https://ntrs.nasa.gov/search.jsp?R=19890001550 2020-03-20T06:04:59+00:00Z Introduction

wo successivejourneys will soon outersolar system , towardthe ultimate offera newperspective on theorigin goalof understandingthe natureof our of thesolar system - andperhaps provide origin. Toobtain the data needed to newclues to theorigin of life aswell. addressthat goal , theCRAF and Cassini Traveling on a newgeneration of plane­ missionscarry different s cientific taryexploration spacecraft , thefirst of instrumentsthat can be flownon thesemissions will examinethe most spacecraftof thesame design primitivebodies of theso lar system, comets and asteroids. Oneyear later , a CRAFand Cassini are exciting planetary spacecraftnearly identical in designwill missions. Theobjectives that they share , venture to theou ter solarsystem to study theregion of thesolar system in which therich diversityof theSaturn ian system , comets, asteroids, andthe Saturnian includingi ts ringsand satell ites, the system haveevolved and now reside , and surfaceand atmosphere of its principal the spacecraftthat wi II carryboth sets of moon, Titan,a nd thenature of fieldsand experiments to theirtargets in theouter particlesin Saturn's magnetosphere.The solar systemare described in thepages NationalAeronautics and Space thatf ollow. Administration, in cooperationw ith the

EuropeanSpace Agency and the F ederal Republic of Germany, is preparingto embark on thesetwo journeys in 1995 Cover: Voyagerphoto­ and1996 . polarimeter view of Saturn's F ring. Themi ssions, the CometR endezvous Inset: CometWest , AsteroidF lyby(CRAF) and Cassini (t he 1976_ (Photob y Saturnorbiter(Tit anprobe), combine to D. di Cicco, Sky & formt hefi rst initiativeof theMar iner Te/escope) MarkII program,a series of planetary Above: The formation of missonswhose common objec ive is i t to planetesimals in the explorep rimitive bodiesand the solar nebula. (Painting by William K. Hartmann) 2

Goalsof Solar System Exploration

nderstandingthe birth and nucleus. Cassiniwill also fly by oneor evolutionof our planetary system two asteroids, orbit the planet Saturn,

hasbeen a goal of the UnitedStates repeatedlyfly nearmany 01 Saturn's civilian spaceprogram for the three moons,and send an instrumentedprobe

decadesthat NASAhas been in into the atmosphere01 Saturn's moon existence. In pursuitof this goal, NASA Titan. hasbeen guided by the Committeeon Planetaryand LunarExploration Theexplorations to be conducted by (COMPLEX)of the NationalAcademy of theset wo missionsare complementary Sciences, whosest rategyfor planetary becausetheir subjects - comets, Titan, explorationcalls for a three-fold andthe Saturnian system- havea progressionfrom reconnaissance, which commonorigin in the outersolar is accomplishedby fastflybys , to system. Thesevo latile-rich objects exploration, whichis achievedby preservea uniquer ecord of dillerent key orbitersand probes, to intensivestudy , phasesin thefo rmationand evolution o f whichis implementedby landersand the solarsystem . They range Irom the sample returnmiss ions. COMPLEX mostprimitive con ditions in interstellar further recommendsthat this strategybe clouds, to the chaotic andcomplex

appliedto each01 t he threeregions of processesof nebularcol lapsea nd the solarsystem : the innersolar system planetaryformation (accretion), to the (terrestrialplanets), the primitivebodies final sweep-upof interplanetarydebris. Above: Limbof Saturn's (cometsand asteroids),and the outer moonTitan, showingits solarsys tem (the gasgiants) . high-altitude layer of haze. TheM arinerM ark II programrep resents Facing page: A region NASA'sapp licationof this strategyto of star formation in the the primitivebodies and the outersolar

Orion Nebula. system.Us ing a newgene ration 01 Insets: The CRAF cost-ellective,modular spacecralt that spacecraft (top) and the caneas ily be modified to accomplisha Cassini spacecraft varietyof missionsto theseta rgets, the (bottom). programwill focuson critical questions bearingon the origin andevolution of our planetarysystem.

TheCRAF /Cassiniinitiative begins this program CRAFwill fly by at leastone asteroid, orbit a comet, anddrive an instrumentedpenetrator into the comet's

April 1989 October1989 February1990 Magellar Launch Galileo Launch Galileo VenusFlyb y 1 _---1....-1__ Primitive Bodiesand the Outer Solar System: Cluesto Origin and Evolution

luespertaining to theorigin andevolution of theplanets can be foundthroughout the solarsystem . Duringmost of the 4.6 billion years sinceits origin, thesolar system has beenrecovering from the violent , dynamicprocesses that created it. On

Earth andon otherterrestrial planets , virtuallyall evidenceof theearliest epochshas been destroyed by continuingplanetary processes. In contrast, comets, asteroids, andthe planetarybodies of theouter solar

systemhave suffe red lessof these modifyingprocesses and have thus retainedsome record of earlyplanetary formation.

It is commonly believedthat the terrestrialplanets and the cores of the outerplanets grew through the accretion of countlesssmall bodies called plane­ tesimals, whichwere sim ilar to present­ dayasteroids and comets. O negroup o f theseoriginal small bodies has been preservedin theasteroid belt , a rela­ tively stableregion between the o rbits of Mars andJu piter Disturbancesby Jupiter'sgravity probably prevented the asteroidsfrom accreting into a single bodyof planetarysize , andthus they retainmany of their primitivechemical andphysical characteristics. One of the mostscientifically interesting features of asteroidsis their apparentdiversity, and understandingthe origin of this diversityrequires the study of a large numberof asteroids.The asteroid flyby investigationscarried out by CRAFand

August1990 Magellan Arrival _ _ at Venus1_- 4

Cassiniwill build on the information Thering systemsa roundthe outer that the Galileospacecraft will provide planetspermit us to studythe dynamic whenit encountersits two asteroid interactionsof small co-orbitingbodies , targetsen routeto Jupiter. whichare similar to the interactions amongplanetesimals that led to the Just as asteroidsand their meteoritic accretionof planets Thesatellites of fragmentsmay be samplesof the the outerplanets a re cold, frozenworlds buildingblocks of the innerso lar that recordan evolu ti on that in some system, so maycomets be icy remnants waysparalleled the solarsys temas a from theaccretion of the outerp lanets. whole. Rangingfrom the highly Accordingto this view, the newly modifiedto the nearly pristine, these formedouter planets gravitationally satellitesmay contain records of scatteredthe cometsto the mostremote modificationsfrozen on their surfaces part of the solar system- the distant for our inspection Oortcloud , halfwayto the neareststars . Therethe comets remain , in the deep Anotherimportant g oal of the CRAF/ freezeof space, until a passingstar, a Cassiniinitiative is to gatherevidence giant molecularcloud , or the tidal forces concerning the processof prebiotic of the galaxy perturbt hemonto new molecular evolution in the solarsystem. pathsthat bring themback into the Theissue to be add ressedis how the planetaryregion An alternative chemical makeupo f solarsystem Above: Comet Mrkos, scenariois thatcomets are icy plane­ materialchanged as it evolvedfrom observedin 1957. tesimalsthat formedin the solar nebula interstellarcloud to solarnebula and (Photo courtesyof Hale beyondthe orbit of eptune In either finally throughplanetary formation to Observatories) N . case, comets havemost likely been assumeits presentf orm. Themolecular

Facing page: Voyager preservedin a conditionsimilar to their andmineralogical compositions of view of Saturn's initial state, relativelyfree from t he mod­ cometarymate rial will yield information atmosphereand inner ifying processesthat occurred in larger aboutthe physical and chemical rings. bodiesand in bodiescloser to the Sun. conditions and the earliest processesof Insets: Mosaic of Viking changein the solar nebula. The dark images of the Martian Thegiant planets of the outersolar materialon comets,on carbonaceous moonPhobos , which system- Jupiterand Saturn in asteroids, and on Saturn's moonI apetus may be a captured particular- areso massivethat they will yield informationon surface asteroid (top). Saturn's haveretained essentially all the material processing. moonTitan and its from whichthey were originally made . opaque atmosphere Thus,these planets are expected to Scientistsare espec ially intriguedby the (bottom). containa representativesample of the possibility that prebioticorgan ic originalnebular material , alteredin chemicalevo lution mayhave t aken chemicalform but retainingthe placein icy coatingsof interstellardust elementaland isotopicsignatures of the grainsbefore these grains were incor­ primordial solar nebula. poratedinto cometary nuclei; similar

October1990 April 1991 October 1991 UlyssesLaunch MagellanEnd 01 MIssion Galileo GaspraF lyby I I I chemicalevolution may still be occurringon Titan . Bothcometary ices andTitan 's atmospherecontain many of the sameorganic molecules observed in interstellarclo uds. Recentresearch suggeststhat Earth 's earlyatmosphere mayhave been inhospitable to the evo­ lution of organicmaterial and that the necessarycomplex , prebioticmolecules mayhave been delivered to Earthfrom the outersolar system by cometsand asteroids. Analternative theory is that the primitiveterrestrial atmosphere may havebeen similar to Titan's present atmosphere. Thus, cluesto understand­ ing the originof life on Earthmay lie in the investigationof both the organic materialfrozen in cometsand the contemporaryorganic synthesis in Titan's atmosphere.

TheCRAF/Cassini initiative also seeks to enhanceour understandingof a wide rangeof astrophysicalprocesses by studyingexamples of thoseprocesses that canbe foundwithin the solar system.One such example is the useof Saturn'srings as a miniatureanalog of collisionaland accretional processes in the solarnebu la . In a similar manner, studiesof theinteraction of solarwind plasmawith cometary gas and dust , and of Saturn's magnetosphericplasma with the moonsand ri ngs, wiII reveal processesthat heavily influenced the chemicaland physical processing of dust,gas , andplanetesimals in the solar nebula. Suchprocesses must also be importantin manyother astrophysical settingsin whichgas , dust, andplasma interactin thepresence of ionizing radiation.

February1992 Ulysses Jupiter Flyby CometRendezvous Asteroid Flyby

Above: CometBennett Spacecraftstudies of the outersolar uring its 1O-year jo urney, photographedover the systemwill also yield informationabout CRAF will explore a cometand at Alps in 1970. (Photoby solarsyste m evolutionan d comparative leastone asteroid . To seeho w a comet C. Nicollier) planetology. The icy satellites extend was formedan d how it comesap art under Inset: The nucleusof the rangeo f conditions and material the influenceof sunlight, CRAFw ill fly comet Halley as seen properties of solid bodies for compara­ along witha typical short-period comet as from the EuropeanSpace tive studiesof geological processes it movesaro und theSun f romits farthest Agency's Giotto space­ Themagne osphees andatmospheres craft. (@Max-Planck­ t r to its closestpoint CRAF will determine Institut fur Aeronomie, of the giant planetssuch as Jupiterand theelemental, isoto pic, molecular, and 1986. CourtesyDr . H. U. Saturn, and theatmospheres and mineralogical compositionof cometary Keller) perhapsoceans on moonssuch as materialas well as i nvestigatethe comet's Saturn's Titanand N eptune's Triton, microscopiCto large-scalephysical providepoints of comparisonwith the structure. Further, CRAFwi ll studythe terrestrial planets The unifyingtheme physicsand chemis try of thecomet's for exploring primitive bodies and outer atmosphereand its interactionwith sun ­ planets is the study of our origin and light and thesolar w ind. Finally, CRAF evolution, includingthe analysisof will surveythe phys ical propertiesof one possible prebiotic organicchemistry or twoasteroids , includingtheir s izes, in this frozen, volatile-rich partof the shapes, rotationperi ods and poles , and solar system. surfacem orphologies, andi t

August 1993 September1992 Mars Observer August1993 MarsObserver Launch Arrivalal Mars Galileo IdaF lyby will gatherpreliminary information Therendezvous mission deSigned for closeencounter with Jupiterin 1974. about theirchemical compositions , in CRAFrepresents a substantialadvance Wild 2 is a desirabletarget because its addition to measuringthe asteroids' beyondthe ultrafastHalley flybys. In a interiorprobably has not yet reached bulk densities. rendezvousmission, a spacecraft thermalequilibrium with its newsur­ followsan orbit aroundthe Sunthat roundings, andso it haslower internal The Halleymissions gave us a fi rst preciselymatches the comet'sorbit. temperaturesthan other short-period quickglimpse of a cometnucleus and Hence, aftera rendezvousmaneuver , cometsin similarorbits . the first in situ measurementsof thespacecraft and the comettravel cometarygas and dust. In many togetherindefinitely. Amongthe many To reachWild 2, thes pacecraft will be respects, however,the Halleyflybys significantadvantages of a rendezvous launchedon an orbit that brings it back raisedmore questions than they missionover a flyby areits ability to to Earthafter two years for a gravity answered. We learned, for example, producemore detai led mapping, its assistthat w ill thenfling it out toward that cometscontai n a largeamount of longerobservation times and lower the orbit of Jupiter Alongthe journey organicmaterial , but we wereunable to relativespeeds, its useof a penetrator, the spacecraftw ill encounterone of the determineprecisely which organic andits capacityfor adaptiveexploration . largermain belt asteroids, Eunomia, moleculeswere present. We learned , which hasa diameterof 270 kilometers. too, thatthe n ucleusof a cometis a Thetarget for CRAF's rendezvousis the Afterthis asteroidencounter , and dark, irregularlyshaped body , but we short-periodcomet Wild 2, whichwas perhapsone other depending on couldnot determineits densityor the perturbedinto its presentorbit from availablefuel , thes pacecraftwill arrive physicalca use(s)of its irregularshape . fartherout in the solarsystem by a at Wild 2 nearaphelion wher e the comet is relativelyinactive.

October1993 August1994 Mars Observer Ulysses Over April 1995 Mapping Orbit Sun'sSouth Pole CRAFLaunch I From a distanceof 50 kilometers, the spacecraftwill use remote-sensing instrumentsto studythe comet. Imagingat a resolutionof betterthan onemeter will provide detailsabout the size, shape, andsu rfacemorphology of the cometnucleus . By analyzingthe spectrum of reflected sunlight, we will be ableto identifyices andm ineralson the comet'ssurface . Temperatureswill be mappedto determinethermal propertiesand energy ba lanceas well as to aid in the identificationof surface materials. Accurateradio-tracking data will be usedto determinethe massof the cometnucleus . Combinedwith the size,as estimatedfr om the imaging data,this information will allowthe calculationof the comet'sbulk density. Takentogether, these measurements will enablescientists to establishwhe ther the interior of a cometnucleus more closelyresembles a hard-packed snowballor a newlyfallen , fluffy bankof snow. This will thentell us howthe cometnucleus accreted and what processing it might haveunde rgone sinceits formation.

Abouta yearafter the spacecraft'sarrival at the comet, a penetratorcarrying a set of instrumentswill be fired directly into the surfaceof thecomet nucleus to 9

measurethe elemental composition of A completeset of plasmainstruments the nucleusand to determineits wiII studythe interactionsof cometary temperatureprofile andsurface strength , gasand dust with sunlightand with the as well as the natureand composition of solarwind in effortsto clarify the the icy organicmix . Thepenetrator will formationand constantly changing transmitdata back to the CRAForbiter shapesof comettails, the acceleration for aboutone week . of energeticparticles , andother plasma processes Followingperihelion , a Whenthe cometnears the Sunand 50,OOO-kilometerexcursion down the becomesactive , the spacecraftwill move comet'stail will explorethe comet-solar in and out throughthe atmosphere , wind interactionin furtherdetail. collectingdust for onboardanalysis of the elemental, chemical,and micro­ Followingthe tail excursion, the CRAF scopicprope rties of the dust. Emitted spacecraftwill returnto the vicinity of gaseswi II beanalyzed for their the nucleusas its activity wanes.As the composition andtemperatures and for cometonce again begins to journey the velocity at whicht hey flow out. The outwardfrom the Sun, CRAFwill cometaryatmosphere and nucleuswill concludethe missionby studyingthe be studiedas Wild 2 movesa long the changestha t the nucleushas undergone eccentric orbit, reachingmaximum duringperihel ion. activityat perihelion Facing Page: Cross section of the instrumentedpenet rator for the CRAFmission , implanted in the nucleus of comet Wild 2. ---" ( Wifd 2 Orbit Spacecraft Maneuver (­ Mar 1999 Trajectory Eunomia Above: Magnetic field ". Flyby vectors, measuredby Oct 1997 Endof Mission the ICE spacecraft in the tail of comet Giacobini­ ~\oeC:i:3: Zinner, superimposedon launch _ _ Excursion an image obtained at the Apr 1995

,\ Canada-France-Hawaii I Perihelion _ Sept 2003 Telescope. (Courtesyof B. Goldbergand J. A. Earth Flyby / Feb 1997 Slavin)

... Maneuver Apr 1996 Left: The trajectory and Perihelion Phase missionphases of CRAF.

Penetrator Delivery June 2002

Near-Nucleus Science

September 1995 September 1995 Mars Observer December 1995 Ulysses End 01M ission End 01M ission GafileoAr rival al Jupiler I Cassini

Above: Saturn's rings. assiniwill explorethe Saturnian Cassiniwill useon board radarto peer Inset: Artist's concept system, whichcontains a hosto f throughTitan 's clouds andd etermine if of the Cassiniprobe volatile-rich bodieswith a recordof the there is liquid on the surface.E xperi­ entering the atmosphere processesthat have modifiedthese mentson botht he orbiter andthe entry of Titan. (Paintingby M. bodies. Themission will becomposed probewill investigatethe chemical Carroll) of a Saturnorbiter spacecraft built by processesthat producethis unique NASAand a detachableTitan entry atmosphere. Facingpage: The probesupplied by the EuropeanSpace interplanetarytrajectory Agency. TheSaturn orbiter will deliver As the probedescends to Titan's surface of the Cassini spacecraft. theprobe to Titanand will makeclose by parachute, it will beginmeasure­ Ilybys01 Titan on everyorbit to allow mentsjust abovethe cloudtops . The intensivestudy o f this mostunusua l probewill carrya well-equipped robotic moonof Saturn. laboratory with whichto makechem ical analysesof Titan'satmosphere and Oneof the mostintriguing aspects of clouds. Otherinstru ments on the probe Titanis the possibility that its surface will measurethe temperature, pressure, maybe coveredwith oceansor lakesof density. andenergy balance in the liquid hydrocarbonsthat result from atmosphereall the way to the surface. photochemicalprocesses in Titan's Propertiesof the surfacewill be upperatmosphere . Thesehydrocarbons measuredremotely , anda camerawill condenseto form a globalsmog layer takepictures of the Titanpanorama as andeventually rain downt o the surface. the probebreaks thr ought he cloud

April 1996 June 1996 February1997 CassIn!Launch Galileo Ganymede Flyby CRAFEarth Flyb y

1_ 1 1 deck. Theprobe may survive the landing,in whichcase it will relay measurementsfrom the surfacebefore the orbiterflies beyondthe horizon.

Throughthe c louds,the orbiter's radar will takepictures of Titan's surface, Maneuver Jan 1998 mappingthe majority of it to a resolutionof severalkilometers and portionsof it at 2DD-meterresolution . \ Theorbiter will alsocarry remote­ sensinginstruments to measurethe J compositionand structure of Titan's atmosphereon a globalscale. The " Mala Flyby orbiterwill comeso closeto Titanon Mar 1997

manypasses that it will actuallyfly Maneuver Feb1997 throughthe upper atmosphere. At those times, severalaeronomy instruments will makein situ measurementsof atmosphericcomposition

March 1997 October1997 October1997 CassiniMa ia Flyby Galileo End01 Miss ion CRAFEun omlaF lyby Anotherof Saturn's moons, Iapetus, has onehemisphere that is six timesdarker thanthe other; manyscientists suspect that this verydark material is organic in nature. Howthis particularmoon , with its darkmaterial and its hemispheric albedoasymmetry , comesto be among the othervery bright , icy moonsin the Saturnian systemremains a mystery. Cassiniwill examinethe darkmaterial on Iapetusto determinethis material's relationshipboth to organicmate rial on Titanand to darkmaterial on comets andasteroids .

Theorbiter will also passwithin a few hundredki lometersof severalof Saturn's othermoons . The orbiter cameraswill takepictu res of the intensely crateredsurfaces of those icy moons, the radarwill maptheir topography, andthe spectroscopic instrumentswill determinetheir compositions.

Besidesthe largemoons , Saturn's equatorialp lanecontains many smaller objectsand considerable dust, including the magnificentring systemclose to the planet Cassiniwill examinein detail the dynamicand en igmatic rings of Saturn, whichwere explored only briefly by earlierflybys No otherplanet in the solar systemcontains as muchorbita l debrisas Saturn, whereseveral moons canoccupy virtual ly thesame orbit Cassiniwill surveyt he small bodies orbitingSaturn and will searchfor

June 1998 Cassini EarthFly by 13

moonlets within the rings. Cassiniwill examples of thesephenomena in the also studythe rings in fine detail to solar system. Themagnetospheric unravelthe mysteries of their structure, interactionsw ith dust and moonlets in dynamics, composition, andfo rmation. the ring planeprovide information about someof the processesinvolving the The structureand composition of interactionsof plasma, dust, and Saturn's atmospherewi ll also be radiationthat were important at the examined. Sensitiveinstrumen ts on the beginningof the solar system, whent he Cassinio rbiter will makeremote planetswere formed . chemical andphysical measuremen ts deepinto theatmosphe re of Saturn. Finally , omtof the benefitsof the The informationthus derived will then Cassin i trajectoryfr om Earthto Saturn be comparedw ith the Jupiter data is that it takesthe spacecraftthrough the obtained by the Galileo mission. The asteroidbelt. Galileoand CRAFwill resulting comparatives tudies of these also visit asteroids, and a complemen­ two giant planetsw ill contribute far tary type will be chosenfor Cassini. more to our understanding of planetary Thus, in the courseo f theset hreegrea t origin and evolution than could be explorations, several asteroids, each gained through the study of either planet with different surfacecompositions, w ill alone. be studied independently

Facingpage : Artist's The interactions of Saturn's magneto­ conceptof Saturn, the sphere with the rings and with Titan's Cassini spacecraft atmospherea re the most complex (background), and the probe on its way into the atmosphere of Titan. Insets: Saturn's moon Iapetus (top), showing its light and dark hemispheres. Saturn's moonEnceladus (bottom)with its plains and heavily cratered terrain.

Above: A stormin Saturn'supper clouds.

Left: The Cassini trajectory after arrival at Saturn, passingTitan on each orbit.

March 1999 CRAFD eep Space February 2000 July 2001 Maneuver CassiniJ upilerF lyby CRAFArri val alW ild 2 I 14

MarinerMark II Spacecraft

he MarinerMark II spacecraft hasbeen designed specifically to carryo ut deepspace missions to comets, asteroids, and the outerplanets . Theyw ill thereforebe equippedto meet a multiplicity of demandsimposed by thenature of thesemissions , including theneed to provideelec trical powerat greatdistances from the Sun, to transmitlarge amounts of datato Earth overequally vast distances , andto providehighly accurate pointing for the preciseaiming and/or delivery of penetrators, probes,and remote-sensing instruments, as well as for the proper orientationof high-gainantennas . Of equalsignificance is the needfor a high degreeof autonomousoperation and reliability.

At the sametime , the cost of the missionsmust be minimizedw ithout compromisingquality . TheMariner MarkI I programwill meetthis challenge by makingthe spacecraftfor its various missionsas nearly uniformin designas possible. For CRAFand Cassini , each spacecraftwill be designedto carry either set of instrumentsas well as to powerthese instruments , point them, andstore and transmit their datawith the fewestpossible changes in spacecraftdesign . Wheredetailed requirementsdiffer , the final designwill be drivenby the requirementsof the moredemanding m ission. Thesize of

June 2002 CRAF Penelralor Delivery I 15

the propellanttanks , for example, will be Thespacecraft will fly with their high­ determinedby CRAF'srequirements ; gainantennas pOinted toward Earth , On Cassiniwil l usethe same tanks , but CRAF, two instrument-ladenplatforms theywill not be completelyfil led with will keepthe telescopes and other fuel, Asanother example , the sizeof the sensorstrained on the targetunder antennawill be definedby the distance study, Cassiniwill useone of the CRAF betweenSaturn and Earth ; a smaller platforms, but the otherwill be replaced antennawould be sufficientfor the by a turntablethat will continuously CRAFmission , Thecost of over­ mapthe dust, plasma,and energetic designingfor the missionwith the less particlesarriving from all partsof the stringentrequirements will be morethan sky, compensatedby savingsderived from the design, construction, testing, and Thespacecraft are bei ng designedand operation of identicalsubsystems on built by NASA, theT itan probeby the bothmissions , EuropeanSpace Age ncy, and the CRAF propulsionsubsystem by the Federal Computer-generateddrawings of thetwo Republicof Germany, Scientific spacecraft, completewith their scientific instrumentswill be suppliedby all of instruments, areshown at left. The the internationalpartners , Boththe principalcommon features are the CRAFand Cassini missions will be centralten-sided spacec raft bus that launchedon theexpe ndable Facingpage: The CRAF holdsmost of the electronics, the large TitanIV/Centaur G ': CRAFin 1995 spacecraft(top) with the propulsion subsystem(f uel tanks,roc ket and Cassini in 1996, penetratorin the foreground. engi and tru e) belowthe us ne, s ctur b , The Cassinispacecraft the high-gainan tenna andra dio feeds (bottom)immediately after abovethe bus, thetwo nuclear-powered release of the probe. radioisotopethermoelectric generators on a boombehind the bus, andthe Above: CometHalley as boomson whichexperiments are photographedfrom the mounted, In the foreground, the figures UKSchmidt T elescopein Australia. (© Royal showCRAF 's penetratorand Cassini 's Observatory, Edinburgh) Titanprobe at the samelocation on each of theseversatile multimission spacecraft.

January2003 October2002 Cassini Tilan September2003 CassiniArr ival at Saturn Probe Delivery CometW ild 2 at Perihelion 16

Summary

hatwe will learnfrom the To accomplishthese miss ions, an CRAF/Cassiniinitiative will exceptionalspacecraft has been greatlyenhance what we havealready designedthat canaccommodate a wide discoveredabout the outersolar system varietyof targetsand scientific throughground-based observations and instrumentswith only minormodifica­ throughour previousmissions - tions in thespacecraft design from one Pioneer10 and 11 andthe Voyager missiont o the next. FutureMariner encounters of Jupiter, Saturn, Uranus, Mark II missionsmay r eturna sample and, soon, Neptune. CRAF will help us froma comet nucleusfo r laboratory explainthe intriguingg limpsesinto the analysison Earth; a spacecraftmay be mysteriesof a cometnucleus first sentto Uranusor Neptune to learnmore offeredby the internationalflotilla of aboutthose distant targets than Voyager spacecraftthat flew by Halley's comet. wasable to tell us; and a spacecraftmay Cassiniwill providea thoroughinvesti­ rendezvouswith one of the largermain gationof the Saturnian systemthat belt asteroids, study it in detailfor complementsthe Galileoinvestigation severalmon ths, and thenmove on to of Jupiter. Our strategyfor an orderly examinea secondasteroid of a different progressionof exploration throught he composition Endless possibilities exist solarsystem leads us to thesenext to satisfyour needf or knowledge and logicalta rgets the primitivebodies and understandingand to unravel the clues, Saturn. hiddenfor billions of years, that the Artist's view of the solar outersolar system holds to the nebula before the With the implementationof the CRAF mysteriesof our origin and our planets formed. andCassini missions , NASAwill fulfill evolution (Painting by Don Davis. . @)1985Time-life Books a substantialpart of our nation's Inc. All rights reserved. commitmentto the explorationof the Reprinted by permission solar systemW e will haveo rbiters at from Time-Life Books Venus(Magel lan), Mars (Mars Inc.) Observer), Jupiter (Galileo), Saturn (Cassini), and a comet (CRAF), and probeswill havebeen delivered into the nucleusof a comet(CRA F), Jupiter's atmosphere(Galileo), and Titan's atmosphere(Cassini) At the same time, Galileo, CRAF, and Cassiniwill havebegun the reconnaissanceof asteroids, with multipleflybys of asteroidsof differentcompositional types.

February2006 December2003 CassIn!Hig h September2006 CRAFEnd 01 M ISSion InclinationOrb its CassiniE nd 01M ission