of Voyager" Murray, and Dr. Edward Stone. Express are accepted. Call video presentation, Soviet Thurs., Aug. 24: Voyager Watch presentation of Phobos mission The Planetary Society's - free to the public - live images results, Science Fiction Writers' office at of the Neptune encounter all nightl Forum. (818) 793-5100. Fri., Aug. 25 - Sun., Aug. 27: Sun., Aug. 27: Symposium #2: Festival events include: Film Beyond Neptune - the future of Hotel accommodations are Festival, Speakers' Forum featuring solar system exploration through still available through the space program leaders, Voyager international cooperation, featuring Pasadena Visitors Bureau, mission specialists, scientists, space program leaders from the educators, writers, etc. , exhibits USSR , France, Japan, and the Telephone: (818) 795-9311 . and displays of planetary space- USA And More . .. PRICE (IN PRICE (IN ORDER C I R (t) PRICE (IN N~RJ'::R • New Items us DOLLARS) N3~~~ • Books (cont.) US DOLLARS) NUMBER· 0 or epros. con. us DOLLARS) 561 Voyager 2 Sportshirt - 100% 152 The Quickening Universe: Cosmic 330 PlaneHest '81 - Saturn and the cotton, navy blue S M L XL $28.00 Evolution and Human Destiny F-ring (two 22"x35" posters) $ 5.00 562 Voyager 2 Sportshirt - 100% by Eugene T. Mallove. 268 pages. $17.00 333 Saturn - full view photograph cotton, berry S M L XL $28.00 157 Starsailing: Solar Sails and (16"x20" laser print) $ 8.00 563 Voyager 2 Frisbee - 9" dark blue, Interstellar Travel 334 Solar System Exploration i red, green (specify color) $ 3.50 by Louis D. Friedman. 146 pages. $ 9.00 (35"x35" map with booklet) $ 9.00 264 Fannypack - silver & black $13.00 158 Space-The Next 25 Years 335 Voyager 1 at Saturn by Thomas R. McDonough. (set of five posters) $16.00 265 Coffee Mug - The Planetary Society, 237 pages. $16.00 dark blue, light blue, white 336 Solar System in Pictures - (specify color) each $ 8.00 165 The Grand Tour: A Traveler's Guide 9 pictures $10.00 I (set of four) $28.00 to the Solar System 337 Uranus - sunlit crescent by Ron Miller and (16"x20" laser print) $ 8.00 670 The Planetary Society Cloisonne William K. Hartmann. 192 pages. $10.00 Pin - gold lettering on black 340 "You Are Here" (23"x29" poster) $ 5.00 background $ 4.00 170 The Home Planet edited by Kevin W. Kelley. 256 pages. $36.00 PRICE (IN 671 PlaneHest '89 Enamel Pin - N~~::R • 35mm Slide Sets US DOLLARS) Planetfest logo on white background $ 4.00 184 The Search for Extraterrestrial Intelligence: listening for life in 205 Chesley Bonestell's Vision 672 Set of Two Pins - Planetfest '89 and the Cosmos of Space The Planetary Society $ 7.00 by Thomas R. McDonough. (40 slides with sound cassette) $15.00 673 Glitter Pencils in a Tube - 256 pages. Soft Cover $13.50 210 Remember Halley's Comet Planetfest '89 logo $ 5.00 185 The Surface of Mars (20 slides with description) $10.00 674 Glitter Pencils in a Tube - by Michael Carr. 232 pages. $20.00 213 Mars (20 slides with description) $10.00 The Planetary Society logo $ 5.00 PRICE (I N 220 Viking 1 & ~ at Mars 675 Glitter Pencils - different colors N~~::R • Videotapes US DOLLARS) (40 slides with sound cassette) $15.00 each $ .25 415 VHS Jupiter, Saturn & Uranus: 225 Voyager 1 & 2 at Jupiter 676 Key Ring - The Planetary Society 416 BETA The Voyager Missions (40 slides with sound cassette) $15.00 logo $ 5.00 417 PAL (60 min. videotape) $30.00 230 Voyager 1 Saturn Encounter 677 Key Ring - Planetfest '89 logo $ 5.00 425 VHS Mars and Mercury (40 slides with sound cassette) $15.00 680 Planetfest '89 T-Shirt - 100% 426 BETA (60 min. videotape) 231 Voyager 2 Saturn Encounter cotton, white S M L XL $14.00 427 PAL $30.00 (40 slides with sound cassette) $15.00 681 PlaneHest '89 Sweatshirt - 50/50 440 VHS Universe 235 Voyager Mission to Uranus blend, white S M L XL $20.00 441 BETA (30 min. videotape) (20 slides with description) $ 7.00 442 PAL $30.00 682 Planetfest '89 Coffee Mug - blue PRICE (I N each $ 8.00 460 VHS Together to Mars? N~RJ'::R • Other Items US DOLLARS) 461 BETA (60 min. videotape) (set of four) $28.00 505 An Explorer's Guide to Mars 462 PAL $15.00 685 Planetfest '89 Decals - (color map of Mars) $ 5.00 (5 for $400) each $ 1.00 PRI CE (I N 515 The Planetary Society Logo - US DOLLARS) N~~::R • Color Reproductions bookmark (6"x2") (2 for $1.50) $ 1.00 Many Planetfest items will not be available until Aug. 23, 1989 305 Apollo - photograph of Earth, full 516 We're Saving Space for You - disk (16"x20" laser print) $ 8.00 bookmark (6"x2") (2 for $1.50) $ 1.00 PRICE (IN N3~~~~ • Books US DOLLARS) 308 Earth at Night (23"x35" poster) $ 6.00 526 Hugg-A-Planet Earth - 129 living in Space-A Manual for 310 Earthprint - photograph of North 14" -diameter pillow $15.00 Space Travellers America (8"x10" laser print) $ 4.00 528 Hugg-A-Planet-Mars by Peter Smolders. 160 pages. $13.50 315 Earthrise - photograph of Earth from 8" diameter pillow. $13.50 133 ~irror Matter: Pioneering the Moon (16"x20" laser print) $ 8.00 540 Men's T-Shirt - white with blue Antimatter Physics 321 Uranus Encounter - 4 pictures of logo. S M L XL $ 9.00 by Robert L. Forward and Joel Davis. Uranus and its moons $ 4.50 543 Mission Stamps - 10 sets 262 pages. $17.00 322 Jupiter - photograph of southern (4 stamps per set) $ 1.00 140 Out of the Cradle: Exploring the hemisphere (16"x20" laser print) $ 8.00 545 Planetary Report Binder - blue with Frontiers Beyond Earth gold lettering (2 for $16.00) $ 9.00 by William K. Hartmann, Ron Miller 323 Mars - landscape from Viking Orbiter and Pamela Lee. 190 pages. $11.00 (16"x20" laser print) $ 8.00 550 TPS Buttons - blue with logo $ .50 148 Planets Beyond: Discovering the 324 The New Explorers (22"x34" 555 Starwatcher's Decoder Set $35.00 poster) Outer Solar System $ 7.00 560 The Voyager Spacecraft by Mark Littmann. 286 pages. $20.50 325 Other Worlds (23"x35" poster) $ 7.00 Paper Model $14.00

All merchandise is subject to availability.

IF YOU NEED MORE ROOM, JUST ATTACH ANOTHER SHEET OF PAPER

ITEM PRICE PRICE NAME NUMBER aUAN DESCRIPTION EACH lOTAL

ADDRESS

CITY, STATE, ZIP

COUNTRY Sales Tax: For faster service on California residents add 6%. DAYTIME TELEPHONE NUMBER ( credit card orders: Los Angeles County residents add Phone: 9 A.M.-5 P.M. an additional 1;2% transit tax. o CHECK OR MONEY ORDER FOR $ ______(Sorry, no C.O.D.s) (Pacific Time) Shipping and Handling: o VISA 0 MC 0 AM/EXP EXPIRATION DATE ------­ (818) 793-1675 All orders add 10% SALES ONLY (maximum $10.00) COMPLETE ACCOUNT NUMBER Non-US add an additional $4.00

Officers of The Planetary Society contribute to SIGNATURE the Society the royalties due them as authors Total Order: of the books advertised in these pages.

MAIL ORDER AND PAYMENT TO: SALES DEPT., THE PLANETARY SOCIETY, 65 N. CATALINA AVE., PASADENA, CA 91106 It's now been 20 years since human beings first left their footprints in the lunar soil. Here, Neil Armstrong and Buzz Aldrin erect the flag in the only photo that Apollo 11 returned of the astronauts together on the Moon. Twenty years later, thoughts of the Apollo era evoke the nostalgia we reserve for a cherished memory. But past is prelude, and we focus our yearnings on the future. Photo: Johnson Spaceflight Center

THE PLANETARY SOCIETY NONPROFIT ORG. U.S. POSTAGE 65 North Catalina Avenue PAID Pasadena CA 91106 THE PLANETARY SOCIETY