irradiancepp43.com

UCLAUCLAPreviouslyPublishedWorksTitleDawn:AjourneyinspaceandtimePermalinkhttps://escholarship.
org/uc/item/932108swJournalPlanetaryandSpaceScience,52(5-6)ISSN0032-0633AuthorsRussell,C.
T.
Coradini,AChristensen,Uetal.
PublicationDate2004-04-01PeerreviewedeScholarship.
orgPoweredbytheCaliforniaDigitalLibraryUniversityofCalifornia1Dawn:AJourneyInSpaceAndTimeC.
T.
Russell(a),A.
Coradini(b),U.
Christensen(c),M.
C.
DeSanctis(d),W.
C.
Feldman(e),R.
Jaumann(f),H.
U.
Keller(c),A.
S.
Konopliv(g),T.
B.
McCord(h),L.
A.
McFadden(i),H.
Y.
McSween(j),S.
Mottola(f),G.
Neukum(k),C.
M.
Pieters(l),T.
H.
Prettyman(e),C.
A.
Raymond(g),D.
E.
Smith(m),M.
V.
Sykes(n),B.
G.
Williams(g),J.
Wise(o),andM.
T.
Zuber(p).
aIGPP&ESS,UCLA,LosAngeles,CA90095-1567;ctrussell@igpp.
ucla.
edubIFSI,ViadelfossodelCavaliere,00133ROMAItalycMPAe,Katlenburg-Lindau,GermanydIAFS,ViadelfossodelCavaliere,00133ROMAItalyeLANL,MSD466,NIS-1,LosAlamos,NM87545fDLRRutherfordstr2,D-12489Berlin,GermanygJPL,4800OakGroveDr.
,Pasadena,CA91109hUniversityofHawaii,2525CorreaRd.
,Honolulu,HI96822iUniversityofMaryland,CollegePark,MD20742jUniversityofTennessee,Knoxville,TN37996-1410kFreieUniversitat,Berlin,MalteserStr.
74-100,12249Berlin,GermanylBrownUniversity,Providence,RI02912mGSFC,MC920,Greenbelt,MD20771nUofArizona,Tucson,AZ85721oNewRoadsSchool,SantaMonica,CA90404pMIT,Cambridge,MA02139Addresstowhichproofstobesent:C.
T.
RussellIGPP/UCLA405HilgardAveLosAngeles,CA90095-1567Offprintrequeststo:C.
T.
Russelle-mailofcorrespondingauthor:ctrussell@igpp.
ucla.
edu9/18/20032ABSTRACTBysuccessivelyorbitingboth4Vestaand1CerestheDawnmissiondirectlyaddressesthelong-standinggoalsofunderstandingtheoriginandevolutionofthesolarsystem.
CeresandVestaaretwocomplementaryterrestrialprotoplanets(oneapparently"wet"andtheother"dry"),whoseaccretionwasprobablyterminatedbytheformationofJupiter.
Theyprovideabridgeinourunderstandingbetweentherockybodiesoftheinnersolarsystemandtheicybodiesoftheoutersolarsystem.
CeresappearstobeundifferentiatedwhileVestahasexperiencedsignificantheatingandlikelydifferentiation.
Bothformedveryearlyinthehistoryofthesolarsystemandwhilesufferingmanyimpactshaveremainedintact,therebyretainingarecordofeventsandprocessesfromthetimeofplanetformation.
Detailedstudyofthegeophysicsandgeochemistryofthesetwobodiesprovidescriticalbenchmarksforearlysolarsystemconditionsandprocessesthatshapeditssubsequentevolution.
Dawnprovidesthemissingcontextforbothprimitiveandevolvedmeteoriticdata,thusplayingacentralroleinunderstandingterrestrialplanetformationandtheevolutionoftheasteroidbelt.
DawnistobelaunchedinMay2006arrivingatVestain2010andCeresin2014,stoppingateachtomake11monthsoforbitalmeasurements.
Thespacecraftusessolarelectricpropulsion,bothincruiseandinorbit,tomakemostefficientuseofitsxenonpropellant.
Thespacecraftcarriesaframingcamera,visibleandinfraredmappingspectrometer,gammaray/neutronspectrometer,magnetometer,andradioscience.
31.
INTRODUCTIONTheDawnmissioninvestigatestwoofthefirstbodiesformedinthesolarsystem.
1Ceresand4Vestaarecomplementaryprotoplanetsthathaveremainedintactsincetheirformation.
Ceresapparentlyincorporatedwatericeduringaccretion,slowingitsthermalevolutionwhileVesta,smallerandclosertotheSun,apparentlymeltedanddifferentiated.
Dawn'sgoalistoorbitbothasteroidstoobtainmeasurementsthatprovideanunderstandingoftheconditionsandprocessesactingatthesolarsystem'searliestepoch.
TodothisDawninvestigatestheirinternalstructure,densityandhomogeneitybymeasuringtheirmass,shape,volumeandspinstatewithradiometrictracking,andimagery.
Itrecordstheirremanentmagnetization,andelementalandmineralcompositiontoinfertheirthermalhistoryandevolution.
ItprovidescontextformeteoritesthatarebelievedtohavecomefromVesta.
Dawnprovidesimagesofthesurfacesofthesetwoobjectstodeterminetheirbombardment,tectonicandpossiblyvolcanichistory;itusesgravity,spinstateandmagneticdataplaceconstraintsonthesizeofanymetalliccore,anditemploysIR,gamma-rayandneutronspectrometrytosearchforwater-bearingminerals.
DawnfocusesonCeresandVesta,notsimplybecausetheyarethetwolargestrockyplanetsthatremainunexploredbutbecausetheyshouldprovideimportantcluestotheprocessestakingplaceintheearliestphaseofsolarsystemformation.
Inaddition,theyformabridgeinourunderstanding,fromtherockybodiesoftheinnersolarsystemtotheicybodiesoftheoutersolarsystem.
Radioisotopechronologyfromthehowardite,eucrite,anddiogenite(HED)meteorites,believedtobefromVesta,suggestsitdifferentiatedinperhapsonly3millionyears[Yinetal.
,2002;Kleineetal.
,2002].
SimilarevidenceindicatesthatMarscontinuedtodifferentiateforcloseto15millionandEarthfor30millionyears.
TheearlycessationofaccretionintheasteroidbeltwaspresumablyduetotheformationofJupiterwhosegravitationalforcingcounteredtheaccretionaryprocess,andtodayiscausingthedisruptionofthebodiesthatdidaccrete.
AlthoughwedonothavesimilarmeteoriteevidencedirectlylinkedtoCeres,ittooisexpectedtohaveformedinthefirstapproximately10millionyears.
Inadditiontheasteroidbeltmayhavebeenscouredbycomets,scatteredbytheformationoftheremaininggasgiants[Gil-HuttonandBrunini,1999].
Todayonlysomeofthelargestasteroidsremainrelativelyundisrupted.
Themostmassiveofthese[Hilton,1999]areCeresandVesta,twomostcomplementaryminorplanets.
Theformerhasaveryprimitivesurface,water-bearingminerals,andpossiblyaveryweakatmosphereandpolarcap.
Thelatterisadry,differentiatedbodywhoseexteriorhasbeenresurfacedbybasalticlavaflowspossiblypossessinganearlymagmaoceanliketheMoon.
Vestahasexperiencedsignificantexcavatingevents,mostnotablyindicatedbythehugecraternearitssouthernpole[Thomasetal.
,1997a].
CosmicrayexposuredatingofHEDsindicatesthatimpactshaveproducedmeteoriticmaterialatleastfivetimesinthelast50millionyears[EugsterandMichel,1995].
Theseimpactsmayhaveoccurredonvestoids,piecesofVestareleasedatearlytimes.
ThemeteoritesthathavereachedtheEarthhavebeenusedtopiecetogetheramostprobablescenarioforVesta'sthermalevolution[e.
g.
GhoshandMcSween,1998].
4NometeoriteshaveunmistakablycomefromCeres.
PossiblytheexcavatingeventsorinterplanetarydynamicsthatprovidedtheHEDmeteoritesdidnotoccuratCeres,butalso,thereflectancespectrumofthesurfaceofCeresdoesnotgiveuniquesignaturesofitscrustalrocks.
MicrowavestudiessuggestthatCeresiscoveredwithadryclay,incontrasttoVesta'sbasalticdustlayerthatreflectsitscrustalcomposition[WebsterandJohnston,1989].
TodetermineifwehaveCeres-derivedmeteoritesandtounderstandCeres'origin,itisnecessarytogothereandobtainspectrainsidefreshcraters.
Meteoritesprovideanincompleteglimpseoftheirparentbodies.
TounderstandthethermalevolutionofVestaandCeresaknowledgeoftheirinteriorstructureisrequired,asisanunderstandingoftheirgeologicalandgeophysicalrecord.
ItisimportanttodeterminethegeologiccontextfortheHEDmeteoritesfromVesta,andsearchforsimilardataforCeres.
Weareespeciallyinterestedincontrastingdry,differentiatedVestawithitswetcounterpart,Ceres,justalittlefurtherfromtheSun.
ItappearsthatarathershortadditionalradialseparationallowedCerestoaccretewetandstaycoolwhileearlyheatsources(26Al)intheaccretingmaterialmeltedVesta.
Mostimportantly,becausetheybothlieneartheeclipticplaneinnear-circularorbits,itispossibletorendezvouswithandstudybothusingasingleDiscoverymission.
Whiletherehavebeenthreepreviousasteroidflybysandonepreviousasteroidrendezvous,noneofthesemissionshavebeenatallcomparabletoDawn.
Galileohasflownbytwosmallasteroids,theS-types,GaspraandIda,obtainingvisibleimagery.
In1997NEARflewbytheC-classasteroid,253Mathilde,obtainingimagesandderivingitsmass.
In2000,NEARenteredorbitaboutthe30-kmlong,S-typeasteroid433EroswithapayloadsimilartothatofDawnbuttheretheresemblancestops.
Erosisaveryhomogeneousbody,likelyafragmentofalargerbody.
Vestashowssignsofametalliccore,asimpliedfromstudiesoftheHEDmeteorites,aMars-likedensityandlunar-likebasalticflows,soMarsandlunardatacomparisonsmaybemorerelevantthanthosewithEros.
FurthertheactionofwateronthesurfaceofMarswillbecomparedandcontrastedwiththepossibleeffectsofwateronCeres.
InadditionVoyager,Galileo,andCassiniprovidecontrastingdataonthestructureofwater-richsmallbodiesintheoutersolarsystem.
Whileourfirstclose-updatafromasteroidswasobtainedonlyslightlyoveradecadeago,remotesensingdatahavebeenobtainedforover200years.
Inthelate18thcenturyitwasrecognizedthattheplanetswerespacedinaregularmanneraccordingtoaformulanowcalledtheTitius-BodeLaw.
BaronvonZach,aGermanastronomer,believedthatthislawpredictedaplanetbetweenMarsandJupiterandinitiatedthefirstinternationalsciencecampaigntodiscoveritslocation.
However,anItalianobserver,notpartofthesearchteam,GiuseppePiazzi,foundCeresattheexpecteddistancefromtheSunonJanuary1,1801.
ItwaswithsomesurprisethatasecondobjectPallas,wasdiscoveredduringthecourseofmonitoringCeres.
Pallas'discoverywasfollowedby3Juno.
Later,onMarch29,1807amemberofVonZach'steam,theGermanastronomer,H.
Olbers,discovered4Vesta.
Becauseoftheirsmallsizeandlargedistanceeventodaywehavelittleinformationonthesetwobodies.
TheHubbleSpaceTelescopeenablesustoresolveonlytheverylargestscalefeaturesontheirsurfacesasshowninFigure1,forVesta.
Todeterminethedetailedelementalandmineralcomposition,the5tectonicandthermalevolution,theinternalstructure,andthepossiblepresenceofmagneticfieldsandametalliccorewemusttraveltothesebodies.
WhilethescientificcommunityhaslongrecommendedmissionstoVestaand/orCeres,theDeepSpace1mission'sdemonstrationofthesolarelectricionthrustershasonlynowenabledamissiontovisitthesebodiessuccessively,orbitingeach,andfittingwithintheDiscoverycostenvelope.
Oursciencestrategyandinstrumentsareoptimizedforthismissionaswebuildonextensiveground-basedandHST-basedstudiesofbothobjectsandstudiesoftheirmeteoriteanalogs.
2.
CURRENTUNDERSTANDING4Vestaand1Ceresareamongthemostmassiveasteroids.
BothorbittheSunandarelargeenoughtohaveexperiencedmanyoftheprocessesnormallyassociatedwithplanetaryevolution.
Theycouldrightlybeconsideredsmallplanetsbut,becauseoftheirlocationswithintheasteroidbelt,theyhavebeenclassifiedasasteroids.
Unlikemostotherobjectsinthemainbelt,whicharebatteredrelictsoflargerbodies,VestaandCereshavesomehowsurvivedintactthrough4.
5billionyearsofcollisionalhistorythatmusthaveseenthedestructionofmostoftheirneighbors.
Theseprotoplanetscarryretrievablerecordsofphysicalandchemicalconditionsandnebularandgeologicprocessesduringtheearlyplanet-formingepoch.
PlanetesimalslikeVestaandCeresformedbytheaccretionofsmallerobjectsovershorttimescales.
AccretioninthemainbeltapparentlywasterminatedbeforetheformationofplanetaryobjectslargerthanCeresandVesta,presumablyduetoJupiter,whosegravitationalforcingcounteredtheaccretionaryprocess.
Aswediscussinmoredetailbelow,CereshasamuchlowerdensitythandoesVesta(about2100kg/m3vsnearly4000kg/m3,respectively)andCeresisinferredtobevolatilerich,whileVestaisdry.
Yettheseverydifferentobjectswereapparentlyformedrelativelyclosetogetherinthesolarsystem.
ItisoneofDawn'smainscienceobjectivestodeterminehowandwhy.
Beforedescribingthemissionwepresentbelowabriefsummaryofwhatweunderstandabouteachobjectbeginningwiththeobjectwevisitfirst,Vesta.
Forthosedesiringevenmoredetailsonourcurrentunderstanding,Keil[2002]hasprovidedarecentcomprehensivereviewofwhatispresentlyknownaboutVesta.
McCordandSotin[2003]haveprovidedarecentcomprehensivereviewofwhatispresentlyknownaboutCeresanditspossiblethermalevolutiontracks.
SeveralotherrecentpublicationsalsogiveusefulsummariesofthegeneralknowledgeofCeresanddiscussandimproveontheknowledgeofitsprincipalcharacteristics[Parkeretal.
,2002;Brittetal.
,2002].
64VestaVestahasatriaxialellipsoidshapewithradiiof289,280,and229±5km,basedonHSTimagery[Thomasetal.
,1997b].
Itsmeanradiusisthus258±12km,equivalenttoavolumeof7.
19±0.
87x107km3.
Vesta'smasshasbeenestimatedat1.
38±0.
12x10-10solarmasses[SchubartandMatson,1979],1.
5±0.
3x10-10solarmasses[StandishandHellings,1989],and1.
36±0.
05x10-10solarmasses[Michalaketal.
,2000].
Thedensitiescalculatedfromthesevaluesrangefrom3200±5050kg/m3.
Vestaisadry,differentiatedbodywhosesurfacehasbeencoveredbypyroxene-bearingbasalticlavaswithcompositionlikeHEDmeteorites[McCordetal.
,1970].
Absorptionbandparametersrevealmineralogicalvariationsastheasteroidrotates.
AsseeninbothEarth-based[Gaffey,1997;CochranandVilas,1998]andHST[Binzeletal.
,1997]spectra,thesurfaceofVestacontainsabundantpyroxenes–Mg-richandCa-poorpyroxenesintheeasternhemisphere,andFe-richandCa-richpyroxenesinthewesternhemisphere.
Thesevariations,inferredtoreflectimpact-excavatedplutonicrocksintheeastandlavaflowsinthewest,havenotbeenpetrologicallyhomogenizedbyregolithformationsorspectrallyobscuredbyspaceweathering.
ThelackofspaceweatheringhasbeenattributedbyHiroietal.
,[1994]toascarcityofolivine,whichmaybetheprincipalmineralalteredbythisprocess[Yamadaetal.
,1999].
However,thesurfaceofthemoonisweatheredanditlacksolivinealso.
Whetherthecrustformedbyserialmagmatismorsolidificationofamagmaoceanisunclear.
Vestahasexperiencedsignificantimpactevents,oneofwhichexcavatedahuge(460kmdiameter)craternearitssouthpole[Thomasetal.
,1997a].
Spectralvariationswithinthislargecrater[Thomasetal.
,1997b]demonstratecompositionalstratigraphy,probablyreflectingamantleand/orlowercrustenrichedinolivinerelativetosurficialflows.
Thelargest(100meteorites)classofachondrites,thehowardite-eucrite-diogenite(HED)association,iscommonlybelievedtohavebeenderivedfromVesta[ConsolmagnoandDrake,1977].
ThespectralcharacteristicsofVestaconformtothoseofHEDs,analmostuniquematch(onlyoneotherbasalticasteroid,1459Magnya,hasbeenidentifiedinthemainbelt[Lazzaroetal.
,2000],anditisnotlocatedinapositionthatshouldbeeasilysampled).
Withthediscoveryofsmall,impact-ejected"Vestoids"spanningthegapbetweenVesta'sorbit[BinzelandXu,1993]andthe3/1meanmotionandν6secularresonanceswithJupiterwhichserveasescapehatches[Wisdom,1985],thehypothesisthatVestaistheHEDparentbodyhasbecomewidelyaccepted.
Vestoidsexhibitspectrasimilartothoseofeucritesandhowardites[Burbineetal.
,2001],althoughsubtlespectraldifferencesexist[Vilasetal.
,2000].
Thesemighthavebeenejectedduringformationofthesouthpolecrater,whichexcavated~1%ofVesta.
However,cosmic-rayexposureagesofHEDsformclusters,suggestingtheywereproducedpossiblyfromvestoids,duringseveralimpactevents[EugsterandMichel,1995;Weltenetal.
,1997].
TheHEDachondritescrystallizedunderdry,reducingconditionsveryearlyinsolarsystemhistory.
Membersofthisgroupdefineauniqueoxygenisotopemassfractionationlinedisplacedbelowtheterrestrialline[ClaytonandMayeda,1996]andexhibitdistinctive7pyroxene(iron/manganese)andplagioclase(potassium/calcium)compositions[Papike,1998].
Cumulateandnoncumulateeucritesareiron-richgabbrosandbasaltscomposedmostlyofpigeoniteandcalcicplagioclase.
Diogenitesarecumulateorthopyroxenites,sometimeswithminorolivine,andhowarditesareregolithbrecciascomposedoffragmentsofeucritesanddiogenites.
Attemptstorelateeucritesanddiogenitesrequirecomplexigneousmodelsinvolvingmeltingscenariosthatgenerateddiverseliquidswhichproducedavarietyofcumulaterocks[Stolper,1977;LonghiandPan,1988;GroveandBartels,1992]orfractionalcrystallizationofanextensivemagmaocean[RighterandDrake,1997;Ruzickaetal.
,1997;Warren,1997].
Assumingserialmagmatism,WilsonandKeil[1996]modeledthesizesandflowratesofconduitsthatcarriedmagmasfromsourceregionstoVesta'ssurface.
Thewidthsofaugiteexsolutionlamellaeinonecumulateeucriteindicateslowcooling,equivalenttoaburialdepthof~8km[MiyamotoandTakeda,1994].
ThismeasurementmaybetakenasaminimumthicknessofVesta'scrust,inagreementwiththelargest(4yearsandamissionlife>10years.
ThedesignfullyaccomplishesDawn'sscientificandmeasurementobjectives.
Thedesignusesadualarmopticalandfocaldesignwithmappingcapabilityto5m.
Themappingspectrometerisanimagingspectrometerthatcombinestwodatachannelsinonecompactinstrument.
Thevisiblechannelcovers0.
25-1.
0mandtheinfraredchannelcovers0.
95-5.
05m.
Theuseofasingleopticalchainandtheoverlapinwavelengthbetweenthevisibleandinfraredchannelsfacilitatesintercalibration.
Itutilizesasiliconchargecoupleddevice(CCD)toimagefrom0.
25mto1mandamercurycadmiumtellurideinfraredfocalplanearray(IRFPA)toimagefrom1mto5m.
Thespectrometerconsistsofthreemodules:opticalsystem(5.
0kgmass);proximityelectronics(3.
0kgand5W);cryocoolerincludingdrivingelectronics(1.
3kgand12.
6W).
Amechanicalandthermalmountingof5.
0kgmassaccommodatesthespectrometersubsystems.
Theopticalsystem,whichincludesforeoptics,dispersiveelements,filters,focalplaneassembliesaswellasthecryocoolerandproximityelectronicsisacompletere-buildoftheVIRTIS-M.
TheopticalconceptisinheritedfromthevisiblechanneloftheCassiniVisibleInfraredMappingSpectrometer(VIMS-V)developedatOfficineGalileoandlaunchedonCassiniinOctober1997.
ThisconceptmatchesaShafertelescopetoanOffnergratingspectrometertodispersealineimageacrosstwoFPAs.
TheShafertelescopeisthecombinationofaninvertedBurchoff-axistelescopewithanOffnerrelay.
Byputtinganaperturestopnearthecenterofcurvatureoftheprimarymirror,comaisvirtuallyeliminated.
Theresultisatelescopesystemthatreliesonsphericalmirrors,yetremainsdiffractionlimitedoveralargespectralrangeandthewholespatialdirection.
Thehorizontalfieldisrealizedbyrotatingthetelescopeprimarymirroraroundanaxisparalleltotheslit.
TheOffnerspectrometerismatchedtothetelescope,anddoesnotrelyonacollimatorandcameraobjective.
Thisispossiblebecausebothtelescopeandspectrometeraretelecentricandthetelescopehasitsexitpupilonthegrating.
TheopticallayoutisillustratedinFigure13.
Thespectrometerdoesnotusebeam-splitters.
Twodifferentgroovedensitiesareruledonasinglegrating.
Thegratingprofilesareholographicallyrecordedintoaphotoresistandthenetchedwithanionbeam.
Usingvariousmasksthegratingsurfacecanbeseparatedintodifferentzoneswithdifferentgroovedensitiesanddifferentgroovedepths.
The"V"regions,whichmakeupthecentral30%oftheconjugatepupilarea,correspondtothehighergroovedensityneededtogeneratethehigherspectralresolutionrequiredinthe"visible"channelextendingfromtheultra-violettothenearinfrared.
Theinfraredchannelutilizestheouter70%ofthegrating,whichisruledwithalowergroovedensity.
Thelargercollectingareaintheinfraredcompensatesforthelowersolarirradianceinthisregion.
Table7liststheopticsspecifications.
18ThevisibledetectorarrayisbasedontheThomson-CSFtypeTH7896CCDdetector.
Itusesaburiedchanneldesignandpoly-siliconN-MOStechnologytoachievegoodelectro-opticalperformance.
MoreoveritincludesaMultiPinnedPhase(MPP)boronimplanttooperatefullyinvertedandsubstantiallytodecreasethesurfacedarkcurrent,residualimagesafterstrongexposureandothereffectsduetoionizingradiation.
TheTH7896Misafullframeimagesensorwith1024x1024sensitiveelements,tworegistersandfouroutputs.
Itwillbeusedasaframetransferdevicewithasensitiveareaandastoragearea.
Thefirsthalfisusedtoacquirethedataandthesecondhalfisusedtosendthedatatotheproximityelectronicstobeconverted.
TheIRdetectorusedinthespectrometerisbasedonabidimensionalarrayofIR-sensitivephotovoltaicmercurycadmiumtelluridecoupledtoasiliconCMOSmultiplexer.
Thedeviceisanarrayof270x435HgCdTephotodiodesmanufacturedbyRaytheonInfraredCenterofExcellence(SantaBarbaraUSA)withaspacingof38mbetweendiodecenters.
Thespectralwavelengthrangeis0.
95to5.
0mandanoperatingtemperatureof70K.
Thedetectorispackagedintoahousingwhichincludesanopticalwindowwhichprovidessuitablemechanical,thermalandelectricalinterfacesforitsintegrationonthefocalplane.
Furthermore,thewindowfunctionsassubstratefortheorder-sortingfilters.
Thesefiltersareusedtostopthesuperimpositionofhigherdiffractionorderscomingfromthegratingandalsotoreducethebackgroundthermalradiationfromtheinstrumenthousing.
Thetransmissioncharacteristicsofthewindowareoptimizedforeachcorrespondingdetectorposition,sothatforeachfilterzoneonlythedesignedwavelengthrangecorrespondingtothefirstdiffractionorderisallowedtopass.
Sixsegmentfiltersarecoatedinthewindowwiththefollowingbandpasses:0.
9-1.
6,1.
2-1.
9,1.
9-2.
5,2.
4-3.
75,3.
6-4.
4,4.
3-5.
0m.
InordertominimizethethermalbackgroundradiationseenbytheIR-FPA,thespectrometeritselfneedstobecooledtolessthan135Kbyradiatingatleastone,orpossiblytwoofitssurfacestowardcoldspace.
SuchaconfigurationalsoprovidestheoperationaltemperatureneededfortheCCD.
TheIR-FPArequiresanoperatingtemperatureof70Ktominimizedetectordarkcurrent,whichisachievedbyusingaStirlingactivecoolerdrivenbydedicatedelectronics.
TheStirlingcoolerthatbestmeetsMSrequirementswithoff-the-shelfproductsistheRICORK508tacticalcooler.
Itisanintegralcoolerinwhichtheregenerator,wheretheheatexchangesatwarmandatcoldtemperaturesoccur,isdirectlyconnectedtothecompressor.
Withoutthetransferlinecharacterizingthesplitcooler,lessheatlossesoccurandmoreefficiencyisreached.
Ontheotherside,duetotheinternalbalancingdeviceandthereducedheatflowfromthecompressortothecoldfinger,vibrationandheattransmittedtotheregeneratorandtothecoldend(wheretheFPAisconnected)areverylimited.
Acoverinfrontoftheopticsentranceapertureprotectsagainstcontaminationfromexternalsources.
Dedicatedheatersonthefocalplaneremovepossiblecondensingcontaminantsandprovideforannealingofthedetectortoreduceradiationdamage.
Thecover-insideiscoatedandusedascalibrationtargetincombinationwithtwointernalcalibrationlamps(onefortheVIS-FPAandonefortheIR-FPA).
19ElementalCompositionPlanetaryobjectswiththinatmospheresemitgammarayswithenergiescharacteristicoftheemittingnucleusthroughthedecayofnaturallyradioactiveelements,principallyTh,UandK.
Gammaraysalsostemfromnuclearreactionsinducedbyneutronsgeneratedbygalacticcosmicrayinteractionswiththenuclearconstituentsofallmatter,whichforDawnincludesboththeasteroidsandthespacecraft.
Underlyingthegamma-raylinespectrumisacontinuumcomingfromtheasteroids,thespacecraft,andthegalaxy.
Thegamma-ray/neutrondataareobtainedequallywellforallsolarlightingconditionsandincreasemodestlywithincreasingheliocentricdistance.
With130daysofobservationsatanaltitudeof130kmorlessaboveVestaandCeres,wheretheasteroidfillstheGN/RSfieldofview(FOV),wemeasuretheabundancesofFe,Ti,O,Si,Ca,U,Th,K,H,Al,Mg,GdandSm.
Thisresultholdsbothfortheasteroidsasawholeandseparatelywithinmajorgeologicregions.
Thissetconstitutesallofthemajorrock-formingelementsaswellasseveralimportanttraceelements.
Table8showstheexpectedstatisticalprecision(ε),expressedasarelativestandarddeviation,forthedeterminationofmajor-andradioactiveelementalabundancesforthreedifferentdrymaterials:eucrite,ferroananorthosite,andbasalt.
Notethatεisexpressedasapercentageofthelistedabundanceandistobemultipliedby,notadded,tothepercentabundanceslisted.
Themeasurementsareassumedtakenover120hours(5days)atanorbitalaltitudeof130km.
TheeucritecompositionisrepresentativeofVesta.
TheexpectedprecisioninelementalabundancesfortheeucritecompositionindicatethatthesensitivityoftheGR/NSissufficienttodeterminewhetherVestaistheparentoftheHEDmeteorites.
Theelementslistedhereconstituteover99%ofthemassofHEDmeteoritesandcanallbedetectedintheexpectedstaytimesatVestaandatCeres.
Wecreatecompositionmapsofbothasteroidsforlatitudeandlon-gitudeboundariesdeterminedfromtheGR/NSmeasurementsalone,aswellasforgeographicalregionsdeterminedfromthecameras.
BecausetheasteroidsfilltheGR/NSFOV,itseffectivesensitivitygreatlyexceedsthatobtainedfromNEARatErosatsimilaraltitudes.
Theneutronmeasurementsenhanceourabilitytodetecthydrogenand,byinferencewater,morethanafactorof3indepthandafactorof100inconcentrationoverthatderivedfromgamma-rayanalysisalone.
Amassfractionofwatergreaterthan0.
02%canbedetectedataneutronmeasurementsensitivityof1%.
Thissensitivityistobecomparedwiththemassfractionof3%H2Ocontainedinmartianbasalts.
NeutronobservationsusingtheLunarProspectorNeutronSpectrometer(whichhadasimilarsensitivitytothatforDawn),yieldedahydrogenabundanceofabout50ppmnearthelunarequator,150ppmnearbothpoles,and1700ppmwithinthepermanentlyshadedcratersnearthesouthpoleoftheMoon[Feldmanetal.
,2000].
DawnneutronmeasurementsshouldthereforedeterminethelevelofhydrationofCeres'crust.
Thesemeasurementsalsoprovideanindependentmeasureoftheaverageatomicmassofsurfacesoils,sufficientlyrobusttodiscriminatebetweenbasalticandfeldspathiclithologies.
20Adiagnosticindicatorofthedegreeofvolatileelementdepletioninplanet-formingmaterialintheinnersolarsystemistheK/Uratio,measuredbythegamma-rayspectrometer.
Figure14showstheK/UratioplottedversusKconcentrationformeteoritesandlunarandterrestrialsamples[Taylor,1992].
Themeteoritesclearlyfallintospecificclassesinthisdisplay.
RocksfromthesurfacesofEarth,Venus,andMarsappeartobesimilartoeachotherandquitedif-ferentfromthevariousmeteoritetypesandthelunarsurface.
GammaRay/NeutronSpectrometerTheDawngamma-rayandneutronspectrometer(GR/NS)mapsthemajor(O,Si,Fe,Ti,Mg,Al,andCa)andtraceelement(U,Th,K,H,Gd,Sm)composition.
ItdrawsondecadesofexperienceatLANLinmeasuringneutronsandenergeticphotonsandisanimprovedversionofthehighlysuccessfulGR/NSonLunarProspector(LP),andthepresentlyoperatingNeutronSpectrometeraboardMarsOdyssey(MO).
ThedesignofthespectrometeranditsexpectedperformanceisdescribedindetailbyPrettyman,etal.
,2003.
Thegamma-raysensorissegmentedintotwopartsandtheneutronsensorintofourparts.
Onboardclassificationofthemultiplesignalsfromeacheventthenallowsdirectionalitydeterminationthatcandiscriminateradiationofasteroidandspacecraftorigin.
Thegamma-raysensorisasquared-offversionoftheLPscintillator,inlaidwitha4x4arrayof1cm3cadmiumzinctelluride(CZT)sensorsonthesidefacingupwardfromthedecktowardtheasteroid.
TheCZTsensorsareanewtechnologydemonstration[Prettyman,etal.
,2002].
Thissensorissurroundedbyfoursegmentsofananticoincidenceshield(ACS)madeusingaboratedplasticscintillator(BC454).
Theupward(asteroid)anddownward(spacecraft)facingsegmentsoftheACSarelaminatedwitha6Liloadedglassscintillator(GS20)toprovideaseparationbetweenincomingthermal(GS20)andepithermal/fast(BC454)neutrons.
TheepithermalandfastneutronsareseparatedelectronicallyaswasdonefortheLunarProspectorandMarsOdysseyinstruments.
TheDawnGR/NSdataatVestaandCeresareofcomparablequalitytothatoftheLPneutronspectrometersandhavebetterthanafactorofthreehigherspectralresolutionthanthatoftheLPgamma-rayspectrometer.
Simulationsverifythatourpresentdesignprovidesarobustneutronandgamma-raysignalstrengthatVestaandCeres,adequatelycontrolsspacecraftbackgrounds,yieldsarobustbismuthgermanate(BGO)spectrumthatisasgoodorbetterthanthatmeasuredusingtheLPGRSBGOdetectorandiseffectiveinsuppressingthebackgroundfromthespacecraft.
IftheCZTdetectorfailstoachieveoptimalresolutione.
g.
becauseofradiationdamageinthespaceenvironment,theDawnGR/NSscienceobjectivesarestillmet.
WehaveshownthatannealingofCZTatmoderatetemperatures(40to60C)forshortperiodsoftimecanfullyrestoreresolutionfollowingradiationdamage.
TheexposurepredictedfortheCZTbehindtheACSislowenoughthatwedonotanticipatesignificantdegradationinperformanceduetoradiationdamage.
However,toensureperformance,ourdesignincludesthecapabilitytoannealtheCZTarrayifdamageisobserved[Prettyman,etal.
,2003].
21TheDawnGR/NSconsistsofa7.
6cmwide,by7.
6cmlong,by6cmhighrectangularslabofBGOthatisviewedbya7.
6cmdiameterphotomultipliertube(PMT).
A4x4squarearrayof16cm3CZTsensorelementspositionedabovetheupwardBGOface(awayfromthespacecraftdeckfacingtheasteroid)asshowninFigure15.
TheBGOactsasanactiveshield,minimizingspacecraftcontributiontotheresponseoftheCZTarray.
Thiscompoundgamma-raysensorissurroundedonfivesidesbyarectangularboratedplasticACSthatiscomposedoffourseparateelements,eachviewedbyitsown2.
5cmdiameterPMT.
Thethicknessofeachoftheplasticscintillatorelementsis2.
5cm.
ThetopandbottomACSelementsarelaminatedby2mmthicklithium-6glassscintillatorsheets,whichareviewedbythesamePMTsthatviewtheplastictowhichtheyareopticallycoupled.
ThetopandbottomACSfacesaresurroundedontheirsidesby1cmthicksheetsof6Li-loadedpolyethylenetopreventaccessbythermalneutronsfromthespacecraft.
Thefront-endelectronicsfortheBGOandACSportionsoftheGR/NSareconfiguredtoclassifyeachdetectedeventintooneoffivecategories.
ThesecategoriesareidenticaltothatusedonLPandMO.
Thefivecategoriesare:1)anisolatedBGOinteraction,2)asinglecoincidentBGOandACSinteraction,3)asubsetofthesecoincidentinteractionswheretheenergydepositedintheBGOandBC454aredefinedbywindowdiscriminatorstofallwithinnarrowrangescenteredon478keVintheBGOand93keVintheBC454,4)asingleACSinteraction,and5)atime-correlatedpairofACSinteractionsthatoccurwithin25.
6s.
AdditionoftheCZTsensorsaddsfouradditionalcategories:1)anisolatedCZTevent,2)acoincidentCZTandBGOinteraction,3)asubsetofthesecoincidenteventswheretheenergydepositedintheBGOis0.
511+/-0.
075MeV,and4)acoincidentCZTandACSinteractionwheretheenergydepositedintheCZTandBC454aredefinedbywindowdiscriminatorstofallwithinnarrowrangescenteredon478keVintheCZTand93keVintheBC454.
SimulatedspectrahavedemonstratedthecapabilitiesofthesehybridCZT-BGOdetectoroperationmodes.
Allinformationispackagedina3kb/sdatastringwithanaccumulationtimeof60s.
A478keVgammarayfrom7Li*providesacontinuouscalibrationofgainasprovenonLP.
Gamma-rayspectra,bothacceptedandrejectedbytheACSarerecordedseparatelyandtelemeteredtoEarth.
ThermalneutronsfromtheasteroidaremeasuredusingtheGS20oftheupward-facingACSelementandthosefromthespacecraftusetheGS20ofthedownward-facingelement.
SeparationoftheLi-glassfromtheBC454willbeimplementedusingtime-domainfiltersthathavebeendevelopedatLosAlamos.
Epithermalneutronshavingenergyintherangebetweenabout0.
2eVand0.
5MeVfromtheasteroidaremeasuredusingtheBC454oftheupwardACSelementincoincidencewitha478keVCZTorBGOinteraction.
ThosefromthespacecraftusethedownwardACSelementincoincidencewitha478keVBGOinteraction.
Fastneutronshavingenergiesbetweenabout0.
5MeVand8MeVaremeasuredusingtheACSbyisolatingdoubleinteractionevents.
Thepulseheightofthefirstpulseofthepairprovidesameasureoftheenergyoftheincidentfastneu-tron.
22Topography,GeodesyAndGeomorphologyPlanetarytopographyisafundamentaldatasetforstudiesofthesurfacesandinteriorsofsolarsystemobjects.
Itisrequiredtointerpretgravitydata,providesacrucialthirddimensiontosur-facefeatureimagesandyieldsquantitativeinformationonlocalsurfacepropertiessuchasroughness.
Thesemapsprovidefundamentalinsightintovolcanism,tectonicsandcratering,andhelpconstrainstudiesoftheformationandevolutionofthecrust.
Gravitystudiesmapandmodeltheinternaldensityvariationswithinthebodyusingsignalsarisingfromthesumofthecontributionsfromtheinteriordensityvariationsandsurfacetopography.
High-qualitytopographicinformationintheplanetarycenterofmassreferenceframeisessentialtointerpretgravitydata.
Thecombinationofgravity,andsurfacecompositiondataiskeytomodelingtheentirecrustalcomposition.
WhilethetopographydataforVestashowninFigure16isthebestavailableforanymainbeltasteroid,itisstillfarfromsufficienttobegintointerpretgravitydataandtheinternaldensitydistributionoftheasteroid.
Thelatterrequiresaprecisedeterminationoftopographytoremoveitseffectsfromtheobservedgravitysignal.
Thisisobtainedbyamappingofsurfaceelevationswiththeframingcamera.
IntrinsicAndInducedMagneticFieldsItisclearthatmanyofthesmallerobjectsinthesolarsystemcangenerateorhavegeneratedintrinsicmagneticfields.
ThisistruefortheEarth'sMoon[Russelletal.
,1975],Mercury[Nessetal.
,1975],Ganymede[Kivelsonetal.
,1996].
MoreoverastronglymagnetizedcrusthasbeendetectedatMars[Acuaetal.
,1998].
ThemeteoritesassociatedwithVesta,thehowardites,eucritesanddiogenitesexhibitnaturalremanentmagnetizationthatperhapswasproducedinasurfacemagneticfieldequaltoorgreaterthanthatoftheEarth[CollinsonandMorden,1994].
TherendezvouswithVestaandCeresbyDawnallowsustosurveytheseobjectsataltitudeswellbelowonebodyradius,anddetermineiftheypossessnaturalremanentmagnetizationand,throughgeologiccorrelations,whenitwasproduced.
Themagnetometeralsomeasurestransientmagneticfieldsandhowtheyareaffectedbytheasteroids,providingconstraintsontheelectricalconductivityoftheinterior.
TheresponsetimeoftheMoontosteptransientsinthesolarwindmagneticfieldis80s[DyalandParkin,1973],andatVestashouldbeintherange2to8s,easilyresolvablebythe10Hzbandwidthand0.
1nTresolutionofthemagnetometer.
DetectionofremanentmagnetizationoranelectricallyconductinginterioratCereswouldleadtoamajorreassessmentofourpresentunderstandingofthebody.
TheMagnetometerThesescientificobjectivesandourexperienceintheexplorationofothersolarsystembodiesenablesustodevelopasetofrequiredspecificationsfortheDawnmagnetometer.
Intheasteroidbelttheambientmagneticfieldisabout3nT.
BasedonthemaximumfieldstrengthsobservedonthesurfaceoftheMoononApollo(300nT),abovethepolesofGanymedeby23Galileo(1000nT)andabovethecrustofMarsbyMarsGlobalSurveyor(1000nT),weexpectthatthefieldatVestaandCeresatthealtitudeofDawnisunder500nT.
Wehaveconservativelychosena±1000nTrangeforthemagnetometer,sampledat20Hz.
Thedataaredigitizedto16bitsproviding±0.
015nTdigitization.
MagneticcleanlinessisofconcernontheDawnmissionbecausethemagneticfieldoftheasteroidscouldbesmallovermostoftheorbitofDawn.
Dualsensorsattheendofand1/3waydownthemagnetometerboomprovideredundancyandameasureofspacecraftmagneticfields.
Sincethethrusterscontainstrongpermanentmagnetsandsincesignificantcurrentsflowfromthesolararraytothethrusters,weusea5mboomextendedawayfromthethrusterstominimizespacecraftfields.
Wereversethepolarityofthemagnetsinoneofthethrustersandrotateeachabouttheiraxesofsymmetrytoachievetheminimumfieldatthesensors.
Thisprocedurewiththehigh-ordernatureofthethrusterfieldleadstoanexpectedsteadyfieldoflessthan10nTatthesensorsdespitetheirratherhigh(5000nTmaximum)fieldat1m.
Keycomponentsareheldatfixedtemperaturestoeliminatetemperature-dependenteffects.
Currentlevelsonboardaremonitoredandtelemeteredataratesufficienttoenabletheremovaloftheeffectsofvaryingcurrentlevels.
Inadditionthetwosensorsaresampledsimultaneouslytoprovideagradiometermeasurementthatverifiesthatthetime-varyingfield(e.
g.
solararraycurrents)hasbeencompletelyremoved.
Techniquesusedtoremoveunknownspacecraftandsensor-zerolevelsusingthepropertiesoftheinterplanetarymagneticfieldasdevelopedforPioneerVenusandGalileoareusedtomaintaintheinstrumentbaselinetobetterthan0.
1nT.
Thetime-varyingfieldismeasuredto3x10-5nT2/Hzat1Hz.
Thesuccessofthemagneticsinvestigationdependsnotjustonthequalityofthemagnetometersbutalsooftheentiresystem,thusthemagnetometerteamleadsalow-costmagneticcleanlinessprogramthatbothidentifiespotentialmagneticproblemsinthedesignstageandverifiesthesuccessofthemagneticcleanlinessprogramafterlaunch.
TheUCLAmagnetometer[e.
g.
Russelletal.
,1995]derivesfromalonglineofmissionsincludingOGO5(launchedin1968);ISEE1and2(1977),PioneerVenus(1978);Galileo(1989);Polar(1996)andST5(infabrication).
ThemainelectronicsunitandablockdiagramareshowninFigure17.
Theelectronicsunitdrivesthering-coresensorsshowninFigure18atafrequencyof12kHz.
Theamountofsecondharmonicsignalinquadraturewiththedrivefrequencyisdetected.
Afeedbackcircuitnullsthesecondharmonicsignalbyapplyingsufficientcurrenttokeeptheringcoreinzerofields.
Thiscurrentisameasureofthestrengthoftheexternalfield.
Themainelectronicsboardcontainsapowersupply,drive,senseandfeedbackcircuitryforthethreesensorstogetherwithdigitalconversionandcommandandcontrolcircuitry.
Themainelectronicsandthesensorsarecompletelyredundant,asinglerangeanddatarateandcontinuousoperation.
Theonlycommandstotheanalogmagnetometerareonandoff.
Athirdnewtechnologymagnetometerwithadesignbasedonthesigmadeltamodulatoraddsfurtherredundancy.
Itisnotneededtomeetbaselinerequirements.
Thisdesignwaschosenbecauseofitslow-noiselevel,simplicity,lowcostanditshighinheritancefromrecentmissions.
Theelectronicsunitweighs2.
35kg;thesensorsandcableweigh700g;theunitdraws3.
0Wofpower.
Themagnetometerisaccuratelycalibratedonthegroundandrecalibratedinflightwithapreciseinternalcalibrationsource.
24GravityScienceTheobjectivesofthegravityinvestigationaretodeterminethemassesoftheasteroids,theglobalgravityfieldofVestaandCerestothe12thharmonicdegreeandorder,theprincipalaxes,therotationalaxis,andthemomentsofinertia.
Themasstogetherwiththeshapemodeldeterminesthebulkdensity.
Theshapeandgravitymodelscharacterizecrustalandmantledensityvariations[Zuberetal.
,1999;Zuber,2001],andtogetherwithadetectablewobbleinrotation,thepossibledifferentiationandformationofametalliccore.
PresentestimatesofthefractionalradiusofthemetalliccoreofVestarangeupto50%,correspondingtoafractionalmassofupto20%.
IfCeresaccretedwatericeduringformationasitpresentlyappears,wedonotexpectittobedifferentiated.
Theprincipalaxesaredetermineddirectlyfromtheseconddegreeharmonicsofthegravityfield.
Thenormalizedpolarmomentofinertia(homogeneityconstant)constrainstheradialdensitydistribution.
PresentlythedensitiesofCeresandVestaareknownto2%and3%respectivelyfromperturbationsonotherasteroidsandtheiropticallydeterminedshapes[Hilton,1999,Konoplivetal.
,2002].
Thescienceobjectiveistomeasurethebulkdensitytobetterthan1%andDawnwillachieverelativeaccuraciesnear0.
1%.
Thegravityfieldisdeterminedinloose,mediumandtightorbitstates.
Intheinitial"loose"orbit,onlythemassisdeterminedwhileopticalimagesdeterminetherotationalstate.
Intheopticalmapping(medium)orbit,thegravityfieldisdeterminedtoaboutdegree4.
Inthelowaltitudemappingorbitataltitudesbelowonebodyradius,thegravityfieldisdetermineduptothe12thdegree.
Witha12thdegreegravityfield,correlationswithsurfacefeaturesonVestaof65kmorgreatercanbeinvestigatedincludingthelarge460kmimpactbasinnearthesouthpole[Thomasetal.
,1997a].
Thishigherresolutiongravityfieldallowsforcomparativemodelingwithlunarimpactbasins[Zuberetal.
,1994].
ThishigherresolutiongravityfieldisattainablesinceDawnusesreactionwheelsforattitudecontrol,thuseliminatingdisturbancesduetothrusterfirings,exceptduringorbitalchanges.
Tousethebulkdensitytomodeltheinteriorstructure,wemeasurethepolarmomentofinertia.
Figure19showsthenormalizedpolarmomentofinertia,C,versuscoresizeforthreedensitiesandaVesta-sizedobject(C=0.
4impliesahomogeneousbody).
A1%determinationofCwouldsignificantlyconstraincoresizeandcomposition.
ThedetectionofawobbleofftheprincipalaxesisnecessarytoenableCtobedetermined.
FortheNEARmissionthedetectionofa0.
1wobblewouldhavedeterminedCto1%for433Eros[Milleretal.
,1995].
WenotethatatEROS,nowobblewasdetectedthatwasgreaterthan0.
01°[Konoplivetal.
,2002].
AsforNEAR,apossibleVestaandCereswobblewillbemeasuredbyprecisephotogrametricmappingandradiotracking.
Whetherwobbleispresentisdeterminedbytherecentimpacthistoryoftheobject.
Thenavigationeffortisacollaborationoftheradioscience,andimagingteamsandinvolvesprocessingdatafrombothinstruments.
Itiscloselycoordinatedtooptimizethesciencereturnandminimizeduplicationofeffort.
Theopticalimagesforlandmarkobservationsarerequired25foraccuratenavigation.
Shapeobservationsareneededformissionplanning,andgravityfielddeterminationisneededfororbitdetermination.
4.
DATAANALYSISANDARCHIVINGTheDawnscienceteamiscommittedtoprovidingfullydocumentedandcalibrateddataproductsoflong-termusetothecommunity,withminimallatency.
Duringthemission,dataflowsfromthespacecrafttoUCLA,whereitisvalidatedanddistributedautomaticallytothescienceteams.
Theraw(Level0)dataarealsosenttothePlanetaryDataSystem(PDS);fulldocumentationandsoftwaretoaccessthedataanddisplaythemisincluded.
ThescienceteamproducesandvalidatesLevel1calibrateddata,andimmediatelydeliversthemtothePDS.
TheLevel2dataproductsdescribedinTable9arereleasedwithseveralmonthsofdataacquisition.
ExistingscienceanalysissoftwaredevelopedontheMGS,LP,Rosetta,NEARandGalileoprogramswillbeusedtoobtainthemeasurementsanddeductionstoachievethemissionobjectives.
UCLAisresponsibleforcreatinganintegratedmissiondatabasewithscience,navigationandNAIF/Spicedata,accessiblebythecommunityovertheweb.
FullydocumentedimageswillbereleaseddailyovertheWorldWideWebformedia,educatorsandthepublicalike.
Duringthemission,aparticipatingscientist(PS)programwillbeconducted.
TheobjectiveofthePSprogramistoprovidepostdoctoralandseniorscientistswithopportunitiesforresearchonprojectsoftheirownchoicethatarecompatiblewiththeinterestsandgoalsoftheDawnmission,thuscontributingtotheoverallsciencereturnofthemission.
Alistofresearchopportunities,eachwithanassociatedadvisor/collaboratorwhoisamemberoftheDawnscienceteam,willbeofferedforeachcompetition.
Proposedinvestigationsthatareunsolicitedwillbesupervisedby,orperformedincollaborationwith,theDawnPI.
TheselectedscientistswillworkcloselywiththeDawnScienceTeam,gainingvaluablemissionexperienceandsharingintheexcitementofitsdiscoveries.
ThetermsofPSscientistswillbefortwoyears.
DataanalysisbythescienceteambeginsinAugust,2010,uponarrivalatVesta,andendsoneyearafterthereceiptofthelastCeresdata,nominallyJuly,2015.
AdataanalysisprogramwillbeginattheendoftheVestaencounterandcontinueuntiltwoyearsbeyondtheCeresencounter.
ToaddresstherangeofscientificquestionsaddressedbythelargevolumeofdiversedatatobeacquiredbyDawnatVestaandCeresweenvisionadataanalysisprogrammoreanalogoustotheMarsdataanalysisprogramthantheprogramforanalysingErosdatafromtheNEARmission.
MostsmallasteroidslikeErosexhibitrelativelyhomogeneouscompositionandposequestionsintheareasofcratermechanicsandstructuralhomogeneity(e.
g.
,rubblepileorcoherent).
VestaandCeressimilarlyposequestionsregardingcrateringmechanics,yettheypresentcrateringrecordsextendingovertheageofthesolarsystem(whichsmallerbodiesdonot),andheterogeneitiesofcompositionandstructureoveralargerangeofspatialscales.
Theseprotoplanetsalsoposequestionsofplanetaryevolution,suchasdifferentiation,volcanism,watermobilizationandtransport,coreformationandmagnetic26dynamophysics.
Instrumentshavebeenchosenanddataproductshavebeendesignedtoaddressthesequestions.
AnextendedmissionatCeresispossible,extendingtheobservationsthereforanadditionalsixmonths,tocapturevaryinglightingconditions.
IfsignificantfuelremainsaftertheVestaandCeresencounters,thespacecraftmaybetaskedtovisitadditionalasteroidsinalongerextendedmission.
6.
EDUCATIONANDOUTREACHDawn'sEPOprogramsupportsNASA'sstrategicplanto"Communicatewidelythecontent,relevancy,andexcitementofNASA'smissionanddiscoveries".
Furthermore,theprogramwillinvolvetheeducationcommunityinourendeavorstoinspireandtrainthenextgenerationofthenation'sscientistsandengineers.
Usingcurrentnationalstandardsandpracticesineducation,theEPOteamwillproducecontentmodulesconsistingoflearningandexplorationtools.
Thesemoduleseachcoverdifferentscientific,historicalandtechnologicalaspectsofthemission.
Thelearningtoolsfeatureemerging,classroominnovationssuchasCalibratedPeerReview,PerceptualLearning,andothers.
Thesetoolshavebeenshowntoimprovestudentlearningandcomprehension.
Theexplorationtoolsprovideuniqueopportunitiesforstudentsandthegeneralpublictoparticipateinthescienceofthemission.
Forexample,studentswillstudythemotionandrotationalpropertiesofCeresandVestaanalyzingimagesfromground-basedtelescopes.
TheTelescopesinEducationprogram(TIE)willbeapartnerinthisendeavor,usinganexistinginfrastructureofaccesstotelescopestostudyphysicalparametersofourtargetsthatareimportanttothesuccessofthemission.
AnotherexplorationtoolisprovidedbyourpartnershipwithClickworkers.
Studentswillstudyandclassifycratersfromexistingimagesofotherplanets.
ThenatureandnumberofcratersonCeresandVestaareacriticalpartofthescientificanalysisofimagesreturnedbytheFramingCamera.
Ifstudentsbegintheirstudiesofcratersasseventhgraders,theywillbegraduatingfromhighschoolbythetimetheDawnspacecraftreachesVestaandpossiblyenteringgraduateschoolwhenwegettoCeres.
ThesurprisesgleanedfromcratersonVestaandCereswillbeappreciatedbythosewhohavestudiedcratermorphologyandfrequenciesonotherplanets.
AthirdmajorexplorationtoolinvolvestheSouthwesternIndianPolytechnicInstitute'sMeteoriteIdentificationLaboratory.
Studentsaretaughtaboutthedifferenttypesofterrestrialrocksandareencouragedtofindsamplesofthem.
Theyarealsotaughtthedifferencebetweenterrestrialandextraterrestrialmaterial.
Thoughtheprobabilityoffindingameteoriteissmall,thisprogramoffersanopportunityforstudentstomakescientificobservations(characterizetherocks),andtounderstandhowtheinstrumentsoftheDawnmissionwillsearchfor27evidencerelatingVestaandCerestothemeteorites.
Makingsuchaconnectionwillexpandourunderstandingoftheevolutionofprotoplanetsinthesolarsystem.
Dawn'spropulsionsystemisanemergingtechnology.
WewillpartnerwithtechnologydevelopmenteffortsatJPLinbringingknowledgeofionpropulsiontostudentsandthepublicacrossthecountry.
ThehistoryofexplorationoftheseasteroidsandthebiographiesofthoseinvolvedinourcurrentexplorationinvitestudentstolearnandfindcommoninterestsbetweenthemselvesandthosemakingtheDawnmissionpossible.
Dawn'sEPOprogramwascreatedfromthefollowingconsiderations:ProductsmustreflectcurrentandbesteducationpracticesDisseminationofmaterialsispractical,efficientandmakesgooduseoftechnology.
ActivitieswillleverageexistingprogramsthroughpartnershipsestablishedbytheEPOteam.
Theteamconsistsofscientists,teachersandexperiencededucators.
Ascienceteammemberleadstheprogramwithresponsibilityforscientificaccuracyandalignmentwiththemission.
Dailyoperationsaremanagedbyanexperiencededucatorandmanager.
Acoreplanningteammakesdetailedplans,whichareimplementedbyateamofexperiencededucatorsfromtheMid-ContinentResearchforEducationandLearninglaboratory(McREL).
Materialsarereviewedandtestedinschoolsacrossthecountry.
McREL'sevaluationdivisionwilldevelopandcarryoutformalassessmentsofthematerialsproducedbytheEPOteam.
OurmaterialsandprogramstargetkeyaudiencesincludingK-14students,teachers(bothintrainingandasprofessionaldevelopment),thegeneralpublic,andunderservedandunderutilizedpopulations.
WereachtheseaudiencesthroughactivitiesoftheOSSSolarSystemExplorationforum,nationalmeetingsofscience,mathandtechnologyteachersandtheworld-wideweb.
PartnershipswithmuseumsandinformalsciencecentersofferanotherimportantavenuetoaninterestedpublicasdoestheAmbassador'sprogramsponsoredbyJPL.
Theprintandtelevisionmediaalsobringpeopletoourwebpagewheretheycanfindupdatedinformationaboutthemissionaswellasaccesstooureducationalproducts.
TheEPOprogramwillpreparethegeneralpublicfortheexcitementofseeingthesenewplanetaryworldsandlearninghowtheyformed.
7.
CONCLUDINGREMARKSDawnisclearlyanambitiousmissionbothscientificallyandtechnically.
Atthesametimewebelievethatthroughmaintainingsimplicityandredundancy,byusingprovendesignsandbykeepingamplemargins,wehaveproducedaveryrobustmission.
DawnbuildsupontheNewMillennium'sProgram'stechnologicaldevelopments,transferringNASA'ssolarelectricion28propulsiontechnologytothecommercialsectorsandputtingittoworkinsolarsystemexploration.
Themissionhasmuchpotentialtogarnerthepublic'sattention,beginningjustover200yearsfromthediscoveryofCeresandVesta,anditcanbeusedasavehicletoenrichbothpureandappliedsciencecurricula.
Finally,themissionbuildsondecadesofasteroidandmeteoriticstudies,providingthefirstdetailedassessmentofthetwolargestasteroidsinthemainbelt,andsettingthestageforfutureexplorationofthemainbelt.
Acknowledgements:TheDawnteamwishestothankthefollowingpeoplewhoplayedearlycriticalrolesinassistingwiththedevelopmentoftheDawnmission:M.
HickmanfromGlennResearchCenterandhiscolleaguesinthenavigationsection,G.
VaneandM.
ShirbachehatJPL,G.
RawlsofMcRELandA.
McGlynnofUCLA.
Theauthorsarealsoverygratefulformanyconstructivecommentsfromthetworeferees:R.
P.
BinzelandM.
Gaffey.
ThiseffortissupportedbytheNationalAeronauticsandSpaceAdministrationaspartoftheDiscoveryProgram.
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1Vestashapemodel[ZellnerandThomas,1997]Fig.
2TwoviewsofCeresasobservedbyHubbleSpaceTelescope[J.
Parkerpersonalcommunication,2000]Fig3Numberofasteroidsversussemi-majoraxisinastronomicalunitswiththerangeofheliocentricdistancesoverwhichVestaandCerestravelFig.
4PictorialsummaryoftheDawnmissionFig.
5(Top)SubsolarlatitudeduringDawnplannedoperationsatVesta(Bottom)Heliocentricrange.
Fig.
6SubsolarlatitudeduringDawnplannedoperationsatCeresFig.
7Dawnspacecraftandpayload.
NotethatthesolararrayandthemagnetometerboomhavebeencroppedFig.
8CeresandVestareflectanceinthevisibleandnearinfraredwithfilterbandsoftheframingcamerasuperimposedFig.
9ThesensorheadoftheframingcameraFig.
10Reflectanceofdifferentmaterialsillustratingthevariationinabsorptionfeaturesindifferentmaterials[PietersandMcFadden1994]Fig.
11Normalizedreflectancespectraofmajorasteroids.
Figurefrom"Reflectancespectroscopyandsurfacemineralogy"byM.
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Gaffey,J.
F.
BellandD.
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CruikshankeditedbyRichardP.
Binzel,TomGehrelsandMildredShapleyMathews.
1989,TheArizonaBoardofRegents.
ReprintedbypermissionoftheUniversityofArizonaPress.
Fig.
12SpectrumofCeresillustratingabsorptionnear3m[Rivkin1997]Fig.
13OpticallayoutofthevisibleandinfraredmappingSpectrometerFig.
14Potassiumtouraniumratioversuspotassiumconcentration[Taylor,1992]Fig.
15Gammaray/neutronspectrometerFig.
16VestatopographyrevealedbyHubbleSpaceTelescopeimagery[Zellner,1997]Fig.
17Magnetometermainelectronicsunitblockdiagram(singletriad)togetherwithST5mainelectronicsboard37Fig.
18RingcoresensorsdevelopedforST5tobeusedonDawnmissionFig.
19PolarmomentofinertiaversuscoreradiusofVesta38Table1.
RecentradiusdeterminationsofCeresTechniqueEquator[km]Polar[km]ReferencesStellarOccultation480±2453±5Millisetal.
[1987]HST/FOC485±5466±6Parkeretal.
[2003]AdaptiveOptics508±5,473±7445±5Drummondetal.
[1998]AdaptiveOptics499±20469±20Saint-Peetal.
[1993]Average489±6458±639Table2.
ExamplesofthemanyflybyopportunitiesalongDawn'sflightpathBodyRadius(km)Rel.
Vel.
(km/s)Distance(Gm)Date(mo/yr)197Arete152.
13.
23/12149241994VZ2.
23.
54.
53/09182722495T-31.
13.
42.
17/09204841989NL412.
51.
94.
010/1140Table3.
OrbitalparametersatVestaandCeres.
Foreachphase,oneinstrumentcontrolspointingandthereisnocontentionforspacecraftattitude.
BodyPeriod[Hr]SMA[Km]Altitude[Km]FOV#[Km]Pixel#[m]PointingoptimizedforVesta12.
1950661-7216968Opticalmapping*3.
037586-1461212Topography12.
230415-7555Hi-resimaging+Ceres11.
01360880-9068989Opticalmapping*3.
3610130-1561414Topography12.
752949-7566High-resimages+*VisibleandIR,1gamma-rayandmagneticfieldincluded+sharedcontrolbutgenerallynadirpointing#Framingcamera41Table4.
Imagesobtainedineachphaseofthemission.
FramingCameraMappingSpectrometerPhaseRes[m]Clearimages%surfaceFilteredimages%surfaceRes[m]2-Dim.
SpectraCruise1*NA*7000NA0NANA0VSurvey2502180NA0NANA0VOpticMap691200700600$3501407100VHiResMap52400150NA103500Cruise2*NA7000NA0NANA0CSurvey4502650NA0NANA0COpticMap891518430759#2161785500CHiResMap6250070NA123650*Excludingtargetsofopportunity.
NA–Notapplicable#-ineachof3colors$-ineachof7colors42Table5.
Dawntelemetryrequirementsbyphase.
TotalScienceBits(Gbits)+Track.
Hrs/WkTele.
MarginPhaseDuration(days)FCMSGR/NSMAGNAVTelem.
*%Cruise1152518---43.
514VSurvey185.
6---11217686VOptMap3013.
2124.
81.
6564040VLowAlt13014-299.
72417.
537VHiRes25966.
52.
2563655Cruise2114418---43.
710CSurvey165---11211935Coptmap309.
39.
34.
91.
6564427CLowAlt14318-37122417.
537CHiRes2596.
26.
52.
2563655+Assumedcompressionratios:FC6:1,MS2.
5:1,*include1kbpsengineeringdataand15%relativeoverheadindownlink.
43Table6.
Baseline/FloorMissionComparison.
TheperformancefloormissionperformsarendezvouswithVestaonlyandlasts5.
3years.
BaselineMissionPerformanceFloorLaunchDateMay27,2006May27,2006TargetsVesta,CeresVesta,50Virginia3MissionEndDateJuly26,2015September16,2011Missionduration9.
2years5.
3yearsTimeinorbit(days)338(V)+340(C)426(V)FuelUsed(Xe)1428kg380kgDrymass1,2680kg615kgHydrazine1,256kg27kg1includingreserveandmargin2formaximuminjectionmass3candidate44Table7.
MappingspectrometeropticalspecificationsParameterMappingSpectrometerPupildiameter(mm)47.
5ImagingF#5.
6Visand3.
2IREtendue(m2sr)3.
610-11Vand7.
510-11IRSlitdimension38mx9.
53mmSpectralrange(m)0.
25-1.
00(Vis)0.
95-5.
05(IR)Fieldofview(FOV)64mrad(slit)x64mrad(scan)ModulationTransferFunction(MTF)@(cy/1mrad)>50%FWHM(LinearSpreadFunction(LSF)slitpix)<60mSpectralresolution100-500Spectrometermagnification145Table8.
DetectabilityofmajorelementsEucrite(Mason,1979)FerroanAnorthosite(Lunar)Hi-TiBasalt(Lunar)AbundanceεAbundanceεAbundanceεO42.
4%1.
54%45.
6%1.
35%41.
5%1.
61%Si22.
8%0.
96%20.
7%1.
09%18%1.
28%Ti38%0.
28%0.
08%41.
36%7.
8%1.
00%Al6.
6%5.
41%17.
6%1.
86%4.
6%8.
01%Fe14.
6%0.
91%1.
5%2.
47%14.
2%0.
92%Mg4.
6%4.
13%0.
5%32.
71%6%3.
22%Ca7.
2%3.
38%13.
6%1.
81%7.
4%3.
29%Th0.
45ppm5.
29%0.
8ppm3.
12%10ppm0.
27%U0.
1ppm17.
47%0.
3ppm5.
95%3.
7ppm0.
51%K400ppm4.
79%200ppm9.
32%3300ppm0.
63%46Table9.
DataProductsInstr.
ProductGR/NSK/Th/UmapsOmapSimapCamapAlmapFemapsMg/TimapsHmapMAGMagneticmapFCGlobalclearatlasGlobalcoloratlasGlobalmosaicShapemodelMSPyroxenemapOlivinemapSpinelmapGeologicmapRadioSci.
Gravitycoeff.
&covar.
FreeairgravitymapGeoid&uncertaintymapsBougermap47Fig.
1Vestashapemodel[ZellnerandThomas,1997]48Fig.
2TwoviewsofCeresasobservedbyHubbleSpaceTelescope[J.
Parkerpersonalcommunication,2000]492.
01.
52.
53.
03.
540801201602002400Semi-MajorAxis[AU]NumberofMinorPlanetsVestaCeres280Fig3Numberofasteroidsversussemi-majoraxisinastronomicalunitswiththerangeofheliocentricdistancesoverwhichVestaandCerestravel50Fig.
4PictorialsummaryoftheDawnmission512010201120122.
62.
42.
22.
0020-20VestaRendezvousArrival30Jul2010Departure3Jul2011HeliocentricDistance(AU)SubsolarLatitudeFig.
5(Top)SubsolarlatitudeduringDawnplannedoperationsatVesta.
(Bottom)Heliocentricrange.
52CeresRendezvousArrival8Aug2014EndofMission26Jul20152014201520163.
03.
22.
82.
62.
4020-20HeliocentricDistance(AU)SubsolarLatitudeFig.
6SubsolarlatitudeduringDawnplannedoperationsatCeres535mMagnetometerBoomHighGainAntennaXenonThruster(1of3)ThrustTubeHydrazineThruster(1of12)StarTrackers(2)MappingSpectrometerGR/NSFramingCameraInstrumentBenchRadiatorPanelSolarArrayLaunchVehicleSeparationPlaneFig.
7Dawnspacecraftandpayload.
Notethatthesolararrayandthemagnetometerboomhavebeencropped540.
81.
01.
24006008001000nmRelativeReflectanceCeresVesta1234567Fig.
8CeresandVestareflectanceinthevisibleandnearinfraredwithfilterbandsoftheframingcamerasuperimposed55Fig.
9Thesensorheadoftheframingcamera560.
300.
600.
901.
201.
501.
802.
102.
40SpinelPyroxeneOlivineSerpentineIronNickelAsphaltiteTroiliteMagnetiteGraphiteWavelengthinMicrons0.
400.
200.
100.
300.
100.
000.
100.
200.
100.
00Reflectance0.
700.
500.
300.
100.
10Fig.
10Reflectanceofdifferentmaterialsillustratingthevariationinabsorptionfeaturesindifferentmaterials[PietersandMcFadden1994]57NormalizedSpectralReflectanceWavelength(m)0.
51.
01.
52.
02.
532116Psyche15Eunomia8Flora4Vesta1CeresFig.
11Normalizedreflectancespectraofmajorasteroids[Gaffeyetal.
,1989]580.
60.
81.
02.
43.
02.
82.
63.
23.
43.
61.
2ScaledReflectanceWavelength(m)Ceres,joinedspectrumRe-scaledNH4-montmorilloniteRivkin1997Fig.
12SpectrumofCeresillustratingabsorptionnear3m[Rivkin1997]59TelescopeOpticalAxisFoldingMirrorGratingSpectrometerSlitXZ+YUpward14.
2cm22.
1cmIRChannelFocalPlane(SpectralDirection)Fig.
13OpticallayoutofthevisibleandinfraredmappingSpectrometer60EarthSurfaceVenusNakhlaVenusMORBShergottitesLunarSurfaceEucritesBulkMoonBulkEarthClCM,CVLHE105104103104103102K(ppm)K/UFig.
14Potassiumtouraniumratioversuspotassiumconcentration[Taylor,1992]61Fig.
15Gammaray/neutronspectrometer62Fig.
16VestatopographyrevealedbyHubbleSpaceTelescopeimagery[Oner,1997]63Fig.
17Mainelectronicsunitblockdiagram(singletriad)togetherwithST5mainelectronicsboard64Fig.
18RingcoresensorsdevelopedforST5tobeusedonDawnmission650.
300.
340.
38NormalizedPolarMoment80160200120CoreRadius[km]MantleDensityRange2700to3500kg/m3BulkDensity3700kg/m3900kg/m4100kg/m333Fig.
19PolarmomentofinertiaversuscoreradiusofVesta