spacesciencereviewsmanuscriptNo. (willbeinsertedbytheeditor) ARTEMIS Mission Design TheodoreH.Sweetser · StephenB.Broschart · VassilisAngelopoulos · GregoryJ.Whiffen · David C.Folta · Min-KunChung · SaraJ.Hatch · Mark A.Woodard Received:date/Accepted:date Abstract The ARTEMIS mission takes two of the five THEMIS spacecraft beyond their prime missionobjectivesandreusesthemtostudytheMoonandthelunarspaceenvironment.Al- thoughthespacecraftandfuelresourcesweretailoredtospaceobservationsfromEarthor- bit,sufficientfuelmargins,spacecraftcapability,andoperationalflexibilitywerepresentthat withacircuitous,ballistic,constrained-thrusttrajectory,newscientificinformationcouldbe gleanedfromtheinstrumentsneartheMoonandinlunarorbit.Wediscussthechallengesof ARTEMIStrajectorydesignanddescribeitscurrentimplementationtoaddressbothhelio- physicsandplanetaryscienceobjectives.Inparticular,weexplainthechallengesimposed bytheconstraintsoftheorbitinghardwareanddescribethetrajectorysolutionsfoundinpro- longedballisticflightpathsthatincludemultiplelunarapproaches,lunarflybys,low-energy trajectorysegments,lunarLissajousorbits,andlow-lunar-periapseorbits.Weconcludewith a discussion of the risks that we took to enable the development and implementation of ARTEMIS. Keywords ARTEMIS·THEMIS·low-energytransfer·Lissajousorbits·lunarscience· lunarmission·heliophysics·magnetosphere 1 Introduction TimeHistoryofEventsandMacroscaleInteractionsduringSubstorms(THEMIS)isavery successful NASA Explorer mission launched in February of 2007 to advance our under- standing of magnetic substorms, a space weather phenomenon in the Earth’s magneto- T.Sweetser JetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDr.,M/S:301-121,Pasadena, CA,91109. Tel.:818-354-4986E-mail:[email protected], S.Broschart,V.Angelopoulos,M.-K.Chung,S.Hatch,G.Whiffen, JetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDr.,Pasadena,CA,91109 D.FoltaandM.Woodard GoddardSpaceFlightCenter,Greenbelt,MD. (cid:13)cCopyright2011.Allrightsreserved. 2 sphere(Angelopoulos2008).ThemissionconsistsoffiveidenticalEarth-orbitingspacecraft (probes)equippedwithparticleandfieldinstruments(Harveyetal.2008).Asofthetimeof thiswriting,thebaselinemissionscienceobjectiveshavebeenachieved,andallfiveprobes (andtheirinstruments)arefullyfunctional. InFebruary2008ARTEMIS,theAcceleration,Reconnection,TurbulenceandElectro- dynamicsoftheMoon’sInteractionwiththeSunmission,wasproposedtotheNASAHe- liophysicsSeniorReview(AngelopoulosandSibeck2008)asanextensiontotheTHEMIS mission.ItwasapprovedfordevelopmentinMayofthatyear.TheARTEMISmissionpro- posedtosendthetwooutermostTHEMISprobes,P1andP2(alsoreferredtoasTHEMIS-B andTHEMIS-C),tolunarorbitsbywayoftwocircuitoustransfersthattakeaboutoneand ahalfyearseach.Thegoalsofthemissionasproposedin2008weretousetheMoonasan anchorfortheARTEMISprobestoconductstudiesofEarth’smagnetotailandsolarwind fromapproximately60Earthradiiandtostudythelunarwakeanditsrefillingasafunction oftheupstreamsolarwind.ARTEMIStwo-pointmeasurementsopenanewvantagepointto phenomenapreviouslystudiedbysingle-spacecraftmissions.Inparticular,whensolarwind measurementsaremadesimultaneouslybyoneprobeinthelunarwakeandthesecondfrom variouslocationsjustupstreamofthelunarwake,accuratecomparisonsofwakephenomena withupstreamvariationscanbemade. TheARTEMISproposalrepresentedthecombinedeffortsoftheTHEMISscienceteam ledbythePIatUCLA,theTHEMISMissionOperationsteamledbytheMissionOpera- tionsManagerattheUniversityofCaliforniaBerkeley’sSpaceScienceLaboratory(UCB- SSL),theNASAGoddardSpaceFlightCenter(GSFC),andtheJetPropulsionLaboratory attheCaliforniaInstituteofTechnology(JPL).Twoearlierreports(Broschartetal.2009; Woodardetal.2009)describethepreliminarymissiondesignasproposedin2008;portions ofthispaperaretakenfromthosereports.Thispaperpresentstheevolutionofthetrajectory designtothetrajectorybeingflowntoday,onlyafewmonthspriortolunarorbitinsertion. Numerous challenges were inherent to the ARTEMIS mission’s trajectory design be- causeoftheconstrainedcapabilitiesoftheTHEMISprobes.Limitedfuelremainedafterthe THEMIS baseline mission was completed. Thruster configuration limits thrust directions toonehemisphere.Additionally,anon-offthrusterdutycycleimposedduetothespinning of the probe bus restricts effective thrust to less than a newton in the spin plane, i.e., for maneuverdirectionsneartheeclipticplane.Maneuverscannotbedoneinshadowbecause accuratepulsetimingreliesonsun-sensordata.Telecommunicationswiththeprobeswere limitedtoarangeofabouttwomillionkilometers.Finally,theprobescanonlywithstand uptoa4-hourshadow.HadnothingbeendoneattheendoftheTHEMISbaselinemission, longeclipses(>8hr)wouldhaveneutralizedP1byMarch2010(Angelopoulos2010).This becameaverysignificantdriverforproposingtheARTEMISmission. InSection2wedescribethecapabilitiesandorbitconfigurationoftheTHEMISprobes attheendoftheirbaselinemission.InSection3weoutlinethehistoryoftheARTEMISmis- siondesignconceptasitfollowedthemission’sprogrammaticevolution.Section4outlines thesciencegoalsandorbitdesigngoalsofthemission.Theremainderofthepaperdescribes thedesignofthetrajectoriesthataretakingP1andP2fromeccentric,high-altitudeEarth orbits into lunar orbits that satisfy the science objectives. Figure 1 shows the ARTEMIS trajectory design used to send P1 and P2 from their respective Earth orbits at the start of ARTEMIS maneuvers into lunar Lissajous orbits. Section 5 presents the most up-to-date ARTEMISmissiondesign.Section6describesthecurrentmissionstatus,includingongo- ingtradestudies.Section7isaretrospectiveonthechallengesandenablingattributesofthe missiondesigneffort. 3 Fig.1 ARTEMIStrans-lunartrajectoriesintheeclipticplane.Thecoordinateframehererotatessuchthat theSunisalwaystotheleft.TheredlineshowstheP1trajectory;thebluelineshowstheP2trajectory.The Earthisatthecenterofthefigure,andtheMoon’sorbitisshowningray.ThebluedotsaretheSun-Earth L1andL2Lagrangepoints;thegraydotsaretheMoonandtheEarth-MoonL1andL2pointsataparticular epoch. 2 SpacecraftOverview OnFebruary17th,2007,thefiveTHEMISprobeswerelaunchedonaDelta-II7925rocket intoa1.3-dayEarthorbitwithperigeeat437kmaltitudeandapogeeat∼87500kmaltitude (Angelopoulos2008).Basedoninitialon-orbitdata–inparticular,betterlinkmarginper- formance–THEMIS-Bwasassignedtoa4-dayorbitanddesignated“P1”,andTHEMIS-C wasassignedtoa2-dayorbitanddesignated“P2”.THEMIS-D,E,andAwereassignedto 1-day orbits, becoming P3, 4 and 5, respectively, per the mission design plan (Frey et al. 2008)requiredtoachieveTHEMISmissionsciencegoals(Figure2)(Angelopoulos2008). After29monthsinorbit,thetwooutermostprobes,P1andP2,werecalledontojourneyto theMoonaspartoftheARTEMISmission. ThefiveTHEMISprobeswereidenticalatlaunchwith134kgmass(including49kg ofhydrazinemonopropellant).Eachmeasuresapproximately0.8×0.8×1.0meters(Har- vey et al. 2008). On orbit, each has deployed a number of instrument booms and is spin- stabilizedat∼20RPM.Figure3(a)showsaTHEMISprobewithboomsdeployed.Figure3(b) showsaschematicofthebusdesign.Thebluearrow,whichindicatesthespinvector,shall bereferredtoastheprobe+Zdirection. Each probe has four thrusters, nominally 4.4 N each, with locations indicated by the blackarrowsinFigure3(b).Twoprovideaxialthrust(accelerationin+Zdirection)forlarge ∆V maneuversandattitudecontrol.Theothertwoprovidetangentialthrustinthespinplane forsmall∆V maneuversandspinratecontrol.Notethattheprobescannotapplyacceleration inthe−Z direction.DuringthenominalTHEMISmission,P1andP2wereflownwiththe −Z axis close to the ecliptic north pole, i.e., in an “upside-down” configuration relative 4 Pristine Solar Wind Fig.2 THEMISmissionorbitconfiguration.FilledcirclesrepresentTHEMISprobelocationsduringaday- sideconjunction(Pink:P14-dayorbit,Blue:P22-dayorbit,Red:P31-dayorbit,Green:P41-dayorbit, Black:P51-dayorbit).Theorbitgeometriesareindicatedbyblacklines. (a) (b) +Z C A D B Fig.3 THEMIS/ARTEMISprobeconfiguration.TheprobebusesweremanufacturedbyATKSpaceSys- tems(formerlySwalesAerospace),andtheinstrumentsweremanufacturedundertheleadershipoftheUni- versityofCalifornia,BerkeleywithbothUSandinternationalcollaborators.(a)On-orbitconfigurationwith booms deployed, adapted from Auslander et al. (2008): A – four 20 m long radial EFI booms; B – two 5mlongaxialEFIbooms;C–1mlongSCMboom;D–2mlongFGMboom(http://www.nasa.gov/- images/content/164405mainTHEMIS-Spacecraftbus2.jpg),(b)probebusschematic.Blackarrowsindicate locationsofthe4.4Nhydrazinethrusters.Bluearrowindicatesspinaxis. to ecliptic north and opposite the inner three probes. This was done to aid the main orbit correction maneuvers in the second year of THEMIS, which were designed to counteract lunar perturbations on the orbit plane (Frey et al. 2008). ARTEMIS would maintain the sameorientation,asitisquitefuel-intensivetoimpartspin-axischangestotheprobes.Thus, maneuverstowardseclipticnorthcouldnotbeincludedintheARTEMIStrajectorydesign. Atlaunch,eachprobehad960m/stotal∆V capability(Harveyetal.2008).Atthestartof 5 ARTEMISmaneuverstheremaining∆V (approximately320m/sforP1and467m/sforP2) wereavailablefortheARTEMIStrajectorydesign.Duetofueltankdepressurization(Sholl etal.2007;Freyetal.2008),eachthrusterisexpectedtoproducebetween2.4Nand1.6N forceduringtheARTEMISmission. Because the spacecraft is spinning the effective thrust of a sideways burn is further reduced,soamaneuverinaparticulardirectioninthespinplaneisperformedbypulsing thethrustersonandoffduringeachrevolution.Witha60degpulseduration,thethrusters areononlyone-sixthofthetime(16.7%dutycycle).Becausethrustersareswingingthrough anarc,thethrustinthedesireddirectionisfurtherreducedto95.5%effectivethrust;witha 40degdutycyclethethrustersaverageonlyone-ninththrust,butloseonly2%inefficiency averagedthroughthearcofeachpulse.Onlythesecondreductionineachcaseinfluences theeffectiveI ,soa40degdutycyclewouldbepreferredtoa60degoneexceptthatlower sp thrustmeanslongerburnsduringperiapsepassages,whichwouldincreasegravitylosses. The thermal and power systems have been designed to withstand shadowing from the Sunforuptothreehours(Harveyetal.2008).ItwasdemonstratedinMarchof2009,how- ever,thata4-hourshadowissurvivablewithappropriateprecautions.Thislimitistherefore beingusedasthemaximumallowableshadowdurationfortheARTEMISmissiondesign, where“shadow”isdefinedtobelessthan50%sunlight. 3 ARTEMISConceptDevelopment The baseline THEMIS mission design included the expectation that P1 would experience inordinatelylong(>8hr)shadowsbyMarch2010.AlthoughtheapoapsealtitudeoftheP1 orbitcouldhavebeenreducedtopreventthis,THEMISscientistsandJPLmissiondesigners cameupwiththeideaofsendingP1“up”insteadof“down”in2005.WithTHEMISinstru- mentation,compellingsciencecouldbeconductednearorattheMoonwithasingleprobe. According to initial trajectory studies, a direct transfer from P1 Earth orbit to a 1500 km altitude by 18000 km radius polar orbit at the Moon would require ∼500 m/s of ∆V (not including margin or losses associated with long thrust arcs). This was well beyond P1’s expected∆V capabilityattheendofthebaselinemission.However,theremainingfuelap- pearedsufficienttotransferP1fromitsEarthorbittothedesiredeccentricpolarlunarorbit bywayofalunarswing-byandlow-energytransfer(Chungetal.2005).Wheninitiatedby alunarswing-by,thistypeoftransferdoesnotrequireanyless∆V toleaveEarth,butsaves essentiallyallthe∆V costofgettingintoaLissajousorbitaroundoneoftheEarth-Moon Lagrange points. It does this by using solar gravity tidal perturbations to make the three- body energy change on the trajectory that would otherwise have to be done propulsively atarrivalneartheMoon.ThefuelreservesonP2offeredsimilarcapability,suggestingthe possibilityofsendingtwoTHEMISprobestotheMoon. With the encouraging initial trajectory design results in hand, proposals for funding tosupportadetaileddesignstudyoflow-energytrans-lunartrajectories,feasibilitystudies relatedtotheTHEMIShardware,andoptimizationoftheremainingTHEMISmissionfor P1 and P2 were made in 2006 and 2007. Although these proposals were not selected for funding,thescienceteamcontinuedconceptdevelopmentastimepermitted. Inthesummerof2007,internalJPLfundingbecameavailabletosupportanExplorer programMissionofOpportunityproposalforaTHEMISmissionextensionthatwouldbe- comeARTEMIS.AteamfromtheJPLInnerPlanetsMissionAnalysisgroupwasconvened todesigntrajectoriestotheMoonforP1andP2.Buildingontheworkdonein2005,the 6 JPLteam(workingcloselywiththeTHEMISscienceandmissionoperationsteams)devel- opedaworkabletrajectorywithinTHEMISprobeconstraintsthatprovidedtheopportunity forahighlyrewardingscientificmission.Thisformedthebaselinetrajectoryofthecurrent ARTEMISmission.Midwaythroughthispreliminarydesigneffort,NASAheadquartersad- visedtheARTEMISteamthatthenewmissionwouldbemoreappropriatelyproposedasan extendedmissionforTHEMIS,ratherthanasamissionofopportunity.Ataroundthesame time,themissionoperationsteamatUCB-SSLwasaugmentedbynavigatorsandmaneu- verdesignersatGSFCwhocontributedoperationsexperiencewithLissajousandtranslunar orbitstothedesigneffort. ThecompletepreliminarydesignfortheextendedmissionwaspresentedtotheSenior Review Board for the Heliophysics Division in February 2008 (Angelopoulos and Sibeck 2008); approval to proceed with detailed design was given in May of that year. The pre- liminary trajectory design that was presented to the Senior Review Board is described in Broschartetal.(2009).Thispaperisanupdateofthatearlierdesignpaper;thedesignde- scribed there has changed significantly since the approval to proceed. As was understood atthetime,theseriesofEarthorbitsleadinguptotheinitiallunarflybysneededtobesig- nificantly redesigned. More recently, a number of changes have been made in the science operationsphaseofARTEMIS. In2009itwasrecognizedthatsignificantadditionalscientificbenefitsfromARTEMIS couldbeobtainedforthePlanetaryDivisionofNASA’sScienceMissionDirectorate.The teamwasinvitedtoproposeanamendmenttoitsHeliophysicsplanthataddressedPlanetary objectives.TheproposalwasreturnedbyNASA/HQ,andtheinvitationwasre-extendedfor submissioninthe2010SeniorReviewcycle,sobothHeliophysicsandPlanetaryaspectsof the ARTEMIS proposal could be evaluated by a joint panel. ARTEMIS/Heliophysics was giventhego-aheadtocontinueoperationsinJune2010.TheARTEMIS/Planetarydecision, though delayed until December 2010, was also positive. The 2008 preliminary design of ARTEMIS’s lunar orbits needed to be modified to accommodate planetary objectives by loweringaltitudeperiapses,raisinginclinations,andadjustingthelinesofapsidesforbetter overlapofmeasurementswiththoseofNASA’sLunarAtmosphereandDustEnvironment Explorer(LADEE)mission. The Planetary Division’s decision to execute the planetary objectives of the mission cameonly3monthspriortothebaselineARTEMISlunarorbitinsertion(originallyslated for April 2011). This did not leave sufficient time for performing the necessary lunar or- bit optimization to meet the expanded science objectives. Therefore, the team decided to postponeinsertiontoJune-July2011toenablefurtherstudyoftheplanetaryaspectsofthe investigation.ThispostponementinturnentailedmodificationstoboththeLissajousphase andthetransitiontolunarorbits. The ARTEMIS science objectives and the characteristics of orbits that would satisfy them(forbothHeliophysicsandPlanetaryDivisionsoftheScienceMissionDirectorate)as proposedandacceptedbythe2010SeniorReviewweredescribedinAngelopoulos(2010). Therevisedmissiondesigndescribedinthispaperrepresentsthemostup-to-dateARTEMIS orbitexecutionplan. 4 ARTEMISScienceGoals Angelopoulos(2010)givesacomprehensiveoverviewofARTEMISmissionscienceobjec- tivesanddescribeshowthemissiondesignandoperationsarestructuredtomeetthem.Here wedescribeaspectsofthemissionthatdrivemissiondesign. 7 Eachprobeisequippedwithasuiteoffiveparticleandfieldinstrumentsusedtostudy geomagnetic substorm activity during the nominal THEMIS mission. These instruments include a Fluxgate Magnetometer, a Search Coil Magnetometer, an Electric Field Instru- ment, an Electrostatic Analyzer, and a Solid State Telescope (Angelopoulos 2008). This instrumentationsuiteallowstheprobetomeasurethe3Ddistributionofthermalandsuper- thermal ions and electrons and the AC and DC magnetic and electric fields to study the interactionbetweentheEarth’smagneticfieldandtheSun’smagneticfieldandsolarwind. ByexpandingthespatialextentofTHEMIS’smultiple,identically-instrumentedspacecraft, ARTEMISallowsustostudyplasmoidsinthemagnetotail,particleaccelerationandturbu- lenceinthemagnetotailandthesolarwind.Furthermore,ARTEMISwillstudylunarwake formationandevolutionforthefirsttimewithtwoidentical,nearbyprobes,therebyresolv- ingspatio-temporalambiguities.Theaforementionedheliophysicsobjectivesofthemission can be addressed by inter-spacecraft separations and wake downstream crossings that are initially as large as 20 Earth radii and are progressively reduced to 1000 km or less. This goalisachievedinitiallybyhavingtheARTEMISprobesatlargeseparationsinLissajous orbitsaroundtwo(andlaterone)oftheEarth-MoonLagrangepoints,andsubsequentlyby insertionofprobesintolunarorbitswith∼18,000kmapoapseradiusandhighlyvariable angularseparationbetweentheirlineofapsides. ARTEMISalsooffersauniqueopportunitytocontributetoplanetaryscience.Fromits uniqueorbitsARTEMISwillstudythe“sourcesandtransportofexosphericandsputtered species; charging and circulation of dust by electric fields; structure and composition of thelunarinteriorbyelectromagnetic(EM)sounding;andsurfacepropertiesandplanetary history,asevidencedincrustalmagnetism.Additionally,ARTEMIS’sgoalsandinstrumen- tationcomplementLRO’s[LunarReconnaissanceOrbiter’s]extendedphasemeasurements ofthelunarexosphereandofthelunarradiationenvironmentbyprovidinghighfidelitylo- calsolarwinddata.ARTEMIS’selectricfieldandplasmadataalsosupportLADEE’sprime goalofunderstandingexosphericneutralparticleanddustparticlegenerationandtransport” (Angelopoulos2010). To achieve these objectives, ARTEMIS requires both high- and low-altitude measure- mentsbyonespacecraft,whiletheothermeasuresthepristinesolarwindnearby.Lowpe- riapsesareveryimportantinincreasingtheabilityofARTEMIStomeasuresputteredions andcrustalmagnetisminsitu.Forthisreasonperiapsealtitudeslessthan50kmarehighly desired.Additionally,thelatitudeofperiapsisisanimportantconsiderationforlunarcrustal magnetism–increasedperiapsislatitudeprovidesopportunitiesforcoveringalargerportion ofthelunarsurface.Alatitudegreaterthan10deg(goal20deg)ishighlydesirable.Finally, conjunctions with LADEE at the dawn terminator necessitate that one of the ARTEMIS probes have its periapsis positioned near the dawn terminator and pass through periapse closetothetimeofLADEEpassagethroughthatregion.Thesedesignconsiderationshave beenincorporatedintothecurrentplanningfortheupcominglunarorbitinsertions(LOIs). 5 ARTEMISTrajectoryDesign Figure 1 shows the ARTEMIS trajectory design that sent P1 and P2 from their respective orbits at the end of the THEMIS primary mission to insertion into lunar Lissajous orbit. TheP1trajectoryisshowninred,andtheP2trajectoryisshowninblue.Thedesignsuc- ceededinmeetingboththetrajectoryconstraintsimposedbytheprobecapabilitiesandthe requirementsderivedfromthescienceobjectives. 8 Inthefollowingsubsections,thetrajectoryisbrokenupintophasesfordetaileddiscus- sion.TheseincludetheEarthorbitphase,thetrans-lunarphase,theLissajousorbitphase, andthelunarorbitphase.AnintegratedtimelineoftheeventsforP1andP2inthesefour missionphasescanbefoundinTable4. 5.1 EarthOrbitPhaseTrajectories WhenthepreliminarydesignwasbeingdevelopedtoshowthefeasibilityofARTEMIS,the orbitraisedidnotappeartopresentanyparticularchallenge,sothisphasewassimplified to a single impulsive velocity increase at perigee, followed by a number of Earth orbits including lunar approaches that modified the orbit and culminated in the lunar flyby that begins the low-energy transfer to the Moon. This simplification allowed one track of the designefforttofocusmoststronglyonthelunarflybyandtransfer;theseriesoffiniteorbit raisemaneuvers(ORMs)toraisetheEarthorbitcouldbedevelopedlaterinparallelona separatedesigntrack. Figure4showstheARTEMISP1trajectoryfromtheendofthenominalTHEMISmis- sionthroughthefirstcloselunarflyby.Inthefigure,theredlinerepresentstheARTEMISP1 trajectorystartingwithitsorbitattheendoftheTHEMISprimarymission,andthegraycir- cleindicatestheMoon’sorbit.TheplotiscenteredontheEarthandshownintheSun-Earth synodiccoordinateframe,whichrotatessuchthattheSunisfixedalongthenegativeX axis (totheleft)andtheZaxisisalignedwiththeangularmomentumoftheEarth’sheliocentric orbit. As time passes, the line of apsides of P1’s geocentric orbit rotates clockwise in the mainfigure.TheinsertinthebottomleftshowsP1’smotionoutoftheeclipticplane,where thelargestplanechangewascausedbyalunarapproachinDecember2009.Thelabelson theplotprovideinformationaboutkeyeventsduringthisphaseofthemission. The design of the P2 Earth orbits phase was similar, as shown in Figure 5, but lasted twomonthslongerbecauseitstartedfromasmallerEarthorbitandalongerseriesoffinite maneuversneededtobeincludedtoraisetheorbit. Aswegraduallycametorealize,thereferencetrajectorydesignfortheEarthorbitphase ofbothP1andP2wouldturnouttobesignificantlymorecomplexthanasimpleseriesof maneuvers to replace the preliminary design’s impulsive orbit raise maneuver. This com- plexity stemmed from: (1) probe operational constraints, (2) the tight ∆V budget, (3) the precision phasing required to reach the designed low-energy transfers to the Moon, and (4)theactualinitialstatesforARTEMISP1,P2inthesummerof2009.Theseactualstates endedupsignificantlydifferentfromtheinitialstatesthatwerepredictedin2005-2007;this change was due to deterministic orbit-change maneuvers that occurred in 2008, mid-way throughtheTHEMISmission,toimprovescienceyieldforthesecondTHEMIStailseason (Figure6showsthisdifferencefortheP1orbit).Asexpected,theactualorbitraiserequired perigeeburnsonmultipleorbitsduetothesmallthrustcapability.Thedesignoftheseburns waschallengingbecausegenerallyanoptimaldesignofhighlyellipticaltransfersisnumer- icallydifficult,andbecauselunarapproachescreatedacomplexthree-bodydesignspace. During the refinement of the orbit design, it was recognized that several factors con- spiredtofurthercomplicatethedevelopmentofthereferencetrajectory: 1. Earth’sshadowcoversperigeeformuchoftheorbitraiseseason,prohibitingthrusting at/nearperigee.Thedesignnecessitatedsplittingmostperigeeburnsintotwo(AandB) burnarcsbracketingtheshadow,furtherincreasingburnarclengthandgravitylosses. 2. Theinitialpropellantloadof∼50%forP2forcedalargefractionofthemaneuversto beperformedatalowerdutycycle(shorterpulse)duetothepropellantloadbeingnear 9 Fig.4 EarthorbitportionoftheP1trajectorydesign.Distancesquotedarerangesmeasuredfromthecenter ofmassoftheEarthorMoon. Fig.5 EarthorbitportionoftheP2trajectorydesign.Distancesquotedarerangesmeasuredfromthecenter ofmassoftheEarthorMoon. 10 Fig.6 a).InitialorbitoftheEarthorbitportionoftheP1preliminarytrajectorydesign.Theinitialcondition forARTEMISP1predictedwhenARTEMISwasproposedisingreen;theactualstartingorbitisinred. b).End-onviewofa). a “slosh resonance” (Sholl et al. 2007; Auslander et al. 2008; Frey et al. 2008). This further exacerbated gravity losses, necessitating more maneuvers to obtain the same totalorbit-raise∆V.ThiswasaddressedbystartingtheORMsequenceforP2asearly asJuly20,2009. 3. Sidethrustingfororbit-raisemaneuversalsoresultsinasmallreorientation(precession) ofthespinaxisduetoasmalloffsetofthethrustdirectionrelativetotheprobecenterof mass.Thecumulativeeffectofsidethrustinghasbeensignificantspin-planeprecession oftheprobesindirectionsthateitherviolatedoperationalconstraintsorincreasedlosses fromvector-thrusting.Spinaxisreorientationmaneuverswereincludedinthemission designtoaccountforthateffect. 4. Thrustrestrictionsduetotheabsenceof“up”thrustingcapabilityposedanon-traditional restriction to the mission design. The usual intuition that 1 burn allows targeting of 3 elements and 2 burns separated in time allows for the targeting of 6 elements is not correct for ARTEMIS. In fact, even 3 separated burns can fail to provide 6-element targetingwhenallmaneuversareconfinedtoasingleplane. 5.1.1 Orbit-raiseDesignProcess TheP1andP2orbit-raisedesignswereconstructedusingMysticsoftware(Whiffen1999, 2006). Mystic wasable toaccommodate allmission constraintsoutlined above.However, thecomplex(andoftentreacherous)designspaceresultingfromnumerouslunarapproaches duringtheorbit-raisephasemadesimpledesignstrategiesimpossible.Toprovidesomero- bustness against missed burns, and sufficient tracking data for orbit/maneuver reconstruc- tion, perigee maneuvers were double-spaced, i.e., two orbits apart. On occasion it proved advantageoustoseparateburnsevenfarthertotakeadvantageoforavoidstronglunarin- teractions. Most perigee burns were divided into and modeled as two separate burn arcs, oneoneithersideoftheEarth’sshadow.Thedurationandpointingofeachburnwasfully optimizedusingMystic,withtheconstraintthattheendstatesofthisphasewouldbeonthe translunartrajectoriesalreadydesigned. Severaldifferentend-to-endorbit-raisestrategieswerethusattemptedforbothP1and P2,withthedesiredtranslunarinjectionasagoalandtheinitialARTEMISstateasastart- ingpointasearlyasneeded,i.e.,withanascendstartdateunrestrictedbyTHEMISscience