The Calathus Mission Alpbach, 25.07.18 D. Barros Caballero, N. Berge, C. Convenevole, Z. Dionnet, G. Duvieusart, F. Enengl, G. Fedorets, F. Hessinger, B. Kedziora, A. Kiss, L. Kissick, J. Navarro Montilla, M. Novak, H. Petersson, K. Pump Tutors: Günter Kargl and Cecilia Tubiana ABSTRACT With recent missions to asteroids, comets and icy moons, we have realized that the formation conditions of small bodies in the solar system may be more complex than previously thought. To know where and under what conditions an asteroid or comet formed, we would need to know its precise composition. The Dawn mission discovered that the Occator Crater on Ceres has areas of salts ejected from its interior. The composition of these salts and their surroundings provides valuable information about the formation of small bodies in the solar system, as well as on the possibility of favorable conditions for life on Ceres in the distant past. Calathus is a mission designed to travel to the Occator Crater to perform mass spectroscopy and take high resolution images of the surface, but most importantly it will bring samples of the salty surface material back to Earth. The spacecraft consists of an orbiter to map the crater, a lander equipped with a drill, a mass spectrometer, a thermal mapper and a second camera to be left on the surface as the collected sample rejoins the orbiter to return to Earth for analysis. The Calathus mission aims to return a sample of a main asteroid belt body to contribute in research of the solar systems origin by characterizing Ceres’ white spots. 1 INTRODUCTION We here propose a sample return mission to the dwarf planet Ceres, to acquire a sample from a bright spot in the Occator Crater. We name the mission Calathus, after a basket used by the agriculture and fertility goddess Ceres, which she used to deliver grain to the people. 1.1 SCIENTIFIC BACKGROUND The dwarf planet Ceres holds a unique position among the bodies of the inner solar system. Not only is it the largest body in the main asteroid belt, making up 30% of the entire belt’s mass (Pitjeva & Pitjev, 2016), but it is also, to our knowledge, the only differentiated body in the inner Solar system comparable to carbonaceous bodies. Among the many discoveries made by the Dawn mission mapping the surface of Ceres, exceptionally bright spots in the Occator Crater called faculae have received special attention (e.g. de Sanctis et al., 2016) due to their unusual chemical composition, see Figure 1. These faculae are associated with impact-related fractures and cryovolcanism- associated domes (Buczkowski et al., 2016), both suggesting that their origin might be a result of upwelling of brines from beneath the crater surface. The abundance of organic material on Ceres' surface and the presence of salts in liquid water at some point during its formation also raises the question of whether the conditions for the formation of life may ever have been present in the dwarf planet. These kinds of questions have also arisen for both Jupiter and Saturn's icy moons, although these are heated by tidal forces, whereas Ceres only relies on its leftover core heat. Figure 1: A close-up view of the Occator Crater with its faculae as imaged by the Dawn mission (NASA). The surface of Ceres is similar to carbonaceous chondrites and can be classified as a C-type asteroid from spectroscopic observations (Larson et al., 1979). It also has a low bulk density, which implies high water content (McCord & Zambon, in press), and a combination of carbonates with other salt ions at the same sites (Carrozzo et al., 2018). This, in particular the presence of carbonates, is a strong indicator of Ceres having an aqueous nature with a subsurface cryosphere. Cryospheres of various types have already been inferred for Jupiter's moons Europa,
ABSTRACTWithrecentmissionstoasteroids,cometsandicymoons,wehaverealizedthattheformationconditionsofsmallbodiesinthesolarsystemmaybemorecomplexthanpreviouslythought.Toknowwhereandunderwhatconditionsanasteroidorcometformed,wewouldneedtoknowitsprecisecomposition.TheDawnmissiondiscoveredthattheOccatorCrateronCereshasareasofsaltsejectedfromitsinterior.Thecompositionof these salts and their surroundings provides valuable information about the formation of small bodies in the solarsystem,aswellasonthepossibilityoffavorableconditionsforlifeonCeresinthedistantpast.Calathus is amission designed to travel to the Occator Crater to performmass spectroscopy and take high resolutionimagesofthesurface,butmostimportantlyitwillbringsamplesofthesaltysurfacematerialbacktoEarth.Thespacecraftconsists of an orbiter tomap the crater, a lander equippedwith a drill, amass spectrometer, a thermalmapper and asecondcameratobeleftonthesurfaceasthecollectedsamplerejoinstheorbitertoreturntoEarthforanalysis.
1 INTRODUCTIONWe here propose a sample return mission to the dwarfplanetCeres,toacquireasamplefromabrightspotintheOccator Crater. We name the mission Calathus, after abasketusedby theagricultureand fertilitygoddessCeres,whichsheusedtodelivergraintothepeople.1.1 SCIENTIFICBACKGROUNDThedwarfplanetCeresholdsauniquepositionamongthebodiesof the inner solar system.Notonly is it the largestbodyinthemainasteroidbelt,makingup30%oftheentirebelt’s mass (Pitjeva & Pitjev, 2016), but it is also, to ourknowledge, theonlydifferentiatedbody in the innerSolarsystem comparable to carbonaceous bodies. Among themanydiscoveriesmadebytheDawnmissionmappingthesurface of Ceres, exceptionally bright spots in theOccatorCratercalledfaculaehavereceivedspecialattention(e.g.deSanctis et al., 2016) due to their unusual chemicalcomposition, see Figure 1. These faculae are associatedwith impact-related fractures and cryovolcanism-associated domes (Buczkowski et al., 2016), bothsuggesting that theiroriginmightbea resultofupwellingofbrinesfrombeneaththecratersurface.The abundance of organicmaterial on Ceres' surface andthepresenceof salts in liquidwaterat somepointduringits formation also raises the question of whether theconditions for the formation of life may ever have beenpresentinthedwarfplanet.Thesekindsofquestionshavealso arisen for both Jupiter and Saturn's icy moons,
although these are heated by tidal forces, whereas Ceresonlyreliesonitsleftovercoreheat.
Figure1:Aclose-upviewoftheOccatorCraterwithitsfaculaeasimagedbytheDawnmission(NASA).ThesurfaceofCeresissimilartocarbonaceouschondritesand can be classified as a C-type asteroid fromspectroscopicobservations(Larsonetal.,1979).Italsohasa low bulk density, which implies high water content(McCord & Zambon, in press), and a combination ofcarbonateswithothersaltionsatthesamesites(Carrozzoetal.,2018).This,inparticularthepresenceofcarbonates,is a strong indicator of Ceres having an aqueous naturewithasubsurfacecryosphere.Cryospheresofvarioustypeshave already been inferred for Jupiter's moons Europa,
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Callisto,andGanymede,whiletheNASACassinispacecraftdirectly observed water vapor plumes and salts frombeneath thesurfaceofSaturn'smoonEnceladus (Waiteetal.,2009).WhilethesaltcarbonatesonCeresareconcentratedintheOccatorCrater, ammoniatedphyllosilicates areubiquitousacross its surface (de Sanctis et al, 2015). Their presencesignifies abundant chemical contact of surface mineralswiththevolatileammoniaatthedawnofthesolarsystem(de Sanctis et al., 2015, 2016). The dynamical models ofVokrouhlickýetal.(2016)arguethatthemigrationofCeresto the main belt is improbable but not impossible. Thiswouldrequirespecificcircumstancesduringtheplanetaryinstability scenario in the early formation stages of thesolarsystemknownasthegrandtack(Walshetal.,2011).It would also imply that many of the current C-typeasteroids in the main asteroid belt could have migratedalongwithCeres.It should also be noted that the dating of Jupiter'sformation provides evidence of two spatially andcompositionally distinct reservoirs forming on either sideof the gas giant – hydrated carbonaceous bodies furtherout,anddehydratednon-carbonaceousbodiesclosertotheSun(Morbidellietal.2016;Kruijeretal.,2017).Ceres,withasemi-majoraxisof2.77AU,currentlyliesjustoutsidethedehydrated realm of the primordial snowline at 2.70 AU.After Jupiter's formation, destabilisation and re-organisationofitsorbitleadtoamixingofthesereservoirstothepresent-dayhomogeneity(Kruijeretal.,2017).ThiscouldexplainCeres'volatile-richcrustdespite itspositionrelativelyclosetotheSuninthemainasteroidbelt.As a summary, we here identify two scientific domainswherefurtherinvestigationofCeresisrequired:
1. Were the ingredients for life present in thesubsurfaceofCeres?
2. WheredidCeresforminthesolarsystem?1.1.1 PreviousandupcomingmissionsOur proposed Calathus mission to Ceres stands on theshoulders of half a century of in situ and sample returnmissions. The Dawnmission, which one could consider aprecursor or scout mission for Calathus, has collectednearly all currently known physical information aboutCeres.TherecentRosettamissionwithitslanderPhilaetothe comet 67P/Churyumov-Gerasimenko serves as asource of both scientific and technological inspiration.Other results that are scientifically interesting for theCalathus mission have been obtained by the Cassinimission during its flight through the plume of Enceladus(Waite et al., 2017). We also anticipate results from theupcomingJupiterIcyMoonExplorer(JUICE)missiontotheJoviansystem.There are multiple lessons to be learned about sampleacquisition fromthePhobos-GruntandupcomingLuna-27and InSight missions. Other counterpart missions are theOSIRIS-RExsamplereturnmissionfromasteroid(101955)
Bennu,themissionHayabusa2toasteroid(162173)RyuguandanticipatedMarssamplereturnmissions.1.2 MISSIONPROFILEThegoalofourmissionistolaunchaprobefromEarthtoCeres. The spacecraft Calathus consists of four segments:the orbiter, the orbiting sample (OS), the propulsionplatform and the surfacemodule, see Figure 2. The latterthreepartscomprisethelanderPiazzi.Wewillbeusingionthrusters and a Mars fly-by to reduce the required fuelmass andmake the launch feasible. Ceres’ orbit averages2.77 AU from the Sun, and 110 m2 of solar panels willpowerthespacecraftonthewaythere.
Figure 2: Exploded view of the Calathus spacecraft and itssegments.WhentheprobeentersCeres’proximity,itwillperformanorbital insertion and map the Occator Crater faculae,sendingthegathereddatatoEarthforanalysisandlandingsite selection. Once an adequate landing spot is selected,the lander will decouple from the orbiter. It will usechemicalenginestodeorbitandsafelylandonthesurface,photographingthesurfaceduringthedescent.On the surface, batteries will power the lander while itperformsseveraloperations.Thefirstandmostimportantoperationwillbe theacquisitionof four samplesofCeres’surface. The necessary instruments will be located at theend of a robotic arm, which will also carry a camera forEarth-based decision of the exact sampling location. Wewillusetitaniumbrushestoremovetheuppermostspace-weathered layer of the surface that may have beencontaminated by the engines’ exhaust, and a drill withdifferentpowerincrementstocollectthesamples.Oncethesampleiscollected,thearmwillplaceitintoabasketinthebus.Theprocess is thenrepeatedthreetimes forasmanysamples,withafourthrepetitionplacingasampleintotheonboardmassspectrometertodetermineCeres’D/H-ratioand relative abundances of volatile subsurface material.When finished, the arm will perform a 360° horizontalrotationtocaptureapanoramafromthelandingsite.Once all operations on Ceres’ surface are done, thepropulsion platform will launch the OS with the samplebasket on board, detaching from the landing structure,lifting off and leaving the rest of the instrumentation onCeres.TheOSwillperformarendezvousanddockwiththeorbiter,whichwillcapturethebasketwithanarmandput
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it into the reentry capsule. The orbiter will then leaveCeres’ orbit to return to Earth, againwith ion propulsion,ending with an atmospheric reentry of the capsule onEarth.Thecapsulewillberetrievedontheground,broughttoacurationfacilitywherepreliminarytestswillbedone,before distributing samplematerial to laboratories acrosstheworld.Thetotalspacecrafthasavolumeof2.5x2.5x3.3mandawetmassof5569kg.
2 SCIENCE2.1 SCIENTIFICQUESTIONSThescientificquestionsidentifiedfortheCalathusmissionareasfollows:1. What is the nature of the bright material on Ceres’surface?Theexactmineralogical compositionof the faculaeon thesurface of the Occator Crater cannot be resolved throughremote sensing observations. The spectroscopicobservations from Dawn indicate the presence of variouscarbonatesrangingfromnatrite,natron,magnesite,calciteand dolomite, to even more exotic species such asrhodochrosite (Carrozzo et al., 2018), but theirspectroscopicbands inseparablyoverlap.A sample returnfromthefaculaewillresolvetherespectiveabundancesofthecomponentspresentinthebrightmaterial,fromwhichthe nature of their subsurface parent reservoir can becharacterized.2.Are the ingredients for lifepresent in the subsurfaceofCeres?The faculae in the Occator Crater are among the mostaccessible retrievable materials that originate fromsubsurface cryospheric reservoirs, falling into the samecategory as Europa and Enceladus. By investigating theresidual material from this reservoir, we can probe theorganic and chemical composition of an unchartedprimordialenvironment.3.Whatroledosmallbodycryospheresplayinthesearchforlife?With the discovery of subsurface oceans on Europa andEnceladus,ouroriginal ideaofsmall,hydrosphere-bearingworldsasanomalieshaschanged.Thetraditionaldefinitionofthehabitablezonebasedonliquidwateronthesurfaceis challenged. Ceres' cryosphere only has minor surfaceactivity,whichmightindicateanolongeractiveworld.Stillit is one of the most accessible examples of these icyworlds,makingitaprimetargetfortheexplorationofthisclassofbodies.4.WhatisthenatureofCeres'carbonaceousmaterial?Characterizing the carbonaceous material on the surfacethrough thorough mineralogical analysis will give moreinsight concerning the pressure and temperature underwhich Ceres was formed. The precise birth conditions ofthese elements will pinpoint Ceres in context of theformationofthesolarsystem.
5.WheredidCeresandotherC-typeasteroidsform?If Ceres indeed formed in the gas giant region, theplanetary migration process must have resulted in asubstantial portion of current C-type asteroids migratinginwardstothemainasteroidbelt.Learningabouttheexactformation circumstances of Ceres would constrain theacceptable variations of the grand tack early instabilityscenarioofgiantplanetsintheSolarSystem.6. Did C-type small bodies contribute to the delivery ofEarth'swater?Various measurements of deuterium/hydrogen (D/H)ratios from objects around the solar system have notrevealed themysterybehind theoriginofwateronEarth.Based on previous measurements, C-type asteroids havetheD/H-ratiomostsimilar to thatofEarth.TheD/H-ratioofvolatilesnearCeres'surfaceprovidesalinkbetweenicymoonsandC-typeasteroids.2.2 SCIENTIFICOBJECTIVESThe identified science questions have been quantified toincludethefollowingscientificobjectives:
12. TocomparethepropertiesofCerestoicymoons.2.3 SAMPLERETURNIn order to fulfill all but the last two of the scientificobjectives, we require the following sample analysismethods:
1. X-ray diffraction for determining the mineral orchemicalstructureofthesample.
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2. Gas chromatography mass spectroscopy for theidentificationoftheinsolubleorganicphase.
3. Infrared spectroscopy for identifying spatialdistributionsoforganicsandmineralsandthelinkbetweenthem.
All mentioned methods require large, massive andextremely high precision instrumentation, which is notfeasible to accomodate on a spacecraft. Sample returnallowsthesecomplexmethodstobecarriedoutonEarthtoa scientificallypermissible levelofaccuracy.Furthermore,returned samples allow the material to be characterizedand compared by research laboratories across the world.Returning samples also provides material for futuregenerationsofscientiststoutilizemethodsandtechniquesnotyetinvented.Notableforthismissionisthepossibilitythat Ceres is a parent body for certain clans ofcarbonaceous chondrites, as spectra of Ceres made fromEarth and by Dawn are similar to the spectra of suchmeteorites found on Earth (e.g., Larsen et al., 1979;Ehlmannetal.,2018).Returnedsampleswouldenablethehighprecision techniques used for studyingmeteorites tobe applied to samples from Ceres, contextualizing theoriginofcarbonaceouschondritemeteorites.Themain priority of the sample return is the retrieval ofthewhite carbonatematerial from theOccator faculae.Asdark colororganicmatter andammoniatedphyllosilicatesare ubiquitous on the surface, it is highly likely that theywillalsobepresentatthelandingsite.Intheunlikelycaseof the sample containing exclusively white material, weneverthelesshavethecapabilityofansweringthemajorityofourscientificquestions.2.4 INSITUMEASUREMENTSInadditiontothesamplereturn,somemeasurementsneedtobeperformedon-site.Thispertainsprimarilytovolatilesubstances, as they are not transportable to Earthwithinthescopeofthismission.TheonboardmassspectrometerwillcharacterizetheD/H-ratioofanyvolatilesandrelativeabundances of gases exposed during the drilling of thesamples. Detailed mapping of the landing site will beperformed,whichcanbeusedtocreatetopographicmaps.A thermal mapper will perform temperature variationmeasurementsofthelandingsite.Wewillalsocharacterizethelandingandsamplesiteswithcamerasandthesurfacestrength of the sampling site jointly with the drillingoperation.
3 PAYLOAD3.1 ORBITER3.1.1 OrbitercameraThe Calathus orbiter will have a high resolution, narrowangle camera for choosing a landing site, as well as toprovide us better information about Ceres’ surface. TheDawnmissionalreadymappedCereswitha resolutionupto3.3m/pxachievedonit’sclosestfly-bys.Thecameraontheorbiterwillbuildonheritage fromtheOSIRISNarrowAngle Camera (NAC) on the Rosettamission (Keller et al.2007).Sinceourmissionwillbemore than tenyears intothe future, we will replace the CCD from the NAC with amoremodern onewith 4096 x 4096 pixels. This gives anangular resolution of 9.3 μrad/px for the NAC’s 2.20° x2.22° field of view (FOV). Thiswill allow us to perform amappingof theOccatorCraterat a resolutionof less than1.1 m/px from a distance of 100 km. This is animprovement of factor three from the Dawn images. Wewould then go to lower orbits, mapping the faculae andlanding site in even higher resolution. We would notrequireasmany filtersas the14theNACcarried,butwilldo with 3 filters in the visible spectrum, as well as theHydro and Fe2O3 filters from the NAC. The mechanicalshutteron theNAC is tooslowfor takingclearpicturesatthepredictedorbital speeds, soweenvisionexchanging itfor an electrical shutter capable of exposure times downtoward1ms.3.2 THELANDERPIAZZI3.2.1 LandercamerasTo map the terrain around the sample during descent,heritage fromtheRosettaLander ImagingSystem(ROLIS)isused,previously carriedbyRosetta’s landerPhilae.ThecameraismountedonthebottomofPiazzi,andconsistsofaminiaturizedCCDcameraandfourindependentarraysoflight emitting diodes (LEDs) in visible and near-infraredwavelength ranges to illuminate the image field. Thedetector,aThomsonTH7888Awith frontside illuminatedframe transfer CCD, has a 1024 x 1024 pixel active area.Onemoveableandfourfixedlensesprojectasquarefieldofviewof 57.7° x 57.7° on theCCD,which corresponds to aresolution of 0.98 mrad/px. Therefore, an object 30 cmawaycanbemappedwitharesolutionof0.33mm/px.TheROLIS Imaging Main Electronics (IME) is the interface tothelanderandrestrictstheoperationaltemperaturerangeto-50°Cto+60°C,whereasitscameraheadelectronicscangodownto-150°Cwithminimumpowerconsumption.A second camera set on the robotic arm of the lander isused to put the immediate sample area in context. Themodel is heritage of the InSight Mars lander InstrumentDeploymentCamerawhichisaflightspareofMarsScienceLaboratory (MSL)NavcamSerialNumber210.TheseMSLcameraswereconvertedfromgrayscaletocolourbyusingaBayer colour filterarrayversion insteadof thepreviousdetector.Itwillthenoperateinthevisiblelightrange.Thedetectorhas a1024x1024pixel active area.Theangularresolutionis0.82mrad/pxatthecenteroftheFOV,witha45°x45°FOV.Since thiscamerasystemhasbeenutilized
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previously on Mars landers, it is qualified for thetemperaturerangesonamissiontoCeres.3.2.2 ThermalmapperAn infrared mapper is used to complement thecharacterization by the cameras of the bright materialaroundthelandingsite.Theinfraredmappermeasuresthetemperature variations, so a temperature profile can bemade of the immediate area around the sample. Thethermal mapper is based on heritage of the MASCOTradiometer MARA on the Hayabusa2 mission. Theinstrument’sFOVis10°.Thesensorinuseisaminiaturizedthermalradiationsensor,modelTS-72M,bytheInstituteofPhotonic Technologies, Jena. The expected absolutebrightness temperature accuracy of the instrument is 2 Koperatingatlowtemperatures(100K–400K).3.2.3 SamplerThe requirements for the Calathus mission are to collectthreesamples,eachwithavolumeof4cm3(roughly10g)thatshallreturntoEarth.Thesamplesshallbecollectedataminimumdepthof50mm.Inaddition,onesampleof0.5cm3 shall be delivered from the surface to the on-boardmass spectrometer. The sample collecting process mustmaintainthetemperaturebelow-20°Ctokeepthevolatilematerials. Due to the limited information available aboutCeres’ surface, the system will be capable to work indifferent typesof soil: solidwith compressive strengthupto20MPa, and loosewithhighand lowadhesion.Finally,all sampling areas will be cleaned from materialscontaminated by the solar wind, space dust or landerthrusters.
Figure3:Hammeringdrillcross-sectionThe sampling system is equippedwith the four followinginstruments: a hammering drill to collect samples, acamera to provide feedback, a mechanical brush to clean
theimmediatesamplingareaandamanipulatortooperatealldeclaredinstruments.Thehammeringdrill,seeFigure3,isbasedontheprincipleof a reluctant electromagnet, located inside the device,generating strokes whose energy is transformed totorsional and linear movement of the drill. Therefore itgeneratesmuchlessheatonthedrillingendincomparisonto a conventional driller and can still penetrate highlycompressed regolith, rock and ice. By splitting the deviceintothreeparts,itcanoperateonmicrogravitybodiesandrequires support only during the initial phases ofoperation.A detachable sampling container made of hardenedtitanium with diamond inserted at the bottom also playstheroleofadrillingbit.ThecontainerwillreturntoEarthwiththesampleinside.ADCmotorwilldrivethebrushandhavebristlesmadeoftitanium in order to remove even solid contaminatedmaterial. The 2-meter long manipulator will be based onalready available systems with profound space heritagehavingTechnologicalReadinessLevel(TRL)9.3.2.4 MassspectrometerPiazzi will have amass spectrometer to perform analysison volatiles (such as water, carbon monoxide and noblegases), light elements (carbon, nitrogen and oxygen) andlight organic compounds. It is crucial to do thesemeasurementsinsitu,aspartofthevolatileswouldbelostduringthetravelbacktoEarth.Afterthecollectionofthreesamplesforthereturncapsule,wearegoingtocollectonelast sample and carry out mass spectrometrymeasurements.Asmalllaserwillheatthissample,andtheresultant gas will be analyzed to extract its chemicalcompositionandisotopicratio.Forinstance,theD/Hratiowillbeobtainedtobecomparedtothatofotherbodiesofoursolarsystem(Hallis,2017).Wewillalsotakeadvantageofthemeasurementoflightelements.WearegoingtouseaninstrumentdevelopedforMercuryexploration(Rohneretal.,2017).Asthissetupwasdesignedforaspacemissionit has a low mass (0.5kg) and power consumption (3W),but alsohighperformance. It can reach amass resolutionbetter than Δm/m = 600 and 30‰ for isotopiccompositions.
5 MISSION5.1 LAUNCHTheCalathusmissionwillbeginwithalaunchfromKourou,FrenchGuiana,usingtheAriane64rocket.Thespacecraft’swetmassatlaunchis5569kg,andthedimensions,seeninTable 1, allows for a dual launch configuration. ThetargetedΔvis10.6km/sintheEarthinertialframe,whichallowsusforagoodstartonthetrajectorytoCeres.
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InnerdimensionsofAriane64
Outerdimensionsofspacecraft
Diameter[mm] 4460 2000x3000 mm, diagonal 3606
Height[mm] 3660 3500
Table1:DimensionsofAriane645.2 ORBIT
Figure 4: The outbound (top) and inbound (bottom)trajectory.TheΔvforaLamberttransferfromEarthtoCeresisabove11.6 km/s plus escape velocity, which is far above thecapabilities of chemical propulsion. Therefore ionpropulsion was selected, and a trajectory made with thePagmo Optimizerwas chosen using amaximum thrust of1.2Nat1AUandaMarsgravityassist.Wecanseethefull
missionorbitinFigure4.Theredandgreenalternatearcsare the part of the trajectory in which we have electricpropulsion.Thelightbluearcsarewhenwedonotfireourionengines.AfterthejourneytoCereswehaveaninsertionintoacircularorbit fromaparabolicone.WeincreasetheinclinationgraduallyuntilwereachCeres’inclination.Thenwe lower our orbit twice with a Hohmann maneuver totakehigh-resolution imagesofCeres’surface.Afterhavingdone ancillary science for 404 days,we descendwith thelander and collect the samples. Then we turn on the ionenginestoprovidethrustfor149days.Thefinalpartofthejourney will not use propulsion. We catch the Earth andhaveareentryatarelativespeedof11km/s.5.2.1 MappingWhile orbiting Ceres, the orbiter camera will first spendthree months mapping the whole Occator Crater at analtitude of 100 km,with amapping distance ofmaximum120km.Theresolutionatthisheightisatleast1.11m/px,which results in approximately 4600 images of around4000mx4000m.Afterthis,mappingisdoneatanaltitudeof40kmfortwomonths to cover the faculae with a better resolution forselectingalandingsite.Theresolutionoftheseimageswillbe less than 0.56m/px.When a landing site is chosen, afinalmappingisdoneover5dayswithresolutionssmallerthan0.28m/pxatanaltitudeof20km,maximumdistance30km,collectingabout50imagesoftheimmediatelandingarea.5.3 LANDINGONCERESThelanderPiazziwilldetachfromtheorbitertodescendtothe surface of Ceres. The spacecraftwill be orbiting at analtitude of 20 km, making the Δv requirement of thisprocedure 419 m/s. Piazzi will determine its altitude byusinganonboard radar.A landing camerawill beused tocontextualizetheobtainedsample.OncePiazzihaslanded,the orbiter fires the electric propulsion engines with a36.23m/sΔvtoreacha100kmorbit.When all the scientific measurements are performed, thebusfiresitschemicalpropulsionengineswitha371m/sΔvtoreachthesameorbitastheorbiter.Forsecurityreasons,thebusandtheorbiterwillbeseparatedbyatleast10kmwhich corresponds to an orbit angle phase of 91° orgreater. Then the OS is separated from the propulsionplatform,whichfirestheenginesonelasttimetoseparatefromtheOSorbit.Forthemoment,weenvisageoneoftheflatterareasinthefaculaeVinaliaandCerealiaforthelandingsiteonCeres,aswe are concerned about avoiding steep slopes andfractures.Thefinaldecisionwillbetakenoncewehavetheresultsofthemappingfromtheorbiter.5.4 RENDEZVOUSTherendezvousphasetakesplaceduringthefollowing24hours. OS radio beacons allow the orbiter to approach it
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using the ADCS. Figure 5 shows the rendezvous system(RVS) based on a Mars Sample Return mission designwhichallowstakingtheorbitersample(OS)fromtheorbitand fixing it inside the Reentry Capsule (RC). Lidars willdetect the OS with less than 0.5 m accuracy during therendezvous.Once theOS is inside the cone, a robotic armblockstheentry.
Figure5:TherendezvoussystemofMarsSampleReturnSpacecraftSystemsArchitecture(Priceetal.,2000).5.5 RETURNTOEARTHAreentrycapsule(RC)containingtheOSisusedtoassurea-20°C thermal environment of the samples during Earthreentry.The reentry sequence is done in four different stages.Beyond 106 km from Earth, the orbiter tracks the Earthusing its navigation systems and the NAC. Then attitudecorrections aremade to reach themostprecise trajectoryfor landing. At less than 40,000 km, the RC is separatedfromtheorbiter.During theseparationtheRC isspineupforstabilizationreasonswithareentryvelocityof11km/s.Finally,whiletheorbiterdisintegratesinthehighestpartoftheEarth’s atmosphere, theRC starts a calculated reentrytrajectorytoreachgroundintheAustralianOutbackwithavelocityof less than41m/s.During the reentry, theRC istrackedby two0.7Wradiobeaconspoweredby260goflithiumbatteries.The design proposed is a reentry capsule based on thetechnologydevelopedfortheMarsSampleReturnmission.AsshowninFigure6,athermalprotectionsystemisolatesthe samples from the temperatures greater than 2500°Cthat will be reached on the surface of the RC. Energyabsorption material will protect the OS from thedeceleration of more than 2500 G at the impact withground.No parachute or attitude control is used in this design.Tests already done shows that even in the worst-case
Figure6:Thereentrycapsule(Priceetal.,2000).5.6 SAMPLECURATIONOnce the sample has landed on Earth, itwill be retrievedand brought to the EuroCare facilities in the UK, viarefrigeratedtransportattemperaturesbelow-20°C.AttheEuroCarefacilities,thebasketandtwoofthethreesamplecapsuleswill be opened in a refrigerated and atmospherecontrolledcontainmentchamber.Thechamberwillbebuiltto Planetary Protection standards to avoid anycontamination of the sample by terrestrial life,contamination of Earth by Cererianmaterial, or chemicalalteration of the sample. When analysis has mapped anyriskandensured there isnodangerof contamination, theopened sample material will be characterized andcataloged, before half of it is distributed to laboratoriesafterareviewoftheirproposedutilization.
6 SPACECRAFTDESIGNThespacecraftCalathusconsistsofthefollowingsegments:theorbiter,and the landerPiazzi,madeupof theorbitingsample (OS), the propulsion platform and the surfacemodule.TheorbitinsertionatCereswillbefollowedbytheseparation of the orbiter from the rest of the spacecraft,which descends to Ceres’ surface. After the completion ofall measurements and the successful acquisition of thesample, the propulsion platform will use its chemicalpropulsionsystemtoleaveCeres’surfaceandto take the sample intoorbit,where itbe capturedby theorbiter and transferred into the reentry capsule. This isfollowedby a transfer back to Earth and the atmosphericreentry.6.1 THERMALCONTROLIn order to meet the temperature requirements of on-board electronic components, instruments and thecollectedsample, thespacecraftCalathus isequippedwithtemperature sensors, carbon-fibre radiators, louvers andelectricheaters.Italsofeaturesmulti-layerinsulationandagolden coating to be able to deal with the very differentthermicsituationsonthejourneytoCeres.
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Duringtheso-called“hotcase”,whichdescribesasituationinwhichCalathus is only 1 au away from the Sun and itssolarpanelsareperpendiculartotheincomingsunlight,theradiators protect the spacecraft from overheating byradiating off up to 46 kW. During the “cold case” theelectric heaters compensate the negative heat balance byproducing more than 230 W of thermal energy – forexampleduringeclipseatCeres.6.2 ATTITUDEANDDETERMINATIONCONTROL
(ADCS)TwoseparateADCSsystemsare required for theCalathusmission, one on the orbiter and on the Piazzi lander. Theattitude control of both modules necessitates the use ofthrusters and reaction wheels together. MonopropellantRCSthrustersareusedtodesaturatethereactionwheels.Themaindrivingfactorfordeterminingthereactionwheelfeaturesisthescanningspeedrequiredtomapthesurfaceof Ceres from the orbiter, as the cameramust be pointedtowards the crater.Basedon the trajectoryof theorbiter,its inertia and the characteristics of Ceres’ motion it wascalculatedthat theorbiterwillhaveanangularvelocityof0.53mrad/s,andasaresultanangularmomentumof2.39Nms. Subsequently, an example of reaction wheel waschosen: the model RSI 4-75/60 by Rockwell Collins. Foractuatingthethreerotationaldegreesof freedomwehaveutilized six reaction wheels, making the system highlyredundant.In the case of the lander, themain driving factorwas thenecessityof a90° tilt before landingwithin a timespanofone hour, which determines the angular velocity of 0.44mrad/s.Forthiscasethecalculatedangularmomentumisaround 0.018Nms, and a potentialmodelwas found: theRWP050 by Blue Canyon Technologies. This model shallproviderequiredangularvelocitywithinafewseconds.6.3 TELECOMMUNICATIONANDLINKBUDGETThe communication system is driven by the payload datarate requirements, and for deep space missions also bymassrequirements.OngroundwewilluseESA’sESTRACKnetwork.Forup-anddownlinkofstatusandtelecommanddata we use low gain antennas (LGA) in the X-band. Forefficiency the LGAs are also used to get the data fromPiazzi.FordatadownlinktheKa-bandisusedtoenablefastdatatransferwiththehighgainantennawithgimbal.Theoperationhas twocriticalphasesconcerning thedatarate: First the mapping phase requiring 5.9 Gb data perorbit with a visibility window of 1.633 hours per orbit,resulting in 1.27 Mb/s *[1] of needed data downlink.Second the 69minute descent of Piazziwith 2.32 Gb and2.71Gbofdata resulting in0.55Mb/s and0.67Mb/s*forPiazzi and the orbiter respectively, resulting of a datatransmission rate through the orbiter of 1.22 Mb/s†. For
Table2:Datatransferrates.6.4 ONBOARDCOMPUTINGData handling is one of the key aspects to control thespacecraft and deliver sufficient data rates for scientificdata to be sent to Earth. The estimated amount of datagenerated per orbit is expected to be from 1.008 Mbit/sduringmappinganduntilthedescentofPiazzi.DuringthistimethedatabusoftheorbiterandPiazzimusthandle2.01Mbit/s and 1.12 Mbit/s respectively. Therefore, bothsystemsareequippedwithaSpaceWirebus.DespitetheADCSandallocationofincomingdata,Calathusdoes not need fast processing. Due to this almost allradiationproofprocessorscanbeusedofftheshelf.Fortheorbiter a RAD750 is sufficient, for Piazzi GR LEON3FT isused.During descent the data rate created by the orbiter andreceived from Piazzi is, as seen above, higher than themaximumdataratetheorbitercansend.Therefore,partofthe incomingdata is stored in the internalmemoryof theorbiter. The needed capacity is 0.3 Gb during the actualdescent. As a worst case scenario, it is assumed that theorbiter has no contact with Earth for four orbits, whichresultsinatotalof2.63Gbofdatatobestored.Thereforeweusea3DPlus4GbFLASHNANDforCalathus.6.5 POWERMODESFor the Calathus mission, the main power consumptionmodesarehighlightedinTable3.DuringthelaunchonanAriane64rocket,theorbiterandPiazziwillbeinalaunchmode limiting their power consumption. Two operationalmodesarehighlightedfortheorbiter.Operationalmode1where Ceres surface characterization is done using theNAC,andoperationalmode2wherePiazzihaslanded.The Piazzi spacecraft has two main consumption modes,the landing mode and the operational mode. Theoperationalmodeconsistsof theclose-upcharacterizationofthesurface,thesamplingandtheinsitumeasurements.Also,boththeorbiterandPiazzihaveasafemodeincaseoffailures compromising the mission. In this mode the
TheCalathusMission
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spacecraft turns off all the systems except the on-boardcomputer, the thermal and telecommunications and onlysendshousekeepinginformation.
ofsolararraysatitsdisposal,producingmorethan20kWofelectricpowernearEarthand4kWwhenorbitingCeres.Also,15kgof lithiumbatteriesdeliver3kWhofelectricalenergy to the different subsystems. For Piazzi, 30 kg oflithiumbatteries thatdeliver6kWhenable the spacecrafttoperformalltechnicalandscientificoperations.
7 RISKANDCOST7.1 RISK HerewediscussthemainrisksoftheCalathusmissionandclassify them into severity from 1 to 5 (5 for the worstcase) and likelihood from remote (A) to near certain (E).The combination of these two classifications lead to aratingfromverylow(green)toveryhigh(red),seeTable4.The fatal case for thismissionwould be to not bring thesample back to Earth safely. In this scenario the missiongoalwouldnotbeachieved.ThelikelihoodisCthatoneofthe steps of collecting and handling the sample fails or isnot correctly performed. The severity of this would be 5.The combination relates to a high risk. ESA is currentlyworking on a Mars sample return and new technologieswill bedeveloped in the coming years.We anticipate thatthe likelihood classification will lower to level B beforeCalathuslaunches,whichwouldleadtoareclassificationoftherisktoamediumlevel.Asthesamplingsitecannotbemappedinadvance,thereisapossibilityofnotbeingabletoretrievematterotherthansalts fromtheCeres faculae.Werate thisas likelybutnotsevere (1C), because evenwith only salts it is possible tofulfillthemajorityofthescientificrequirements.
Anotherriskisthattheionengineswillnotworkproperly,whichcouldleadtoanincorrectorbit.Thiscouldalsoleadto not achieving the mission goal of bringing back asample.Thisriskisratedwithaseverityof5andalikelihoodofA,which gives a summarized low risk. This technology isalreadyatatechnicalreadinesslevel(TRL)of7,andbeforeCalathus will launch it might have become TRL 9, as ionenginesareflownbycurrentmissions(e.g.BepiColombo).InadditionthereistheriskofcontaminatingtheEarth.Werate thisa lowrisk (3B).Planetaryprotectionneeds tobeplannedaccurately, and laboratories areworkingon theirresearch techniques.Missions likeHayabusa2andOSIRIS-RExwilllikelyimproveourcontainmentprocedures,whichwilllowertherisklikelihood.
Table4:Riskclassificationmatrix.7.2 COST Aroughestimationof the costs leads to a1.5billioneuromission,which by ESA classificationswould be an L classmission,seeTable5.
Table5:Missioncostestimate. The most expensive part is the orbiter, because the costestimate is calculated by multiplying correction factors(depending on complexity) by the dry mass. The costslowerwhenpayloador techniques are already tested andused.7.2.1 DescopingoptionsTo lower the mass and costs, it would be possible toremovethethermalmapperorthemassspectrometer.Thisleads to less context information, but the mission goal isstill achievable.However, thiswilldecrease thecostsonlyvery little, as the costs are driven by the structure andelectricalarchitectureoftheorbiter,especiallybythesolarpanels. It is possible towork on a lighter structure of thesolar panels but this could add a safety risk.
7.3 PLANETARYPROTECTIONAccording to theplanetaryprotection (PP) guidelines, theCalathus mission is a class V-RER (Restricted EarthReturn). This implies that our spacecraft must bethoroughlysterilizedbefore launch, that launch frombothEarthandCeres,aswellasEarthreentry,mustbereviewedandapprovedbyaPPofficer.Anyreturninghardwarethathasdirectlyor indirectlycome incontactwithCeresmustbeeithersterilizedorcontainedforreentry.Moreover, the samplewill be required to be curated in afacility capable of preventing any risk of Earthcontamination during sample analysis and distribution.ThisisplannedtobedonebythecurationfacilityEuroCare,whichwillbereadybytheendofthismission.
8 OUTREACHOutreach for space missions is an important for publicacceptance and support. It is necessary to inform peopleaboutwhatisplannedandwhatisgoingonatthemoment.In addition, good outreach can arouse interest andpersuadeyoungpeople tobecomeengineersor scientists.TheCalathusmissionwillthereforebepresentondifferentsocialmediaplatforms(Facebook,Instagram),andwewillhave live tweets and messaging during every importantmissionevent.In addition, it is possible to prepare lessons forschoolteachers, visiting schools and offer open-door days.Onesuchactivitywepreparedwas‘TasteCeres:ahands-onactivitywhere younger audiences can ‘create’ the interiorstructure of Ceres through analogous materials. Theseinclude chocolate sauce in place of a mud-rich mantle,caramel cereal in place of rocks rich in complex organicmatter and an edible clay in place of ammoniatedphyllosilicates. This provides an entertaining andeducationalmeans of explaining the structure of Ceres tosmallchildren.
9 CONCLUSIONThe discovery of bright spots on Ceres with the Dawnmission has raised a lot of questions concerning thecompositionofcarbonaceousmaterialandtheissueofthepast astrobiological potential of Ceres. On the other handthemodels for formationandevolutionof smallbodies inthesolarsystem,inparticulartheoriginofCeres,andmoregenerally C-types asteroids, could be constrained bycharacterizing the organicmaterial. The proposed samplereturn mission Calathus could lead to a breakthrough inthesetwoscientificdomains.From a technological point of view, the Calathus missioncouldserveasaprecursortoasamplereturnmissionfroma Galilean moon of Jupiter. In particular, Europa isspecifically mentioned in ESA’s Cosmic Vision as a targetfor the future due to its astrobiologically exceptional
interest. While such a mission is currently unfeasible,stepwise maturing of technology would make thatendeavorclosertoactuating.
