LessonsforInterstellarTravelfromtheG&CDesignoftheNEAScoutSolarSailMissionAndrewHeaton,NASAMSFC

BenjaminL.Diedrich,JacobsESSSA(DynamicConceptsInc.)

AbstractNASAisdevelopingtheNearEarthAsteroid(NEA)Scoutmissionthatwilluseasolarsailtotraveltoanasteroidwhereitwillperformaslowflybytoacquirescienceimagery.Aguidanceandcontrolsystemwasdevelopedtomeetthescienceandtrajectoryrequirements.TheNEAScoutdesignprocesscanbeappliedtoaninterstellarorprecursormissionthatusesabeam-propelledsail.Thescientificobjectivesaremetbyaccuratelytargetingthedestinationtrajectorypositionandvelocity.Thedestinationistargetedbyunderstandingtheforceonthesailfromthebeam(orsunlightinthecaseofNEAScout)overthedurationofthethrustmaneuver.Thepropulsivemaneuverismaintainedbyaccurateunderstandingofthetorqueonthesail,whichisafunctionofsailshape,opticalproperties,andmassproperties,allofwhichapplytoNEAScoutandbeampropelledsails.NEAScoutusesactivecontrolofthesailattitudewhiletrimmingthesolartorque,whichcouldbeusedonabeamedpropulsionsailifnecessary.ThebiggestdifferenceisthatNEAScoutcancorrectforuncertaintiesinsailthrustmodeling,spacecraftorbit,andtargetorbitthroughouttheflighttothetarget,whilebeamedpropulsionneedsaccurateoperationfortheshortdurationofthebeamedpropulsionmaneuver,makingaccurateunderstandingofthesailthrustandorbitsmuchmorecritical.

IntroductionTheNearEarthAsteroid(NEA)Scoutmissionhastwoprimarygoals.OneistoimageandcharacterizeaNEAduringaslowflyby.Theotheristodemonstratealowcostmethodofperformingasteroidreconnaissance1.Toachievethisgoal,aspacecrafthasbeendesignedanddevelopedthatdeploysan86m2solarsailfroma6UcubesatbusthatwillsailtotheasteroidafterlaunchasasecondarypayloadonthefirstSpaceLaunchSystem(SLS)launch.NEAScoutthenusesaseriesofpropulsivemaneuvers,sailthrusting,andlunargravityassiststodeparttheEarth-Moonsystem.TheNEAScoutmissionrequiresanend-to-endspacecraftandmissiondesignthataccountsforalltheuncertaintiesinnavigation,attitudedeterminationandcontrol,thrustandtorqueofthesail,andtargetorbitwhilestillallowingasuccessfulsciencecampaign.

https://ntrs.nasa.gov/search.jsp?R=20170012413 2020-02-01T01:36:31+00:00Z

Figure1.NEAScoutsolarsailandspacecraftbusshowingbodyaxesthatareinplane(XandY)andoutofplane(Z).

Precursorandinitialinterstellarmissionsemployingbeamedpropulsionsimilarlyrequireanunderstandingofhowuncertaintiesinthevariouselementsofthesystemaffecttheabilitytoperformascientificreconnaissanceofthetarget.Earlyinterstellarmissionsmaybetocharacterizeatargetstarorstarsandtheirplanets,sothatfollow-onmissionscanpursuemorecomprehensiveortargetedscientificobjectives.Interstellarprecursormissionsmaydemonstrateinterstellarcapabilitiesby,forexample,surveyingKuiperBeltorOortCloudobjects.Ineithercase,thespacecraftneedtoaccuratelytargetspecificbodies,andflybywithinarangeoftimes,distances,orvelocitiesthatallowsuccessfulobservation.Todothis,theuncertaintiesofthebeamedpropulsionsystem,spacecraftorbitdetermination,sailforcesandtorquesinresponsetothebeam,gravitationalandotherperturbationsonthespacecraftorbit,andknowledgeofthetargetbodiesneedtobeaccountedforindesigningthespacecraftandmission.

Precursormissionstoexplorethespaceenvironmentwheretheouterheliosphereinteractswithinterstellarspacearelessconstrainedbytheirdestination.Theystillrequireaccurateenoughguidanceofthebeamonthesail,accountingforallthesameuncertaintiesasaflybyexceptforthetargetorbit.

ScientificObjectivesNEAScoutThescientificobjectivesofNEAScoutaretoperformlongrangeopticalnavigationofthetargetasteroid,mediumrangereconnaissance,andcloseproximityimaging.Thetargetasteroidis1991VG.Itisasmall~5-12mdiameterasteroidwithunknownalbedoandanEarth-likeorbitthatisreachablebyNEAScoutwithina2.5yearmissionduration.1

Opticalnavigationisperformedstartingat50,000kmfromtheasteroid,andisusedtoupdatetheorbitestimateoftheasteroidasthesailapproaches.Duringapproach,opticalnavigationdataisdownlinkedtoEarth,theorbitoftheasteroidisupdated,andthesteeringguidanceofthesolarsailisupdatedtomoreaccuratelytargetthecloseapproach.Mediumrangeobservationswithin100kmoftheasteroidand50cm/pixelresolutionover80%oftheasteroid,willcharacterizethevolume,shape,spinproperties,andlocalenvironment.Closeproximityimagingat10cm/pixelresolutionover30%ofthesurfacewillcharacterizetheasteroid’slocalmorphologyandregolithproperties.Theminimumscientific

objectiveswillbemetwithaclosestapproachof28km,althoughthegoalistotargetunder1km.Achievingthesegoalswillfullydemonstrateanabilitytoperformasteroidreconnaissancewithasmallsolarsailspacecraft,employingcubesattechnologies,andallowtheidentificationofhighsciencevaluetargetstovisitwithfollow-onmissions.2

Figure2.PhasesofNEAScoutscience,fromtargetdetectionandopticalnavigation,mid-rangereconnassaince,andclose

proximityscienceimaging.

Thekeydriversforthedesignofthemissionaretotargetacloseenoughdistancetotheasteroidataslowenoughrelativevelocity,withknowledgeofwheretopointthesciencecamera,toacquiretheimagesrequiredtoresolvethephysicalproperties.Additionally,theattitudecontrolmustbeaccurateandstableenoughtopointattheasteroidwithoutjitterblurringtheimages.

InterstellarandPrecursorMissionsInterstellarandprecursormissionstoperformreconnaissanceofanotherstaroranoutersolarsystembodywillhavesimilarscientificobjectives.Theserequirementsneedtobedevelopedinordertodetermineifthetechnologyanddesignoftheinterstellarmissionaresufficienttoachieveusefulresults.Ifnot,lessambitiousgoalscanbepursuedfirst,togainexperienceforfollow-onmissions.

Aprimarygoalofafirstinterstellarmissionmaybetocharacterizetheorbitsandgeneralphysicalpropertiesofthetargetstarsandplanets,sothatfollow-onmissionscanperformmorecomprehensiveobservations.ThiswouldbelikeperformingtheopticalnavigationandlimitedmediumrangephysicalobservationsofNEAScout.Oncetheorbitalandphysicalpropertiesofthetargetsystemarebetterunderstood,followonmissionscouldmoreaccuratelytargettheplanetsandstarsthemselvesforin-depthobservations.

BecauseNEAScoutisaninterplanetarymissionthatoperatesat~1AUfromthesun,itstrajectorycanbecontinuouslyupdatedwithsteeringguidancefromgroundcontrollersonEarthtomoreaccurately

flybythetargetbody.Earlyinterstellarandprecursormissionsmaynothavethiscapability,duetoonboardmassconstraints,propulsiontechnologylimitations,andtheseveretimelagtoEarth.

PrecursormissionstosurveyKuiperBeltandOortCloudobjectsmayperformmorein-depthstudiesofthosebodies,becausetheirorbitsmaybemoreaccuratelydeterminedbyobservationsfromEarththanexoplanets,anderrorsinthetrajectorymaybesmallerthaninterstellarmissionsbecauseoftheshorterdistancefromEarthandlowervelocityrequiredtoreachtheminareasonablemissionduration.

Precursormissionstoexploretheheliospheretypicallycarryin-situsensorstomeasurethemagneticfield,chargedparticlepopulation,andplasmapropertiesofthespaceenvironment.Thesesensorsdonotrequirepreciseattitudetopointataparticulartarget,andcanbetakenoveramuchlargerareaofspacethanimagingasolarsystembody.Forthisreason,missionstostudytheouterheliospherewhereitinteractswithinterstellarspacemaybeaneasiertestbedforinterstellarprecursormissions,toemploytechnologieslikebeamedsailpropulsion,whilestillgeneratingusefulscienceresults.

NavigationandGuidanceNEAScoutTheNEAScoutmissionmeetstheposition,velocity,andpointingsciencerequirementsbycontinuouslyupdatingthesteeringguidanceofthesailbasedontrajectoryestimatesofthespacecraftandorbitestimateupdatesofthetarget.Thiscanbereferredtoas“closedloop”navigationofthesail,andallowsforuncertaintiesinthesailforcemodel,navigationaccuracy,perturbations,andattitudecontroltobecorrectedforthroughoutthemission.Radiometricrangeobservationsduringflightareusedtoupdatethetrajectoryestimateandupdatetheforcemodelusedtopredictsailacceleration.Theseareusedtosolvefornewsailsteeringguidancetotargettheapproachtotheasteroid.Duringthemonthlongapproachphase,thenavigationincludesbothradiorangingandopticalnavigationrelativetotheasteroid,withobservationsoftheasteroidmadetwiceaday,andthreetimesadayduringthefinalapproach.

InterstellarandPrecursorUnlikeNEAScout,earlyinterstellarorprecursormissionswillhavelimitedopportunitiesfor“closingtheloop”betweenobservationsoferrorsinthetrajectory,sailthrust,andbeampropertieswithchangestothebeamguidanceorsailattitude(assumingthesailhasactiveattitudecontrol).Forthesakeofdiscussion,thefollowingassumptionsaremadeaboutanearlybeamedpropulsionsystem.Itconsistsofalaserorotherfrequencybeamsystem,locatedonEarthorinorbit,andaspacecraftwithasailonit.Thespacecraftmayhaveactiveattitudecontrolduringbeamedpropulsion,orrelyonpassivestabilityonthebeam.Duetomassconstraints,theremaybelittleornoonboardactivetrajectorymaneuvercapability.Whilethespacecraftcouldsailoncloseapproachusinglightfromthetargetstar,thetrajectorymustfirstbeaccurateenoughtogetcloseenoughforusefulsolarpressure.

Unlessabeamedsailmissionisdoneinmultiplephaseswithanopportunitytoperformtrajectoryestimationandupdatebeamguidancebetweenpropulsionphases,therewillbeonlyonechancetoperformthethrustphaseaccuratelyenoughtoreachthedestination.Onceallpropulsionphasesarecomplete,thesailwillcoasttoitsdestination,subjectonlytothegravitationalandotherenvironmentalperturbations.Theaccuracyofthefinalorbitstateatcompletionofthepropulsionphasemustbehighenoughtoensuretheposition,velocity,andtimingrequirementsofthetargetflybyaremet.

Beforefiringthelaser,theorbitofthespacecraftneedstobeknownwithenoughaccuracytotargetitwiththebeam.Thebeamsteeringneedstobeaccurateenoughtopointitatthepredictedlocationofthesail.Becauseofthespeedoflightandtimeittakestoperformorbitdetermination,therewillnotbetimetoupdatethebeamsteeringinresponsetotheobservedtrajectoryofthesail.Thesailtrajectoryinresponsetothebeamneedstobeaccurateenoughthatsailstaysonthebeam,andthethrustoverthecourseofthemaneuverisaccurateenoughtoputthesailoncoursetothetarget.

ForceandTorqueModelingNEAScoutTheNEAScoutforceandtorquemodelpredictsthethrustrelativetothedirectionofsunlightafunctionofthesailshapeandopticalproperties.Thesedeterminelinearandangularaccelerationsthesailexperienceswhenilluminatedbysunlight.Thismodeliscurrentlyanestimate,andisbasedonmeasurementsofsailmembraneopticalpropertiesandfiniteelementmodelingofthesailboomsandmembranewhensubjecttothermalheatingfromsunlight.Imperfectreflectionresultsinasignificantin-planeforcecomponentwhenthesailisatanangletotheincomingsunlightbecauseofthedifferencebetweenabsorbedandreflectedlight.Thisin-planecomponentresultsinasignificantsolartorquethatmustbecontrolled.3

Afterdeployment,thesailtorquemodelwillbecalibratedbyslewingtoavarietyofsailattitudes,andobservingthereactionoftheattitudecontrolactuatorstosolartorque(seesection“AttitudeDynamicsandControl”).Duringtheearlyphaseofthemission,thesailthrustwillbeestimatedbyusingradiorangingtoobservetheactualtrajectoryandestimatingtheaccelerationrequiredtoproduceit.Differencesinthrustwillshowupindeviationsoftheactualtrajectoryfromthatpredictedusingthepre-launchthrustmodel.Theserefinementswillbeusedtomakethesteeringguidancemoreaccurateduringtheflighttotheasteroid.

Wellbeforelaunch,expecteddeviationsintheshapeandopticalpropertiesneedtobeaccountedfor,toensurethatthesailasmanufacturedissufficienttocompletethemissiontotheasteroid.Toensurethisisthecase,missionplanningassumesareductionfactorinthethrustduetovariationsinsailsize,shape,opticalparameters,andtimespentpointingtowardsEarthforcommunication.Inthisway,thereisconfidencethatthesailhasmorethrustthanisneededtocompletethemission,tocoverunforeseenproblemswiththesaildeploymentormissionoperationsthatresultinlessthrustthanexpected.Sailmodelsthatweredeformedorhadholesinthematthecornersofthesailwereanalyzedtoensuretheattitudecontrolsystemhadenoughmargintocontrolsailswithunexpecteddeploymentproblems.

InterstellarandPrecursorBecauseofthelimitedopportunitiestocorrectforuncertaintiesinthethrustofthebeamedpropulsionsail,thethrustonthedeployedsailshouldbeestimatedbeforelaunchwithenoughaccuracytoensurethemanufacturedsailcombinedwiththebeamsystemcanachievethetrajectorytothetarget.LikeNEAScout,thesailthrustandtorquemodelandbeampowerpropertiescanbecalibratedafterdeploymentinspacewithalowpowertestusingthebeam.Withoutcalibration,thebeamedpropulsionsystemcouldproducetoomuchortoolittleacceleration,resultinginatrajectorythatistoofarfromthetarget,movingtoofastortooslow,orpossiblyresultinthesailfailingtostayilluminatedbythebeamforthefullduration.Withouttorquemodelcalibration,thesailcouldbecomeunstableonthebeam,orhaveoscillationsthataretoolargeandreducetheaverageaccelerationonthesail.

Stabilityofthesailpositiononthebeamisneededtokeepthebeamonthesailforthedurationofallpropulsionmaneuvers.Thisrequiresarestoringforcethatsendsthesailbacktothecenterofthebeam.NEAScouthasnosuchrequirement;itjustrequiresaccurateknowledgeoftheforcemodeltoplansailingmaneuversusingapredictionofthethrustmodelbeforelaunchandcalibrationaftersaildeploymentinresponsetothesourceofillumination.Bothwillbecriticaltoensurestabilityofabeamedpropulsionsail.

AttitudeDynamicsandControlNEAScoutNEAScoutsteersthesailusinganactiveattitudecontrolsystem.Itusesreactionwheelsforpreciseattitudecontrol,whicharesmallflywheelsthatspinupoppositethedirectionthespacecraftneedstosteer.Astartrackercamerameasuresattituderelativetothebackgroundstars.Solartorqueswouldeventuallydrivethereactionwheelstohitthespeedlimitoftheirbearings,soanActiveMassTranslator(AMT)shiftsthecenterofmassinordertokeepwheelspeedsandsolartorqueundercontrolaboutthetwobodyaxesthatarealignedwiththesailbooms.Areactioncontrolsystem(RCS)usessmallthrusterswithcoldgaspropellanttomanagetorqueaccumulationabouttheaxisnormaltothesail(ZaxisinFigure1),whichismuchsmallerthantorquesaboutthebooms(axesXandYinFigure1).Designofthiscontrolsystemrequiresthesolartorquemodelofthesail.Thecontrolactuatorsneedtobesizedwithenoughmargintohandlereasonableuncertaintiesinthesolartorque,duetothesamefactorsthataffectsailthrust–size,shape,andopticalproperties–aswellasshapedifferencesnotreflectedinsailthrustandlocationofthecenterofmass.6

Figure3.ActiveMassTranslator(AMT)forshiftingthecenterofmassofNEAScouttocontrolthesolartorqueonthevehicle.

Japan’sIKAROSsolarsailusedliquidcrystaldiode(LCD)reflectivecontroldevices(RCDs)tocontrolsolartorqueaboutthetwobodyaxesthatlieintheplaneofthesail,liketheAMTdoesforNEAScout.4TheRCDsworkbyelectronicallycontrollingtheamountofsunlightreflectedalongthedifferentedgesorcornersofthesail,resultinginatorqueonthesail.NASAMSFCisworkingwiththeUniversityofMarylandtodevelopsimilarRCDsforpossibleuseonfutureNASAsolarsailmissions.5Withoutactivecontrolbythereactionwheels,AMT,orRCS,NEAScoutisnotpassivelystable,andtheattitudewoulddeviateifuncontrolled.

InterstellarandPrecursorProposalsfornear-termbeamedpropulsiondonotassumeactiveattitudecontrol,tokeeponboardmasslow.Rather,thesailisdesignedtobepassivelystable,bygeneratingrestoringtorquestoreturntotheneededattitudeonthebeamforgeneratingthrusttoachievetherequiredtrajectoryaccuracy.Ifthesailisnotstabilized(activelyorpassively),itcouldfailtogeneratethethrusttostayonthebeamandreachthetarget.Evenifthesailisstable,oscillationsaboutthestablepointwillreducetheaveragethrust,andmayfailtomeetthetrajectoryrequirementsiftheyaretoolarge.

Onelimitationofapassivelystablesailisthatitmaynotbeabletogeneratetheattitudeneededtothrusttowardsthetargetdestination.SolarsailslikeNEAScoutregularlyneedtopointatanangletotheincidentsunlighttogeneratesidewaysthrustforchangingtheorbitenergy.Ifabeamedpropulsionsailcansteertoattitudeatanangletothelaser,itmayopenupmuchmoreflexibilityforgeneratingthethrustneeded.Thisisanareathatrequiresfurtherstudy.Actuatorslikecenterofmassshiftingorreflectivitycontrolmaybeconsideredforactivelymaintainingsailattitudeandpossiblypositionofthesailonthebeam.Evenwithactivecontrol,passivestabilityhelpsmaintainthecommandedattitudewithlesseffortbythecontrolsystem.

ConclusionsThedesignandimplementationoftheNEAScoutsolarsailmissiontovisitanasteroidhighlightsseveralaspectsofsailpropelledmissionsthatareincommonwithbeamedpropulsionmissionstotheoutersolarsystemandtonearbystars,despitethelargedifferencesbetweenthemissions.Accurateknowledgeofthespacecraftandtargetorbitsareneededforbothmissions,whiletheaccuracyforinterstellarandprecursormissionsarefarhigher,becauseofthelimitedopportunitiestocorrectthetrajectorytoaccountforuncertainties.CalibrationoftheforceandtorquemodelimprovesthenavigationandattitudecontrolperformanceofNEAScout,andmaybeessentialtomeetingnavigationrequirementsforaninterstellarmission.

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