the laser interferometer space antenna: unveiling the millihertz … · 2019-07-23 · the laser...

The Laser Interferometer Space Antenna: Unveiling the Millihertz Gravitational Wave Sky

An Astro2020 White Paper

lisa.nasa.gov

John BakerJillian BellovaryPeter L. BenderEmanuele Berti

Robert CaldwellJordan Camp

John W. ConklinNeil Cornish

Curt CutlerRyan DeRosa

Michael EracleousElizabeth C. Ferrara

Samuel FrancisMartin Hewitson

Kelly Holley-Bockelmann Ann Hornschemeier

Craig HoganBrittany Kamai

Bernard J. KellyJoey Shapiro KeyShane L. Larson

NASA Goddard Space Flight CenterCUNY-Queensborogh Community CollegeUniversity of ColoradoJohns Hopkins UniversityDartmouth CollegeNASA Goddard Space Flight Center University of FloridaMontana State UniversityNASA Goddard Space Flight Center NASA Jet Propulsaion LaboratoryThe Pennsylvania State UniversityUniversity of Maryland, College ParkNASA Jet Propulsion Laboratory Albert Einstein Institute HannoverVanderbilt UniversityNASA Goddard Space Flight Center University of Chicago / FermilabCalTech, Vanderbilt UniversityUniversity of Maryland Baltimore CountyUniversity of Washington BothellNorthwestern University

TheLISAconceptisaproductofdecadesofworkbyaninterna9onalteamofscien9stsandengineers.Thisdocumentwaspreparedbyrepresenta9vesoftheUSLISAcommunityonbehalfoftheglobalcommunityandinsupportoftheESA-ledLISAmission.

Jeff Livas Sridhar Manthripragada

Kirk McKenzieSean T. McWilliams

Guido MuellerPriyamvada Natarajan

Kenji NumataNorman Rioux

Shannon R. SankarJeremy Schnittman

David ShoemakerDeirdre Shoemaker

Jacob SlutskyRobert Spero

Robin StebbinsIra Thorpe

Michele VallisneriBrent WarePeter WassAnthony Yu

John Ziemer

NASA Goddard Space Flight CenterNASA Goddard Space Flight CenterNASA Jet Propulsion Laboratory West Virginia UniversityUniversity of Florida Yale UniversityNASA Goddard Space Flight CenterNASA Goddard Space Flight Center University of Maryland, College ParkNASA Goddard Space Flight CenterMassachusetts Institute of TechnologyGeorgia Institute of TechnologyNASA Goddard Space Flight CenterNASA Jet Propulsion Laboratory JILA / University of ColoradoNASA Goddard Space Flight CenterNASA Jet Propulsion Laboratory NASA Jet Propulsion Laboratory University of Florida NASA Goddard Space Flight Center NASA Jet Propulsion Laboratory

The Laser Interferometer Space Antenna An Astro2020 APC Whitepaper

1 Executive SummaryThe first terrestrial Gravitational Wave

(GW) interferometers [1, 2] have dramat-ically underscored the scientific value ofobserving the Universe through an en-tirely different window – and of foldingthis new channel of information with tradi-tional astronomical data for a multimessen-ger view [3, 4, 5, 6]. The Laser Interferom-eter Space Antenna (LISA) will broadenthe reach of GW astronomy by conductingthe first survey of the millihertz GW sky,detecting tens of thousands of individualastrophysical sources ranging from white-dwarf binaries in our own galaxy to merg-ers of massive black holes (MBHs) at red-shifts extending beyond the epoch of reion-ization. These observations will inform– and transform – our understanding ofthe end state of stellar evolution, MBHbirth, and the co-evolution of galaxies andblack holes through cosmic time. LISAalso has the potential to detect GW emissionfrom elusive astrophysical sources such asintermediate-mass black holes as well as ex-otic cosmological sources such as inflation-ary fields and cosmic string cusps1.

LISA is now in Phase A as a EuropeanSpace Agency (ESA) led mission with sig-nificant contributions anticipated from sev-eral ESA member states and NASA. Themission concept retains all essential fea-tures of the NASA/ESA LISA mission thatwas ranked as the 3rd priority for Large-class missions in the 2010 Decadal Sur-vey [7], including the full three-arm trian-gular configuration that measures GW po-larization and improves robustness. Sincethat ranking, LISA’s technical readinesshas been greatly advanced through twoflight demonstrations: the ESA-led LISAPathfinder mission (2015-2017) and the

1LISA Astro2020 Science whitepapers availableat: lisa.nasa.gov/documentsCWP.html

Box 1 - LISA Mission Overview

Science Objective: All-sky survey ofmillihertz gravitational waves

Measurement Concept: Long-baseline optical interferometrybetween drag-free test masses

Orbit: Heliocentric 2.5 Mkm triangu-lar constellation, 20◦ Earth-trailing

Launch: Early/mid 2030s, Ariane 6.4

Lifetime: 1.5 yr transfer, 1 yr com-missioning, 4 yrs science, ≤ 6 yrsextension

Cost: Total mission: Large (>$1.5B);US share: Medium ($500M - $1.5B)

Partners: European Space Agency(lead), ESA Member States, NASA

Laser Ranging Instrument on board theUS/German Gravity Recovery And Cli-mate Explorer Follow-On mission (2018-).The Midterm Assessment of the 2010Decadal Survey recommended that the USparticipate as a “strong technical and sci-entific partner” in an ESA-led LISA mis-sion [8]. NASA is currently supporting pre-project activities to support a range of po-tential contributions to LISA including in-struments, spacecraft elements, and scienceanalysis. The currently envisioned scaleof these contributions is at the lower endof the medium-scale cost range identifiedby Astro2020 ($500M - $1.5B). A recom-mendation for an upscope in US partic-ipation in LISA would provide opportu-nities to more fully exploit heritage fromprior US investments, balance technicaland programmatic risks across the partner-ship, and expand opportunities for futureUS leadership in this new field of astron-omy.

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lisa.nasa.gov/documentsCWP.html


2 Key Science Goals and ObjectivesThe scientific case for GW observations in

the millihertz band is well-summarized inThe Gravitational Universe [9], the documentwhich formed the basis for ESA’s selectionof GW astronomy as the science theme forthe 3rd Large-class mission of the CosmicVision Programme in 2013. While someelements of the LISA science case overlapwith those for GW observations in otherbands [10], notably higher-frequency obser-vations with terrestrial interferometers andlower-frequency observations with PulsarTiming Arrays, a space-based facility suchas LISA is uniquely capable of answering anumber of pressing and fundamental ques-tions in astrophysics.

The LISA Science Objectives are formallydocumented in the Science RequirementsDocument (SciRD) [11]. The SciRD iden-tifies eight LISA Science Objectives (SOs,see Box 2) – broad questions in astrophysicswhich can be addressed through GW as-tronomy. The SOs are used to derive mis-sion and instrument requirements includ-ing requirements on the sensitivity of LISAto GW strain as well as other factors includ-ing total observing time and data latency(for certain classes of EM counterpart inves-tigations).

The broad range of scientific targets ismade possible by the abundance and diver-sity of astrophysical sources expected forLISA. Figure 1 shows a comparison of sev-eral representative LISA sources with thesensitivity limit of the instrument. Data areplotted as spectral amplitudes of GW strain– a dimensionless number characterizingthe amplitude of the spacetime stretchingcaused by GWs passing through the detec-tor. The ‘characteristic strain’ is used to ac-count for variations in observation time be-tween transient and persistent signals [12].Sensitivity to astrophysical sources is pri-marily limited by instrument noise (green

Box 2 - LISA Science Objectives

SO 1: Study the formation & evo-lution of compact binary stars in theMilky Way

SO 2: Trace the origin, growth &merger history of MBHs

SO 3: Probe the dynamics of densenuclear clusters using EMRIs

SO 4: Understand the astrophysics ofstellar origin black holes

SO 5: Explore the fundamental na-ture of gravity & black holes

SO 6: Probe the rate of expansion ofthe Universe

SO 7: Understand stochastic GWbackgrounds & their implications forthe early Universe and TeV-scale par-ticle physics

SO 8: Search for GW bursts and un-foreseen sources

trace), which will be discussed in more de-tail in Section 3. The representative sourcesin Figure 1 are, in order of typical distancefrom nearest to most distant.

Compact Binaries in the Milky WayLISA will be sensitive to millions of binarysystems of compact objects (white dwarfs,neutron stars, or black holes). Tens of thou-sands will be individually resolved withmeasured masses, orbital parameters, and3D locations [14, 15] (blue dots in Figure1). The remaining systems contribute toan unresolved foreground (grey shaded re-gion) which limits sensitivity to other GWsources (black dashed line). Several “verifi-cation binaries” are already known throughEM observations [16], providing guaran-teed multimessenger sources (large blue as-terisks). LISA observations will provide in-

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Figure 1: Representative examples of LISAsources compared with the instrument sen-sitivity. Sources and instrument sensitivityare plotted as frequency spectra of charac-teristic GW strain. All sources are observedsimultaneously and individually extractedthrough a global fit of the LISA time-seriesdata. Figure 1 from [13].

sight on stellar populations and evolutionas well as the dynamics of compact binaries[17].

Black Hole BinariesBlack hole binaries with component massesin the approximate range of 10-100 M�,such as have been observed with terrestrialGW observatories [2], will also be observ-able by LISA at earlier epochs in theirevolution. Unlike lighter binary systems,these systems evolve appreciably duringLISA’s observing lifetime (grey and blacktraces, blue trace is GW150914), occasion-ally exiting the LISA band before rapidlyevolving to merger in the ∼ 100Hz band,raising the intriguing possibility of multi-band GW observations [18]. Applicationsinclude tests of gravity and fundamentalphysics through cross-comparison of GWmeasurements in the millihertz and audiobands [19].

Extreme Mass-Ratio Inspirals (EMRIs)A unique class of signal in the millihertzband is the capture of black holes orneutron stars by MBHs in the local Uni-verse (z ≤ 2). The large difference inmass between the two objects results ina highly complex orbit with multiple fre-quency components simultaneously evolv-ing (five reddish traces representing a sin-gle EMRI at z=1.2). LISA will observe tensto hundreds of these EMRI events, yield-ing one of the most precise possible testsof General Relativity in the strong-fieldregime and also providing unique insightinto the demographics and dynamics of thehigh mass end of the population of objectsin nuclear clusters of galaxies like the MilkyWay [20]. LISA may also be able to detectGWs from the capture, and eventual dis-ruption, of individual WDs by MBHs in thenearby Universe, leading to an exciting newmultimessenger source [21].

Massive black hole mergersLISA will detect hundreds of black holemergers with signal-to-noise ratios of10−104 and redshift of 1−30. Multi-coloredtraces show three example equal-massMBH mergers at z=3 which sweep acrossthe LISA band from low to high frequencieswith time before merger, as indicated onthe track. LISA will provide opportu-nities to probe the birth and growth ofmassive black holes and their host galax-ies at redshift ranges and for halo massranges that are not readily accessible withother techniques [22, 23]. Sky localiza-tion to O(10 arcmin) by merger for thehighest-SNR (and most nearby) systems,supporting multimessenger observationsto provide insight into the astrophysicalenvironments of merging MBHs as well asindependent measurements of cosmologi-cal expansion via standard sirens [24].

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Exotic SourcesThe potential for discovery may be thestrongest motivation for making observa-tions in this as-yet-unobserved window onthe Universe [25]. Possibilities include bothastrophysical sources (e.g. intermediate-mass black holes [26]) and cosmologicalsources (e.g. GW backgrounds from in-flation or early-universe phase transitions,and cosmic string bursts, etc. [27]).

3 Mission OverviewObserving in the millihertz band requires

a space-based facility, much as observing inparts of the infrared electromagnetic spec-trum requires going to space. TerrestrialGW detectors are limited to higher frequen-cies by gravitational coupling to seismicdensity fluctuations that are increasinglysevere at low frequencies. More fundamen-tal is the physical size of the detectors them-selves, which are not sufficiently sensitiveto the long wavelengths of millihertz GWs.In contrast, LISA can be placed in an orbitfar from Earth where the thermal, magnetic,and gravitational environment is far morestable and the observatory can be expandedto the million-km baselines that maximizesensitivity to the GW signals of interest.

Earth

Sun1 AU (150 million km)

19 – 23°60°

2.5 million km

Figure 2: Orbital configuration of the LISAmission. The 2.5Mkm triangular constella-tion is inclined to the ecliptic by 60◦ and un-dergoes a cartwheeling motion once per or-bit.

3.1 LISA mission designAt the most basic level, the LISA mea-

surement concept parallels that which hasbeen successfully employed on LIGO andother terrestrial GW interferometers [34,35]. A set of test masses are arrangedacross widely-separated baselines and theproper distance between these masses ismonitored using optical interferometry forfluctuations caused by passing GWs. Inthe case of LISA, the test masses are ar-ranged in three pairs, with each pair hostedin a spacecraft placed at one vertex of anapproximately-equilateral triangle with aside-length of 2.5 Mkm. The resulting con-stellation forms a triangle that is inclinedat 60o with respect to the ecliptic plane andundergoes a cartwheeling motion with onerotation in the constellation plane per orbitabout the Sun. This configuration, whichis depicted in Figure 2, remains passivelystable over the lifetime of the LISA mis-sion (1yr commissioning & calibration + 4yrs science operations + 6 yrs potential ex-tended operations) [36].

3.2 Performance DriversThe sensitivity of the observatory is

chiefly determined by two performancemetrics: the level of imperfection of thetest-mass free-fall and the precision withwhich changes in the test mass separa-tion can be measured. The requirementson each of these are defined in the LISAMission Requirements Document (MRD).Flowing these top-level mission require-ments down to requirements on individualsubsystems and components is a major fo-cus of the current mission formulation ac-tivities. This process begins with a detailedperformance model that accounts for eachphysical effect that contributes to either testmass acceleration or displacement metrol-ogy noise. These models can then be usedto evaluate different instrument architec-

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Box 3 - Near-perfect free-fall in LISA Pathfinder

LISA Pathfinder placed a sin-gle 2kg Au-Pt test mass in aLISA-like drag-free configura-tion and employed a secondidentical test mass and an op-tical interferometer as a low-noise witness sensor. The fig-ure shows the amplitude spec-tral density of the equivalentsingle test mass residual accel-eration noise for Pathfinder inthe nominal configuration [28](red curve). Also shown is an analogous measurement made over a shortertime and with correspondingly decreased statistics in a configuration where theNASA-supplied colloidal micronewton thrusters were used in place of the cold-gas thrusters [29] (light blue curve). Both of these spectra outperform the LISAMRD requirement (black dashed line). In addition to characterizing accelerationnoise performance, Pathfinder provided experience in sub-picometer interferomet-ric metrology of free-flying test masses, non-contact charge control [30], precisiondrag-free control using micropropulsion [31, 32, 29], in-flight thermal diagnostics inthe LISA band [33], and other techniques and technologies relevant to LISA.

tures and develop an error budget at eachlevel of the system. The key performancedrivers for test mass acceleration noise areresidual gas pressure in the test mass cav-ity, control of electrostatic charges on thetest mass, stability of the electrostatic sus-pension used to control the test mass inthe non-measurement degrees of freedom,and careful control of the magnetic, ther-mal, and gravitational environment of thespacecraft. The displacement measurementis fundamentally limited by photon shotnoise, which is in turn determined by laserpower, telescope diameter, and arm length.Reaching this fundamental limit requiresmitigation of technical noises, the largest ofwhich is laser frequency noise which cou-ples into the measurement through the un-equal arms of the LISA constellation. Lasernoise is mitigated through a combination

of active stabilization of the laser frequencyand application of a post-processing tech-nique known as Time-Delay Interferome-try [37, 38, 39]. Beyond laser frequencynoise, the key performance drivers for thedisplacement performance are thermome-chanical stability of the optical structures,mitigation of scattered light, and geometri-cal errors that lead to coupling of spacecraftjitter into the displacement measurement.

LISA’s technical readiness has takensignificant steps forward since the 2010Decadal Survey. Much of this progress isdue to two in-flight demonstrations thatvalidated key aspects of LISA’s measure-ment concept and several critical technolo-gies. LISA Pathfinder (2015-2017) wasan ESA-led mission with the express pur-pose of increasing technical readiness forLISA. The instrument included a pair of

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representative LISA test masses, one ofwhich was placed in a LISA-like config-uration of drag-free flight and the otherof which was used as a low-noise wit-ness for measuring residual accelerationsof the primary test mass (see Box 3). TheGravity Recovery And Climate ExplorerFollow-On (2018-) is the replacement forthe highly successful US/German GRACEmission. Most relevant to LISA, GRACE-FO includes the Laser Ranging Instrument(LRI), a laser interferometer which mea-sures the inter-satellite distance in paral-lel with the primary microwave ranginginstrument. Recent results from the LRIhave demonstrated nanometer-level inter-ferometric ranging over a 210 km link [40],meeting the design goals of LRI which arerelaxed from LISA due to the larger sizeof GRACE-FO’s geodesy signal relative toLISA’s GW signals. Nevertheless, LRI pro-vides flight heritage for key LISA compo-nents such as photoreceivers, phase mea-surement systems, and laser control sys-tems as well as valuable experience withoperational activities such as link acquisi-tion.

3.3 Science AnalysisThe combined effects of LISA’s all-sky

sensitivity and the long observational du-ration of typical LISA sources results inO(104) individual signals with signal-to-noise ratios (SNRs) above the detectionthreshold existing simultaneously in theLISA data stream. Extracting each of thesesources accurately and efficiently is a crit-ical part of the LISA measurement effort.The primary tools for addressing this chal-lenge are matched-filtering and Markov-Chain Monte Carlo techniques, which havebeen successfully employed by terrestrialGW interferometers to extract and charac-terize signals. LISA adds the complexityof overlapping signals but benefits from

the fact that each of the signals accumu-lates many cycles in GW phase during themeasurement, helping to resolve overlap-ping signals and more precisely measureastrophysical parameters [41]. Addressingthe LISA data analysis challenge requiresa coordinated effort of a team with a di-verse set of expertise including source as-trophysics, gravitational waveforms, deepknowledge of the instrument, and exper-tise in matched-filtering searches. The in-ternational LISA community has worked tobring individual efforts together in a setof data challenge activities, in which simu-lated LISA data are generated with a knownset of sources using a common set of simu-lation tools, and various groups work to an-alyze that data before gathering together tocompare and discuss results. The first seriesof such exercises, known as the Mock LISAData Challenges (MLDCs), were carried outby the joint NASA-ESA science team in thelate 2000s [42, 43]. LISA Data challenge ex-ercises have recently been renewed2, incor-porating lessons learned from the MLDCsas well as advances in the understandingof both the astrophysical signals and theLISA instrument, that allow for more faith-ful simulated data sets.

4 Organization and Current StatusESA’s Science Programme Committee se-

lected The Gravitational Universe (millihertzGW astronomy) as the science theme for the3rd large-class mission in the Cosmic Vi-sion Programme (L3) in 2013. Following theearly successes of LISA Pathfinder [46] andthe historic first observations of GWs byLIGO [1], a call for mission concepts was is-sued by ESA in 2016. A European-US teamof scientists responded with a proposal forLISA in early 2017 [13], which was subse-quently selected by ESA in June 2017. As

2lisa-ldc.lal.in2p3.fr/ldc

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lisa-ldc.lal.in2p3.fr/ldc


Box 4 - NASA-supported Technology Development for LISA

NASA is currently supporting the development of five separate technologies forpotential contribution to LISA. The strategy for selecting these technologies, whichis laid out in detail the Interim Report of the L3 Study Team [44], is to balance im-pact and insight into the LISA system, heritage from prior NASA investments, andtractability of interfaces. The NASA Study Office Technology Plan [45] describes indetail the development strategy for each of these technologies to reach ISO TRL 6by Mission Adoption (2023). The table below briefly introduces them:

Technology Role in LISA Development StrategyTelescope Efficiently deliver

optical power acrosslong baselines

Control dimensional stabilityusing low-expansion materi-als and stable thermal design.

Laser Provide light for pri-mary interferometricmeasurement

MOPA architecture utilizingNPRO technology from LISAPathfinder and LRI

Charge Man-agement

Control electriccharge on the testmasses using UVlight

Build on Pathfinder heritage;replace Hg lamps with UVLEDs as light source

Micropropulsion Precision attitude/-position control ofthe spacecraft

Leverage ST7 heritage, in-crease reliability and lifetime

Phase Measure-ment Systems

Acquire primary sci-ence, auxiliary, andcontrol-loop error sig-nals.

Build off LRI experience; addLISA-specific functionality

is common for ESA missions, contributionsto both flight hardware and science supportare anticipated from ESA Member States aswell as international partners. The LISAConsortium3 was subsequently formed tosupport the development of the payloadand to coordinate the efforts of the interna-tional research community in areas of dataanalysis and science exploitation.

In 2015, NASA convened a team of USexperts in LISA science and technologyknown as the L3 Study Team to study po-tential opportunities for US contribution

3www.lisamission.org

to an ESA-led LISA mission [44]. Fol-lowing the strong recommendations for in-creased US participation in LISA by the2016 Midterm Assessment of New Worlds,New Horizons [8], NASA established theNASA LISA Study Office (NLSO) to coor-dinate technical interchange with the ESAStudy Team and its partners, consolidatetechnology development activities support-ing potential US contributions, and engagewith the scientific community in the US andEurope. The L3ST was replaced by theNASA LISA Study Team (NLST) after the2017 mission selection.

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www.lisamission.org


Figure 3: An overview of the current LISA schedule including major project milestones.

The NLSO is located within the Physicsof the Cosmos Program Office of the NASAAstrophysics Division and is led by theGoddard Space Flight Center with substan-tial contributions from the Jet PropulsionLaboratory, the Marshall Space Flight Cen-ter, and the University of Florida. The near-term goal of the NLSO is to identify a setof potential US contributions to LISA andassess the merit, risk, and cost of each con-tribution. Part of this effort includes de-veloping a set of technologies to a levelthat makes them viable, low-risk contribu-tions at the time that a final arrangement ismade (Box 4). The Study Office will tran-sition to a Phase A project once a final setof roles and responsibilities has been nego-tiated between ESA, NASA, and the partnerESA Member States. This agreement willbe made at or before ESA’s adoption mile-stone (see additional discussion in Section5) and will be influenced by technical readi-ness, suitability of interfaces, and availablebudgets among all partners.

A critical aspect of LISA is that it is effec-tively a single instrument distributed acrossa constellation of three spacecraft. While itis common in space missions to have dif-ferent organizations providing different in-struments, it is not common to have theseseparately-sourced units interacting withone another in as intricate a way as LISAdemands. All of the LISA partner organi-zations recognize this fact and are closelycooperating at both technical and program-matic levels. Under a Letter of Agreementsigned between NASA and ESA in 2019, the

NASA and ESA Study Offices, and by ex-tension ESA’s European partners, are ableto share technical information that allowsthe LISA design to proceed in a coordinatedfashion. NLSO personnel have been in-vited to participate in a number of ESA andLISA Consortium-led activities, includingprogress meetings with industrial contrac-tors performing detailed design studies ofthe spacecraft and payload. A Systems En-gineering Office (SEO) has been establishedto facilitate technical interchange among ex-perts from across the partnership, and toharmonize processes for the developmentof requirements and interfaces across pro-grammatic boundaries.

5 ScheduleA simplified LISA schedule is shown in

Figure 3. LISA is currently in the earlystages of formulation as a Phase A Studyat ESA and a pre-Phase A Study withinNASA. ESA is currently conducting a midPhase A review intended to close major ar-chitectural trades and identify any criticalneeds in technology development that re-quire attention. This will be followed by aMission Formulation Review as the gate re-view to Phase B. The next major milestonefor LISA is Mission Adoption, a critical de-cision point in the ESA framework whichauthorizes the mission to proceed throughthe final stages of formulation and the im-plementation phase. In this sense it is anal-ogous to the Confirmation milestone in theNASA framework. Mission Adoption iscurrently targeted for 2023. After Mission

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Adoption, industrial contracts for the im-plementation phase will be negotiated andthe final formulation and implementationactivities can begin. These phases typi-cally last 8-10 years for an ESA L-class mis-sion, leading to a launch date in the early2030s, consistent with, or perhaps slightlyin advance of, the target 2034 launch datefor L3 in the ESA Cosmic Vision planningdocument. As a junior partner, the NASAproject schedule will track the ESA sched-ule where possible. The most significantdiscrepancies are in the formulation phase,when NASA will remain in pre-Phase A un-til the final set of NASA roles and responsi-bilities are negotiated with ESA and otherEuropean partner agencies. Input fromthe Decadal Survey, assuming it adheres toits late-2020/early-2021 estimate for release,will have an opportunity to influence thisnegotiation. Most importantly, all NASAtechnology development activities are be-ing managed to a schedule that is consistentwith ESA’s guidelines, namely achieving anISO technology readiness level (TRL) of 5/6on all critical items prior to Mission Adop-tion. Further details on NASA’s develop-ment schedule can be found in the Technol-ogy Development Plan [45].

6 CostAs a junior partner contributing to a mis-

sion led by an international partner, the costconsiderations for LISA are somewhat dif-ferent than for other missions under con-sideration by Astro2020. While total-projectcost estimates were previously made byboth the NASA LISA project of the 2000sas well as by independent cost estima-tors [47, 7], those cost estimates are of lim-ited value for assessing the costs of an ESA-led mission, which operates under differentfinancial conditions. Based on the ESA L-class cost cap of €1.05 B and the expectedlevel of contributions from European Na-

tional agencies and NASA, it is reasonableto assume that the total LISA mission costwill lie in the “large” category identified inthe Astro2020 APC guidelines (> $1.5B).The size of the NASA contribution un-der any conceivable partnership arrange-ment would be in the “medium” category($500M − $1.5B). This cost would includeboth the value of the hardware deliverablesto Europe as well as contributions to the sci-ence ground segment, US Guest Investiga-tor programs, and NASA project overheadincluding management, systems engineer-ing, project science, and mission assurance.The NLSO has developed and continues torefine cost estimates for the project lifecyclecost (LCC) of each potential hardware con-tribution as well as the science participa-tion and project overhead activities. TheseLCC estimates are used to validate potentialcontribution scenarios against budget con-straints. A memo describing the LCC es-timates and their underlying assumptionscan be provided to the Astro2020 commit-tee upon request.

7 Scenarios for US ParticipationLISA is a single scientific instrument that

is distributed across a constellation of threespacecraft and which conducts an all-skysurvey resulting in a single data set con-taining a mixture of all sources. Further-more, the LISA science and mission require-ments have been established and formula-tion activities are proceeding. The motiva-tions for an upscope to the US LISA contri-bution are not to make the instrument moresensitive but rather to contribute to over-all mission success and to boost US par-ticipation in a compelling new field in as-tronomy. The currently envisaged US con-tribution to LISA is at the lower end ofthe “medium” scale identified by Astro2020($500M-$1.5B), which will enable the US toprovide a subset of the instrument compo-

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nents described in Box 4 as well as con-tribute to the LISA science analysis efforts.An upscope would expand the range ofpotential hardware contributions, allowingmore complete utilization of the significantUS-based investments in LISA and relatedefforts. Additionally, it would enable in-creased participation by the US communityin LISA data analysis and science exploita-tion. The specific set of US contributionswill depend on a number of factors includ-ing technical readiness, compatibility withEuropean partners, and available budgets.Here we present three broad categories ofcontributions which could be enabled by anupscope.

Engineering of Instrument SubsystemsCareful and deliberate systems engineeringis key to the success of precision measure-ment apparatus such as LISA. While LISAis a distributed instrument, there are ar-rangements of instrument subsystems thatminimize complexity of interfaces. Takingresponsibility for one of these subsystemswould be the most effective way for the USto mitigate risk through hardware contri-butions. The most logical approach wouldbe to build such a subsystem around oneof the component-level technologies underdevelopment by NASA (see Box 4). In sucha scenario, the US could additionally pro-vide component-level contributions to theadditional, European-led instrument sub-systems.

US Science FacilitiesAnalysis and scientific exploitation of mis-sion data is typically funded by ESA Mem-ber States for ESA missions, and the samearrangement is being planned for LISA. Animportant part of the NASA LISA Projectwill be a Guest Investigator / Science Cen-ter facility which will fulfill a similar role forthe US-based research community. In ad-dition to implementing the US role in the

project-level data analysis (e.g. similar tothe arrangement on the ESA-led Planck andHerschel missions), such a facility wouldprovide outside users with access to mis-sion data at a variety of levels, as well astools to facilitate working with LISA dataand for combining LISA data with otherfacilities in multimessenger investigations.The scope of such a facility, as well as GuestInvestigator grants to carry out research us-ing LISA data, will be directly influenced bythe scale of the NASA effort.

Propulsion AlternativesIn addition to GW strain sensitivity, themost determinative factor in LISA’s sci-ence performance is mission lifetime. Thethree major determinants of mission life-time are stability of the orbits, reliabil-ity of the spacecraft and instrument sys-tems, and amount of propellant in themicropropulsion system. The NASA-developed colloidal micronewton thrusters(CMNTs), performed successfully on LISAPathfinder [29] and may enable a signif-icantly lower system mass. Provided amission architecture that effectively incor-porated CMNTs could be developed, sucha contribution could significantly enhanceLISA science return by reducing systemmass and increasing the propellant-limitedlifetime.

In many ways, LISA represents a uniqueopportunity for NASA as the junior part-ner – full participation in the science ofa flagship-scale mission with a medium-scale investment. A robust US contribu-tion to LISA will further the success of thisgroundbreaking mission and will provideengagement and leadership opportunitiesfor current and future members of the USscience community.

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