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2020-03-20T06:04:59+00:00Z Introduction https://ntrs.nasa.gov/search.jsp?R=19890001550 2020-03-20T06:04:59+00:00Z Introduction wo successive journeys will soon outer solar system , toward the ultimate off er a new perspective on the origin goal of understanding the nature of our of the solar system - and perhaps provide origin . To obtain the data needed to new clues to the origin of life as well. address that goal , the CRAF and Cassini Travel ing on a new generation of plane­ missions carry different scientific tary exploration spacecraft, the first of instruments that can be flown on these missions will examine the most spacecraft of the same design primitivebodies of the so lar system , comets an d aste roids. One year later, a CRAF and Cassini are exciting planetary spacecraft nearly identical in design will missions . The objectives that they share, ventu re to the ou ter solar system to study the region of the solar system in which the rich diversity of the Saturn ian system, comets , asteroids , and the Saturnian includi ng its rings and satell ites , the system have evolved and now reside , and surface and atmosphere of its principal the spacecraft that wi II carry both sets of moon , Titan, and the nature of fields and experiments to their targets in the outer particles in Saturn 's magnetosphere. The solar system are described in the pages National Aeronautics and Space that fol low. Adm in istrati on, in coo peration with the Eu ropean Space Agency and the Fede ral Repub lic of Germany, is preparing to em ba rk on these two journeys in 1995 Cover: Voyager photo­ and 1996. polarimeter view of Saturn 's F ring. The mi ssions, the Comet Rendezvo us Inset: Comet West , Asteroid Flyby (CRAF) and Cassini (the 1976_ (Photo by Saturn orbiter(Titan probe), combine to D. di Cicco , Sky & form the fi rst initiative of the Mariner Te/escope) Mark II program, a se ries of planetary Above: The formation of missions whose common objective is to planetesimals in the explore prim itive bodies and the so lar nebula . (Painting by William K. Hartmann) 2 Goals of Solar System Exploration nderstanding the birth and nucleus. Cassini will also fly by one or evolution of our planetary system two asteroids , orbit the planet Saturn , has been a goal of the United States repeatedly fly near many 01 Saturn's civilian space program for the three moons, and send an instrumented probe decades that NASA has been in into the atmosphere 01 Saturn's moon existence . In pursuit of this goal , NASA Titan . has been guided by the Committee on Planetary and Lunar Exploration The explorations to be cond ucted by (COMPLEX) of the National Academy of these two missions are complementary Sciences , whose strategy for planetary because their sub jects - comets , Titan , exploration calls for a three-fold and the Saturn ian system - have a progression from reconnaissance , which common origin in the outer solar is accomplished by fast flybys , to system. These vo latile-rich objects exploration , which is achieved by preserve a unique reco rd of dillerent key orbiters and probes , to in tensive study, phases in the fo rmation and evolution of which is imp lemented by landers and the solar system. Th ey ran ge Irom the samp le return miss ions. COMPLEX most primitive con ditions in interstellar fu rther recommends that this strategy be clouds , to the chao tic and complex applied to each 01 the three regions of processes of nebular col lapse and the solar system: the inner solar system planetary formation (accretion), to the (terrestrial planets), the primitive bodies fina l sweep-up of interplanetary debris. Above: Limb of Saturn 's (comets and asteroids), and the outer moon Titan, showing its solar system (the gas giants) . high-altitude layer of haze. The Mariner Mark II program rep resents Facing page: A region NASA's app lication of this strategy to of star formation in the the primitive bodies and the outer solar Orion Nebula. system. Us ing a new generation 01 Insets: The CRAF cost-ellective, modular spacecralt that spacecraft (top) and the can eas ily be mod if ied to accomplish a Cassini spacecraft variety of missions to these ta rgets , the (bottom). program will focus on critica l questions bearing on the orig in and evolution of our planetary system. The CRAF/Cassini initiative begins this program CRAF will fly by at least one asteroid , orbit a comet , and drive an instrumented penetrator into the comet's April 1989 October 1989 February 1990 Magellar Launch Ga lileo Launch Ga lileo Venus Flyby 1 _---1....-1__ Primitive Bodies and the Outer Solar System: Clues to Origin and Evolution lues pertaining to the origin and evolution of the planets can be found throughout the solar system. During most of the 4.6 billion years since its origin , the solar system has been recovering from the violent, dynamic processes that created it. On Ea rth and on other terrestrial planets, virtually all evidence of the earliest epochs has been destroyed by continuing planetary processes. In contrast, comets, asteroids , and the planetary bodies of the outer solar system have suffe red less of these modifying processes and have thus retained some record of early planetary formation . It is common ly believed that the terrestrial planets and the cores of the outer planets grew through the accretion of countless small bodies called plane­ tes imals, which were sim ilar to presen t­ day asteroids and comets. One group of these original small bodies has been preserved in the asteroid belt, a rela­ tive ly stable region between the orb its of Mars and Jupiter Disturbances by Jupiter's gravity probably prevented the asteroids from accreting into a single body of planetary size , and thus they retain many of their primitive chemical and physical characteristics. One of the most scientifically interesting features of asteroids is their apparent diversity, and understanding the origin of this diversity requires the study of a large number of asteroids. The asteroid flyby investigations carried out by CRAF and August 1990 Magellan Arriva l _ _ at Venus1_- 4 Cassini will build on the information The ring systems around the outer that the Galileo spacecraft will provide planets permit us to study the dynamic when it encounters its two asteroid interactions of small co-orbiting bodies, targets en route to Jupiter. which are similar to the interactions among planetesimals that led to the Just as asteroids and their meteoritic accretion of planets The satellites of fragments may be samples of the the outer planets are cold , frozen worlds building blocks of the inner so lar that record an evol uti on that in some system , so may comets be icy remnants ways paralleled the solar system as a from the accretion of the outer planets. whole. Ranging from the highly According to this view, the newly modified to the nea rly pristine, these formed outer planets gravitationally satellites may contain records of scattered the comets to the most remote modifications frozen on their surfaces part of the solar system - the distant for our inspection Oort cloud , halfway to the nearest stars. There the comets remain , in the deep Another important goal of the CRAF/ freeze of space , until a passing star, a Cassini initiative is to gather evidence giant mo lecular cloud , or the ti dal fo rces concernin g the process of preb ioti c of the ga laxy perturb them onto new molecular evolutio n in the solar system. paths that bring them back into the The issue to be ad d ressed is how the planetary region An alternative chemica l makeup of solar system Above: Comet Mrkos, scenario is that comets are icy plane­ material changed as it evolved from observed in 1957. tesimals that formed in the solar nebula interstellar cloud to solar nebula and (Photo courtesy of Hale Observatories) beyond the orbit of Neptune. In either finally through planetary formation to case , comets have most likely been assume its present form . The molecular Facing page: Voyager preserved in a condition similar to their and mineralogical compositions of view of Saturn 's initial state, relatively free from the mod­ cometary material wi ll yield information atmosphere and inner ifying processes that occurred in larger about the phys ical and chemical rings. bodies and in bodies closer to the Sun . conditions and the ear liest processes of Insets: Mosaic of Viking change in the solar nebula . The dark images of the Martian The giant planets of the outer solar material on comets, on carbo naceous moon Phobos , which system - Jupiter and Saturn in asteroids, and on Saturn's moon Iapetus may be a captured particular - are so massive that they will yield information on surface asteroid (top). Saturn 's have retained essentially all the material processing . moon Titan and its from which they were originally made . opaque atmosphere Scientists are espec ially intrigued by the (bottom). Thus, these planets are expected to contain a representative sample of the possibi lity that prebiotic organ ic original nebular material , altered in chemical evo lution may have taken chemical form but retaining the place in icy coatings of in terstellar dust elemental and isotopic signatures of the grains before these grains were incor­ primordial solar nebula. porated into cometa ry nuclei ; similar October 1990 April 1991 October 1991 Ulysses Launch Magellan End 01 MIssion Ga lileo Gaspra Flyby I I I chemical evolution may still be occurring on Titan . Both cometary ices and Titan 's atmosphere contain many of the same organic molecules observed in interstellar clo uds .
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