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4 InfraredImaging

WilliamDanchi,NASAGoddardSpaceFlightCenter,Chair

PeterLawson,JetPropulsionLaboratory,Co­ChairOlivierAbsil,RachelAkeson,JohnBally,RichardK.Barry,CharlesBeichman,AdrianBelu,MathewBoyce,JamesBreckinridge,AdamBurrows,ChristineChen,DavidCole,DavidCrisp,RolfDanner,PeterDeroo,VincentCoudéduForesto,DenisDefrère,DennisEbbets,PaulFalkowski,RobertGappinger,IsmailD.Haugabook,Sr.,CharlesHanot,ThomasHenning,PhilHinz,JanHollis,SarahHunyadi,DavidHyland,KennethJ.Johnston,LisaKaltenegger,JamesKasting,MattKenworthy,AlexanderKsendzov,BenjaminLane,GregoryLaughlin,OliverLay,RéneLiseau,BrunoLopez,RafaelMillan­Gabet,StefanMartin,DimitriMawet,BertrandMennesson,JohnMonnier,NaoshiMurakami,M.CharlesNoecker,JunNishikawa,MeyerPesesen,RobertPeters,AliceQuillen,SamRagland,StephenRinehart,HuubRottgering,DanielScharf,EugeneSerabyn,MotohideTamura,MohammedTehrani,WesleyA.Traub,StephenUnwin,DavidWilner,JulienWoillez,NevilleWoolf,andMingZhao

4.1 IntroductionThis chapter describes the science and technology of infrared andmid‐infraredmissionscapable of detecting and characterizing exoplanets through the interferometric nulling ofstarlight.

Aprobe‐scalemissionofthistypewouldimagenearbyplanetarysystems,andallowustostudy planet formation, study structure in debris and exozodiacal disks, and detect andcharacterizewarmJupiters,andpossiblySuper‐Earths.Planetsaround~50starswouldbecharacterized.Aprobe‐scalemissionwouldprovide transformativescienceandwould laytheengineeringgroundworkfortheflagshipmissionthatwouldfollow.

AflagshipmissionwouldenablethedetectionofbiosignaturesinthespectraofEarth‐likeplanetsof200ormorenearbytargetstars.Suchamissionwouldhavethehighestangularresolutionforplanet‐findingofanyofthemissionconceptsdescribedinthisReport.

ThemissionsdescribedinthischapterhavegrownfromthelegacyoftheTerrestrialPlanetFinder(TPF),whichwithitscounterpartinEurope,theDarwinmission,havebeenstudiedandreviewedsincethemid‐1990s.TechnologydevelopmentforTPFwasendorsedbytheMcKee‐Taylorreport(NationalResearchCouncil2001)withthegoalofenablingamissionsometimeafter2010.Herewereportonthesuccessofthatinvestment.

Aswillbeshowninthischapter,thetechnologyforaprobe‐scalemissionislargelyinhand.A probe‐scale mission would use a fixed or connected‐structure and operate at infraredwavelengths.Laboratoryexperimentsinstarlightsuppressionhavealreadyexceededflightrequirements for such a mission by a factor of 10, and all of the necessary component

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hardwarehasbeendemonstratedinthelab.Futuretechnologyeffortsshouldincludesomeadditionalsystem‐leveltestingandmodeling,andcryogenicdemonstrationofsingle‐modefibers.Theprobe‐scalemissionthatispresentedistheFourierKelvinStellarInterferometer(FKSI),asdescribedbyDanchietal.(2003).

Furthersustainedeffort isnonethelessrequired toaddressseveralof therisksassociatedwithaflagshipmission.Themajorriskofsuchamissionisdominatedbytherelianceonasimultaneous and coordinated use of five spacecraft flying in formation; four telescopeseachonseparatespacecraftandanadditionalspacecraft forabeamcombiner. Guidance,navigation,andcontroldemonstrationshaveshownthatsuchcontrolisfeasible,butactualhardwaretests inspacehavenotyetbeenperformed. Ontheotherhand, interferometricnullinghasnowreachedtheperformancelevelrequiredforflight,asdemonstratedthroughthe Adaptive Nuller testbed. Future technology development should include cryogenicsystem‐level tests with mature brassboard designs and space‐based demonstrations offormation flying maneuvers using representative sensors and thrusters. The flagshipmissionthatispresentedistheTerrestrialPlanetFinderInterferometer(TPF‐I),identicaltothe Darwin mission concept that was proposed to the European Space Agency’s CosmicVisionprogramin2007.

Althoughwesupportmostof the long‐termgoalsthattheExoplanetTaskForce(ExoPTF)recommendedforaflagshipinfraredmission,werecommendadifferentpathforwardforthenear‐term. Specifically,wearenotconvinced,as theExoPTFreportsuggests, that theproblemofexozodilevelsanddebrisdiskscanbesolvedwithground‐basedobservationstotheextentnecessaryfortheformulationofaflagshipmission.Wearenotconvinced,inpartdue to our own experience with the Keck Interferometer, for which the lower limit onexozodis is100–200 times thatof theSolar Systemzodi level.Weexpect that thenullinginstrument on Large Binocular Telescope Interferometer (LBTI) will reduce this limitsubstantiallyforarelativelysmallsampleofstars.Wediscusshowaprobe‐classmissionintheinfraredcanmeasuretheexozodilevelsdowntothelevelofoneSolarSystemzodiforessentiallyall of thepotential target stars for theeventual flagshipTPF/Darwinmissions.Thisstep iscrucial,notonlyfortheflagshipcharacterizationmissions,but isalsoofgreatvaluetoanastrometricmissionbecausethatmissioncanthenfocusitssearchesforEarth‐twinsaroundstarswithlowexozodilevels.Moreover,aninfraredprobe‐classmissionhasahigherdegreeoftechnologyleveragingfromJWST,andcouldbeundertakeninarelativelyshort time without undue cost and technology risk. Our summary of recommendationsfromthischapterfollows:

Recommendation: A vigorous technology program, including component development,integrated testbeds, and end‐to‐end modeling, should be carried out in the areas offormation flying andmid‐infrared nulling,with the goal of enabling a probe‐scale nullinginterferometrymissioninthenext2to5yearsandaflagshipmissionwithinthenext10to15years.

Recommendation: The fruitful collaborationwith European groups onmission conceptsandrelevanttechnologyshouldbecontinued.

Recommendation: R&A should be supported for the development of preliminary scienceandmission designs. Ongoing efforts to characterize the typical level of exozodiacal lightaroundSun‐likestarswithground‐basednullinginterferometryshouldbecontinued.

Inthischapter,weoutlinethescientificparameterspaceaffordedbythenextgenerationofmid‐infraredinterferometersinthecontextofground‐basedarrays,includingtheAtacamaLarge Millimeter Array (ALMA), the Keck Interferometer (KI), the Large Binocular

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Telescope Interferometer (LBTI), and theVery LargeTelescope Interferometer (VLTI), aswellascooledspacetelescopes(Spitzer,Herschel,JWST).

The remainder of this chapter is divided into self‐contained sections on Science Goals(§4.1), Observatory Concepts (§4.2), Technology (§4.3), and Research & Analysis Goals(§4.4).

4.1.1 SensitivityandAngularResolutionBoth high‐angular resolution and high sensitivity in the mid‐infrared are key to futureprogress across all major fields of astronomy. Figure 4‐1 shows the parameter space ofspatialresolutionversussensitivity,illustratingthetrade‐offsbetweenthehighsensitivitypossibleinspaceandthehighangularresolutionachievablefromtheground.Thisdiagramhighlights the rich and varied science goals made possible by continued incrementaltechnicaldevelopmentsforadvancedimaginginthemid‐infrared.

Processesoccurringat0.5–5AUinprotoplanetarydisksenabletheformationofgiantandterrestrialplanetsandarerevealedbyobservationsinthemid‐infrared,whicharesensitivetodustemissionfrom100–1000K.

Figure4­1.Relationshipbetweenresolutionandsensitivityas itpertainstoadvancedimagingintheinfraredforplanetformation,debrisandexozodialdisks,anddirectdetectionofexoplanets.(W.C.Danchi,NASAGSFC)

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Achieving the necessary angular resolution at infrared and mid‐infrared wavelengths tostudy protoplanetary disks requires long‐baseline interferometry with ~100–250‐mbaselinestoprobe1‐AUscalesfortheneareststar‐formingregionsatadistanceof140pc.Suchbaselinesarealreadypossiblefromtheground.TheKIandtheVLTIarenowabletoprobe the dust and temperature structure around luminousHerbig Ae stars and T Tauridisks. The study of planet formation is being revolutionized by mid‐IR interferometry’ssensitivitytothewarminnerdiskandsoonwillbeaugmentedbyALMA’sprobeofthecoldouter disk. Although thenext‐generationmillimeter‐wave interferometerALMAwill lackthe angular resolution to probe the terrestrial planet formation zone, mid‐infraredinterferometrywith>250meterbaselineswillbesensitivetowarmdustemissionwithintheinnerfewAUsofplanet‐formingdisks.

However, thermal emission from the atmosphere and telescope limits the sensitivity ofground‐basedobservations,drivingmostscienceprogramstowardsspaceplatforms.Evenverymodest sized cooledapertures canhaveordersofmagnitudemore sensitivity in thethermal infrared than the largest ground‐based telescopes currently planned or inoperation.

4.1.2 Probe­ScaleMissionScienceGoalsAdvanced Imaging in the Mid­infrared: Planet Formation, Debris and Exozodiacal Disks,WarmJupiters,andSuper­Earths

Themoremodestbaselinesof~10–20m,attainablewithaprobe‐scalemission,wouldbecapableofmeasuringsolar‐systemfeaturesaroundnearbystars.

Aprobe‐scalemissionwouldfocusonthreemainsciencegoalsforfurtherstudyofgreatestimportance to the direct detection of exoplanets in the infrared, namely (1) measureresonantstructuresinexozodiacaldisks,theirfluxlevels,andthechemicalevolutionofthematerial they contain, (2) detect and characterize the atmospheres of nearby exoplanets,including thosecurrentlyknownandthose thatmaybediscoveredby this technique,and(3)surveyprospectivetargetstarsforlevelsofexozodicaldust.

MeasureResonantStructuresinExozodiacalDisksTheoptically‐thickandgas‐richdisksoutofwhichplanetsformarebright,dominatingoverthe emission from the central star. As the system evolves, the dust grains grow intoprogressivelylargerbodies,andthegasdissipates.Smallparticlesofduststillpervadethesystem, created in collisions between planetesimals, and occasionally large collisionswillproduce dramatic structure in these “debris disks.” Here, mid‐infrared imaging inconjunctionwithnulling(toblockoutthecentralstar),allowsthespatialdistributionandspectralenergydistribution(SED)ofthedebrisdiskstobescrutinized.Ground‐andspace‐based interferometers both have important roles here, since ground systems can probebrighterdebrisdiskswithgreaterangularresolution(necessarytonulloutthestarwithoutnulling out the dust) and the space systemswithmodest~20‐m baseline can sensitivelylookfordebrisaroundnearbystars.

Exozodiacal disks are already under intensive study because the mid‐IR emission andscattered light can mimic or hide the signal of terrestrial planets around nearby stars.Chapter5isdevotedtothiscrucialtopic,andwereferthereadertothatchapterforfurtherdetails.

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DetectandCharacterizeNearbyExoplanetsPlannedmid‐infrared facilitiesarealsobeingdesigned todirectlydetectexoplanetsusingnulling interferometry. Ground‐based systems can contribute at shorter wavelengths(<5µm) but exoplanet detection in the mid‐infrared (6–20 µm) is not expected to bepractical from the ground because thermal emission from the warm sky and telescopeswamps the faintplanetarysignal (exceptperhaps in somerare scenarios).Aprobe‐scalemissioncouldbeusedtocharacterizetheatmospheresofa largenumberof thecurrentlyknownmassiveexoplanetsandpossiblySuperEarths(e.g.,Danchietal.2003a,b),whereasaflagshipformation‐flyinginterferometercouldalsodetectterrestrialplanetsandsearchforatmosphericbiomarkers.

CharacterizetheAtmosphericConstituentsofGiantPlanetsAprobe‐scalemissionwouldhavesufficientsensitivity todetectandcharacterizeabroadrangeofexoplanets.Manymolecularspecies,suchascarbonmonoxide,methane,andwatervapor, have strong spectral features in the 3–8µm region, as can be seen in Figure 4‐2,whichdisplaysmodelatmospheres forplanetsatvariousdistances fromthehoststar, forexoplanets as calculated by Seager et al. (2005). The red curve shows the theoreticalspectrumofaveryhot,close‐inplanetat0.05AU,andthebluecurvedisplaysthespectrumfor amuch cooler planet ten times further out, at 0.5 AU. Also displayed are sensitivitycurves for the IRAC instrument onSpitzer (light blue) and the curve for a representativeprobe‐scalemission,FKSI(purple).

Figure4­2.TheFKSIsystemcanmeasurethespectraofexoplanetswithawiderangeofsemi‐majoraxes.

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CharacterizetheAtmospheresofSuperEarthsTheapparentlycommonplacenatureoflowermassplanetssuggestsanintermediateclassofplanets,SuperEarths,maybeobservablewithaprobe‐scalemission.Aprimeexampleofthis is theplanetarysystemofGL581. Theouter twoplanetsof thissystemarenear thehabitable zoneof the star. Selsis et al. (2007)haveanalyzed this systemandsuggest theouterplanet(planetd,a=0.25AU,R~2R⊕,M~8M⊕) is themost likelytobehabitable.Since the star is at 6.3 pc, the planet's angular separation, 0.04 arcsec, is at the firstmaximum of the fringe pattern of FKSI. At four times the flux of an Earth‐size planet,GL581dwouldbewithin the sensitivity limit of FKSI. Thus, there is alreadyoneknown,possibly Earth‐size planet that could be observed with FKSI. As more Super Earths arediscovered, it is likely thatFKSIwillhaveat leasta small sampleofobjects thatmightbecharacterizedfortheatmosphericconstituentsandcouldindicatehabitability.

CharacterizetheDynamicalEnvironmentoftheHabitableZoneInadditiontoobservationsof theatmospheresofgiantplanets,aprobe‐scalemissioncancontribute to our understanding the typical distribution of giant planets in wide orbits(>5‐10AU).Suchplanetscandominatethedynamicalenvironmentofthehabitablezone,andaffectthedeliveryofvolatilestoterrestrialplanets(Lunine2001).Planetsoneccentricorbitsmay disrupt the habitable zone or affect the composition by perturbation of outerplanetesimals into crossingorbitswith thehabitable zone. Understanding theplacementand orbital parameters of outer planets is an important prerequisite to searching forterrestrialplanetsinthesesystems.Suchobservationsaredifficulttoobtainfromground‐basedtelescopes.Massiveoryoungplanetscanbedetected(cf.Lafrenièreetal.2008;Hinzetal.2006),butplanetsthatareolderthan~0.5GyrorarelessmassivethanJupiterarenotdetectablewith8‐mclassground‐basedtelescopes.

SurveytheExozodiacalDustofTargetStarsKnowingwhichnearbymain‐sequencestarshave the lowestamountof zodiacaldustwillallowforamoreefficientstrategyofspectralcharacterizationofEarth‐sizedplanets.Thisinformationcouldalsobeusedtooptimizethesearchstrategyforanastrometricmission.Aprobe‐scale infraredmissionwouldenableanexozodiacaldust survey (<1 zodiat<1 to>4AU) including infrared spectra to elucidate grain chemistry and evolution, diskstructure,andbrightness,forallthelikelytargetstarsofaflagshipmissionwithin30pcoftheSolarSystem.

4.1.3 FlagshipMissionScienceGoalsandRequirements

SearchforHabitabilityandSignsofLifeAflagshipmissionwouldbedesignedtodetectterrestrialexoplanetsaroundnearbystarsandmeasuretheirspectra(e.g.,Beichmanetal.1999,2006;Fridlund2000;Kaltenegger&Fridlund2005).Thesespectrawouldbeanalyzedtoestablishthepresenceandcompositionof the planets’ atmospheres, to investigate their capability to sustain life as we know it(habitability),andtosearchforsignsoflife.Aflagshipmissionwouldalsohavethecapacitytoinvestigatethephysicalpropertiesandcompositionofabroaderdiversityofplanets,tounderstand the formation of planets, and to search for the presence of potentialbiosignature compounds. The range of characteristics of planets is likely to exceed ourexperiencewith the planets and satellites in our own Solar System.Moreover, Earth‐like

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planetsorbitingstarsofdifferentspectral typemayalsobeexpected toevolvedifferently(Selsis2000;Seguraetal.2003,2005).

Biomarkers are detectable species whose presence at significant abundance requires abiologicalorigin(DesMaraisetal.2002).Theyarethechemical ingredientsnecessary forbiosynthesis (e.g., oxygen [O2] and CH4) or are products of biosynthesis (e.g., complexorganic molecules, but also O2 and CH4). Our search for signs of life is based on theassumption thatextraterrestrial lifeshares fundamentalcharacteristicswith lifeonEarth,inthatitrequiresliquidwaterasasolventandhasacarbon‐basedchemistry(Owen1980;DesMaraisetal.2002).Therefore,weassumethatextraterrestrial life issimilarto lifeonEarth in its use of the same input and output gases, that it exists out of thermodynamicequilibrium, and that it has analogs to bacteria, plants, and animals on Earth (Lovelock1975).Figure4‐3displaystheobservedemissionspectrumfromtheEarthintheinfrared.

Figure4­3.Observedemissionspectrumintheinfrared(Christensen&Pearl1997)oftheintegratedEarth,asdeterminedfromEarthshineandspacerespectively.Thedataisshowninblack,modelandindividualchemicalfeaturecolor.(Kaltenegger,Traub&Jucks2007)

Candidatebiomarkers thatmightbedetectedby a flagshipmissionwith a low‐resolutioninstrument include O2, O3, and CH4. There are good biogeochemical and thermodynamicreasons for believing that these gases should be ubiquitous byproducts of carbon‐basedbiochemistry, even if the details of alien biochemistry are significantly different than thebiochemistry on Earth. Production of O2 by photosynthesis allows terrestrial plants andphotosynthetic bacteria (cyanobacteria) to use abundant H2O as the electron donor toreduce CO2, instead of having to rely on scarce supplies of hydrogen (H2) and hydrogensulfide (H2S). Oxygen and nitrous oxide (N2O) are two very promising bio‐indicators.Oxygen is a chemically reactive gas. Reduced gases and oxygen have to be producedconcurrentlytoproducequantities largeenoughtobedetectable indisk‐averagedspectra

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ofterrestrialplanetatmospheres,astheyreactrapidlywitheachother.N2OisabiomarkerintheEarth’satmosphere,beingproducedinabundanceby lifebutonly intraceamountsbynaturalprocesses.AlthougharelativelyweakfeatureintheEarth’sspectrum,itmaybemorepronouncedinterrestrialexoplanetatmospheresofdifferentcompositionorhost‐starspectral type (Seguraet al. 2005). Currently, efforts areongoing to explore theplausiblerange of habitable planets and to improve our understanding of the detectable ways inwhichlifemodifiesaplanetonaglobalscale.

Inthemid‐IR,theclassicalsignatureofbiologicalactivityisthecombineddetectionofthe9.6‐μmO3band,the15‐μmCO2band,andthe6.3‐μmH2Obandoritsrotationalbandthatextends from12μmout into themicrowave region (Selsis&Despois2002). Theoxygenand ozone absorption features in the visible and thermal infrared, respectively, couldindicatethepresenceofphotosyntheticbiologicalactivityonEarthanytimeduringthepast50%oftheageoftheSolarSystem.IntheEarth’satmosphere,the9.6‐μmO3bandisapoorquantitative indicator of the O2 amount, but an excellent qualitative indicator for theexistenceofeven tracesofO2. TheO39.6‐μmband isaverynonlinear indicatorofO2 fortwo reasons. First, for thepresentatmosphere, low‐resolution spectraof thisband showlittlechangewiththeO3abundancebecauseitisstronglysaturated. Second,theapparentdepth of this band remains nearly constant as O2 increases from 0.01 times the presentatmospherelevel(PAL)ofO2to1PAL(Seguraetal.2003).Theprimaryreasonforthis isthat thestratosphericozone increases thataccompanied theO2buildup lead toadditionalultraviolet heating of the stratosphere. At these higher temperatures, the stratosphericemissionfromthisbandpartiallymaskstheabsorptionofupwellingthermalradiationfromthesurface.

Methaneisnotreadily identifiedusinglow‐resolutionspectroscopyforpresent‐dayEarth,but the CH4 feature at 7.66 μm in the IR is easily detectable at higher abundances(Kalteneggar, Traub & Jucks 2007). When observed together with molecular oxygen,abundantCH4canindicatebiologicalprocesses(seealsoLovelock1975;Seguraetal.2003).Dependingonthedegreeofoxidationofaplanet'scrustanduppermantle,non‐biologicalmechanismscanalsoproducelargeamountsofCH4undercertaincircumstances.

N2O isalsoacandidatebiomarkerbecause it isproduced inabundanceby lifebutonly intraceamountsbynaturalprocesses.TherearenoN2OfeaturesinthevisibleandthreeweakN2O features in the thermal infraredat7.75μm,8.52μm, and16.89μm.Forpresent‐dayEarth,oneneedsaresolutionof23,23and44,respectively,todetecttheseN2Ofeaturesatthermal infraredwavelengths(Kaltenegger,Traub&Jucks2007).Spectral featuresofN2OalsobecomemoreapparentinatmosphereswithlessH2Ovapor.Methaneandnitrousoxidehave features nearly overlapping in the 7‐μm region, and additionally both lie in the redwingofthe6‐μmwaterband. Thus,N2Oisunlikelytobecomeaprimetargetforthefirstgenerationofspace‐basedmissionssearchingforexoplanets,butitisanexcellenttargetforfollow‐upmissions.Thereareothermoleculesthatcould,undersomecircumstances,actasexcellent biomarkers, e.g., themanufactured chloro‐fluorocarbons (CCl2F2 [Freon 13] andCCL3F[Freon12]) inourcurrentatmosphere in the thermal infraredwaveband,but theirabundancesaretoolowtobespectroscopicallyobservedatlowresolution.

Otherbiogenictracegasesmightalsoproducedetectablebiosignatures.Currentlyidentifiedpotentialcandidatesincludevolatilemethylatedcompounds(likemethylchloride[CH3Cl])and sulfur compounds. It isknown that these compoundsareproducedbymicrobes, andpreliminaryestimatesoftheirlifetimesanddetectabilityinEarth‐likeatmospheresaroundstarsofdifferentspectral typehavebeenmade(Seguraetal.2003,2005). However, it isnot yet fully understood how stable (or detectable) these compounds would be in

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atmospheresofdifferent compositionand for starsofdifferent spectral typeand incidentultraviolet flux. These uncertainties, however, could be addressed by further modelingstudies.

ScienceRequirementsTherequirement fora flagship‐classmissiontodetectdirectlyandcharacterizeEarth‐likeplanetsaroundnearbystars implies that themissionmustdetermine the typeofaplanetand characterize its gross physical properties and its main atmospheric constituents,thereby allowing an assessment of the likelihood that life or habitable conditions existthere.

TypesofStars:Onastrophysicalgrounds,Earth‐likeplanetsarelikelytobefoundaroundstarsthatareroughlysimilartotheSun(Turnbull2004).Therefore,targetstarswillincludemainsequenceF,G,andKstars.However,Mstarsmayalsoharborhabitableplanets,andthenearestofthesecouldbeinvestigated.

TerrestrialPlanets:Considering the radii andalbedosoreffective temperaturesofSolarSystemplanets,themissionmustbeabletodetectterrestrialplanets,downtoaminimumterrestrialplanetdefinedashaving1/2EarthsurfaceareaandEarthalbedo.Intheinfrared,theminimumdetectableplanetwouldbeonewithan infraredemissioncorresponding tothesurfaceareaandopticalalbedo,positionedintheorbitalphasespacestipulatedbelowandillustratedinFig.4‐4.

Habitable Zone: A flagship mission should search the most likely range as well as thecomplete range of temperatures within which life may be possible on a terrestrial‐typeplanet.IntheSolarSystem,themostlikelyzoneisnearthepresentEarth,andthefullzoneistherangebetweenVenusandMars.Thehabitablezone(HZ)isheredefinedastherangeof semi‐major axes from 0.7 to 1.5 AU scaled by the square root of stellar luminosity(Kastingetal.1993;Forget&Pierrehumbert1997).TheminimumterrestrialplanetmustbedetectableattheouteredgeoftheHZ.

Orbital Phase Space: The distribution of orbital elements of terrestrial type planets ispresently unknown, but observations suggest that giant‐planet orbits are distributedroughly equally in semi‐major axis, and in eccentricity up to those of the Solar Systemplanets and larger. Therefore, a flagshipmissionmust be designed to search for planetsdrawn from uniform probability distributions in semi‐major axis over the range 0.7 to1.5AUandineccentricityovertherange0to0.35,withtheorbitpoleuniformlydistributedoverthecelestialspherewithrandomorbitphase.

Giant planets: The occurrence and properties of giant planets may determine theenvironmentsof terrestrialplanets.The fieldofviewandsensitivitymustbe sufficient todetect a giant planet with the radius and geometric albedo or effective temperature ofJupiterat5AU(scaledbythesquarerootofstellar luminosity)aroundat least50%of itstargetstars.Asignal‐to‐noiseratioofatleast5isrequired.

Exozodiacaldust:Emissionfromexozodiacaldustisbothasourceofnoiseandalegitimatetargetofscientificinterest.Aflagshipmissionmustbeabletodetectplanetsinthepresenceofzodiacalcloudsatlevelsuptoamaximumof10timesthebrightnessofthezodiacalcloudintheSolarSystem.Althoughtheaverageamountofexo‐zodiacalemissioninthe“habitablezone”isnotyetknown,weadoptanexpectedlevelofzodiacalemissionaroundtargetstarsof 3 times the level in our own Solar Systemwith the same fractional clumpiness as ourSolar System’s cloud. From a science standpoint, determining and understanding thepropertiesofthezodiacalcloudisessentialtounderstandingtheformation,evolution,and

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habitabilityofplanetarysystems.Thus,themissionshouldbeabletodeterminethespatialand spectral distribution of zodiacal clouds with at least 0.1 times the brightness of theSolarSystem’szodiacalcloud.

Spectral range: The required spectral range of the mission for characterization ofexoplanetswillemphasizethecharacterizationofEarth‐likeplanetsandisthereforesetto6.5to18µmintheinfrared.Theminimumrangeis6.5to15µm.

Figure4­4.TheextentoftheHabitableZoneforthesinglemain‐sequencestars(FandGstarsonupperpanel,andM,Kstarsonlowerpanel)within30pcfromtheSun.(Kalteneggeretal.2009)

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Spectrum: Themissionwill use the spectrum of a planet to characterize its surface andatmosphere. The spectrum of the present Earth, scaled for semi‐major axis and starluminosity,isusedasareferenceandsuggestsaminimumspectralresolutionof25withagoalof50. Themissionmustmeasurewater(H2O)andozone(O3)with20%accuracy inthe equivalent width of the spectral feature. Additionally it is highly desirable that themissionbeabletomeasurecarbondioxide(CO2)aswellasmethane(CH4)(if the latter ispresent in high quantities predicted in somemodels of pre‐biotic, or anoxic planets, e.g.,Kasting&Catling2003).

Numberofstarstobesearched:Tosatisfy itsscientificgoals, themissionshoulddetectandcharacterizeastatisticallysignificantsampleofterrestrialplanetsorbitingF,G,andKstars. Although at this time, the fractional occurrence of terrestrial exoplanets in theHabitableZoneisnotknown,asampleof150starswithin30pc(includingasmallnumberofnearbyMstars)shouldsufficebasedonourpresentunderstanding.

Extendednumberof stars: It isdesired to searchasmanystarsaspossible,beyond therequiredcoresample.Weanticipatethatanymissioncapableofsatisfyingtheseobjectiveswill also be capable of searching many more stars if the overall requirements oncompletenessarerelaxed.Itisdesiredthatthemissionbecapableofsearchinganextendedgroupofstarsdefinedasthosesystemsofanytypeinwhichallorpartofthecontinuouslyhabitablezone(seebelow)canbesearched.

Search completeness: Search completeness is defined as that fraction of planets in theorbital phase space that could be foundwithin instrumental andmission constraints.Werequire each of the 150 stars to be searched at the 90% completeness level. For othertargetsinadditiontothe150stars,theavailablehabitablezonewillbesearchedastolimitsinplanet'sorbitalcharacteristics.

Table4­1.FlagshipInfraredImagingMissionRequirementSummary

Parameter Requirement

StarTypes F,G,K,selected,nearbyM,andothers

HabitableZone 0.7–1.5(1.8)AUscaledasL1/2(Note*)

NumberofStarstoSearch >150

CompletenessforEachCoreStar 90%

MinimumNumberofVisitsperTarget 3

MinimumPlanetSize 0.5–1.0EarthArea

GeometricAlbedo Earth’s

SpectralRangeandResolution 6.5–18μm;R=25[50]

CharacterizationCompleteness Spectraof50%ofDetectedor10Planets

GiantPlanets JupiterFlux,5AU,50%ofStars

MaximumTolerableExozodiacalEmission 10timesSolarSystemZodiacalCloud

*Therearetwodefinitionsintheliteraturefortheouterlimitofthehabitablezone.Thefirstis1.5AUscaledtotheluminositytothe½powerbasedonKastingetal.(1993).Thesecondis1.8AUscaledinthesamewayfromForget&Pierrehumbert(1997).

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Characterizationcompleteness:Whileitwillbedifficulttoobtainspectraofthefainterorlesswellpositionedplanets,werequirethatthemissionbecapableofmeasuringspectraofatleast10(~50%)ofthedetectedplanets.

Visitations:Multiplevisitsper starwillbe required toachieve requiredcompleteness, todistinguishitfrombackgroundobjects,todetermineitsorbit,andtostudyaplanetalongitsorbit. Themissionmustbecapableofmakingatleastthreevisitstoeachstartomeetthecompletenessandotherrequirements.

MultiplePlanets:Afterthecompletionoftherequirednumberofvisitationsdefinedabove,themissionshouldbeabletocharacterizeaplanetarysystemascomplexasourownwiththree terrestrial‐sized planets assuming each planet is individually bright enough to bedetected.

OrbitDetermination:After thecompletionof therequirednumberofvisitationsdefinedabove,themissionshallbeabletolocalizethepositionofaplanetorbitinginthehabitablezonewithanaccuracyof10%ofthesemi‐majoraxisoftheplanet’sorbit.Thisaccuracymaydegradeto25%inthepresenceofmultipleplanets.

4.2 ObservatoryConceptsAninterferometerisacompellingchoicefortheoveralldesignofaflagshipmission.Atmid‐infrared wavelengths the angular resolution needed to resolve an Earth‐Sun analog at adistanceof10pc,wouldbe~50mas,sothataconventionalsingle‐telescopedesignwouldneed a primarymirror with a diameter larger than 40m. By using separated apertures,baselines of hundreds of meters are possible. Nulling interferometry is then used tosuppress the on‐axis light from the parent star, whose photon noise would otherwiseoverwhelm the light from the planet (Bracewell 1978).Off‐axis light ismodulated by thespatial response of the interferometer: as the array is rotated, a planet produces acharacteristic signal,which canbedeconvolved from the resultant time series (Bracewell1978;Légeretal.1996;Angel&Woolf1997).Imagesoftheplanetarysystemcanbeformedusinganextensionoftechniquesdevelopedforradiointerferometry(Lay2005).

Aprobe‐scalemissionwouldhave collecting aperturesdeployedona single ‘structurally‐connected’ spacecraft, and a flagshipmissionwould have the very large formation‐flyingsystems to achieve the strategic goals of life detection and planet imaging. We brieflydescribe concepts for both a small structurally connected interferometer and a largerformation‐flyingsystem.

4.2.1 Probe­ScaleMissionConcept

Structurally­ConnectedInterferometerAn example of a small structurally‐connected infrared interferometer concept is theFourier‐Kelvin Stellar Interferometer (FKSI), which is a two‐telescope passively coolednulling interferometer operating with observing wavelengths between 3 and 8 µm (seeDanchietal.2003a,b),showninFigure4‐5.TheFKSIobservatorywilloperateatthesecondSun‐EarthLagrangepoint(L2)inalargeamplitudeLissajious(orhalo)orbit.Thebaselinedesign employs 0.5‐m apertures. The telescopes employ two flat mirrors (siderostats)mounted12.5m apart on composite support booms,minimizing alignment requirementsforthebeamsthatentertheinstrumentpackage.Theboomssupportsunshadesthatallowpassivecoolingofthestructureto60–65K,reducingthermalnoiseinthetelescopesystem

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to a level that is negligible compared to that from the local zodiacal cloud (zodiacalbackgroundlimitedperformance)overmostoftheinstrumentpassband.

ThemechanicaldesignisappropriateforthesmallestofthetheAtlasVlaunchvehicleswitha 4‐m diameter fairing. The design minimizes the number of deployments. Thedeploymentsarelistedasfollows:(1)and(2)thetwosiderostatsmountedontheendsofthe booms, separated by 12.5 m, deployed using the well proven MILSTAR hingetechnology. The booms aremade from composite structures for an optimal strength‐to‐weightratio,andthefixedsunshadesaremountedtothem;(3)ahigh‐gaingimbaledS‐bandantennaforthedatadownlink;(4)asolararrayforpower;and(5)aradiator.

Figure4­5.Asmallstructually‐connectedinterferometermissionconcept.Twotelescopesandpassivecoolingareusedforobservationsinthe3–8µmband.(W.C.Danchi,NASAGSFC)

A schematic diagram of the optical design is shown in Figure 4‐6. There are five majorsubsystems in the instrument payload, which is mounted through a low‐mechanical‐frequency coupling and gamma‐Al struts to the spacecraft bus. These include: (1) theboom‐truss assembly that holds the siderostatmirrors and the non‐deployed sunshields;(2) themain instrumentmoduleandopticalbenchassembly,which includesapairofoff‐axis parabolic mirrors for afocal beam size reduction, and also a fast steering mirrorassemblyforthecontroloffinepointing,twoopticaldelaylines(onetobeused,theotherisa spare), and two dichroic beam splitters, one for the angle tracker and fringe trackerassemblies (0.8–2.5µm), the other for the science band of 3–8µm; (3) the angle trackerassemblyforfinepointing;(4)thefringetrackerassemblyforstabilizingthefringesforthenullinginstrumentassembly;and(5)thenullinginstrumentassemblyitself.

The nulling instrument is based on a modified Mach‐Zehnder beamsplitter design thatmaximizesthesymmetryofthetwobeams,helpingtoensureadeepnull.Otherelementsinthe system include shutters for alignment and calibration purposes, an assembly foramplitude control of the fringe null, and optical fibers forwavefront cleanup. The fibershelp achieve the desired null depth with reduced tolerances on the preceding optical

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components(inordertoreducemanufacturingcosts).Thedarkandbrightoutputsaresentto their respective Focal Plane Arrays (FPAs), which could be long wavelength HgCdTearraysfromTeledyne(previouslyRockwell)operatingat35K,orSi:Asarraysoperatingat8K, based on those for theMIRI instrument on JWST. The FPAs for the fringe and angletrackers are operated at 77 K and are based on theHgCdTe arrays used on theNICMOSinstrumentonHST.Cryogenicdelaylinesareusedtoequalizethepathlengthsbetweenthetwosidesoftheinterferometer.

Figure 4­6. Schematic design of the boom and instrument module subsystem for FKSI. The varioussubassembliesareasnotedinthefigure.(W.C.Danchi,NASAGSFC)

Whentheinstrumentisinoperation,itrotatesslowlyaroundthelineofsighttotheobject.Insodoing,aperiodicsignal isgeneratedat theoutputof thenuller instrument,which isnon‐zeroifthereisaplanetorspatiallyresolvedsourcewithinthetransmissionpatternofthenuller.Datafromthebrightportisalsotakenprovidingmeasurementsofthevisibilityof the object. Simulations have been performed, both for nulling operation as well asimaging (Danchi et al. 2003a; Barry et al. 2005, 2006). Table 4‐2 lists the types ofmeasurements that can bemadewith such as system; the companion figure, Figure 4‐2,shows thatasmall interferometer iscapableofmeasuring theemissionspectraofknownexoplanetswithalargerangeofsemi‐majoraxes.

Theconceptdiscussedabovehasundergonea thorough integratedanalysisandmodelingstudyof thestructureandoptics tovalidate thedesignandensure itmeetsrequirements(Hydeet al. 2004). Amajor concernwas thepropagationof reaction‐wheeldisturbancesfrom the spacecraft bus through the bus‐instrument package coupling up to the boomsystem. Contributionstotheperformanceerrorbudgetfortherequired10–4nullarewell

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understood and have been reported in Hyde et al. (2004). The null depth requirementdrivestheinstrumentperformance.OthersourcesofnoiseincludetheFringeTracker(FT),theOpticalDelayLine(ODL),theAttitudeControlSystem(ACS),andtheboomsthemselves.Theboomsystemhadthelowestordermodesat5.6and7.3Hz.Thismodelingverifiedthatthe closed‐loop performance requirement could be met or exceeded at all wheel speedsfrom0.1to100Hz(Hydeetal.2004).

PerformanceTable 4‐2 lists the planetary characteristics andwhat can be determined using amodestnulling interferometer. This list includes planetary temperature, radius, mass density,albedo,surfacegravity,andatmosphericcomposition,aswellasthepresenceofwater.Todate, progress has been made on the physical characteristics of planets largely throughtransiting systems, but a small planet‐finding interferometer can measure the emissionspectraofalargenumberofthenon‐transitingones,aswellasmoreprecisespectraofthetransitingones.

Table4­2.CharacteristicsofexoplanetsthatcanbemeasuredusingFKSI

Parameters WhatFKSIDoes

Removessin(i)ambiguity MeasurePlanetCharacteristics

Temperature MeasureTemperaturevariabilityduetodistancechanges MeasurePlanetradius MeasurePlanetmass EstimatePlanetalbedo CooperativeSurfacegravity Cooperative

Atmosphericandsurfacecomposition Measure

Timevariabilityofcomposition Measure

Presenceofwater Measure

PlanetarySystemCharacteristics

Influenceofotherplanets,orbitco‐planarity Estimate

Comets,asteroids,zodiacaldust Measure

Manymolecularspecies(suchascarbonmonoxide,methane,andwatervapor)havestrongspectralfeaturesinthe3–8µmregion,ascanbeseeninFigure4‐2,whichdisplaysmodelatmospheres for exoplanets calculated by Seager (2005), for planets at various distancesfrom the host star. The red curve shows the theoretical spectrumof a very hot, close‐inplanetat0.05AU,whilethebluecurvedisplaysspectrumforamuchcoolerplanettentimesfurther out, at 0.5 AU. Also displayed are sensitivity curves for the IRAC instrument onSpitzer (light blue) and FKSI (purple). Clearly such a mission concept has sufficientsensitivitytodetectandcharacterizeabroadrangeofexoplanets.

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Figure4­7.ExpectedperformanceforPegaseandFKSIcomparedtotheground‐basedinstruments,for30minintegrationtimeand1%uncertaintyonthestellarangulardiameters.(Defrèreetal.2008)

Figure4­8.Skycoverageafter1yearofobservationofGENIE(darkframe),ALADDIN(lightframe)andPegase(shaded area) shown with the Darwin/TPF all sky target catalogue. The blue‐shaded area shows the skycoverageofaspace‐basedinstrumentwithaneclipticlatitudeinthe[–30˚,30˚]range(suchasPegase).TheskycoverageofFKSIissimilartothatofPegasewithanextensionof40˚insteadof60˚.(Defrèreetal.2008)

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Initialstudieswithveryconservativeassumptionsofan8‐mboomlength,140exoplanets(known at that time), 120‐s on‐source integration time, 15‐nm RMS path length error,telescopetemperatureof65K,andasmallsunshieldwitha±20degreefieldofregard;gave~25detections.Currrently,thereareabout250knownplanets,sowiththoseassumptionsandthesamedetectionratioofabout18%,itshouldbepossibletodetectandcharacterizeabout 45 exoplanets, and a larger sunshadewith a field of regard of ± 45 degreeswouldapproximatelydoublethisnumberto~90planets.

Recentstudiesof thedetectabilityofdebrisdisks (Defrèreetal.2008)demonstrated thatthe residualpathlengtherror fora small system likeFKSI shouldbeof theorderof2nmRMS, which would significantly increase the number of exoplanets. As a conservativeestimate,weexpectthatasmallsystemcoulddetect(e.g.,removethesin(i)ambiguity)andcharacterizeabout75–100knownexoplanets.

These recent studies also have shown that a smallmission is ideal for the detection andcharacterizationofexozodialanddebrisdisksaroundallcandidatetargetstarsintheSolarneighborhoodasseeninFigures4‐7and4‐8.(Defrèreetal.2008).Indeed,theperformanceleveloftheFKSIsystemisoftheorderof1SolarSystemZodiin30minutesofintegrationforaG0starat30pc.Withasmallsunshade,thissystemcouldobserveoftheorderof450starsintheSolarneighborhood,andwithalargerfield‐of‐regard,suchas±45degrees,theexozodisanddebrisdisksofabout1000starscouldbestudied.

Although the telescopes are somewhat larger than has been discussed in some of theexistingmissionconcepts (e.g.,1–2m)andaresomewhatcooler (e.g.,<60K)so that thesystem can operate at longerwavelengths, it is possible for a small infrared structurallyconnectedinterferometertodetectandcharacterizeSuperEarthsandeven~50–75Earth‐sized planets around the nearest stars. This is especially important now that there isevidencethatlowermassplanetsmaybeverycommon,basedonthedetectionofthe5.5‐Earth‐massplanetusing themicrolensingapproach(e.g.,Beaulieuetal.2006;Gouldetal.2006).

Furtherstudiesofthecapabilitiesofasmallinfraredstructurally‐connectedinterferometerarenecessarytoimproveuponourestimatesofsystemperformance.

4.2.2 FlagshipMissionConcept

FormationFlyingInterferometerFigure 4‐9 depicts amission concept capable of detecting a large number of Earth‐sizedplanets. This Emma X‐Array formation‐flying design is the culmination of more than adecade of study by NASA and ESA. Light from the target star is reflected from the fourcollectormirrors (~2mdiameter) and focusedonto the input apertures of the combinerspacecraft approximately 1 km away. To observe a target system, the collector array isrotated slowly about the line‐of‐sight, using a combination of centimeter‐precisionformationflyingandcarefulattitudecontroltomaintainthepointingofthebeams.Thelightenteringthecombinerpassesthroughaseriesofoptics(Figure4‐10)thatperformsbeamsteering and compression, delay compensation, intensity and dispersion correction (see“theAdaptiveNuller”below),pairwisebeamcombinationwithπphaseshift(thenullers),cross‐combinationofthenulledbeams,single‐modespatialfiltering,and,finally,dispersionanddetectionof thenulled light on the science camera.The time series of outputdata issynthesized into an image. A key advantage of the X‐Array configuration is its very highangularresolution,asillustratedbythesimulatedimageshowninFigure4‐11.

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Figure4­9.‘EmmaX‐Array’formationflyingconcept.

Figure4­10:Schematicofbeamcombineroptics.

PerformanceThe planet‐finding capability for a formation‐flying Emma X‐Array design is shown inFigure4‐12. Amissionwith2‐mdiametercollectorswouldbecapableofdetectingmorethan100Earth‐sizedplanetsaroundnearbystars,primarilyF,G,andKspectraltype,overaperiodof2years,assumingη⊕=1.TheperformancemodelisdescribedinLayetal.(2007)and includes the dominant sources of noise: photon noise from local zodi, exo‐zodi andstellar leakage, and instability noise (Lay 2004) from fluctuations in the phasing andintensitybalancewithintheinstrumentoptics.Followingacandidateplanetdetectionandfollow‐up confirmation, a longer period of observing time is dedicated to spectroscopiccharacterization,usingthesameobservingprocedureasfortheinitialdetection.DetailsofadesignstudyforthearrayaregivenbyMartinetal.(2008a).

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Figure 4­11: Simulated ‘dirty’ image from a 400 × 67‐m Emma X‐Array, prior to deconvolution. Angularresolution is 2.5 mas. Planet locations are indicated by red arrows. Simultaneous full resolution (R ~ 100)spectroscopyforallobjectswithinthefieldofviewisanaturalby‐productofinterferometricobserving.Whiledata from the detection and orbit‐determination phases should be sufficient for a coarse spectrum, a deepcharacterization will require significant integration time. Detection of CO2 for an Earth at 5 pc using 2‐mcollectorswill require ~24 hours of integration (SNR of 10 relative to the continuum). The narrower ozoneabsorption line requires 16 days at 5 pc. For ozone at 10 pc, integration times as long as 40 days could beneeded,fallingto~6dayswith4‐mdiametercollectors.(O.P.Lay,JPL)

4.3 Technology

4.3.1 ExperimentsinNullingInterferometry,1999–2009Results obtained in the lab havemet the requirements for infrared nulling needed for aprobe‐scalemissionandareclosetomeetingthoseofaflagshipmission.Laboratoryworkwith the Adaptive Nuller has indicated that mid‐infrared nulls of 1.0 × 10–5 are nowachievablewithabandwidthof34%andameanwavelengthof10µm;thissuggeststhatatthesubsystem‐levelthenullingperformanceforaflagshipmissionisnearTRL4(cryogenictesting would be needed for TRL 5). These results also show that the achromatic phaseshifters,theAdaptiveNuller,andthemid‐infraredsingle‐modefibersthatarecontributingto these results are nowmature technology. A summary of the past decade’s research innullinginterferometryisgivenintheparagraphsthatfollow.

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Figure 4­12: Predicted number of Earths detectable by Emma X‐Array architecture as a function of elapsedmissiontimeandcollectordiameter.(O.P.Lay&S.Hunyadi,JPL)

Deepnullsofnarrowbandlighthavebeenachievedusingbothpupil‐planeandfocalplanenullingtechniques,andtheextensionoftheseresultstobroadbandandrandomlypolarizedsources is now the remaining challenge. To achieve deep nulls in an interferometer, theoptical properties of the combined beamsmust be quite similar. The properties includeintensity balance, polarization angle, polarization dispersion, and wavefront differences.Someoftherequirementsforthebalanceinthesepropertiescanbemitigatedbytheuseofsingle‐mode spatial filtering at the detector, for example by using a single‐mode fiber toreceive the light. The components of the optical system will introduce defects into thepristine wavefront received from the stellar source, adding wavefront deformations andpolarizationchanges. Ifthesedefectsaremoreorlessthesameineachinputbeamoftheinterferometer, the system will still be capable of nulling. For a broadband source, theeffects of the defects need to be controlled across the entire nulling waveband, and thisrequirementhasbeenaddressedwiththedevelopmentoftheAdaptiveNuller(Petersetal.2008), which is a system designed to correct intensity and phase differences across thebandforbothpolarizations.

Since 1999whenOllivier (1999) reported laser nulls of~1.7 × 10–4 and Serabyn (1999)reported 5.0 × 10–5, progress has been made in both null depth and bandwidth. Deep,single‐polarization laser nulls in the mid‐infrared waveband of order 3.0 × 10–6 werereportedbyMartinetal.(2003),andSamueleetal.(2007)reportednullsoforder1.1×10–7atvisiblewavelengths. Similarly, sinceWallaceetal. (2000) reportedbroadbandnullsof7.4×10–5at18%bandwidth,Samueleetal.(2007)achieved1.0×10–6at15%bandwidthinthe visible and Peters et al. (2008) have shown through narrowbandmeasurements that1.0× 10–5 nulls at 34% bandwidth in the mid‐infrared are attainable. These results andothers of note are included in Figure 4‐13,which illustrates the experiments undertakensince1999.

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10-7

10-6

10-5

10-4

10-3

0 5 10 15 20 25 30 35 40

Null D

epth

Bandwidth (%)

Brachet 2005

Samuele et al. 2007

Peters 2009

Gappinger 2009

Gappinger 2009

Mennesson et al. 2003

Martin et al. 2003

Martin et al. 2005

Haguenauer and Serabyn 2006

Samuele et al. 2007

Serabyn et al. 1999

Mennesson et al. 2003 Wallace et al. 2000

Ollivier 1999

Serabyn et al. 1999

Martin et al. 2003

Mennesson et al. 2003

Schmidtlin et al. 2006

Buisset et al. 2006

Ergenzinger et al. 2004

Vosteen et al. 2005

MAII breadboards

Visible wavelengths H band K band

N band Results of Laboratory ExperimentsIn Nulling Interferometry (1999-2009)

Figure4­13.Chartofnulldepthsachievedsince1999.Theblueshadedareahighlightsthenullingperformanceneededtobedemonstratedinthelabinpreparationforaflagshipmission.(P.R.Lawson,JPL)

Serabyn’s early nuller, known as the SIM roof‐top nuller, was used for both laser andbroadband nulling tests near the visible waveband. It used a geometric field inversiontechniquebutwasasymmetricinthatabeamsplitter‐compensatorplatearrangementwasused.AmoresymmetricalnullerwasthenbuiltfortheKecktelescopes,themodifiedMach‐Zehnder (MMZ) nuller (Crawford et al. 2005) in which field inversion was performedoutsidethenullerandthebeamstraversedtwobeamsplitterseach,allowingsymmetricaltreatment(Figure4‐14).

Figure 4­14. (left) MMZ nuller built for Keck telescopes, (right) single‐polarization laser null at 10.6 µmshowingnulldepthof5×10–7.

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Figure4­15.(left)conceptforanullerusingafiberbeamcombiner,(right)unpolarizedbroadbandnullof18%bandwidthat1.65mmshowingnulldepthbetterthan1×10–4.

TheKecknuller (Serabynetal.2004)employs twoMMZnullers tonull four inputbeamsandoperates in themid‐infrared.Morerecently, laboratoryworkhasusedMach‐Zehnderschemes to produce symmetrical nulling systems for the Planet Detection Testbed (PDT)(Martinetal.2006)andtheAchromaticNullingTestbed(ANT)(Gappingeretal.2009),forexample.ThePDThasnulledfourlaserbeamsat10.6µmwavelengthatthe2.5×10–5level.

Asomewhatdifferentinterferometricbeam‐combinationschemehasalsobeenproposedinEurope(Wallneretal.2004;Karlsonetal.2004).Bycombiningbeamsdirectlyonasingle‐mode fiber, the complex interaction of the light with beamsplitter and antireflectioncoatings can be eliminated. Conceptually, such a beamcombiner can be both achromaticandpolarization insensitive,at thecostofsomeoptical throughputefficiency. Small‐scaletestbedsforthistypeofbeamcombinerarebeingdevelopedandhaveachieveddeeplasernulls of 2.0 × 10–4 (Mennesson et al. 2006) in the visible and broadband nulls of order1.0×10–4at18%bandwidthatJ‐band(Martinetal.2008b)(Figure4‐15).

4.3.2 TechnologyforProbeandFlagshipMissions

Mid­IRSpatialFiltersSpatialfiltersareanessentialtechnologyfornulling interferometry.Theycanbeused toreduce complex optical aberrations in theincomingwavefrontstosimpleintensityandphase differences (which are more readilycorrected), thus making extremely deepnulls possible. Spatial filters may beimplemented inavarietyofways, includingsingle‐mode fiber‐optics made fromchalcogenide glasses, metallized waveguidestructures micro‐machined in silicon, orthrough the use of photonic crystal fibers.Two types of fiber spatial filters have beensuccessfullydeveloped:polycrystalinesilverhalide fibers have been developed by Prof.AbrahamKatziratTelAvivUniversity (TAU) in Israel; andchalcogenide fibershavebeendevelopedbyDr.JasSangheraattheNavalResearchLaboratory(NRL).

Figure4­16.Single‐modeChalcogenidefibersdevelopedbytheNavalResearchLaboratory

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The20‐cmlongchalcogenidefibersdevelopedbytheNavalResearchLaboratory(showninFigure4‐16)wereshowntodemonstrate30‐dBrejection(afactorof1000)ofhigherordermodes andhave an efficiency of 40%, accounting for both throughput andFresnel losses(Ksendzov et al. 2007). The transmission losses weremeasured to be 8 dB·m–1, and thefibersareusableuptoawavelengthofabout11µm.ThechalcogenidefibersdevelopedattheNavalResearchLaboratorywereused successfully in theAchromaticNullingTestbedandarecurrentlyinuseintheAdaptiveNullertestbed.

The 10–20 cm long silver halide fibers thatwere developed by Tel Aviv Universitywereshown to demonstrate 42‐dB rejection (a factor of 16,000) of higher order modes withtransmissionlossesof12dBm–1(Ksendzovetal.2008).Thishighrejectionofhigher‐ordermodeswasaccomplishedwiththeadditionofaperturingoftheoutputofthefibers,madepossible by the physically large diameter of the fibers. Silver halide fibers should inprinciplebeusableuptoawavelengthofabout18µm,althoughthe labtestsat JPLwereconducted only at 10µm.This is the first time silver halide fiberswere demonstrated tohavesingle‐modebehavior.

Figure4­17. Schematic of theAdaptiveNuller. Uncompensated light that enters the device is split into twolinearpolarizations,andeachpolarizaionistreatedseparatelythenrecombined.Lightisdispersedandimagedontoadeformablemirror(DM).PistonofelementsintheDMadjustsphaseataparticularwavelength,andtiltofthesamepixeladjustsrelativeintensitybyshearingtheoutputpupilatthatwavelength.

AdaptiveNullingandAchromaticPhaseShiftersThevariationsinamplitudeandphasethatmaybepresentintheincomingwavefrontmustbecorrected inordertoachievedeepnullsateverywavelength.TheAdaptiveNullerwasdesigned to correctphaseand intensityvariationsas a functionofwavelength, in eachoftwo linear polarizations. In principle this should allow high‐performance nullinginterferometry, while at the same time substantially relaxing the requirements on thenullinginterferometer’sopticalcomponents.Thegoalofthetestbedwastodemonstrate,ina3‐μmbandcenteredatawavelengthof10μm,thecorrectionoftheintensitydifferencetolessthan0.2%RMS(1σ)betweentheinterferometer’sarms,andatthesametimecorrect

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the phase difference across the band to < 5 nm RMS (1 σ). This overall correction isconsistent with a null depth of 1 × 10−5 (1 part in 100,000) if all other sources of nulldegradationcanbeneglected.

TheAdaptiveNullerwasdesigned to correct these variations,matching the intensity andphase between the two arms of the interferometer, as a function ofwavelength, for bothlinear polarizations. A deformable mirror is used to adjust amplitude and phaseindependentlyineachofabout12spectralchannels. Aschematicoftheadaptivenullerisshown inFigure4‐17.Twoprismssplit the incoming light into its two linearpolarizationcomponentsanddivideitintoroughlyadozenspectralchannels.Thelightisthenfocusedontoadeformablemirrorwherethepistonofeachpixelindependentlyadjuststhephaseofeach channel. By adjusting the angle of the reflected beam so that the beam is slightlyvignetted at an exit aperture, the intensity is also controlled. The adaptive nuller uses abroadbandthermalsourcetogeneratelightinthe8–12μmwavelengthband.AphotographofthetestbedisshowninFigure4‐18.

Figure 4­18. The Adaptive Nuller Testbed (AdN). The Adaptive Nuller demonstrated phase and intensitycompensationofbeamswithinanullinginterferometertoalevelof0.12%RMSinintensityandlessthan5nmRMSinphase.ThisversionoftheAdNoperatesoverawavelengthrangeof8–12µm.Thelong‐focusparabolas,usedinAdNareseenontheupperright.

The initial results of this research have been published by Peters et al. (2008). InMarch/April 2007, the testbed achieved its primary goal of demonstrating phasecompensation to better than 5 nm RMS across the 8–12 μm band and intensitycompensation tobetter than0.2%RMS. Narrow‐bandmeasurementsofnulldepth in thechannels across the passband indicated a null depth of 1.0× 10–5, would be attainable.When demonstrated using the full bandwidth of the Adaptive Nuller, it would be thedeepestbroadbandnulleverachievedbyamid‐infrarednullinginterferometerandwoulddemonstratetheflightrequirementforaflagshipmission.

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CryocoolersWiththeAdvancedCryocoolerTechnologyDevelopmentProgram(ACTDP),theTerrestrialPlanetFinder(TPF)andJWSTprojectshaveproduceddevelopmentmodelcoolersthathavemetorexceededtheirperformancerequirements,whicharetoprovide~30mWofcoolingat 6 K and ~150 mW at 18 K. This demonstrates the approach to cooling the sciencedetectortoatemperaturelowenoughtorevealtheweakplanetsignals.ThisactivityisatTRL6.

CryogenicdelaylinesTheDutchcompanyTNOScienceandIndustryledaconsortiumthatdevelopedacompactcryogenicOpticalDelayLine(ODL)foruseinfuturespaceinterferometrymissionssuchasESA'sDarwinandNASA'sTPF‐I(Figure4‐19).Theprototypedelaylineisrepresentativeofa flightmechanism. The ODL consists of a two‐mirror cat's eyewith amagnetic bearinglinearguidingsystem.TNOanditspartnershavedemonstratedthataccurateopticalpath‐length control is possiblewith the use ofmagnetic bearings and a single‐stage actuationconcept.Activemagneticbearingsarecontactless,haveno frictionorhysteresis,arewearfree,andhavelowpowerdissipation.ThedesignoftheDarwinbroadbandODLmeetstheESA requirements, which are an OPD stroke of 20 mm, stability of 0.9 nm RMS (with adisturbance spectrum of 3000 nm RMS, < 20 mW power dissipation (2 mW with flightcabling), outputbeam tilt<0.24microradians, output lateral shift<10‐µmpeak‐to‐peak,wavefront distortion < 63 nm RMS at 40 K, and wavelength range (0.45–20 µm). TheDarwinODLisrepresentativeofafutureflightmechanism,withallmaterialsandprocessesused being suitable for flight qualification. Positioning is donewith a voice coil down tosubnanometerprecision.Theverificationprogram,includingfunctionaltestingat40K,hasbeencompletedsuccesfully.ThistechnologyisatTRL6(seeFridlundetal.2008).

Figure 4­19. Overview of the Optical Delay Line with the outer fixed structure holding the active parts formagnetic levitation and axial motion, and the inner moving structure that includes the optical components.(CourtesyofESAandTNO)

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ThermalshieldsScience requirements for JWST have driven the need for a deployable, low areal‐density,high thermal‐performance efficiency (effective emittance, e* of 10–4 to 10–5) sunshield topassively cool the Observatory Telescope Elements and Integrated Science InstrumentsModule(OTE/ISIM).Thethermalperformancedictatedtheneedforasunshieldconsistingofmultiple,space‐membrane layerssuchthatthe~200kWoftheSun’senergy impingingon the Sun‐facing layerwould be attenuated such that the heat emitted from the rear orOTE‐facing membrane would be < 1 W. This would enable the OTE/ISIM to operate atcryogenictemperaturelevels(<50K)andhaveatemperaturestabilityof0.1to0.2Koverthefield‐of‐regardpointingre‐alignments.

Figure4­20.TheJamesWebbSpaceTelescope.(CourtesyNASA/JWST)

Basedon these top‐levelmissionrequirementsasunshieldwasdevelopedemploying fivemembrane layers with pitch and dihedral separation angles of 1.6 and 2 degrees,respectively.Throughtheuseofalowsolarabsorptance‐to‐emittanceratio(αS/εH)coatingontheSun‐facingsurface(layer1)andaninfraredhighlyreflectivecoatingonthemajorityoftheothermembranes’surfaces,theSun’sheatloadabsorbedbytheSun‐facingmembraneis reduced and the resulting infrared energy re‐emitted to the other layers would bereflectedlaterallyouttheopensidesofthesunshield,thusgreatlyattenuatingtheresidualinfraredenergyemittedbytheOTE/SIM‐facingmembranesurface(layer5).Fromavarietyof trade studies the Sunshield evolved into a five‐membrane system as shown in Figure4‐20withmembranes140to150m2insizethataresupportedbysixspreaderbar/boomassembliesandarelightlytensionedinadeployedconfigurationbyconstant‐forcespring‐loadedcatenarycableassembliesattachedtotheperipheryofeachmembrane.Thepresentsunshadedesign for JWST isatTRL6andmeets thesimilarrequirements for the flagshipmissionandismorethanadequateforasmallmission(Kurland2007).

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DetectorTechnologyDetectorsorSensorChipAssemblies(SCAs)fromtheJWSTMid‐InfraredInstrument(MIRI)have been developed by Raytheon Vision Systems (RVS) having 1024 × 1024 pixelsfabricated from arsenic‐doped silicon (Si:As) andwhich consist of a detector layer and areadoutmultiplexer.Thedetectorstructureisabackside‐illuminatedsiliconsubstratewithanactive‐detectorlayergrownontopofthat,followedbyablockinglayer(tominimizethedark current), and then indium bumps. The readout multiplexer is a cryogenicallyoptimized electronic circuit chip having a source follower (transistor) for each detectorpixel,whichthenmultiplexesthosepixelsdowntofourvideooutputsthatarereadbytheremaining instrument electronics. The two components of the SCAs aremated togetherthroughmatching indiumbumps. The requiredperformance levels are dark current lessthan 0.03 electrons per second, readout noise less than 19 electrons, and quantumefficiency>50%. Thesedetectorshavebeenrigorously testedandhavemetorexceededthese requirements for JWST, giving aTRLof 6 (Ressler 2007). Thesedetectorsmeet orexceed the requirements for both a small structurally connected interferometer missionandtheflagshipmission.SeeFigure4‐21foranexampleofacompletefocalplanemodulefortheMIRIinstrumentonJWST.

Figure4­21.TheMIRIFocalPlaneModulecontainstheSCA,afanoutboard,andthemountingstructureneededtopositionandisolatethesensor.(CourtesyNASA/JWST)

IntegratedModelingObservatorySimulation:Demonstrateasimulationoftheflightobservatoryconceptthatmodels the observatory subjected to dynamic disturbances (e.g., from reaction wheels).ValidatethismodelwithexperimentalresultsfromatleastthePlanetDetectionTestbedatdiscrete wavelengths. Use this simulation to show that the depth and stability of thestarlightnullcanbecontrolledovertheentirewavebandtowithinanorderofmagnitudeofthe limits required in flight todetectEarth‐likeplanets, characterize theirproperties,andassesstheirhabitability.ThisactivityisatTRL5(Lawsonetal.2007).

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4.3.3 AdditionalTechnologyforaFlagshipMission

Four­beamnulling:Dual­beamchoppingandspectralfilteringThe Planet Detection Testbed (PDT) is a breadboard system intended to demonstratetechniquesthatwillberequiredfordetectionofexoplanetsusingamid‐infraredtelescopearray.Asabreadboardsystemoperating inanormal roomenvironment, itdiffers fromaflight‐demonstrationsysteminbothits layoutandinthefluxlevelsthatituses,whicharemuchhigher thanwouldbe encountered in space. It does, however, embody the controlsandsensorsnecessaryforoperationinspace.Furthermore,thedifferencesdonotaffectthekey goal of the testbed, which is to demonstrate faint planet detection using nullinginterferometry,phasechoppingandaveraging.

Figure4­22.PhotonratesfortheplanetdetectionprocessforflightandPDT.

Key exoplanet‐detection techniques are being tested and demonstrated in the PDT. Toreachtheflightgoalofdetectionofaplanet107timesfainterthantheparentstar,aseriesofsteps(illustratedinFigure4‐22)mustbetaken:

• Thestar’sapparentintensityisreducedrelativetotheplanetbyafactorof100,000.Thisisdonebyinterferometricnulling.

• Theinterferometerarrayisrotatedaroundthelineofsighttothestartosearchthewholeregionaroundthestarforacharacteristicplanetsignature.InthePDTthisisdonebycontrollingtheopticalpathrelationshipsbetweentheplanetsourcebeams,simulatingthephasechangescausedbyrotationofthetelescopearrayaroundthelineofsighttothestar.Duringthisrotation,stablenullsneedtobemaintained;thePDThassystemsandcontrolloopsanalogoustothoseneededforflightinordertocontroltheinstabilitynoiseandmaintaindeepnulls.

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• Theplanetsignalismodulatedagainstthebrightradiationbackground.Thisisdoneusingphasechopping.Thecombinationofrotation,phasechopping,andaveragingovertimereducesthenoiselevelbyafactorof100to10–7ofthestellarintensity.

• Thetechniqueofspectral fittingusescorrelationsbetweennull fluctuationsacrossthespectralbandtoreducetheinstabilitynoise.Thisyieldsafurtherfactorof10inreductionofthenoiselevel.

Thus, the combination of these four techniques yields the necessary performance. Thecurrentobjective is todemonstratethe first threepartsof thisprocess; thestablenulling,thearray(orplanet)rotationandthephasechopping.Atthetimeofwriting,thePDThasdemonstrated detection of a planet 2 million times fainter than the star using somesimplifiedmethods.Thetestbedisnowbeingpreparedforfaintplanetdetectionusingthefirstthreetechniques.Thesuppressioneffectachievedusingthesetechniquesinwhichthenulled starlight at 10–5 is reduced to 10–7 will be representative of flight performance.Validationofthefourthtechnique,spectralfitting,willbelefttoafutureplan.

Figure 4­23.The Formation Control Testbed (FCT). Shown here are the two robots of the FCT. Each robotcarriescannistersofcompressedairthatallowthemtofloatoffapolishedmetalfloor.Thefloorisflattowithin2one‐thousandthsofaninchandspansamuchlargerareathanshownhere.Therobotsserveasthehardwareinterfaceandtestinggroundforflightsoftwaredevelopedforspaceapplicationsinformationflying.(CourtesyCaltech/JPL/NASA)

Guidance,Navigation,andControlThe Formation Control Testbed (FCT) was built to provide an end‐to‐end autonomousformation‐flying system in a ground‐based laboratory. The FCT provides an environmentfor system‐level demonstration and validation of formation‐control algorithms. Thealgorithms are validated using multiple floating test robots that emulate real spacecraftdynamics. The goal is to demonstrate algorithms for formation acquisition, formation

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maneuvering, fault‐tolerant operations, as well as collision‐avoidance maneuvering. (SeeLawsonetal.2008andreferencestherein.)

TheFCTiscomprisedoftworobotswithflight‐likehardwareanddynamics,aprecisionflatfloorfortherobotstooperateon,ceiling‐mountedartificialstarsforrobotattitudesensingand navigation, and a “ground control” room for commanding the robots and receivingtelemetry.TherobotsandpartoftheflatfloorareshowninFigure4‐23.ThelayoutoftheFCT emulates the environment of a formation of telescopes that is restricted tomaneuveringinthesameplaneinspace,normaltothedirectionofthetargetstar.

To be as flight‐like as possible, each robot is equipped with a typical single‐spacecraftattitude‐control suite of reaction wheels, gyros, and a star tracker. Thrusters are alsoavailable forattitudecontrol.Eachrobothasa lower translationalplatformandanupperattitudeplatform.Theattitudeplatformisthe“spacecraft”andiscompletelydisconnectedfromthetranslationalplatform.

• Theattitudeplatform/spacecrafthousestheavionics,actuators,sensors,inter‐robotand“ground”‐to‐robotwirelesscommunicationantennas,andspacecraftprocessors.

• Thetranslationalplatformprovidesbothtranslationalandrotationaldegreesoffreedomtotheattitudeplatformvia(i)linearairbearings(theblack,circularpadsatthebaseofeachrobot)thatallowtheentirerobottofloatfreelyontheflatfloor,and(ii)asphericalairbearingatthetopoftheverticalstage(theblack,verticalcylinders).

In2007theFCTdemonstratedprecisionmaneuversusingtworobots,showingautonomousinitialization, maneuvering, and operation in a collision‐free manner. The key maneuverthat was demonstrated was representative of TPF‐I science observations. Repeatedexperimentswiththerobotsdemonstratedformationrotationsthroughgreaterthan90°attentimestheflightrotationratewhilemaintainingarelativepositioncontrolto5cmRMS.Although the achievable resolution in these experiments was limited by the noiseenvironmentofthelaboratory, itwasnonethelessdemonstratedthroughmodelingthat intherelativelynoise‐freeenvironmentofspace,theperformanceofthesealgorithmswouldexceedTPF‐Iflightrequirements.

PropulsionSystemsMissionsemployingPrecisionFormationFlying (PFF) requirepropulsion capabilities thatexhibit low plume contamination, high thrust precision, and high power and propellantefficiency.Ionthrusterstypicallydeliverlow‐contaminationplumesandhighefficiencybyusingnoblegaspropellants,butconventionalthrustersprovidethrustlevelsoveranorderofmagnitude greater than the sub‐milli‐Newton (mN) tomN levels needed forprecision‐controlledformation‐flyingspaceinterferometermissions.

Aminiatureionthruster,theMiniatureXenonIon(MiXI)thrusterdevelopedatJPL,isanewoptionthatmeetsmissionneeds.OneparticularlyusefulcharacteristicoftheMiXIthrusterisitsincrediblylargethrustrangethatprovidessmoothamplitudemodulatedthrustinthe0.1–3.0mN in theamplitudemodulatedmodeand0.001–0.1mNof thrust inpulse‐widthmodulation(PWM)mode(patent‐pending).Withaminimumon‐timeoflessthan1ms,theMiXIthrustercanprovide impulsebitsof lessthan1µN⋅sat lowpower levels. ThePWMmodeoftheMiXIthrusterisachievedbyprecisioncontrolofthethrustervoltages.

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4.3.4 FutureMilestones

Probe­ScaleMissionTable4‐3isasummaryofthetechnicalreadinesslevels(TRLs)ofkeytechnologiesusedinthe probe‐scale mission (FKSI). These levels are estimated using NASA standardterminology(Mankin1995).

Many of the key technological hurdles for FKSI, as understood in the original studiesperformed in 2002–2003, have been solved by NASA flagship projects. In particular, theinvestments in JWST and TPF‐I/Darwin technologies can be directly used in the FKSIsystem.Asofearly2007,JWSTpassedallofitskeytechnologymilestones,includingthosefor cryocoolers, precision cryogenic support structures, detectors, cryogenicmirrors, andsunshades.AllofthesetechnologiescanbeadaptedtotheFKSImissionwithlittleeffort.

KeytechnologiesuniquetotheFKSImissionhavealsobenefitedfromNASA’sinvestmentsintheKeckInterferometerNullersystem(nowinoperationattheKeckObservatory)andtheTPF‐ItestbedsatJPL,includingthePlanetDetectionTestbedandtheAchromaticNullerTestbed.ThebroadbandnulldepthsfromthesetestbedsareaboutanorderofmagnitudebetterthantherequirementsfortheFKSImission.

Theoptical fibersneeded forwavefront cleanuphavebeen fabricatedand testedat roomtemperatureaspartof theTPF‐I technologyprogram. Currently two typesof fibershavebeen tested in the laboratory and work sufficiently well for the FKSI mission: these arechalcogenide fibers fabricated at the Naval Research Laboratory (NRL) (Aggarwal &Sanghera 2002) that operate from the near‐ to mid‐infrared, and silver halide fibersdeveloped at Tel Aviv University that operate in the longer wavelength part of themid‐infrared(seeKsendzovetal.2007andreferencestherein). Athirdtypeof fiber,ahollowglasswaveguide fiber developed at Rutgers University (Harrington 2001)may also be ofuse.Furthertestingisplanned,includingcryogenictestingofallfibers.

Table4­3.TechnicalReadinessLevelsforKeySubsystemComponents

Item Description TRL Notes

1 Cryocoolers 6 Source:JWSTT‐NAR2 Precisioncryogenicstructure(booms) 6 Source:JWSTT‐NAR3 Detectors(near‐infrared) 6 Source:HST,JWSTNircamT‐NAR4 Detectors(mid‐infrared) 6 Source:SpitzerIRAC,JWSTMIRIT‐NAR5 Cryogenicmirrors 6 Source:JWSTT‐NAR6 Opticalfiberformid‐infrared 4 Source:TPF‐I7 Sunshade 6 Source:JWSTT‐NAR

8 NullerInstrument 4–5 Source:KeckInterferometerNuller,TPF‐Iproject,LBTILBTIBLINCinstrument*9 Precisioncryogenicdelayline 6 Source:TNO

*Note:TherequirementfortheFKSIprojectisanulldepthof10–4ina10%bandwidth.LaboratoryresultswiththeTPF‐Itestbedshaveexceededthisrequirementbyanorderofmagnitude(Lawsonetal.2008).

Atthesystemlevel,anullerinstrumentfabricatedatJPLhasbeendeliveredtoandisinuseat the Keck Observatory. The FKSI nuller instrument is actually simpler than the Keckinstrument,asthelatterrequireschoppingoutthebackgroundcreatedbyawarmtelescopeandsky(notneededbyFKSI).IndeedthefirstdatafromtheKeckInterferometerNullerhas

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recently been published (Barry et al. 2008). The Bracewell Infrared Nulling Cryostat(BLINC, Hinz et al. 2000) is a cryogenic nuller in use on the MMT, which has achievedsignificantresultsinstudiesofaccretiondisksaroundHerbigAe/Bestars.

AlthoughtheBLINCsystemisavacuumcryogenicnullersubsystem,andinmanyrespectsissimilartowhatisrequiredfortheFKSInullinginstrument,ahigh‐fidelityvacuumcryogenicnulling testbed that provides an environment with the expected space‐like signal andthermalnoise levelsandthatalso includesrepresentativedisturbance forces,wouldbeofgreatbenefit to theFKSImission itself,andcouldbeusedasarelatively lowcoststartingpoint for amuchmore complex cryogenic nulling testbed that is needed for the flagshipTPF‐I/Darwinmissions, as discussedbelow. Such a cryogenic testbedwould reduce costandtechnicalriskforboththeProbe‐classmissionaswellastheflagshipmission.

Finally, FKSI requires a precision cryogenic delay line with a stroke of the order of acentimeter. Fortunately, commercial firms in Europe, under contract from ESA havedevelopedsuchaprototypecryogenicdelay line,which isatahighTRL level. Thisdelaylinehasaprecisionof1.3nmanda strokeof10mm,which ismore thanadequate for asmallinterferometer.Also,twoshort‐strokecryogenicdelaylineshavebeenflownaspartof twoMichelson spectral interferometers, bothofwhichweredevelopedat theGoddardSpace Flight Center. One was the FIRAS instrument on the COBEmission, which won aNobel Prize in Physics for Drs. John Mather and George Smoot for cosmic microwavebackgroundresearch(seeforexample,Matheretal.1994,Wrightetal.1994).Asecondhasbeen flown successfully on the CASSINI mission, as part of the CIRS instrument(http://saturn.jpl.nasa.gov/spacecraft/inst‐cassini‐cirs‐details.cfm).

Insummary,wenotethetwoareasforfurthertechnologydevelopmentforasmallinfraredstructurally‐connectedinterferometer:(1)Atthecomponentlevel,cryogenictestingoftheopticalfibersforwavefrontcleanup;and(2)atthesystemlevel,avacuumcryogenicnullingtestbed.

Single­ModeFiltersDemonstratedinaCryogenicVacuum,6–20µmThesingle‐modefibersusedintheAdaptiveNulleraremadeofchalcogenidematerialandhave been demonstrated to yield 25 dB ormore rejection of higher‐order spatialmodes,thusmeetingtheflightrequirements(Ksendzovetal.2007).Becausethismaterialbecomesopaque at wavelengths above 12 µm, other materials must be used in the 12–20 µmwavelengthband. Silverhalide single‐mode fibershavebeendemonstrated tomeet flightrequirementsat10µm,andshouldalsoworkwellat longerwavelengths(Ksendzovetal.2008).Althoughthisperformanceissufficientforflight,itwouldbegreatlyadvantageoustoimprovethethroughputofthesedevices,totestthemthroughoutthefullwavelengthrangetheyareintendedfor,andtotestthemcryogenically.SpatialfiltertechnologywouldthenbeatTRL5.Itwould,furthermore,beadvantageoustoimplementmid‐infraredspatialfiltersandbeamcombinersusing integratedoptics, so as to reduce the risk associatedwith thecomplexityofthescienceinstruments.

CryogenicAdaptiveNullingAdaptivenullingisstraightforwardtogeneralizeoverafullscienceband,anditshouldbedemonstratedwithin a cryogenic vacuum, bringing the technology to TRL 5. Thiswouldnecessitatethesuccessfulvalidationofcryogenicspatialfilters(above)andthetestingofacryogenicdeformablemirror.

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PlanetDetectionSysteminaCryogenicVacuumThefinaldemonstrationforthefeasibilityofaprobe‐scalemissionwouldbetointegrateallthenecessarycomponentsinavacuumcryogenictestbed.Thiswoulddemonstratethefullsystem complexity and include flight‐like servo systems and brass‐boards of thecomponentsdescribedpreviously.

FlagshipMissionThe technology program for the Terrestrial Planet Finder Interferometer is now close toachievingallofitsmilestonesinstarlightsuppression.Thebasiccomponenttechnologyforstarlightsuppressionatmid‐infraredwavelengthsisnowatTRL4.TRL5requirestestingina relevant environment,which for TPF/Darwinwould be a cryogenic vacuumnear 40K.Mostresearchsofarhasbeenundertakeninairatroomtemperature.Aswillbediscussedbelow,sufficientprogresshasnowbeenmadethatthegreatestadvanceinthisareawouldbe to proceed to brass‐board designs of already successful components and subsystems,andtoimplementsystem‐levelcryogenictesting.

Ground‐baseddemonstrationswiththeFormationControlTestbed(FCT)haveshownthattheGuidance,Navigation,andControlalgorithmsnowexisttoexecuteprecisionmaneuverswith two telescopes (Scharf 2007; Scharf & Lawson 2008). Additional testing should becarried out to validate collision avoidance and fault tolerant algorithms, but the greatestadvancewould be to transition to space‐based demonstrations in collaborationwith ourEuropeancolleagues.

Futuretechnologydemonstrationsarediscussedasfollows.

PlanetDetectionwithChoppingandAveragingFurther noise suppression is required in addition to starlight suppression. Nulling canreduce the glare of starlight to a level fainter than the warm glow of local zodical dust(surroundingourSun)andexo‐zodiacaldust(aroundthetargetstar),buttheplanet itselfmay still be 100 times fainter. The technique for detecting and characterizing planetsthereforereliesonseveralotherstrategies.

The first step is to suppress the response to any thermal emission that is symmetricallydistributed around the star. In principle this will remove the detected glow of local andexozodiacaldust.Byrotatingthearrayandaveragingtheresponse,theplanetsignalcanbefurther enhanced. The beam combiner for TPF/Darwin therefore combines two pairs ofbeamstonullthestarlight,andthebeamcombiningsystemmodulatestheresponseonthesky(keepingthestarnulled)bychoppingbackandforthbetweenthesenulledpairs.ThismilestonehasbeendetailedintheWhitepaperforthePlanetDetectionTestbed(Martinetal.2008c).

BroadbandSystematicNoiseSuppressionAnadditionalstepisrequiredtosuppressthenoisedownbelowthetypicalbrightnessofanEarth‐like planet. After nulling and chopping, the dominant source of noise is due toresidualinstabilitiesinthenulldepth,whichgeneratesawhite‐noiseofsimilarintensitytothe planet signal. This noise can be suppressed by appropriate choice of interferometerbaselines and by filtering themeasured data from a complete rotation of the array. WeexpectthismilestonewillbecompletedbythePlanetDetectionTestbedin2009.

Additionaltechnologyprogressbeyondthismilestonewoulddependoncryogenictestingofcomponentsandsystems.

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PlanetDetectionDemonstrationinaFlight­likeEnvironmentLaboratory work is well advanced to demonstrate the detection of planet light with thePlanetDetectionTestbed.Simulatedplanetstwomilliontimesfainterthanastarhavebeendetected in early trials, and the work is ongoing. Although these tests represent the fullcomplexityofbeamcombination,thisresearchisbeingconductedatroom‐temperatureinair,andthenoisepropertiesofthetestbedarethereforeunlikethosethatwouldbemetinflight. Following the completion of this work, the next step would be to build a vacuumcryogenic beam combiner with flight‐like servo systems, including the brass‐boardcomponentsdescribedabove.

Guidance,Navigation,andControl:CollisionAvoidance,FaultDetectionAdditional tests that could be carried out with the FCT include demonstrating newcapabilities suchas (1) reactivecollisionavoidance, (2) formation faultdetection,and (3)autonomousreconfigurationandretargetingmaneuvers.Also,usingareal‐timesimulationenvironment would allow the demonstration of performance with full formation‐flightcomplexity,with five interacting spacecraft showing synchronized rotations, autonomousreconfigurations,faultdetection,andcollisionavoidance.

In­flightTestingofFormationAlgorithmsNationalagenciesinEuropeareactivelyadvancingthetechnologyofformationflying,withflight missions starting in 2009–2014. The European Space Agency and national spaceagencies inEuropehaveaprogramofprecursormissionstogainexperience in formationflying.In2009theSwedishSpaceAgencywilllaunchthePrismamission.Thisisprimarilyarendezvousanddockingmission,butitwillalsotestRFmetrologydesignedforDarwin.In2012 ESA plans to launch Proba‐3, which will include optical metrology loops for sub‐millimeter range controlovera30‐mspacecraft separation.TheFrenchand Italian spaceagenciesareplanningtolaunchSimbol‐Xin2015.Simbol‐XisanX‐raysciencemissionwithseparatedetectorand lens spacecraftwitha20‐mspacecraft separation.Simbol‐X shouldenterPhaseBofdevelopmentinearly2009.

Although two‐spacecraft precision maneuvers were demonstrated in 2007, a major goalthat remains is to demonstrate robust algorithms for three or more spacecraft, such aswouldbeusedbyTPF‐I(Lawsonetal.2008).Theopportunitynodoubtexiststoleveragetheexpertisedeveloped forTPF‐I incollaborationwithEuropeancolleagues.Thegreatestadvance in maturing technology for formation flying would be to have a modest‐scaletechnologymission devoted to verifying and validating guidance and control algorithms.Such in‐flight testing would be made even more meaningful if it could include theinterferometriccombinationofstarlightfromseparatedplatforms.

Aground‐based facilitysuchas theFCTshouldcontinuetoprovide themeans to testandimprove real‐time formation‐flying algorithms as the technologymatures, evenwhile thetechnologyisbeingproveninspace.

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4.4 Research&AnalysisGoals4.4.1 Ground­BasedInterferometryGround‐basedinterferometryservescriticalrolesinexoplanetstudies.Itprovidesavenuefor development and demonstration of precision techniques including high‐contrastimaging and nulling, it trains the next generation of instrumentalists, and it develops acommunity of scientists expert in their use. With the highest priority, nulling facilitiesshouldberefinedandoperatedforthecharacterizationofdebrisdisksandexozodiacallightandtheimpactoftheseondirectdetectionmissions.

Ground‐based interferometry can also provide unique support to exoplanet studies. RVdetections have been vetted for the possibility of stellar companions on highly inclinedorbits (Baines et al. 2008a). Transit planets can be studied more precisely with directmeasurementof thestellardiameters(Bainesetal.2008b).KeckandVLTIwillsoonofferprecisionnarrow‐angleastrometry,intermediatebetweenHSTandSIM,thatwillopennewareas of exoplanet parameter space, and will improve the characterization of knownexoplanet systems, contributing to construction of well‐understood target lists for directdetectionmissions.Ground‐basedinterferometersmaysoonbecapableofdirectdetectionandstudyofsomeoftheverybrightestexoplanets(Monnieretal.2006).

We endorse the recommendations of the “Future Directions for Ground‐based OpticalInterferometry”Workshop(Akesonetal.2007)andtheReSTARcommitteereport(Bailynet al. 2008) to continuing vigorous refinement and exploitation of existing ground‐basedinterferometricfacilities(Keck,NPOI,CHARA,andMRO),wideningoftheiraccessibilityforexoplanet programs, and continued development of interferometry technology andplanningforafutureadvancedfacility.

ThenatureofAntarcticplateausites,intermediatebetweengroundandspaceinpotential,offers significant opportunities for exoplanet and exozodi studies by interferometry andcoronagraphy(Kenyonetal.2006;CoudéduForestoetal.2006).Balloonaltitudesarealsopromising for high‐contrast imaging (Traub & Chen 2007). Further evaluation of theseopportunitiesshouldbeencouragedandsupported,withtheexpectationthattheymaybecompetitiveforfutureimplementation.

4.4.2 TheorySupportIn order to achieve our most ambitious goals of relating exoplanet atmospherescompositiontoplanet formation,systemevolution,surfacechemistryandbiology,wewillrequiresustainedsupportofstrongastrobiologyandatmosphericchemistryprograms.

4.4.3 Space­BasedInterferometrySpace‐basedinterferometryservescriticalrolesinexoplanetstudies.Itprovidesaccesstoaspectralrangethatcannotbeachievedfromtheground,anditcancharacterizethedetectedplanets in terms of atmospheric composition and effective temperature. Sensitivetechnology has already been proven formissions like JWST, SIM, and Spitzer, andwithinNASA’s preliminary studies of TPF. With high priority, technology for nullinginterferometers in space should be advanced to make the next generation spacecraft todetectandcharacterizetheclosestplanetsflightready.

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4.4.4 AgencyCoordinationandProgrammaticSynergiesTheverybroadscientificandpublicinterestinexoplanets,andthedirectrelevancetobothNASAandNSFgoals,makesitanidealtopicforcoordinationbetweentheagencies,andweurgeNASAandNSFstafftoleveragethisrelationshiptocoverthefullbreadthofexoplanetscienceandtechnology.

4.4.5 InternationalCooperation,Collaboration,&Partnership

Inpastyears(2002–2006)aNASA/ESALetterofAgreement(LOA)hasallowedtheUSandEuropean communities to develop a joint TPF and Darwin mission concept based on aconvergence of science requirements, design studies, and shared technology goals. TherelationshipsforgedbetweenUSandEuropeancollaboratorsshouldagainbefosteredoverthenextdecade to furtherstudiesofsmallmissionand flagshipmissionconcepts. Anewletterofagreementisnecessarytorenewthesecollaborations.

4.5 ContributorsOlivierAbsil,UniversitédeGrenoble,JosephFourier

RachelAkeson,Caltech,NASAExoplanetScienceInstitute

JohnBally,UniversityofColorado

RichardK.Barry,NASAGoddardSpaceFlightCenter

CharlesBeichman,NASAExoplanetScienceInstitute

AdrianBelu,UniversitédeNiceSophiaAntipolis

MathewBoyce,HeliosEnergyPartners

JamesBreckinridge,Caltech,JetPropulsionLaboratory

AdamBurrows,PrincetonUniversity

ChristineChen,SpaceTelescopeScienceInstitute

DavidCole,Caltech,JetPropulsionLaboratory

DavidCrisp,JetPropulsionLaboratory

WilliamC.Danchi,NASAGoddardSpaceFlightCenter

RolfDanner,NorthropGrummanSpaceTechnology

PeterDeroo,Caltech,JetPropulsionLaboratory

VincentCoudéduForesto,ObservatoiredeParis

DenisDefrère,UniversitédeLiège

DennisEbbets,BallAerospaceandTechnologyCorporation

PaulFalkowski,RutgersUniversity

RobertGappinger,JetPropulsionLaboratory

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IsmailD.Haugabook,Sr.,DigitalTechnicalServices

CharlesHanot,Caltech,JetPropulsionLaboratory,UniversitédeLiège

PhilHinz,StewardObservatory,UniversityofArizona

JanHollis,NASAGoddardSpaceFlightCenter

SarahHunyadi,JetPropulsionLaboratory

DavidHyland,TexasA&MUniversity

KennethJ.Johnston,U.S.NavalObservatory

LisaKaltenegger,HarvardSmithsonianCenterforAstrophysics

JamesKasting,PennsylvaniaStateUniversity

MattKenworthy,StewardObsevatory,UniversityofArizona

AlexanderKsendzov,JetPropulsionLaboratory

BenjaminLane,DraperLabs

GregoryLaughlin,UCOLick

PeterLawson,Caltech,JetPropulsionLaboratory

OliverLay,Caltech,JetPropulsionLaboratory

RéneLiseau,StockholmUniversity

BrunoLopez,ObservatoiredelaCoted’Azur

RafaelMillan­Gabet,Caltech,NASAExoplanetScienceInstitute

StefanMartin,Caltech,JetPropulsionLaboratory

DimitriMawet,Caltech,JetPropulsionLaboratory

BertrandMennesson,Caltech,JetPropulsionLaboratory

JohnMonnier,UniversityofMichigan

NaoshiMurakami,NationalAstronomicalObservatoryofJapan(NAOJ)

M.CharlesNoecker,BallAerospaceandTechnologyCorporation

JunNishikawa,NationalAstronomicalObservatoryofJapan(NAOJ)

MeyerPesesen,Caltech,JetPropulsionLaboratory

RobertPeters,Caltech,JetPropulsionLaboratory

AliceQuillen,UniversityofRochester

SamRagland,W.M.KeckObservatory

StephenRinehart,NASAGoddardSpaceFlightCenter

HuubRottgering,UniversityofLeiden

DanielScharf,Caltech,JetPropulsionLaboratory

EugeneSerabyn,Caltech,JetPropulsionLaboratory

MotohideTamura,NationalAstronomicalObservatoryofJapan(NAOJ)

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MohammedTehrani,TheAerospaceCorporation

WesleyA.Traub,Caltech,JetPropulsionLaboratory

StephenUnwin,Caltech,JetPropulsionLaboratory

DavidWilner,Harvard­SmithsonianCenterforAstrophysics

JulienWoillez,W.M.KeckObservatory

NevilleWoolf,UniversityofArizona

MingZhao,UniversityofMichigan

4.6 ReferencesAggarwal,I.D.,&Sanghera,J.S.2002,“Developmentandapplicationsofchalcogenideglass

opticalfibersatNRL,”J.Optoelectron.Adv.Matter,4,665–678

Akeson,R.,Allen,R.,Angel,R.,etal.2007,WorkshopontheFutureDirectionsforGround­basedOpticalInterferometry:FinalReport,NationalOpticalAstronomyObservatory,Tucson,AZ,http://www.noao.edu/meetings/interferometry/Workshop‐report.pdf

Angel, J. R. P.,&Woolf,N. J. 1997, “An imagingnulling interferometer to study extrasolarplanets,”ApJ,475,373–379

Baines, E. K., McAlister, H. A., ten Brummelaar T. A., et al. 2008a, “The search for stellarcompanionstoexoplanethoststarsusingtheCHARAArray,”ApJ,682,577–585

Baines, E. K., McAlister, H. A., ten Brummelaar T. A., et al. 2008b, “CHARA Arraymeasurementsoftheangulardiametersofexoplanethoststars,”ApJ,680,728–733

Bailyn,C.,Barnes,T.,etal.2008,RenewingSmallTelescopesforAstronomicalResearch,ReSTARReport,NationalOpticalAstronomyObservatory,Tucson,AZ,http://www.noao.edu/system/restar/files/ReSTAR_final_14jan08.pdf

Barry,R.K.,Danchi,W.C.,etal.2005,“TheFourier‐KelvinStellarInterferometer(FKSI):aprogress report and preliminary results from our nulling testbed,” Proc. SPIE, 5905,311–321

Barry, R. K., Danchi,W. C., et al. 2006, “The Fourier‐Kelvin Stellar Interferometer: a low‐complexitylow‐costspacemissionforhigh‐resolutionastronomyanddirectexoplanetdetection,”Proc.SPIE,6265,62651L

Barry,R.K.,Danchi,W.C.,Koresko,M.C.,etal.2008,“High‐ResolutionN‐BandObservationsoftheNovaRSOphiuchiwiththeKeckInterferometerNuller,”ApJ,677,1253–1267

Beichman,C.A.,Woolf,N. J.,&Lindensmith,C.A., eds.1999,TheTerrestrialPlanetFinder(TPF):aNASAOriginsProgramtoSearchforHabitablePlanets,JPLPublication99‐3,JetPropulsionLaboratory,Pasadena,California

Beichman, C.A., Fridlund,M., Traub,W.A., Stapelfledt,K.R.,Quirrenbach,A.,& Seager, S.2006, “Comparative planetology and the search for life beyond the Solar System,”ProtostarsandPlanetsV,editorsRiepurth,B.,etal.,UniversityofArizonaPress,Tucson,AZ,915–928

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Booth,A.J.,Martin,S.R.,&Loya,F.2008,“ExoplanetExoplorationProgramPlanetDetectionTestbed:latestresultsofplanetlightdetectioninthepresenceofstarlight,”Proc.SPIE,7013,701320

Beaulieu, J.‐P., Bennett, D. P., et al. 2006, “Discovery of a cool planet of 5.5 Earthmassesthroughgravitationalmicrolensing,”Nature,439,437–440

Bracewell, R. N. 1978, “Detecting nonsolar planets by spinning infrared interferometer,”Nature,274,780–781

Christensen, P.R.&Pearl, J. C. 1997, “Initial data from theMarsGlobal Surveyor thermalemission spectrometer experiment:Observations of the Earth,” J. Geophys. Res., 102,10875–10880

Cockell,C.S.,Herbst,T.,Léger,A.,etal.2009,“Darwin—AMissiontoDetect,andSearchforLifeon,ExtrasolarPlanets,”Astrobiology,inpress,preprint,arXiv:0805.1873

Coudé du Foresto, V., Absil, O., Swain, M., Vakili, F., & Barillot, M. 2006, “ALADDIN: anoptimizednullingground‐baseddemonstratorforDARWIN,”Proc.SPIE,6268,626810

Crawford,S.L.,Colavita,M.M.,Garcia,J.I.,etal.2005,“FinallaboratoryintegrationandtestoftheKeckInterferometernuller,”Proc.SPIE,5905,59050U

Danchi,W.C.,Deming,D.,Kuchner,M.,&Seager,S.2003a,“DetectionofClose‐InExtrasolarGiantPlanetswiththeFourier‐KelvinStellarInterferometer,”ApJL,597,L57

Danchi, W. C., Allen, R., Benford, R. J., et al. 2003b, “The Fourier‐Kelvin StellarInterferometer,” inTowardsOtherEarths:DARWIN/TPFand theSearch forExtrasolarTerrestrial Planets, Heidelberg, Germany, 22–25April 2003 (ESA Publication SP‐539,October2003)

Danchi, W. C., Allen, Barry, R., Benford, R. J., et al. 2004, “The Fourier‐Kelvin StellarInterferometer: A Practical InterferometricMission forDiscovering and InvestigatingExtrasolarGiantPlanets,”Proc.SPIE,5491,236

Danchi, W. C., & Lopez, B., 2007, “The Fourier‐Kelvin Stellar Interferometer (FKSI)— Apractical infrared space interferometer on the path to the discovery andcharacterizationofEarth‐likeplanetsaroundnearbystars,”Comptesrendus‐Physique(C.R.Physique),8,396–407

Defrère,D., Absil, O., Coudédu Foresto, V., Danchi,W. C.,& denHartog, R. 2008, “Nullinginterferometry: performance comparison between space and ground‐based sites forexozodiacaldiscdetection,”A&A,490,435–445

DesMarais,D.J.,Harwit,M.O.,Jucks,K.W.,Kasting,J.F.,Lin,D.N.C.,Lunine,J.I.,Schneider,J.,Seager,S.,Traub,W.A.,&Woolf,N.J.2002,“Remotesensingofplanetarypropertiesandbiosignaturesofextrasolarterrestrialplanets,”Astrobiology,2,153–181

Forget, F.,&Pierrehumbert,R. T. 1997, “Warming earlyMarswith carbondioxide cloudsthatscatterinfraredradiation,”Science,278,1273–1276

Frey,B.J.,Barry,R.K.,Danchi,W.C.,etal.2006,“TheFourier‐KelvinStellarInterferometer(FKSI) nulling testbed II: Closed‐loop pathlength metrology and control subsystem,”Proc.SPIE,6265,62651N

Fridlund, M. 2000, DARWIN: The InfraRed Space Interferometer, ESA‐SCI (2000)12, 47EuropeanSpaceAgency:Noordwijk,TheNetherlands

Chapter4

130

Fridlund,C.V.M.,Gondoin,P.,Bavdaz,M.etal.2008,“Feather‐lighttouchallthat'sneededforDarwin'sfrictionlessoptics,”intheESAwebsite;http://www.esa.int/esaCP/SEMD3J8J50F_index_0.html

Gappinger,R.O.,Diaz,R.T.,Ksendzov,A., Lawson,P.R., Lay,O.P., Liewer,K., Loya, F.M.,Martin,S.R.,Serabyn,E.&Wallace,J.K.2009,“Experimentalevaluationofachromaticphaseshiftersformid‐infraredstarlightsuppression,”Appl.Opt.,48,868‐880

Gould,A.,Udalski,A.,An,D.,etal.2006, “MicrolensOGLE‐2005‐BLG‐169 implies thatcoolNeptune‐likeplanetsarecommon,”ApJ,644,L37–L40

Harrington,J.A.2001,InfraredFiberOptics,M.Bass,J.Enoch,E.VanStryland,&W.Wolfe,eds.,(McGraw‐Hill,NewYork).

Hinz, P.M., Angel, J. R. P., et al. 2000, “BLINC: a testbed for nulling interferometry in thethermalinfrared,”Proc.SPIE,4006,349–353

Hinz,P.M.,Heinze,A.N.,etal.2006,“ThermalinfraredconstrainttoaplanetarycompanionofVegawiththeMMTadaptiveopticssystem,”ApJ,653,1486–1492

Hyde,T.T.,Liu,K‐C.,Blaurock,C.,Bolognese,J.,etal.2004,“RequirementsFormulationandDynamicJitterAnalysisfortheFourier‐KelvinStellarInterferometer,”Proc.SPIE,5491,553

Kaltenegger, L., Eiroa,C., Stankov,A., Fridlund,M.2009, “TheDarwin target star catalog,”Astrophys.&SpaceSci.,inpress

Kaltenegger,L.,&Fridlund,M.2005,“TheDarwinmission:Searchforextra‐solarplanets,”AdvancesinSpaceResearch,36,1114–1122

Kaltenegger,L.,Traub,W.A.,&Jucks,K.2007,“SpectralevolutionofanEarth‐likeplanet,”ApJ,658,598–616

Karlson,A.L.,Wallner,O.,Armengol, J.M.P.,etal.2004, “Threetelescopenullerbasedonmultibeaminjectionintosingle‐modewaveguide,”Proc.SPIE,5491,831–841

Kasting, J. F., Whitmire, D. P., & Reynolds, R. T. 1993, “Habitable zones around mainsequencestars,”Icarus,101,108–128

Kasting,J.F.,&Catling,D.2003,“Evolutionofahabitableplanet,”ARA&A,41,429–463

Kenyon,S.L.,Lawrence, J.S.,etal.2006, “AtmosphericscintillationatDomeC,Antarctica:Implicationsforphotometryandastrometry,”PASP,118,924–932

Ksendzov, A., Lay, O., Martin, S., Sanghera, J. S., Busse, L. E., Kim, W. H., Pureza, P. C., &Nguyen, V. Q. 2007, “Characterization of mid‐infrared single mode fibers as modalfilters,”Appl.Opt.,46,7957–7962

KsendzovA.,LewiT.,Lay,O.P.,Martin,S.R.,Gappinger,R.O.,Lawson,P.R.,Peters,R.D.,Shalem, S., Tsun, A., & Katzir, A. 2008, “Modal filtering for midinfrared nullinginterferometryusingsinglemodesilverhalidefibers,”Appl.Opt.,47,5728–5735

Kurland, R. 2007, “JWST Sunshield Silicon‐Coated Membrane Material: Summary‐LevelReport,” JWST Technical Non‐Advocate Review (T‐NAR), January, 17 2007, GoddardSpaceFlightCenter,Greenbelt,MD

Lafrenière,D.,Jayawardhana,R.,vanKerkwijk,M.H.,“DirectimagingandspectroscopyofaplanetarymasscandidatecompaniontoayoungSolaranalog,”ApJL,689,L153–L156,2008

InfraredImaging

131

Lawson,P.R.,Lay,O.P.,Martin,S.R.,Peters,R.D.,Gappinger,R.O.,Ksendzov,A.,Scharf,D.P., Booth, A. J., Beichman, C. A., Serabyn, E., Johnston, K. J., & Danchi, W. C., 2008,“TerrestrialPlanetFinderInterferometer:2007–2008progressandplans,”Proc.SPIE,7013,7013N

Lawson,P.R.,Lay,O.P.,Martin,S.R.,etal.2007,“TerrestrialPlanetFinderInterferometer:2006–2007progressandplans,”Proc.SPIE,6693,669308

Lay,O.P.2004,“Systematicerrorsinnullinginterferometers,”Appl.Opt.,43,6100–6123

Lay, O. P. 2005, “Imaging properties of rotating nulling interferometers,” Appl. Opt., 44,5859–5871

Lay O. P. 2006, “Removing instability noise in nulling interferometers,” Proc. SPIE, 6268,62681A

Lay, O. P., Martin, S. R., & Hunyadi S. L. 2007, “Planet‐finding performance of the TPF‐IEmmaarchitecture,”Proc.SPIE,6693,66930A

Léger, A.,Mariotti, J.‐M.,Mennesson, B., Ollivier,M., Puget, J. L., Rouan, D., & Schneider, J.1996,“Couldwesearchforprimitivelifeonextrasolarplanetsinthenearfuture?:TheDARWINProject,”Icarus,123,249–255

Léger, A., Herbst, T., et al. 2007, “DARWIN mission proposal to ESA,” 23 Jul 2007.arXiv:0707.3385v1[astro‐ph].

Lopez,B.,S.Wolf,S.Lagarde,P.Abraham,etal.2006,“MATISSE:perspectiveofimaginginthemid‐infraredattheVLTI,”Proc.SPIE,6268,31

Lovelock, J. E., 1975, “Thermodynamics and the recognition of alien biospheres [anddiscussion],”Proc.R.Soc.Lond.B189,167–181

Lunine, J. I. 2001, “The occurrence of Jovian planets and the habitability of planetarysystems,”PNAS,98,809–814

Mankins,J.C.,1995,“TechnologyReadinessLevels,”NASAAdvancedConceptsOffice,OfficeofSpaceAccessandTechnology,NationalAeronauticsandSpaceAdministration,Washington,DC,http://www.hq.nasa.gov/office/codeq/trl/trl.pdf.

Martin, S. R., Serabyn, E., Hardy, G. J. 2003, “Deep nulling of laser light in a rotationalshearinginterferometer,”Proc.SPIE,4838,656–667

Martin, S. R. 2005, “The flight instrument design for the Terrestrial Planet FinderInterferometer,”Proc.SPIE,5905,21–35

Martin,S.R.2006,“Progressinfour‐beamnulling:resultsfromtheTerrestrialPlanetFinderPlanetDetectionTestbed,”in2006IEEEAerospaceConference,BigSky,Montana

Martin, S. R., Szwaykowski, P., Loya, F., Liewer, F. 2006, “Progress in testing exo‐planetsignalextractionontheTPF‐IPlanetdetectionTestbed,”Proc.SPIE,6268,626818

Martin,S.R.,Scharf,D.,Wirz,R.etal.2007,“TPF‐Emma:conceptstudyofaplanet findingspaceinterferometer,”Proc.SPIE,6693,669309

Martin, S.R., Scharf,D. P.,Wirz,R., et al. 2008a, “DesignStudy for aPlanet‐Finding SpaceInterferometer,”IEEEAerospaceConf,BigSky,Mt,USA

Martin,S.R.,Serabyn,E.,Liewer,K.2008b,“Thedevelopmentandapplicationsofaground‐basedfibernullingcoronagraph,”Proc.SPIE,7013,70131Y

Chapter4

132

Martin,S.R.,Booth,A.J.,Lay,O.P.,&Lawson,P.R.2008c,ExoplanetInterferometryTechnologyMilestone#4Whitepaper:PlanetDetectionDemonstration,JetPropulsionLaboratoryDocumentD‐47627,http://planetquest.jpl.nasa.gov/TPF‐I/TPF‐I_M4_Whitepaper_Final.pdf

Mather, J. C., Cheng, E. S., Cottingham, D. A., et al. 1994, “Measurement of the CosmicMicrowaveBackgroundSpectrumbytheCOBEFIRASInstrument,”ApJ,420,439–444

Mennesson,B.,Haguenauer,P., Serabyn,E.,&Liewer,K.2006, “Deepbroad‐band infrarednulling using a single‐mode fiber beam combiner and baseline rotation,” Proc. SPIE,6268,626830

Monnier,J.D.,Pedretti,E.,etal.2006,“MichiganInfraredCombiner(MIRC):commissioningresultsattheCHARAArray,”Proc.SPIE,6268,62681P

NationalResearchCouncil,2001,AstronomyandAstrophysicsintheNewMillennium,http://www.nap.edu/openbook.php?isbn=0309070317

Ollivier,M. 1999,Contribution à la Recherche d'Exoplanète: Coronagraphie InterférentiellepourlaMissionDarwin,Ph.D.thesis,UniversityParisXI

Owen,T.1980,“Thesearchforearlyformsoflifeinotherplanetarysystems,”inStrategiesforSearchforLifeintheUniverse,editorsPapagiannis,M.D.,Springer:Dordrecht,TheNetherlands,p.177

Peters,R.D.,Lay,O.P.,Jeganathan,M.2008,“Broadbandphaseandintensitycompensationwithadeformablemirrorforaninterferometricnuller,”Appl.Opt.,47,3920–3926

Ressler,M.2007,“Mid‐InfraredDetectors,”JWSTTechnicalNon‐AdvocateReview(T‐NAR),30–31January2007,GoddardSpaceFlightCenter,Greenbelt,MD

Samuele, R., Wallace, J. K., Schmidtlin, E., Shao, M., Levine, B. M., Fregoso, S. 2007,"Experimental progress and results of a visible nulling coronagraph," 2007 IEEEAerospaceConference,BigSkyMontana,paper1333

Scharf,D.P.2007,TPF­IMilestone#2Whitepaper:FormationControlPerformanceDemonstration,JetPropulsionLaboratory,Pasadena,CA,http://planetquest.jpl.nasa.gov/TPF‐I/TPFI_M2_WhitePaper_Final.pdf

Scharf,D.P.,Lawson,P.R.2008,TPF­IMilestone#2Report:FormationControlPerformanceDemonstration,JetPropulsionLaboratory,Pasadena,CA,JPLPublication08‐11,http://planetquest.jpl.nasa.gov/TPF‐I/TPFI_M2_ReportV3.pdf

Scharf, D. P., Hadaegh, F. Y., Keim, J. A., & Lawson, P. R. 2008, “Ground demonstration ofsynchronized formation rotations for precision, multi‐spacecraft interferometers,” inProc. 3rd International Symposium on Formation Flying, Missions and Technologies,Fletcher,K.,ed.,ESASP‐654(CDROM)

Seager,S.,Richardson,L.J.,Hansen,B.M.S.,etal.2005,“OntheDaysideThermalEmissionofHotJupiters,”ApJ,632,1122–1131

Segura, A., Krelove, K., Kasting, J. F., Sommerlatt, D., Meadows V., Crisp D., Cohen, M., &Mlawer, E. 2003, “Ozone concentrations and ultraviolet fluxes on Earth‐like planetsaroundotherstars,”Astrobiology,3,689–708

Segura,A.,Kasting,J.F.,Meadows,V.,Cohen,M.,Scalo,J.,Crisp,D.,Butler,R.A.H.,&Tinetti,G. 2005, “Biosignatures from Earth‐like planets around M Dwarfs,” Astrobiology, 5,706–725

InfraredImaging

133

Selsis,F. 2000, “Physics of Planets I:Darwin and theAtmospheres ofTerrestrial Planets,”DarwinandAstronomy:TheInfraredSpaceInterferometer,ESASP451,EuropeanSpaceAgency,Noordwijk,TheNetherlands,pp.133–140

Selsis, F., Despois, D., & Parisot, J.‐P. 2002, “Signature of life on exoplanets: Can Darwinproducefalsepositivedetections?”A&A,388,985–1003

Selsis, F., Kasting, J. F., Levrard, B., et al. 2007, “Habitable planets around the star Gliese581?,”A&A,476,1373–1387

Serabyn,E.,Wallace,J.K.,Hardy,G.J.,Schmidtlin,E.G.H.,Nguyen,H.T.1999,“Deepnullingofvisiblelaserlight,”Appl.Opt.,38,7128–7132

Serabyn,E.,Booth,A.J.,Colavita,M.M,etal.2004,“TheKeck interferometernuller:systemarchitectureandlaboratoryperformance,”Proc.SPIE,5491,806–815

Traub,W.A.,&Chen,P.,“Planetscopeprecursorexperiment,”BAAS211,30.06(2007).

Turnbull,M.C.,Traub,W.A.,Jucks,K.W.,Woolf,N.J.,Meyer,M.R.,Gorlova,N.,Skrutskie,M.F., & Wilson, J. C. 2006, “Spectrum of a habitable world: Earthshine in the near‐infrared,”ApJ,644,551–559

Turnbull,M.C.,TheSearchforHabitableWorlds:FromtheTerrestrialPlanetFindertoSETI,Ph.D.Thesis,UniversityofArizona,ISBN0496037129

Wallace, K., Hardy, G., Serabyn, E. 2000, “Deep and stable interferometric nulling ofbroadbandlightwith implications forobservingplanetsaroundnearbystars,”Nature406,700–702

Wallner, O., Armengol, J. M. P., & Karlson, A. L. 2004, “Multiaxial single‐mode beamcombiner,”Proc.SPIE,5491,798–805

Wright,E.L.,Mather,J.C.,Fixsen,D.J.,etal.1994,“InterpretationoftheCOBEFIRASCMBRSpectrum,”ApJ,420,450–456