A Cubesat Centrifuge for Long Duration Milligravity Research

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A Cubesat Centrifuge for Long Duration Milligravity Research
ACubesatCentrifugeforLongDurationMilligravityResearch
ErikAsphaug 1 ,JekanThangavelautham 1 ,AndrewKlesh 2 ,1 ,AmanChandra 1 ,Ravi
Nallapu 1 ,LakshRaura 1 ,MercedesHerreras-Martinez 1 ,andStephenSchwartz 1 1
ArizonaStateUniversity,TempeAZ
2
JetPropulsionLaboratory,PasadenaCA
SubmittedtoNPJMicrogravity,July1,2016
RevisedasPerspective,December19,2016
FurtherRevised,April3,2017
FinalRevision,May5,2017
Sendallcorrespondenceto:
ErikAsphaug
ArizonaStateUniversity
SchoolofEarthandSpaceExploration
POBox876004
Tempe,AZ85287-6004
[email protected]
tel:480-727-2219
fax:480-965-8102
1
Abstract
Weadvocatealow-coststrategyforlong-durationresearchintothe‘milligravity’environment
ofasteroids,cometsandsmallmoons,wheresurfacegravityisavectorfieldtypicallylessthan
1/1000thegravityofEarth.Unlikethemicrogravityenvironmentofspace,thereisa
directionalitythatgivesrise,overtime,tostrangelyfamiliargeologictexturesandlandforms.In
additiontoadvancingplanetaryscience,andfurtheringtechnologiesforhazardousasteroid
mitigationandin-situresourceutilization,simplifiedaccesstolong-durationmilligravityoffers
significantpotentialforadvancinghumanspaceflight,biomedicineandmanufacturing.We
showthatacommodity3U(10x10x34cm3)cubesatcontainingalaboratoryofloosematerials
canbespunto1rpm=2π/60s-1onitslongaxis,creatingacentrifugalforceequivalenttothe
surfacegravityofakilometer-sizedasteroid.Wedescribethefirstflightdemonstration,where
smallmeteoritefragmentswillpileuptocreateapatchofrealregolithunderrealisticasteroid
conditions,pavingthewayforsubsequentmissionswherelandingandmobilitytechnologycan
beflight-provenintheoperationalenvironment,inLow-EarthOrbit(LEO).The3Udesigncanbe
adaptedforuseonboardtheInternationalSpaceStation(ISS)toallowforvariablegravity
experimentsunderambienttemperatureandpressureforabroaderrangeofexperiments.
2
Introduction
Weadvocateflyingsmallcommoditycubesats(3U,10x10x34cm3)aswhole-spacecraft
centrifuges,torecreatetheoff-worldenvironmentsofasteroids,cometsandsmallmoons,the
mostcommonplanetarybodies,inlow-Earthorbit.Theirregionalgeologyappearsvaguely
familiar–dustplains,gravelpilesandboulders,cliffsandlandslides(Figure1)–buttheir
processesoperateundergravitationalstressesanddynamicaltimescalesthatarethousandsof
timesdifferentthanonEarth,theMoonorMars1.Themagnitudeoftheirsurfacegravity,~0.01
cm/s2per1kmradius,issufficienttodefineanunambiguous‘down’direction,butsubtle
enoughthatlandedoperationsaremorelikedockingwithloosematerial.Thisgivesriseto
dramatictopography.Materialsandequipmentcanfloatfreelyoncometsandasteroidsfor
shorttimescales2,asontheISS,butafterminutestohourswillenduponthesurface.
Smallbodygeologyisfundamentallyunknown,andthereforeahazardousenvironment.
Touch-and-gosamplingremainsacutting-edgetechnologicalfeat3,andcontrolledlandinghas
neverbeenachieved.Advancedoperationsarehighlyuncertain:whathappenstolowgravity
regolithduringminingorexcavation?Doesitgointoorbit?Doesitadheretospacesuitmaterial
insteadofsettling?Canaspacecraftbeanchoredtoembeddedrocks,orwilltheypullfree?Are
landformsstable,orwillexplorationandminingactivitiesdisturbthemcatastrophically?
3
(a)
(b)
Figure1.Silicateandicyregolithinmilligravityconditions.
(a)Aone-kilometercliffonthe4kmdiametercomet67P/Churyumov-Gerasimenko(C-G)
imagedearlyintheESARosettamission(12/2014).Four-panelNAVCAMmosaicacquiredfrom
20kmradiusaboutthecometcenter.Surfacegravityg~0.1cm/s2,soaleapingastronautwould
landahalfhourlaterat~1m/s,eitherintosoftmaterialsorasolidicycrust.Bouldersatthecliff
baseareuptotensofmetersdiameter.Materialisacombinationoficesandamorphous
volatilesandsilicatesandorganics,inlooseandcementedforms.
(b)Pondedandburiedcraters,largeandsmallboulders,andslumpsandstreaksonaregionof
433Eros,a~20kmdiameterpotato-shapedrockyasteroid(NASANEARmission,
JHUAPL/Cornell).Gravityg~0.6cm/s2.Imageis600macross,andcamerapixelscaleis2m.
Materialisordinarychondrite(mostlysilicate)composition,grounddownbysmallimpactsto
finesizes.
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Theeffectofmicrogravityonlivingorganismshasbeenstudiedsincethedawnof
spaceflight.Howeverwehavefarlessknowledgewhetherasmallbutconstantdirectional
milligravityvector,imperceptibletohumansontimescalesofsecondstominutes,mighthavea
cumulativeeffectoverlongerperiodsonbiology,comparabletoitspronouncedeffecton
asteroidandcometgeology.Plantgerminationandvegetativegrowth,forexample4-7,or
bacterialfermentation8andotherlifeprocesses9-14mightoperatedifferentlyunderaconstant
directionalgravity(milligravity)thanundernon-directionalmicrogravityconditions.
Ifasmalldirectionalgravityissufficienttoovercomesomeofthepronouncedimpediments
ofmicrogravity(e.g.boneloss13andimmunesystemimpairment15-17),enablinghumansand
theirsupportsystemstofunctionreliablyformonthsoryears,thenasmallspacestationwith
slowrotationcouldsufficetocreatemilligravityconditionsinLow-EarthOrbit(LEO)orindeep
space,throughgentlecentrifugalaction.Accelerationinsideacentrifugeisgivenbya=rw2,
whereristheradiusandwistheangularvelocity,soaspacestation10metersacrossrotating
onceper3minuteswouldproduceanaccelerationequivalenttothesurfacegravityofasteroid
Eros(Figure1b),g=0.6cms-2.Thelowrotationalstresseswouldallowalighterandsafer
spacecraftstructure,comparedtowhatisneededforEarth-likeartificialgravity,andtheslow
rotationwouldminimizeastronautdisorientation.
Concerningresourceutilizationonsmallbodies,milligravityconditionsmightrepresenta
sweetspotinrequirementsandcapabilities.Largeoremassescouldbeliftedandtransported
atlittlecostofenergy,whilethedirectionalgravitycouldbesufficienttosegregate,hold,or
processmaterialsbasedondensity,sizeorcharge.Aminingprocesscouldbeoptimizedfor
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asteroid-likeconditions.Butonsmallairlessworlds,thechallengeisnotonlytheunfamiliar
gravity.Surfaceparticlesareexposedtoionizingradiation,creatingshort-rangeforcesthatcan
vastlyexceedthegravitation18.Pebblesandevensmallboulderscanbehavelikecharged
polystyrenepellets(packingpeanuts)onEarth–grainsadheringtograins,andtosurfaces.Dust
mightcloganddamagemechanisms.
Experimentsinrelevantconditionsarerequiredatthisjuncture.Droptowersandparabolic
flightscanattainmicrogravityandmilligravityconditions19onEarthforshortdurations(~1-10
s),butlong-durationexperimentsrequireanacceleratingframeofreferenceinspace(a
centrifugeorconstant-thrustingrocket)orthesurfaceofasmallbody.Thisleadsustoconsider
alowcostwhole-spacecraftcentrifugeforcreatingproxyasteroid-likeconditions,toenable
repeatedexperimentsinLEO,anenvironmentthatisvastlymoreaccessiblethanthesurfaceof
anasteroidindeepspace.
AWhole-SpacecraftCentrifuge
Theideaofawhole-spacecraftcentrifugeoriginateswithTsiolkovsky20 andPotočnik21 inthe
early1900s,andwaspopularizedbyvonBraun22inthe1950s.Thefirstdemonstrationwasin
1966,whenGemini11astronautsattacheda100-foottetherbetweentheircapsuleandthe
AgenaTargetVehicleusedfordockingpractice23.Thrustingagainstthetether,theyinitiateda
rotation~0.1-0.2rpm,creatinganestimatedcentrifugalacceleration~0.15cm/s2,comparable
tothegravityona10kmasteroid,thatwasimperceptibletoeitherastronautbutcauseda
cameratoslidealongtheinstrumentpanel.Thetetheredconfigurationisscaleable(a700-m
tetherunder1rpmrotationwouldattainMars-likegravityconditions)butinpracticespace
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tetheringisacomplexstudyinnonlineardynamics24.
O’Neillproposedaspinningwheelattachedtoacounter-rotatingcylinder25 toresolvethe
challengeofconservingangularmomentum.BasedonthisapproachNASAdevelopeddetailsfor
arotatingspace-colony26in1975.Practicaleffortssincethenhavebeenmoremodest.Japan’s
CentrifugeAccommodationModule(CAM)wastoflyontheISS27andwouldhaveenabled
relativelylargescaleexperimentsfrom0.01to2gunderambientatmosphericconditions,
whereg=980cms-1isthegravityofEarth.In2011NASAproposedalargeinflatablecentrifuge28
thatwouldbeattachedtotheISSasasleepingmodule,todemonstratecrewedjourneysto
Marsandbeyond.
Whilelarge-scaleandwhole-spacecraftcentrifugeconceptshaveyettoattainfruition,smaller
centrifugeexperimentsareinoperationontheISS.KUBIKbytheEuropeanSpaceAgencyuses
asatest-tubesizedincubatorforseeds,cellsandverysmallanimals29,operatingupto1g.The
EuropeanModularCultivationSystem30isslightlylarger,6cmdiameter,andhasbeenusedto
growplantseedlingswithin1g.Nanorack’sBioRackcentrifuge31isofsimilarcapabilityto
KUBIKandcanhandletest-tubemicrobiologyexperimentsupto1g.JAXAhasasmall
laboratoryformousehabitatexperiments32thatconvertsintoacentrifugeoperatingupto1g,
aswellastheSaiboExperimentRackconsistingoftheCellBiologyExperimentFacilitywithan
incubatorandsmallcentrifuge33.
Asteroidgravityistypicallyordersofmagnitudesmallerthantheseexistingcapabilities.
Relevantexperimentsmustcontendwithvibrationsfromspacecraftpumpsandfans(typically
~0.01cms-2onboardtheISS)aswellasexternalforcescausedbyspacecrafttorquesandtides,
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andairdragandturbulence.Absenceofvibrationisespeciallyimportantforstudyingasteroid
regolithphysics,wheretheinjectionofrandomenergycanfluidizeunconsolidatedmaterials.A
free-flyingcentrifuge,floatinginsidetheISSorindependentlyinspace,isrequiredtoattain
cleanmilligravityconditions,sowereturntotheideaofthewhole-spacecraftcentrifuge.
ASpacecraftProxyforAsteroids
Theasteroids,cometsandsmallmoonsvisitedtodatehaveirregularshapesandsignificant
expansesofregolith(Figure1).AsteroidErosandtheMartiansatellitePhobosaredust-covered
bodies34,35about20kmindiameter,whileasteroidItokawa,only300m,hascentimeter-size
gravels36,itssmallergrainswinnowedbyelectricalloftingandsolarwind18.Ithasbeen
proposed1,35thatbedsoffinematerialscreateanillusionofmonolithicstrengthbyallowing
fissurestodepthsof10–100mormore,atwhichpointgravityexceedsdrycohesion.Ifasteroid
geologyseemsunknownandbizarre,thegeophysicsofcometsisevenweirder,asfoundoutby
theRosettamissiontocomet67PChuryumov-Gerasimenko(C-G;Figure1)duringthe
attemptedPhilaelanding37.
Uncertaintyastowhatmighthappenwhenexplorationsystemsinteractwithasteroidand
cometsurfacematerialsisaseriousimpedimenttospaceexploration.Themisadventuresof
Hayabusa-1onthesurfaceofItokawa3,36,andofPhilaeonthesurfaceofC-G37,showhowbasic
uncertaintiesofsurfacephysicstranslateintoimplementationrisksforflagshipmissions,and
constrainmoreambitiousactivitiesinnear-Earthspaceandbeyond.Thisleadsustoadvocatea
whole-spacecraftcentrifugeapproach,creatingpatchesofasteroidregolithinsideoflab
facilitiesinLEOthatcanbeusedtoraisethetechnologicalreadinesslevel(TRL)ofadvanced
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explorationsystemsandresourceextractiontechnologiestoTRL-9,thatis,flight-proveninthe
operationalenvironment.
Theeffectivegravityofanirregular,fast-rotatingasteroidorcometvarieswithlocationonthe
surface,eveng~0attheequatorinsomecases38.Thesurfacepotentialofthe20kmdiameter
naturalsatellitePhobos,deepinsidethegravitywellofMars,variesfrom0.4cm/s2 atitssubMarspointto0.7cm/s2 atthenorthpole39.Theseeffectivegravityvariationsareanalogous
andcomparabletohowaccelerationvarieswithrinsideasmallcentrifuge.Sowhileartificial
gravityisnotconstantinsideasmallcentrifuge,andCorioliseffectsarenoticeable,thisisinfact
representativeofactualconditionsatsmallbodies.
Application
Howmuchgravityisenough,orjustright,foragivenartificialornaturalprocess?Howdoesa
smallbutconstantginfluencetherestingconfigurationofrocksandairlesssoils?Howdoes
thepresenceorabsenceofgravityaffecttheoperationsofanchors,probesandexcavators?Isa
smallbutconstantgravityofsubstantialbenefittohumans10-19,cropgrowth4-8and
medicine12,40-41?Inwhatwaysismilligravityanimpediment,andinwhatwaysbeneficial,to
hasardousasteroidmitigationandmining?Thesebasicquestionscanbeansweredbyrepeated
accessibleexperimentsinspace.
Arotatingcubesatcanprovideaccesstothreekindsoflow-gravityconditions:zerorotation
(freelyfloatingmaterial),constantrotation(milligravity),andchangingrotation(torque
changingtheg-vector,applyingshear).ThatisthebasisfortheAOSAT-1demonstration
mission42,whosesciencepayloadfeaturesopticalcamerasaimedataregolithchamber,
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returningimagedataforanalysisonEarth,andinertialsensors.Tunablevibratorsprovide
additionalexperiments,andhavethebenefitofshakinggranulesofftheviewingglass.The
cubesathasaspaceflightend,roughly1U(10cmx10cmx11cm)ofthechassis,andamodular
labchamber(Figure2)withthecenterofmassnearthe‘top’ofthechamber.Thisfacilitates
theseparationofengineeringrequirements:forthespacecrafttofunctionandreturndata,and
forthelabchambertorunexperimentsandproducedata.Experimentsincludeformationofa
stablepileattheangleofrepose,reversaloftorquetocreateanavalanche,andvibratorsto
fluidizetheregolith.
Figure2.ExplodeddiagramofAOSAT-1mechanicalstructure42,10x10x34cm(3U).Thelab
chamber(left)isdevelopedandtestedseparatelyandintegratedtowardstheend,facilitating
repeatedexperiments.Twostereocameras(nearandfarfocus)arebehindaglasspartition,
andselectableLEDsilluminatethechamber.Meteoritefragments(regolith)sievedto>3mmare
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releasedfrombehindadoorafterspacecraftdeploymentandsystemscheckout.Illustrationby
A.Chandra.
Experimentsareconductedinaspunstate,lastingminutestohours,andcommunicationswith
thegroundareconductedafterwards,inade-spunstate,trackingthegroundstationfor
severalorbits.Centrifugeconditionsareattainedusingasinglereactionwheelthatiscapableof
spinningthespacecraftaboutitsshortaxis(outoftheplaneofFigure2)toseveralrpm.The
wheelissizedtoapplytherequiredtorquewithoutsaturation.Electromagneticrods
(magnetorquers)areusedtostabilizeoff-axismotionsduringspin-up.Wemodelthistorquein
combinationwithflywheelactionandirregularspacecraftmassdistribution,toshowthe
dynamicalstabilityofAOSAT-1(Figure3).Oscillationsdampquickly,sothat1rpmrotationis
stabilizedin15seconds,assumingaworst-casemassdistribution(theentireregolithpileoffset
atafarcornerofthechamber).Wefindthatshiftingtheregolithmassdistributionduring
dampinghasasmallereffect,soconcludethatAOSAT-1willstabilizeinitsexperimentalmode
inminutes43.Aftereachexperiment,themagnetorquersareusedtostoptherotationsothat
thespacecraftcanpointandcommunicatewithEarth.
11
(b)
(a)
Figure3.DynamicalstabilityofAOSAT-1.
(a)Centrifugalacceleration(artificialmilligravity)calculatedasafunctionofangularvelocity
insidea3Uconfiguration43,assumingr=20cm.
(b)Calculatedspin-upofAOSAT-1usingasinglereactionwheelcreatesawobblestabilizedby
magnetorquers,assumingaworst-caseregolithdistribution.Stable1rpmrotation(2·10-4g)is
obtainedfromanonrotatingstateafter15s,and4rpm(3·10-3g)after~100s.
AOSAT-1experimentsareconductedinvacuum.Cubesatstandardallowforapressurized
laboratoryupto1.2bar,soinprinciplethisapproachallowsforsimilarexperimentsunder
atmosphericornebularconditions.However,giventhesevereconstraintsonpower,the
laboratorytemperaturewouldhavetobepassivelycontrolled.Forscienceexperimentsat
standardtemperatureandpressureitwouldbebettertoinstallafunctionally-similar3Uchassis
insidetheISS(Figure4)withalargermotorizedflywheel,spinningfrom1–40rpmtogenerate
asteroid-toMars-likegravityconditions.Thiswouldprovodesufficientroomformultipletest
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tubes,multiplecellcultures,or(asshown)asmallplant.Unlikethefree-flyingcubesat,these
experimentscanbestoppedandanalyzed,replenished,restarted,andretrievedtoEarth.
Figure4.Conceptualadaptationofthe3Udesign,utilizingsimilarsoftwareandhardware,for
biologyandlife-scienceexperimentsonboardtheISS.Onboardareelectronicsforspincontrol,
cameraimaging,thermalandatmosphericsensing,environmentcontrol,anddataacquisition.
Herethelabchamberfeaturesasmallplantgrowingunderartificiallightandgravity,withsoil
maintainedatsetpointmoisture.IllustrationbyA.ChandraandJ.Thangavelautham.
Discussion
Theimplementationofthewhole-spacecraftcentrifugehaswaitedforanaffordable
technology,cubesats,tomeetasuitableresearchobjective,asteroidgeology.Asteroidgravity
isdifferentfrommicrogravity2inthatitdefinesavectoraccelerationsufficienttocreatethe
13
appearanceofEarth-likeorlunar-likegeology.Forstudyingtheseworldsinaccessibleproxy
environments,weenvisionusing3Ucubesatstoperformanincreasinglydetailedsequenceof
experimentsvitaltosolarsystemresearch,engineeringandmaterialscience.Largercentrifuges
wouldlearnfromthesefirststeps,toattainthemilestoneofvalidatingfull-scaleasteroid
landingandmaterialtransportsystemswithoutleavingLEO.
Milligravityexperimentsforlifesciencesandbiologyrequirepressurized,temperature
controlledfacilities;asimilar3UsystemcouldbemountedinsidetheISS,utilizingcommon
hardware,components,andsoftware.Byemphasizingcommodityhardwareandtechnology,
andbeginningwithmodeststeps,theseresearchenvironmentscanbelowcostandhighly
accessible,therebyincreasingthepaceofscientificandtechnologicaladvancementintothe
novelrealmoflongdurationmilligravity.
14
Acknowledgements
WearegratefulforgenerousadvicefromD.J.Scheeres(CUBoulder)andC.Hartzell(UMD)in
theconceptdevelopmentofAOSAT,andfortheeffortsofthreeanonymousreferees.
Contributions
EAandJTconceivedoftheideaofa3Ucentrifugecubesatforasteroidsresearch,andwrote
themanuscript.JTledtheengineeringeffortandAKadvisedtheprojectandprovidedcritical
reviews;EAledthesciencedefinition.AC,RN,LR,MHMandSScontributedequallytosystem
engineering,development,andprojectscience.
CompetingInterests
Theauthorsdeclarenocompetinginterests.
DataAvailability
Thedatathatsupportthefindingsofthisstudyareavailablefromthecorrespondingauthor
uponreasonablerequest.
Funding
EffortbyEAandMHMwassupportedbyArizonaStateUniversity,CollegeofLiberalArtsand
Sciences,RonaldGreeleyChairofPlanetaryScience.
15
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