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ASTRO2020APCWhitePaper

OrbitingStarshade:ObservingExoplanetsatvisiblewavelengthswithGMT,TMT,andELT

ThematicArea:PlanetarySystems

PrimaryContact:Name:1JohnC.MatherInstitution:NASA’sGoddardSpaceFlightCenterEmail:John.C.Mather@nasa.govPhone:240-393-3879ProposingTeam:1JohnMather,2JonathanArenberg,3SimoneD’Amico,4WebsterCash,1MatthewGreenhouse,5AnthonyHarness,1TiffanyHoerbelt,1IsabelKain,7WolfgangKausch,7,8StefanKimeswenger,9CareyLisse,10StefanMartin,11,12StefanNoll,1EliadPeretz,7NorbertPrzybilla,13SaraSeager,10StuartShaklan,14IgnasSnellen,10PhilWillems(institutionslistedatend)

Abstract:Anorbitingstarshadeworkingwith30-mclassground-basedtelescopeswouldenableobservationsofreflectedlightfromexoplanetsatvisiblewavelengths.Molecularoxygenandwateronanexo-Earthcouldbeclearlydetectedina1-hourspectrumoutto7pc,anditscolorscouldbemeasuredoutto17pc.Thestarshadeprovidestheneededcontrastandthetelescopewithadvancedadaptiveopticsprovidesangularresolution,reductionoftheskybackground,imaging,andspectroscopy.Thenecessarystarshadeorbitisahighlyeccentricellipse,withapogeegreaterthan~185,000km,tomatchtheobservatoryvelocity,andadifferentorbitisneededforeachtargetstar.Thrustisprovidedtomatchtheaccelerationoftheobservatory.BasedonaJPLTeam-XstudyinMay2019,aROMcostis$3B,notincludingrefuelingandthepossiblerequirementforalargerlaunchvehicle.WeaddressthetoprecommendationoftheExoplanetScienceStrategyreport[1],that“NASAshouldleadalargestrategicdirectimagingmissioncapableofmeasuringthereflected-lightspectraoftemperateterrestrialplanetsorbitingSun-likestars.”TheorbitingstarshadeisthenewestmemberofthefamilyofstarshadesunderstudywithsupportfromtheNASAExoplanetsExplorationProgram(ExEP).ThisstudywasinitiatedatGSFCinspring2018,followedbyaJPLTeamAstudyinMay2018,thefirstGSFCsciencemeetingMay14-15,2019,andaTeamXstudyatJPLthefollowingweek.StarshadeshavebeenwellstudiedfortheExo-S[2-4],WFIRST[5-11],andHabEx[8,12-16]missionsrecently,andinthepastforJWST,UMBRAS[17],BOSS[18],NewWorldsExplorer[19,20],andTHEIA[21].Webuildonthatworkwithalargerandmoremaneuverablestarshade.Inprincipletheorbitingstarshadecouldbedesignedforcompatibilitywithalltelescopes,includingfuturespacetelescopeslikeHabExandLUVOIR,thoughthedetailswoulddiffer.

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1.Introduction:Anorbitingstarshadewouldenableground-basedtelescopestoobservereflectedlightfromEarth-likeexoplanetsaroundsun-likestars.Withvisible-bandadaptiveoptics,angularresolutionofafewmilliarcseconds,andcollectingareasfarlargerthananythingcurrentlyfeasibleforspacetelescopes,thiscombinationhasthepotentialtoopennewareasofexoplanetscience.Anexo-Earthat5pcwouldbe50resolutionelementsawayfromitsstar,makingdetectionunambiguous,eveninthepresenceofverybrightexo-zodiacalclouds.Earth-likeoxygenandwaterbandsnear700nmcouldberecognizeddespiteterrestrialinterference,withacontinuumsignal-to-noiseratioof17fora2700secexposureandR=λ/δλ=150.Wheredidwecomefrom,andarewealone?Howdoplanetarysystemsformandevolve?Arethereplanetarysystemsresemblingours:smallrockyplanets,anasteroidbelt,gasgiants,icegiants,andaKuiperbelt?ArethereexoplanetssimilartoEarth,andaretheresignsoflifeelsewhere?Aretheresurfacefeaturesandweather?Toanswerthesequestions,wewishto:

• Obtainmulticolorimagesofentireplanetarysystems,includingouterplanets,• Obtainpreciseorbits,• Measurethetimedependenceofbrightness,colors,andspectra,• Obtainplanetaryspectra,withspectralresolutionoptimizedforeachplanet,

sensitivetokeymolecularspecies(water,oxygen,methane),• Observethestructureofexo-zodiacaldustclouds(warmandcold),andfindplanets

inbrightdustclouds,• ObserveenoughtargetstoprobablyfindanEarth-likeplanet,sincesolarsystem

analogsmayberare.Giventheoneknownexampleoflife,weshouldlookforEarthsaroundSun-like(F,G,K)stars[1].WhileHabitableZone(HZ)planetsaroundsmallMstarscanbestudiedwiththetransittechnique,thehoststarsareverydifferentfromours,withmajorcoronalactivity.2.KeyMeasurementObjectives:Thekeymeasurementobjectiveistoimage~12nearbyexoplanetarysystems,andobtainorbitsandspectraoftheirplanets,ina~3-yearprimemission,atvisiblewavelengthsincludingmolecularbandsofoxygen,water,andmethane.ThewavelengthrangeissetbyEarth’satmospherictransmissionandemission,bythewavelengthsofexoplanetmolecularbands,andbythemaximumsizeofthestarshade.Exoplanetcolorscanimmediatelybecomparedwithknownsolarsystemobjects[22,23].Earthstandsoutinthecolor-colorplot(350/550,850/550)basedontheEPOXImissiondata[22],butspectroscopywillalwaysberequired.Ifwecanobserve12targets,andthefractionofstarshaving~Earth-sizeplanetsinthehabitablezoneisηÅ=0.2,thentherewouldbe12×0.2=2.4±1.6potentialEarthssuitableformolecularspectroscopywithin7pc,andwecouldbegintoanswerthequestionofwhethertheyhaveanatmospherelikeours.Somewouldbehiddenbehindthestarshadeduringobservations,sothisisnotayieldcalculation.Thesignatureoftheexo-moleculeswouldbeincreasedequivalentwidthsintheirabsorptionbands,abovethewidthsduetotelluricinterference.

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2.1Imaging:ThesensitivityandIWA(innerworkingangle)aresufficienttoimageasolarsystemwithVenus,Earth,andMarsoutto17pcin20minutes.TheIWAistheapparentradiusofthestarshadeseenfromthetelescope.Itisalsotheangleatwhichtheexoplanetshadowcrossesthecenterofthetelescopeprimary,andthecollectingareaiscutbyabouthalf.IfweachieveanIWAof0.049”therewouldbe~177suchtargets.Thetelescoperesponseiscalculatedfromtheimagequalityofadaptiveoptics(Strehlratio).Thetelescoperesolvestheshapeofthepetals,butthestarshadeisnotinthefarfield,soitsimageisblurred.Thesensitivitycalculationincludestheskybackgroundatthetelescope,anddiffuselightfromreflected

Earthshineanddiffractedstarlightfromthestarshade.Withthehighangularresolutionoflargeground-basedtelescopes,contrastagainstbrightexo-zodiacalcloudsisincreasedinproportiontothesquareoftheaperture;thiscouldbeimportantintheplanetarysystemswithveryhighexozodiacalbrightness,orveryclumpydust.

Figure2.L:SolarSystemwithV=6starat17pc,20minexposure,400-700nm,exozodi=5xsolarsystemvalue,systeminclined60°,withEarthshinefrom99mstarshade.Marsisat1:00,Venusat2:00,Earthat7:30.R:samewithdifferentangularscale.JupiterandSaturnareat2:00and8:00.AssumedStrehl0.7,δθ=3milliarcsec,seeingdisk0.5”.VenusisnearIWA.

The Astrophysical Journal, 729:130 (10pp), 2011 March 10 Crow et al.

Tomasko et al. 2008). As a result, Titan’s reflectance spectrumdecreases steadily from 650 nm to UV wavelengths. Methaneabsorption dominates Titan’s reflectance spectrum longward of650 nm. Although these features are not as strong as those inthe spectra of Uranus and Neptune, the 850 nm and 950 nmHRI filters show marked decreases in Titan’s reflectance due tomethane absorption.

5. DISCUSSION

The objective of our study was to analyze the colors of theplanets within our solar system and use them to create a baselinefor characterizing extrasolar planets. Traub (2003) discussesthe benefits of using color to broadly characterize the types ofplanets detected by Terrestrial Planet Finder and other similarmissions. Similar to his analysis, we used data from Irvine et al.(1968a, 1968b) and Karkoschka (1994) to reproduce the colorsof terrestrial and Jovian worlds. We additionally presentedphotometric observations of Earth, Moon, and Mars taken withthe DI spacecraft. Our data improve upon Traub’s study, inwhich Earth’s colors were from Earthshine and modeled Earthspectra and Mars’ colors were from ground-based data that arecontaminated by the terrestrial atmosphere.

Traub proposed using a color–color diagram and defined threebroadband filters: 400–600, 600–800, and 800–1000 nm. Al-though his choice of bandpasses separates planets into groups,he acknowledges that the filter selection could be improvedupon. The DI HRI afforded us the opportunities to observeEarth, Moon, and Mars from space, explore a range of filtercombinations, and to determine the optimal filters for distin-guishing between different types of planets. We chose the threefilter combinations that reveal Earth’s unique characteristics andpartitioned the solar system bodies into color groups. Figure 10shows the resulting color–color diagram of the reflectance ofeight planets plus Titan and the Moon through the HRI 350,550, and 850 nm filters. The ratio between the reflectivity in the350 and 550 nm filters is plotted on the vertical axis, and theratio between the reflectivity in the 850 and 550 nm filters isplotted on the horizontal axis. The lines where these ratios areequal to unity are also shown, and a perfectly reflective bodywould fall where they intersect.

The ratio between the reflectivity in the 350 and 550 nmfilter in Figure 10 characterizes blue and UV reflectance. Earth,Uranus, and Neptune fall above or near the line of unity,meaning that they are blue or white. The increase toward UVwavelengths in the reflectance of these worlds is due to Rayleighscattering in their atmospheres. Uranus and Neptune also haveRaman scattering and H2S or hydrocarbon absorption in theiratmospheres consequently reducing the effect of Rayleighscattering. If H2S and other hydrocarbons were not present inthe atmospheres of Uranus and Neptune, the planets would have30%–40% higher reflectance at shorter wavelengths. Earth istherefore the bluest of the planets because it has no absorbingspecies that counter Rayleigh scattering at these wavelengths.This suggests that the ratio of reflectance at 350 and 550 nmcould be used to detect the presence of a Rayleigh scatteringatmosphere. However, the ratio is limited in determining theabsence of Rayleigh scattering. As seen in the spectrum ofSaturn, atmospheric absorption can overpower the effects ofRayleigh scattering and result in a decrease in UV reflectance.When analyzing the colors of extrasolar planet reflectance,higher resolution data will be needed to determine whether aUV color value near unity is due to absorption or an absence ofRayleigh scattering.

0 0.5 1 1.5 2 2.50

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0.4

0.6

0.8

1

1.2

1.4

1.6

850 nm / 550 nm

350

nm / 5

50 n

m

Color − Color Plot of Solar System Worlds

Mercury

Venus

Mars

Moon

Earth

Jupiter

Titan

Uranus

Neptune

Saturn

Figure 10. Color–color plot with ratio of reflectance in HRI 350 and 550 nmfilters on the vertical axis and 850 and 550 nm filters on the horizontal axis.The lines of unity are plotted for reference and the Sun’s radiance resides atthe intersection of the two lines. Bodies with 350:550 > 1 have atmospheresdominated by Rayleigh scattering and those with 350:550 < 1 are dominated byatmospheric or charge-transfer absorption. Although 350:550 is diagnostic ofthe presence of Rayleigh scattering it is not sufficient to determine its absence.The 850:550 groups the bodies into three regions: (1) airless bodies on the right,(2) intermediate cloudy atmospheres near unity, and (3) strong NIR absorbingatmospheres on the left. Earth is the only body that resides in the upper rightquadrant of the diagram due to Rayleigh scattering in its intermediate cloudyatmosphere. Bars for the EPOCh targets demonstrate the range of observedfull-disk reflectance values where as the bars for the other bodies representinstrument error.

On the horizontal axis, the 850–550 nm reflectivity ratiois dependent on the NIR reflectance. Mercury, Venus, Earth,Moon, and Mars all have 850:550 > 1, with Mars being thereddest planet, and Earth and Venus being relatively white.The airless bodies have high relative reflectance in the NIRdue to charge transfer interactions and crystal field interactionswith minerals, mainly iron-bearing silicates and glass, on theirsurfaces. The two terrestrial worlds with atmospheres appearwhite due to clouds and an absence of atmospheric absorbingspecies at longer wavelengths. Conversely, Jupiter, Saturn,Uranus, Neptune, and Titan all have 850:550 < 1 as a result ofstrong methane absorption in their atmospheres. Ammonia alsoabsorbs radiation at longer wavelengths, but is only attributed toa small fraction of the NIR absorption of these atmospheres. Themost common physical process in our solar system responsiblefor relatively low NIR reflectivity is strong methane absorption,so similarly to Rayleigh scattering, our analysis suggests thatthe presence of a CH4 absorbing atmosphere is evident fromplanetary colors.

Traub (2003) produced a similar color–color plot of theplanets in our solar system using their broadband colors. Hisresults vary from Figure 10 because his filters were broaderthan those of the HRI and consequently could not detect slightvariations in color due to differences in surface processes.For instance, Traub’s filters average out the increase at shortwavelengths due to Rayleigh scattering causing Uranus andNeptune to fall further to the blue end of the color–color plot

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Figure 1. Solar System colors by Crow et al., 2011

M

E

V

S

J

Seeing halo

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2.1.1PlanetaryOrbits:Weneedatleastthreeobservationstodetermineanorbit[24,25].ForMandKstarswithsmallHZs,theorbitmaybedeterminedfromasinglemonth-longvisit.ForFandGstarstwoorthreeobservationsuptoamonthapartinasinglevisit,plusanother6monthslater,maysuffice,butwouldreducethenumberobservedperyear.Preciseradialvelocitymeasurementsinadvancewouldbeextraordinarilyvaluable.2.1.2TimeDependence:Weseekevidenceofplanetaryrotation,weather,andsurfacefeaturesbycomparingobservationsatdifferenttimes.Wehaveachoiceofstarshadeorbitperiod,asshortas4days,providingobservationsofminutesto~1hourspacedoveranobservingwindowofamonthormore.Smallvariationsofcolorsandbrightnessesshouldbeevidentandmolecularspectramayalsochange.2.2Spectroscopy:Withthe39mELT,weobtainanR=150visiblespectrumofanEarthat5pcwithacontinuumSNR~17in2700secaround700nm,reducedto~5fora3600secexposureat10pc.Ifthemeasuredequivalentwidthsofthemolecularabsorptionbandsaresignificantlygreaterthanthesamebandsforthestaralone,thenexoplanetmoleculesaredetected.Systematicerrorsaresuppressedbecause:1)Thestarshadeblocksthestarlightgeometrically,withhighcontrast,andthedynamicrangeislowatthelocationoftheexoplanetimage.Thereisnoneedtomeasureaparts-per-millionchangeinbrightnessasintransitspectroscopy,andnoneedtosuppressthestarlightspeckleswithanactivelyadjustedcoronagraph.FortheELT,anEarthat5pcisbrighterthantheskybackgroundthroughoutmuchofthevisibleband,andwecanbringotherdiffusebackgroundsdowntocomparablelevels.2)Theplanetaryimageisseparatedby~50λ/dfromthestarforanEartharoundaSunat5pc.3)Theatmospherictransmissionismeasuredconcurrentlywiththesamespectrometer,usingabeacononthestarshade.4)ReflectedEarthshineisdiffusebecausethestarshadeisnotinfocus,andcanbecompensatedbycomparingtheplanetlocationwithneighboringpixels.WeusedtheonlinePSGplanetaryspectrumgenerator[26],whichincludesmultilayeratmospheres,highresolutionspectrumlinemodeling,andradiativetransfer.WepropagatedthespectrathroughtheEarth’satmosphereusingESOmodelsforParanal,usingESOmodelsforatmospherictransmissionandnight-skyradiance(dominatedbyairglow)atCerroParanal(27,28),andincludeddetectornoise,[27,28],aswellasdiffusecontributionsfromreflectedEarthshine.Inamodelexo-Earth,theabsorptionbandsarestrongerthanforouratmospherenearthezenith,becauseoftheincreasedpathlengthforthereflectedlight.Hence,thereareportionsofeachmolecularbandwheretheEarth’satmosphereispartiallytransparent,andtheexoplanetispartiallyabsorbing.Wedetectbothoxygenandwaterifpresentwithconcentrationandatmosphericpressurelikeours,despiteterrestrialinterference.Themolecularlinesarecollision-broadened,andtheopacityinthelinewingsisproportionaltotheproductofcolumndensityandcollisionrate[29].Additionalfactorsincludehighwinds,Dopplershifts,andfastrotation.Apresent-dayEarthcouldbeeasilyrecognized,butanatmospherewithlowpressure,orwithhighcloudsandhaze,wouldnotshowterrestrialmolecules.2.3TargetNumbers:Fuelisrequiredtoholdthestarshadeonthelineofsightfromtelescopetostar,andtochangetoadifferenttargetstar.Therocketequationlimitsthe

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numberofobservabletargets[30].Weneedhighpowerionengines,largesolararrays,largefueltanks,andsufficienttimeformaneuvers.Observingangleconstraintsandlongmaneuveringtimescombinetogivearateofabout4targets/year,butthisratemaybeincreasedbyfindingbetterorbitaltrajectoriesandobservingstrategies.Itcouldalsobereducedbystayinglongerwithindividualtargetstodetermineexoplanetorbits.Fuelcapacitiesgovernthetotalnumberaccessiblebeforerefueling;webudgetenoughfuelfor12targets.Refueling3timeswouldextendthemissionto12yearsand48targets.

Figure3.Simulatedspectraforplanetsat5pcwithStrehl=0.5.ToppanelR=λ/δλ=2500,bottomR=150.1pixel=λ0/2R=0.14nmforR=2000and2.34nmforR=150atλ0=700nm.RedcurvesareskybrightnessattheELTinChile.Widthsofcurvesare±1σ.Waterandoxygenareseenonexo-Earthandnotonexo-Venus,andmethaneregistersona2AUJupiter.

DesiredObservation Metric Designimplication

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ColorsofplanetsandcometsdowntobrightnessofMarsaroundSun-likestars

Contrast10-11betweenstarandplanet,includingstarshadeandtelescope

Size,distance,design,andtolerancesofstarshade;wavelengthrange

O2,H2OinEarth-likeplanetsat5pcin1hr.

760,720nmbandswithSNR>10andR>150

Telescopesize,Strehlratio,efficiency,exposuretime,Earthshinecontrol

HighRgeneralplanetaryspectra,includingCH4bands

R>2500overatleast500-850nm

Starshadedesign,instrumentdesign

Maximizeobservedtargets

12starsprimemission,upto2.4±1.6EarthspectraObjective:48starsin12-yearextendedmission

IWA~0.049arcsec(forMstars)RangeforEarthspectra7pc(sensitivity,exposuretime)Maneuveringcapability

Exoplanetorbitsandvariability3pointscovering≥90°oforbitto0.002”precisionNumber&frequencyofvisitspertarget

HZEarthinbrightexozodi ~100xcapabilityofWFIRSTAngularresolution,contrast3.TechnicalRequirements–SpaceSegmentThetablebelowsummarizesahypotheticalsetofmissionrequirements,withouttradestudiesoroptimization,inordertoderivearoughcostestimate.Theshortanswerfromthe

TeamXstudyisthatthereisnoknowntechnicalreasonwhysuchasystemcouldnotbebuilt,andthebudgetforthemostchallengingitem,thedeployablestarshadeitself,isasmallfractionofthetotal.Theorbitingstarshadeisdesignedtocastadeepshadowanditspointedsunflowershapeischosenbasedondiffractioncalculations[19-21,31].Thedesignwithacentralhubsurroundedbytaperedpetalsapodizesthediffractionpatternbyapproximatingahyper-Gaussiantaperofopacity.Shapetolerancesaregreatlyrelaxedcomparedtodesignsforsmallertelescopes,becausethetelescopeitselfprovideshighangularresolutionandcontrast.TheshadeisorientedtokeeptheSunofftheEarth-facingsurface,butneednotbeperpendiculartothelineofsight.TheshademustbecoatedorshapedontheEarth-facingsurfacetominimizereflectedEarthshine.Chemicalpropulsionisrequiredtoholdthestarshadeonthelineofsightduringobservations,matchingthetransversecomponentoftheaccelerationofthe

Pre-Decisional Mission Concept. Technical Discreet. The technical data in this document is controlled under the U.S. Export Regulations; release to foreign persons may require an export authorization. For Official Use Only (FOUO)

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Design Configuration - StowedConfiguration

4.6m Dia Fairing Envelope

4.5m10.115m

June 6, 2019

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observatoryaroundtheEarth’saxis.Sincethejetsmightbeluminousenoughtointerferewithobservations,weassumetheymustbecapableofoperatinginpulsedmodewithadutycycle<10%,whichimpliesarequirementforupto5000Nthrust.Solarelectricpropulsion(SEP)isrequiredforretargeting,sinceeachtarget-observatory-timecombinationrequiresadifferentorbit,matchingthepositionandvelocitytothetelescopelineofsightatthebeginningofanobservation.Thestarshadeprovidesabeacontosupportadaptiveopticsonthegroundtelescope,andacontinuumlightsourcetocalibratetheatmospherictransmissionatthemolecularbandsofinterest.Thebeaconisonagimbaltoaimatthetelescopewithinarcseconds,whichprovidesitsownbeaconasanalignmenttarget.ThefoldedsystembarelyfitstheFalconHeavyfairinginthisTeam-Xconcept,andcouldnotaccommodateallofthepropulsionrequirementsinthetablebelow.Item Desiredvalues Margin/Remarks

Orbittype

HighlyellipticalEarthorbitPerigee>1,000kmabovesurfaceDistanceduringobservation>195,000kmN≥4-dayperiodDifferentorbitforeachtarget

Higherorbitsneedlessmaneuveringfuel.Observationsneednotbeatapogee.Integernumberofdaysforrepeatobservations.

MissionClass ClassA

MissionDuration 3yrprimemission,baseline Objective:4x3yrswith3refuelingvisits

Starshadecentraldisk 24.75mradius TeamXbaseline,notoptimizedStarshadepetals 48petals,24.75mlong

Starshadetolerance Edgeshape5mm,petalposition5cm

Looserthanforsmallertelescopes&starshades

Earthshinereflectance 0.5%(equivalentstarmag~22) Limitedbydustcontamination

Chemicalfuelthrusterforce 5000N

~10%dutycycleduringobservationtoallowforthrusterluminosity

Ionthrusterforce 4N 8AEPS0.6Nthrustersatatime

Propellantsandspecificimpulse

Biprop(N2H2+N2O4),5000kg,Isp280secXenon3000kg;Isp2700sec

Redesignobjectives,didnotfitrocketinTeam-Xstudy.Globalproductionofxenonin2015~53,000kg.

DryMassLaunchmass

Dry:14000kgCBE+contingencyWet:22000kg

IncreasedfuelfromTeamX

Maneuveringcapability

775m/schemical,3710m/sXenon

Objectiveforredesign

LaunchVehicle(LV) FalconHeavy,22000kgcapacity PlaceholderforfutureLVFairing 4.6mdiamforFalconHeavy TootightSolarPower 116kW TeamXbaseline

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Communication NEN(NearEarthNetwork)

Navigation HighaltitudeGPS;groundstationsignaltiming

GPSprovenonMMSoutto187,000km;GEONSsoftware

FormationSensing&control

Acquisition:imagingfromobservatorycameraScience:±2musingdiffractedstarlightinpupilplaneimager

RequirementsongroundinstrumentationandcommunicationTypical1-minutedeadbandcycleforpulsedjets

AttitudeControl 3-axisstabilizedwithjets,notrotating

±1degsufficient

Opticalsystems

Laserbeaconforadaptiveoptics,simulate8thmagstar.Continuumlightsourcetocalibrateatmospherictransmission

Likelasercommterminalwithoutthedatasystem,needsgimbal.

Radiationhardness 100kRadbehind100mil.ofAl 2000tripsthroughradiationbelts

ObservingAngleConstraintsItem Targetvalue RemarksSunanglefromzenith >108° AstronomicalnightTargetanglefrommeridian <30° forfuelefficiency,couldbe

extendedZenithangle <60° foradaptiveopticsefficiency

AnglefromplanetoLOS 90°±20° Cosine(Tiltangle)reducesprojectedshadowsize

Sun-Earth-Targetangle <110° Widerthanpriordesignsbasedontilt

Observingwindows 2/year/target1-9monthslong

Dependsontargeteclipticlatitudeandobservatory(NorS)

Observationspertarget 1+(30days/orbitperiod) NeedstradestudyObservationduration 1hourtypical fuellimitedPropulsionRequirementsItem Value RemarksStationkeepingDeltaV(N-S) ~120m/s/hr´sin(δ)cos(θ) δ=declination.Duetoobservatoryacceleration.Zeroatequator.StationkeepingDeltaV(E-W) ~120m/s/hrsin(θ) θ=houranglefrommeridian.Zero

atmeridian.Meananglebetweentargets 60° For12primetargets;sqrt(4π/N)

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Transittimetonexttarget <2months(average) Allows4targets/year

RetargetingDeltaV(smallmoves)

40m/s/degreeforsmallmoves,4-dayorbit

ValueforSEP;smallerDVforlargerorbit

RetargetingDeltaV 300m/sforlargemovesvianear-escapeandreturn

Budget;freeparameteristime;orbitstudyneeded

4.TechnicalRequirements–GroundSegment:Theobservatoryrequiresacustomizedadaptiveopticsinstrumentcompatiblewithalaserbeacon,butisotherwisesimilartodesignsalreadybeingdevelopedforexoplanetstudieswithVLT[32,33],MagellanTelescope[34-36],GMT,TMT,andELT.Theinstrumentmustextractthelaserwavelengthforwavefrontsensing,andblockthelaserlightfromthecamera.Moreover,thestarshadeanditslaserbeaconandcontinuumsourcearenotinthefarfield,soarenotfocusedinthesameplaneasthestarandplanets.Werecommendanadditionalcoronagraphicstopataplaneconjugatetothestarshade,toblockstraylightcomingfromthestarshadeitself(reflectedEarthshine,sunlightscatteredontheedgesormicrometeoroidpuncturesorstiffeningstructure,starlightleakingthroughpunctures,andstarlightleakingaroundtheedge).Ashutterwillcloseaboutonce/minutewhilethestarshadejetsarefiring,buttheirplumeswilldisappearwithinmillisecondswhenthejetsarestopped.AnIRsensorwillimagethepupilplane,wheretheIRstarlightdiffractedaroundthestarshadecanprovideapositionerrorsignal.Anintegralfieldspectrometersurveysanentireexoplanetfieldatlowresolution,andafiber-fedspectrometerselectsplanetsthatarebrightenoughforhigherspectralresolution,orthatdonotfallwithintheIFUfieldofview.Item Baseline Remarks

Telescopes 24mGMT,30mTMT,39mELT

SourcephotonnoisefromanEarthat5pcisdominantforELT

Adaptiveopticsefficiency Strehl0.5at700nm Requireslaserbeacononstarshade.Based

onMagAO-XplansforzenithStrehl=0.7.Blockstarshadelight

Lyotcoronagraph;starshadenotinfarfield

Stopstarlightleakage,sunglints,Earthshine

Shutter Blocklightfromstationkeepingjets

Jetscouldbebright,sojetsarepulsedon~1mincycle.Itemforfuturestudy.

Diffractionlimitedcamerapixels

Nyquistλ/2D=1.3milliarcsec FullspatialresolutionforELT

CameraFoV 7arcsecradius Isoplanaticpatch;~100MpixIntegralfieldspectrometer R=150 LargeFoVtocapturewholeplanetary

system.NeedtradestudyforR.Fiberfedspectrometer R=150&R≥2500 Forselectedplanets

Starshadeoffsetsensor ±1mresolution PupilimagingofstarshadeIRdiffraction

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Laserbeacon Targetforstarshadebeacon

5.OperationalConcept:

1. Launchintolongellipticalorbitaimedtowardstopprioritytargetstar,phasedtolineupwithobservatoryatchosentimeanddate,withcorrectvelocity

2. Setstarshadeorientation,edge-ontoSun,nearlyperpendiculartoLineofSight3. Setupadaptiveopticsonground,lockingontolaserbeacon,measuringrelative

positionofstarandstarshade4. Fineorbitadjustmenttowithin10mat1000secfromencounter5. Commandstarshadetoclosecontrollooponstarshadeposition,usingchemical

thrust,maintainwithin±2mtolerancetokeepshadowdark(usingerrortelemetryfromground)

6. Takemulticolorimagestoidentifyexoplanetlocations7. TakeIFUspectraforcentralregion,ORwithinminutes,automaticallysetupfiber

fedspectrometerforselectedtargets8. Expose~1/2hrdependingonbrightness,geometry,priority;~1hrmaximum9. Commandstarshadetostoppositioncontrol,aimorbitforrepeatobservationor

nexttarget,usingsolarelectricpropulsion,andrepeatasneeded10. Whenfuelisnearlyexhausted,awaitservicingmission,oraimforsafedisposal

6.TechnologyDrivers:Theorbitingstarshadesharesthetechnologydevelopmentitemsofsmallerstarshades,currentlybeingmanagedbytheExEPS5technologyprogram.Additionalitemsarelistedbelow.Item Value RemarksImprovedsurfacetolimitEarthshine

0.5%dustcoverageassumed

Futurestudytopic

Advancedadaptiveoptics Strehl0.5assumed

0.7includedinsingle-conjugateMagAO-XplansforMagellanTelescope

Roboticrefueling Unlimitedlifeextension IndevelopmentwithGSFCRestore-L,DARPARSGS,&commercialmissions

Ultra-lightweightstructure

Existingconceptsexceednecessaryshapetolerances

Alternativespossiblyenabledbyrelaxedtolerances

Akeysupportingtechnologyisanorbitinglaserbeacontosupportadvancedsingle-conjugateadaptiveoptics(SCAO)forallpossibletargets,allobservatories,andallwavelengthsincludingvisibleandpossiblyU-bandultraviolet,dependingonAOprogressandscientificdemand.Withoutactivepropulsionduringanobservation,asinglebeaconcanremainintheisoplanaticpatchofachosentargetforupto9000secondsasshownbyMarlowetal.[37].Thecoming30-mclassground-basedtelescopeswillprovidefacility-leveladaptiveopticsfortheentiresky,butonlyatnear-IRwavelengths,usingmultipleupgoinglaserbeams.Afleetoforbitinglaserbeaconswouldenableanangularresolution12xbetterthanwithHubbleSpaceTelescope,and3xbetterthanwithnear-IRAO,though

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withalowdutycycle.Theweathercouldbemonitoredonalltheplanets,theheartonPlutocouldberesolved,and30thmagnitudestarsinnearbygalaxiescouldbeimagedinminutes.Nootherplannedtechnologywouldprovidethiscapability.Acustom-designedSCAOinstrumentwouldberequired;thebeaconisnotinthefarfield,thelaserwavelength(s)wouldbedifferent,and(unlikethestarshade)theguidestarmovesduringtheobservation.7.Organization,Partnership,andCurrentStatus:TheNASAExoplanetsExplorationProgram,managedbyJPLforNASAHeadquarters,AstrophysicsDivision,isthesponsorofthecurrentwork.TheworkwasinitiatedatGoddardSpaceFlightCenterbyJohnMatherandEliadPeretzandsupportedbyGoddardinternalfunds.ContributorstothediscussionincluderepresentativesoftheGMT,TMT,andELTobservatories.StarshadeperformancewascalculatedbyA.Harness(Princeton)andS.Shaklan(JPL).SimulatedimageswerepreparedbyS.ShaklanandsimulatedspectrabyS.Kimeswengeretal.(UniversitätInnsbruck)withsupportfromS.Noll,N.Przybilla,andW.Kausch.OrbitcalculationsatGSFCweremadebyS.Hur-Diaz,C.Webster,D.Folta,D.Dichmann,R.Qureshi,andR.Pritchett.WethankR.Campbell,M.Cirasuolo,J.Kasdin,M.Greenhouse,M.Lake,N.Lewis,S.HildebrandtRafels,M.Turnbull,G.Villanueva,andK.Warfieldforfruitfuldiscussions.Thecurrenttechnicalobjectiveistocompleteamissionconceptstudyforcomparisonwithothermissions.TheTeamXstudyproducedaMasterEquipmentListasabasisforcostandmasscalculation,butdidnotincludeanactualmechanicaldesign.ThestarshadeitselfwasscaledfromdesignsfortheHabExstarshade.However,evenattheconceptstage,theorbitingstarshadecouldbarelyfittheselectedFalconHeavylaunchmasscapacityandfairingsize,andcouldnotobserveenoughtargetsystemswiththeavailablefuel.Toincreasethenumberoftargetstobevisited,wewillanalyzearefuelingoption,consideralternatelaunchvehiclesliketheSLSandBFR,andattempttoreducethedrymasstoincreasemaneuverability.8.Schedule:TheTeamXcostestimatewasbasedonalaunchin2035.Theassumedschedulewas94monthsfromPhaseAthroughD.Thisallowsfor~8yearsofpre-phaseA.9.Costestimates:ThecostestimatewasperformedbyTeamXatJPLinMay2019.Thebasefiscalyearwas2019,andthetop-levelcostwas$3.0BinFY2019dollars.ThecostestimateincludesPhaseAthroughlaunchand3yearsofoperationwith30%reservesonthedevelopment,and$223MfortheFalconHeavy.ThevalueisaROM(roughorderofmagnitude)withoutadetaileddesign.Theestimatedoesnotincludescienceoperations,sciencecommunitysupport,instrumentdevelopmentfortheground-basedobservatory,thetechnologydevelopmenteffort,orthefleetoforbitinglaserbeaconstodemonstrateadvancedadaptiveoptics.Italsodoesnotincludethecostofrefuelingmissions,orthepossibilityofalargerlauncher.ToestimaterefuelingcostsweconsidertheInternationalSpaceStation.TheSpaceXDragoncapsulehasdockedautonomouslywiththeISSandcostsanaverageof$180M/flight.TheLunarGatewayPowerandPropulsionElement(PPE),withsolarelectricpropulsion,isoncontractfora2022launchat$375M,firmfixedprice.ConsideringallthepossiblechangesfromtheTeam-Xconcept,thepartsnotincluded,andthreerefuelingmodules,thetotalcostcouldapproach$4B.

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Institutions

1JohnMather,2JonathanArenberg,3SimoneD’Amico,4WebsterCash,1MatthewGreenhouse,5AnthonyHarness,1TiffanyHoerbelt,1IsabelKain,7WolfgangKausch,7,8StefanKimeswenger,9CareyLisse,10StefanMartin,11,12StefanNoll,1EliadPeretz,7NorbertPrzybilla,13SaraSeager,10StuartShaklan,14IgnasSnellen,10PhilWillems

1NASA,GoddardSpaceFlightCenter,Greenbelt,MD20771USA.2NorthropGrumman,OneSpacePark,R8/2789,RedondoBeachCA902783AeronauticsandAstronauticsDepartment,DurandBldg.,Room262,496LomitaMall,StanfordUniversity,Stanford,California94305-40354UniversityofColorado,Boulder,CO80309,5MechanicalandAerospaceEngineering,PrincetonUniversity,Princeton,NJ08544,USA7InstitutfürAstro-undTeilchenphysik,UniversitätInnsbruck,Technikerstr.25/8,6020Innsbruck,Austria8InstitutodeAstronomía,UniversidadCatólicadelNorte,Av.Angamos0610,Antofagasta,Chile9JohnsHopkinsUniversityAppliedPhysicsLab,11100JohnsHopkinsRoad,Mailstop200-W230,LaurelMD20723-609910JetPropulsionLab,CaliforniaInstituteofTechnology,4800OakGroveDrive,Pasadena,CA91109,USA.11InstitutfürPhysik,UniversitätAugsburg,86135Augsburg,Germany12DeutschesFernerkundungsdatenzentrum,DeutschesZentrumfürLuft-undRaumfahrt,82234Weßling-Oberpfaffenhofen,Germany13MIT,DepartmentofEarth,Atmospheric,andPlanetarySciences,77MassachusettsAve.,Cambridge,MA0213914LeidenObservatory,OortBuilding,Room439,Postbus9513,2300RALeiden,TheNetherlands

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