ARTEMIS Mission Design - UCLA Institute for Geophysics and

32 Pages·2011·2.85 MB·English

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.:[phone]E-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.
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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
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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