the lisa newsletter- issue 2006:1

8/3/2019 The LISA Newsletter- Issue 2006:1

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In this edition: Managers Editorial Introduction to Extreme-Mass-Ratio Inspirals Status Report on LISA Pathfinder A Year of Breakthroughs in Numerical Relativity Update on LIST Data-Analysis Activities

LISA Formulation Studies at EADS Astrium LISA Micronewton Thruster Technology

2006May1

National Aeronautics and Space Administration

European Space Agency

theLISAn

ewsle

tter

Issue2006-1


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Welcome to the first issue of the LISA Newsletter. This news-letter will keep you updated with the programmatic status andthe scientific and technical progress of the LISA project. As the

ESA and NASA LISA project managers, we would like to takethis opportunity to provide you with some background, pro-

gress, and future plans in the programmatic arena.

As you know, the LISA project enjoys a strong partnership be-tween NASA and ESA. In August 2004, an agreement was

signed between the two agencies that defines the roles and re-sponsibilities of each partner. Since then, the LISA personnel atboth agencies have worked together very closely as a single vir-

tual team to develop the required technologies and to developthe baseline mission architecture. LISA received a strong vote of

confidence from an independent review team in December2005, after formal technology assessments of LISA and

Constellation-X, two proposed flagship missions of NASAs

Beyond Einsteinprogram. That team concluded that LISAs tech-nology requirements are well understood and the plans for

completing development of the critical technologies are sound.

LISAs launch will be preceded several years by the LISA Path-finder (LPF), a mission that will demonstrate some of LISAs

key technologies in space. LPF is managed by ESA, and willcarry both ESA and NASA test packagesthe LISA Test Pack-age (LTP) and the Disturbance Reduction System (ST7-DRS),

respectively. You can read more about this mission in the articleby Stefano Vitale in this newsletter. LPF is well into its imple-

mentation phase and has just completed its Preliminary DesignReview. The mission is fully funded by each agency for launch in

the fourth quarter of 2009, and is moving ahead right on sched-ule.

LISA entered the formulation phase in January 2005. During

this phase, trade studies are being completed that will lead toselection of a baseline mission architecture that meets the LISAscience requirements in the most cost-effective way. The Mis-

sion Architecture Review was completed in October 2005, andthe Mid-Term Review will take place soon, in April 2006. With

the baseline architecture in hand, we will proceed with definingthe lower-level requirements. ESA has engaged in a two-yearindustrial contract with Astrium GmbH to support the mission

formulation activity (see the article by Ulrich Johann).

LISA has survived various funding crises over the past fewyears, on both sides of the Atlantic. On the U.S. side, the latest

language in the Presidents proposed budget for fiscal years 2007through 2011 does not imply a definitive launch date for LISAHowever, the budgetary guidelines distributed by NASA Head

quarters indicate that a wedge will be available starting inFY2009 that will allow at least one facility-class mission in the

Beyond EinsteinProgram to go forward. Options for that missioninclude LISA, Constellation-X and the Joint Dark-Energy Mis-

sion. LISAs advanced design status and the consolidated coop-eration framework between ESA and NASA combine to make

LISA a strong candidate for selection. On the ESA side, whilefinal commitment to LISAs implementation will be influencedstrongly by the success of LPF, work will be underway well be

fore LPFs launch to define the LISA mission and prepare theinvitation to tender for the implementation phase. With NASAs

selection in FY2009 and ESAs final commitment, we expecLISA to enter the implementation phase in 2011, and to launch

in the 20152016 timeframe.

From now through the final NASA selection in 20082009,

because of the limited funds available, priority will be given to

activities that make the LISA mission as competitive as possible These activities cover three principal areas: science capabilitytechnology maturity, and total mission cost. In particular, we wil

focus on achieving the following objectives:

1. Science capability

Maintain substantial support from the community of

scientists who may be interested in the LISA data, be-yond the core fundamental physics community.

Demonstrate that LISAs science requirements are welunderstood.

Demonstrate that the science proposed by LISA can be

accomplished.

2. Technology maturity Demonstrate that the required technologies are at their

highest possible levels of readiness at the time of deci-

sion.

3. Total mission cost

Demonstrate the highest possible credibility for the tota

cost estimate at the time of the decision.

Understand the architecture in sufficient detail that a

credible hardware-acquisition-type (Price-H) estimation model can be developed.

Prove that the selected mission design offers the bestscience return for the cost.

Ensure to the maximum extent possible that the missiondesign is scalable, with clear understanding of the impacon science return of any cost reductions.

We hope that this newsletter will be a good vehicle for providingyou with vital insight into the LISA project activities. We count

on your support and encouragement to make LISA a successfumission that will open a new window to the universe.

Sincerely,

Mansoor Ahmed Alberto GianolioNASA LISA Project Manager ESA LISA Project Manage

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theLISAnew

sletter2006May1 Dear Readers,

Whats Happening...The Sixth International LISA Symposium will be held

June 1923, 2006, at NASA Goddard Space Flight Center

in Greenbelt, Maryland. Topics include: fundamental gravi-

tational physics and astrophysics of LISA, gravitational-

wave data analysis, LISA instrumentation, LISA Pathfinder,

and ground-based gravitational-wave antennas. A tutorial

session on LISA will also be given the first morning of the

Symposium.

More information and registration forms can be found at

the symposium web site, lisa6.gsfc.nasa.gov. The deadline

for registration is May 1, 2006.
http://lisa6.gsfc.nasa.gov/http://lisa6.gsfc.nasa.gov/http://lisa6.gsfc.nasa.gov/


3/12

How do you observe an object made en-

tirely of gravity? The cleanest and simplestapproach is to drop a particle into that

gravitational field and see how it moves.

Extreme mass ratio inspirals (EMRIs) the capture of a stellar-mass body by a

supermassive black hole provide just

such a probe.

In recent years, astrophysicists have come

to the conclusion that supermassive blackholes are ubiquitous in the cores of galax-

ies. As galaxies merge and grow, so too dotheir resident black holes. A given super-

massive black hole may typically have

gone through several mergers, and will

also have grown by feeding off the gas

and stars of its host galaxy. Each of theseproperties will leave its imprint on themass and spin of the black hole; accreting

gas from a thin disk tends to spin the

black hole up to extreme values, while

capturing stars from an isotropic swarm

tends to decrease spin, and a merger oftwo comparable-mass black holes tends to

produce a moderately-spinning black hole.In turn, the spin of the black hole causes

rapid precession in the orbits of objects

deep in the holes gravitational well. Com-

pact stellar remnants, such as whitedwarfs, neutron stars, and stellar-mass

black holes, are expected to remain intacteven as they are swallowed whole by the

supermassive black hole, making themideal test particle probes of the space-

time structure just outside the holes eventhorizon. This is the regime that LISA is

designed to explore.

The value of EMRIs comes from the long

duration and intricate complexity of the

orbits, and of the gravitational waveforms

they produce. LISAs peak sensitivity at3mHz is ideally tuned to detect the final

orbits of objects falling into ~ 106 M

supermassive black holes. The last year of

inspiral will comprise about 3 mHz 1yr

~ 105 orbits. These orbits will typically beboth eccentric and inclined from the holes

equator, producing waves with a richstructure: the effects of frame dragging

due to the spin of the hole will be clearly

observable. With 105 wave cycles to work

with, it should be possible to measure theparameters of the orbit and of the central

black hole to fractional-percent precision.

Enough such detections would give usaccurate demographics of the distribution

and characteristics of supermassive black

holes in the Universe. Furthermore, sinceeach inspiraling orbit is highly sensitive to

the structure of spacetime that it traverses,individual detections ought to provide

stringent tests of the predictions of gen-

eral relativity. That is, it should be possible

to measure whether the supermassive

black hole is in fact a black hole as pre-

dicted by relativity, or instead some otherexotic object with unexpected properties.

Quantifying this expectation is a subject ofcurrent research.

So what are the prospects for detecting

such signals? From empirical relationshipsbetween galactic luminosity, velocity dis-

persion, and central black hole massesmeasured in nearby galaxies, we can esti-

mate the space density of supermassive

black holes: about 7106 black holes of

~106 M per cubic

gigaparsec. Numerical

simulations of theswarm at the center of

our own Galaxy suggestthat its 3106 M black

hole captures a whitedwarf, neutron star, or

stellar-mass black hole

every million years or

so. This gives a capture

rate density of several

per year per cubic giga-parsec give or take an

order of magnitude.On the other side of

the equation, LISAsability to detect these

objects depends criti-

cally on the signal analy-

sis method one adopts.

The gravitational wave-

forms long durationand complexity are cru-

cial in our ability to separate their weak

signal from the instrumental noise, buthese same features rule out simple

matched filter searches: such an opti

mal search would occupy all the worldscomputers for longer than the lifetime of

the Universe. Instead, various other

schemes have been proposed that wouldbe able to see nearly as far as optimal fil-

tering (within a factor of a few), while stilbeing computationally tractable. The fina

technique and its performance are a sub-

ject of ongoing research, but we expect to

achieve an astronomical reach of severa

gigaparsecs (luminosity distance) for

black-hole captures, about a gigaparsec foneutron stars, and at least half a gigapar

sec for white dwarfs. Over a 5-year mission LISA is therefore expected to see

anywhere from tens to thousands o

EMRI events.

Direct observations of black holes have

always proven a singular challenge to as-tronomy. But under the onslaught of so

many high-precision probes, their proper

ties will soon come to light.

3

theLISAnew

sletter2006May1

Final hour of particles falling into a 106 M black hole

having 50% (blue solid line) or 55% (red dotted line) of

maximum spin. The vertical axis is the projection of the

particle position onto an axis in the hole's equatorial

plane. Particle orbits have eccentricity 0.5 and no incli-

nation. Shaded region represents the size of the hole's

horizon. Even over a span of just one hour (a dozen

orbits), a 5% change in spin produces clear differences

in the orbits.

Introduction to Extreme Mass-Ratio Inspirals:

supermassive black holes under the microscope

Teviet Creighton

Jet Propulsion Laboratory


4/12

Like all interferometric detectors of gravi-

tational waves, LISA is based on the pos-sibility of realizing, to some accuracy, aTransverse and Traceless (TT) space-timecoordinate frame. In this coordinate

frame, despite the presence of a weakspace-time curvature associated with

gravitational waves, a free particle initiallyat rest remains at rest, i.e., its space coor-dinates do not change with time; and co-

ordinate time coincides with the propertime of clocks sitting on such resting par-ticles. Even though such free particles

remain still, the proper distances amongthem change with time because the metric

tensor itself changes with time, and this variation can be detected with laser inter-ferometry.

The accuracy with which the referenceparticles (a.k.a. proof masses) can be

known to be still determines thestrength and wavelength of the gravita-tional waves we can detect and measure.

Figure 1 shows the desired accuracy (spec-tral amplitude of displacement) as a func-tion of gravitational-wave measurement

frequency for LISA (10 pm1/Hz be-tween a few mHz and a few tens of mHz)

and for the ground-based gravitational- wave detector LIGO (sub-am2/Hz be-tween a few hundred and ten thousand

Hz).

Electromagnetic forces and locally-generated gravity can drive the proofmasses out of their geodesics, mimicking a

change in the metric tensor. This effectlimits the accuracy of the definition ofreference frames at the lowest frequency,because the conversion from accelerationto displacement spectral amplitude in-volves the inverse square of the frequency.

The comparison between LISA andLIGO is qualitatively reversed when itcomes to requirements for the maximum

tolerable acceleration noise (see Figure 1). At the low frequencies of interest forLISA (0.1 to a few mHz), the maximum

tolerable parasitic acceleration corre-sponds to a spectral amplitude of about

310-15ms-2/Hz, whereas LIGOs mostsevere requirement for acceleration noisespectral amplitude (between a few tens

and a hundred Hz) is about ten times

larger. Figure 2 also shows the accelerationnoise requirements for GOCE, ESAsEarth geodesy mission based on gravita-tional gradient mapping. Although GOCE

requires a high level of thermal dynamicisolation for its proof masses, its require-

ment for acceleration noise is two-to-threeorders less demanding than for LISA overa similar measurement band, a level which

for certain types of disturbances has beenachieved already on the ground [1].

The demonstration of the possibility ofachieving such a high level of accuracy in

the definition of the TT frame has beenincluded in ESAs plans with the LISAPathFinder mission, to which NASA willcontribute. The principle of the test on

LISA Pathfinder is to provide the mini-mum setup needed to define a TT frame

along one axis, mimicking the key aspectsof one of the LISA arms: two free-fallingproof masses and a high-precision

interferometer-based metrology system tomeasure their relativemotion to an accuracy

on the order of 10pm/Hz. The specificgoal of the LPF mis-sion is to demonstratethat the relative accel-

eration between twoproof masses is lessthan 310-14ms2/Hzup to a frequency ofabout 9 mHz, above which the interfer-

ometer measurementerror of 10 pm/Hz begins to domi-nate the signal.

By comparison, LISA

requires a relative ac-celeration between thetwo proof masses ineach arm of 2310-15ms-2/Hz, abouta factor of 7 timesmore demanding. This

slight relaxation per-mits a less demandingset of requirementsfor the LPF spacecraft

and onboard systems,but it is not intended

to bring about any change in design of the

proof masses or their motion sensorsOne further relaxation for LPF is that thislevel of acceleration noise is required onlyabove 1 mHz (as opposed to down to 0.1

mHz). This will accommodate some extradisturbances anticipated to result from the

nature of the test setup unique to LPF which will not exist for LISA. Details othe LPF hardware can be found elsewhere

[2]. Following is a summary of the pri-mary features of the ESA-provided coreinstrument, known as the LISA Technol-

ogy Package or LTP (Figure 3).

As for LISA, the proof masses are 46-mm Au-Pt cubes surrounded by Au-coateelectrodes (see Figure 4). Electrodes areused both for capacitive sensing of proof

mass position and orientation ( 2 nm/Hz resolution) and for applying forcesneeded to keep it in equilibrium. Relativelylarge gaps (4 mm) separate the proofmasses from the closest bodies in order to

suppress electromagnetic interactions.

4

theLISAnew

sletter2006May1 A status report on LISA Pathfinder:

leading the way toward LISA technology

Stefano Vitale

University of Trento, Italy

Figure 1. Positional accuracy desired in the definition of

coordinate frames for LIGO and LISA.

Figure 2. Maximum tolerable level of parasitic force noise

for LISA, LIGO, and GOCE.

1 pm = picometer (10-12m);2 am = attometer (10-18m)


5/12

A laser interferometer reads out the rela-tive displacement between the two LTPproof masses along the line joining theircenters of mass (see Figure 5). In addi-tion, one more interferometer provides an

independent readout of the motion ofone of the proof masses relative to thespacecraft. Progress in the design of LISAhas recently clarified that inference of therelative motion of its two widely separated

proof masses requires measurement ofthe motion of each proof mass relative toits own spacecraft plus the measurementof the relative motion between the space-craft. This extra interferometer on board

LISA Pathfinder thus provides demonstra-tion of another critical part of the LISAinstrument.

As on LISA, the spacecraft will follow oneof the proof masses by using micronew-ton thrusters. Accurate centering of thespacecraft around the proof mass isachieved with a control loop driven by the

motion sensor for one of the proofmasses motion sensor (electrostatic or

interferometric sensor, depending on thespecific experiment). Since the single LPFspacecraft cannot follow two proofmasses at the same time along the same

direction, thesecond proofmass must beforced tofollow thefirst one byan electro-static force,an aspect ofthe setup

which limitsthe accelera-tion noiseperformance

at the lowestfrequencies.

The technol-ogy tested onLISA Pathfinder will be nominally trans-ferred to LISA with no change of design.Specifically, this includes the following:

Proof masses and electrostatic readoutand actuation, a set referred to as the

Gravitational Reference Sensor (GRS). High-vacuum enclosure for the GRS.

High-precision balance masses to cancelthe static gravitational field and gradi-ent.

UV-light charge management system to

keep the proof mass neutral to withinthe required tolerance (


6/12

From the point of view of fundamentalphysics, the direct detection of gravita-

tional waves will be an amazing achieve-ment. However, when it comes togravitational-wave detectors such as LISA,

the scientific community expects to realizemuch more than just the detection ofgravitational waves. One such achievementcomes in the form of the detailed exami-

nation of some of the most exotic objectsin the universeblack holes. From

extreme-mass-rat io scenarios tocomparable-mass supermassive inspirals,black holes figure to play prominently in

the scientific achievement of gravitational-wave detectors.

A necessary ingredient required to trans-form the gravitational-wave detection

capabilities of LISA into a tool for imag-ing black holes is the calculation, withinthe mathematical framework of generalrelativity, of the gravitational-wave signa-

tures of astrophysical events involvingblack holes. Calculation of the waveform

from an equal-mass coalescing black-holebinary remains an unsolved problem. Itsresolution will probably require the syner-

gistic efforts of post-Newtonian solutions(during the quasi-equilibrium, adiabaticregime of the inspiral), perturbation tech-niques (during the ringdown of the final

merged object), and numerical relativity tomodel the highly dynamic, nonlinear

plunge and merger phases ofthe inspiral.

The past year has seen several

breakthroughs in numericalsimulations of binary black-hole

inspirals. The first success, re-ported by Frans Pretorius [PRL95, 121101 (2005)], was achievedwith a code based on harmonic

coordinates first suggested byDavid Garfinkle of Oakland,and implementing modificationssuggested by Carsten Gundlach

(Southampton). The code usessecond-order finite-difference

methods and employs adaptivemesh refinement along withdynamical horizon excision.

Starting from approximatelycircular initial orbits, Pretorius

has been able to track the binarythrough several orbits, to themerger and the beginning of ringdown(see Figure 1).

Numerical-relativity groups based at theUniversity of Texas at Brownsville (Manu-ela Campanelli, Carlos Lousto, and Yosef

Zlochower, working with Florida AtlanticUniversitys Pedro Marronetti) and atGoddard Space Flight Center (John Baker,Joan Centrella, Dae-Il Choi, Michael Kop-

pitz, and James van Meter) used the BSSN(Baumgarte-Shapiro-Shibata-Nakamura)

formulation of the Einstein field equa-tions, along with puncture initial data

(first proposed by Steve Brandt and BerndBruegmann). While the UTB group employs fourth-order finite-difference methods to obtain higher accuracy, the God

dard group uses a hybrid second/fourth-order discretization, along with adaptivemesh refinement, to obtain higher accu-

racy. Interestingly, neither group exciseinside the black-hole horizons, yet both

are able to track the binary through multi-ple orbits in their numerical simulationsThe UTB results [PRL 96, 111101 (2006)

see Figure 2] track the binary through thelast orbit before plunge, demonstratingfourth-order convergence of the extracted

waveform.

Perhaps the most impressive demonstration of waveform calculations to date arereported by the Goddard group [PRD 73104002 (2006), see Figure 3]. They star

from circular-orbit initial data with a variety of initial separations, ranging from theInnermost Stable Circular Orbit (ISCOroughly 6 times the ADM mass of thesystem) to 50% farther than the ISCO

The resulting plunge waveforms are compared, modulo the overall phase, withgood agreement among all simulations.

While the above progress is an excitinstep forward for numerical relativity, thereare still many challenges to be faced

6

theLISAnew

sletter2006May1 Simulating black-hole mergers: a year of

breakthroughs in numerical relativity

Mark Miller


Figure 1. Pretorius: the real part of 4, evaluated

on the orbital plane of the binary black hole inspi-

ral evolution, shortly after merger.

Figure 2. UTB: the real part of r4 at various times during the binary black hole

simulation (sequenced from top left, top right, bottom left, bottom right).
http://link.aps.org/abstract/PRD/v73/e104002http://link.aps.org/abstract/PRD/v73/e104002http://link.aps.org/abstract/PRD/v73/e104002http://link.aps.org/abstract/PRD/v73/e104002http://link.aps.org/abstract/PRL/v96/e111101http://link.aps.org/abstract/PRL/v96/e111101http://link.aps.org/abstract/PRL/v95/e121101http://link.aps.org/abstract/PRL/v95/e121101http://link.aps.org/abstract/PRL/v95/e121101http://link.aps.org/abstract/PRL/v95/e121101


7/12

These challenges include obtaining astro-physically relevant initial data for binarysimulations, as well as ensuring that thesimulations themselves are accurateenough that signal searches and parameterestimations based on waveforms extractedfrom these simulations are dominated bydetector noise, rather than by analysiserrors (e.g., discretization errors) inherentto the numerical relativity simulations. A

casual perusal of the results describedabove indicates that this challenge remainsto be met; at present, relatively large phaseerrors exist over the entire wave trains

extracted from the multiple-orbit simula-tions. The demands on numerical relativitymay well be extremely severe for LISA,which is expected to detect supermassive-black-hole mergers anywhere in the Uni-verse with high signal-to-noise: our theo-

retical models must be of similarly highfidelity in order for LISA to realize itsscientific potential.

At the LISA International Science Team(LIST) meeting of December 2005, in

Pasadena, the Working Group on DataAnalysis (LIST-WG1B) established a task-force to organize several rounds of mock

data challenges, with the dual purpose offostering the development of LISA data-analysis tools and capabilities, and ofdemonstrating the technical readiness al-

ready achieved by the gravitational-wavecommunity in distilling a rich science pay-off from the LISA data output. The LISAMock Data Challenges were proposed and

discussed at meetings organized by theU.S. and European LISA Projects thatwere attended by a broad cross section of

the international gravitational-wave com-munity. These challenges are meant to beblind tests, but not really a contest.

The Mock LISA Data Challenge (MLDC)Taskforce has been working since the be-

ginning of this year to formulate challengeproblems of maximum efficacy, to estab-lish criteria for the evaluation of the

analyses, to develop standard models ofthe LISA mission (orbit, noises) and ofthe LISA sources (waveforms, parameteri-

zation), to provide computing tools suchas LISA response simulators, source wave-form generators, and a Mock Data Chal-lenge file format, and more generally toprovide any technical support necessary tothe challengers, including moderated dis-

cussion forums and a software repository.The activities of the MLDC Taskforce can

be tracked on the WG1B website,(www.tapir.caltech.edu/dokuwiki/listwg1b:home ), where you will find the contact

list, working materials, and teleconferenceminutes. The taskforce welcomes contri-butions and feedback from any interestedparties. Questions and comments can be

sent to the MLDC co-chairs, MicheleVallisneri ([email protected]) and AlbertoVecchio ([email protected]).

Meetings: the LIST working groups will be

meeting immediately prior to the LISASymposium (lisa6.gsfc.nasa.gov ) on Satur-day, June 17 in Greenbelt, Maryland. The

main topic for the WG1B meeting will bethe materials produced by the MLDCtaskforce. The goal of the meeting is toinform interested parties and to gatherfeedback. The recommendations of theMLDC taskforce will be presented for

adoption at the executive session on theLIST on Sunday, June 18.

The first set of challenge datasets will bereleased during the 6th LISA Symposium(June 1923, 2006, at Goddard Space

Flight Center, Greenbelt, Maryland).There will be an afternoon session at thesymposium where the challenges will bedescribed and tutorials will be given on theuse of the datasets and the MLDC tools. The challenges will involve the distribu-

tion of several datasets, encoded in a sim-ple standard format, and containing com

binations of realistic simulated LISA noise with the signals from one or more LISAgravitational-wave sources of parameter

unknown to the challenge participants.

It is envisaged that the results of the firs

MLDCs will be presented to the broad

community and discussed in a dedicatedsession at the 11th Gravitational-Wave

Data Analysis Workshop (December 18 to21, 2006, at the Albert Einstein Institutein Golm, Germany; see the website

gwdaw11.aei.mpg.de ). The second anthird sets of challenge datasets, embodying more ambitious data-analysis prob

lems, will be released in December 2006with target timeframes for the completionof the analyses in June and Decembe

2007.

Informal meetings and/or teleconference

will be scheduled in consultation with theparticipants to discuss progress, issues andpreliminary/final results. The attendance

to the LISA Symposium and to theGWDAWs is therefore not required toparticipate in the challenges, since all chal

lenge materials and results will be availableonline.

Neil Cornish & Bernard Schutz

LIST WG1B co-chairs

7

LIST Working Group 1B (data analysis): update on activities and call for input

Figure 3. GSFC:

paths of binary

black hole nu-

merical evolutions

starting from dif-

ferent initial sepa-

rations; the track

of only one of the

black holes isshown from each

simulation. An

overall phase fac-

tor is applied to

the tracks, chosen

so that the phases

match at the time

of merger (indi-

cated by an aster-

isk in the plot).

theLISAnew

sletter2006May1
http://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://gwdaw11.aei.mpg.de/http://gwdaw11.aei.mpg.de/http://lisa6.gsfc.nasa.gov/http://lisa6.gsfc.nasa.gov/mailto:[email protected]:[email protected]:[email protected]:[email protected]://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://www.tapir.caltech.edu/dokuwiki/listwg1b:homehttp://www.tapir.caltech.edu/dokuwiki/listwg1b:home


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The European Space Agency, operating in

close coordination with NASAs GSFCand JPL, awarded a study contract in Janu-ary 2005 to EADS Astrium, Germany.The objective is to support the Agency asindustrial partner in all aspects of formu-lating a consolidated and optimized LISAmission architecture and design. The studywas structured in two phases. Phase two iswell advanced now and will nominally becompleted in January 2007. The study hasalready matured the LISA mission devel-opment by proposing a significantlyevolved payload architecture incorporatingadvanced conceptual ideas into a consoli-dated baseline design. At the same time,novel payload architectural concept alter-natives have been proposed, with the po-tential for further mission optimization.

Despite the need for focused technologydevelopments, particularly in some pay-load areas, the robustness of the LISAmission concept has been reconfirmed,and no critical item has been encountered.

Astrium began with the results of theirprevious study performed in 2000 and

reconsidered theoretical and technicalwork accomplished since then by variousEuropean and American institutions, as well as progress achieved by the LISAPathfinder mission. Astrium is also indus-trial partner to ESA and the nationalEuropean space agencies for the devel-

opment of the LISA Pathfinder missionand its European payload (LTP).

The mission formulation phase covers allaspects of the LISA mission. It is organ-ized to derive specifications for all missionelements in a flow-down and apportion-ment from top-level science requirements,taking into account operations, constella-tion and control, payload engineering,spacecraft engineering, and mission infra-structure. This comprehensive approach isnecessary in order to optimize the delicateinterdependencies of the technical and

performance requirements imposed ateach level on the various mission ele-ments. Results are used to forge a devel-opment program for critical enablingtechnologies and assemblies. Other impor-

tant tasks include the optimization of themission design in terms of risk reduction,technical maturity, budget, interfaces, andschedule impact. Despite the necessarytop-down approach, the investigations arepayload-oriented, this being the elementthat drives science performance and which

is also is the major focus of the European

contribution.

LISA is a triangular constellation of threespacecraft, separated by 5 million km fromeach other and mutually linked by laser

interferometry in an active transponderscheme, trailing Earth in its heliocentricorbit by about 20. The triangular plane is

revolving over the year, while its normalremains tilted at 30 to the plane of theecliptic, and pointed towards the sun. Thelaser interferometers along each of thethree arms are referenced to free-fallingproof masses (cubes) inside the payload.They are kept in free fall along the associ-ated interferometer axis and within thedesired measurement bandwidth of310-510-1 Hz by a drag-free controlsystem, which minimizes acceleration dis-

tortions from the space environment andfrom the surrounding spacecraft itself.Passing gravitational waves causepicometer-level differential phase modula-tion in the laser interferometers that willbe detected through signal processing ofall of the laser heterodyne phase-meter

signals, derived from superposition oflocal and received laser signals for all inter-ferometers. Detection sensitivity is con-strained in the low-frequency band byproof-mass acceleration noise (e.g. ther-moelastics, electrostatics), and in the me-dium and high ranges by laser photon

statistics (laser power). The LISA missionduration is 6.5 years (including 1.5 yearsfor orbit transfer and commissioning) witha goal of 10 years. The three spacecraftare injected into their individual heliocen-tric orbits via dedicated, jettisonable,chemical propulsion modules, an optionenabled by use of a Delta 4 or Atlas 5launch vehicle.

The primary engineering challenge is, ofcourse, to minimize technically-inducedaccelerations and laser phase fluctuations

within the measurement band. This trans-

lates into demanding requirements onthermal and thermoelastic stability in criti-cal payload elements, sensing of proof-mass relative position and attitude, thrus-ter noise handling and compensation forlocal gravity.

Decisive for LISAs success is the schemefor canceling inherent laser phase noise,since it is orders of magnitude larger thanthe expected gravitational-wave signal. This is accomplished by locking the in-stantaneous laser phase in each interfer-

ometer arm to the phase of the laser

measured at the start of the round trip(about 32 seconds earlier, or multiplesthereof), and by subsequent processing othe signals from all arms, either onboardor on the ground. Classical optical refer-ence cavities might be used to help suppress high-frequency laser phase noiseThis arm-locking scheme is undergoingdetailed modeling, to optimize the signalprocessing approached used for the datalinks and the laser assembly design.

A further technical challenge is caused bythe fact that the triangular constellationbreathes by about 601 within itsplane over each yearly period, due to or-bital distortions. The effect requires slowrelative pointing changes and also causesDoppler shifts of about 20 MHz along

the lines of sight, both features exhibitingsinusoidal patterns. The finite round-triptimes and directional change of the planevector relative to an inertial frame cause slowly, sinusoidally varying offset (pointahead) angle of 6 rad between transmiand received beams, perpendicular to the

constellation plane. This value alreadyexceeds the diffraction-limited beam di vergence of about 2.5 rad. All of theseeffects combine to require designs forsensing and actuating common-mode anddifferential line-of-sight directions, respectively, as well as a sophisticated laser fre-

quency map and mixing scheme.

The study has also uncovered a large clasof geometrical projection effects in thenear and far fields of the laser beams (alocal or remote spacecraft, respectively)that cause crosstalk between pointing jitterand piston/phase jitter that exceeds themeasurement tolerance. Locally, thebackwards-projected line of sight direc-tion that represents the measurement ref-erence (not the laser beam itself) has topass with tight tolerances through theassociated proof-mass center of mass

That holds for both the transmitted anreceived beams. The far-field effect wasformerly known as the flat-spot problem. More precisely, the laser beamsphase centers have to coincide with the

measurement reference point accuratelyenough to keep the crosstalk between thepointing jitter and the piston/phase jitter within acceptable limits. The tight tolerances for the beam alignment and verification are extremely challenging. Astriumproposes to utilize the very accurate in

8

theLISAnew

sletter2006May1 Status and progress of ESAs LISA mission

formulation studyat EADS Astrium

Ulrich Johann

EADS Astrium, Germany


9/12

stantaneous pointing knowledge providedby the differential wavefront sensing ofthe heterodyne phase-meters (200 prad) tocorrect the phase signal accordingly afterin-orbit calibration of the crosstalk func-tion.

Similar crosstalk effects are encounteredin the laser beam to proof-mass interac-tion (acting as active mirror), but can bemitigated again by precise knowledge ofproof-mass attitude and position relative

to the laser-beam fiducial points. Thisinformation is also provided by a dedi-cated differential wavefront-sensing read-out. Interestingly, the problem is alreadypresent in LISA Pathfinder, and it issolved here in the same way.

As part of the payload, a dedicated opticalassembly for each constellation arm isfeaturing, in rigid interconnection, a gravi-tational reference, an optical bench and a

telescope, operated as a monostatictransmit/receive antenna. The laser-link

budget requires a transmitted signal inexcess of 1W in order to have a receivedsignal with a residual power of the orderof 100 pW. Hence, stray light of thetransmitter is a challenge even for a het-erodyne detection scheme and much more

for the non-coherent CCD acquisition.Introducing a slight frequency offset be-tween transmit and local-oscillator laserbeams (laser swap) allows discriminationbetween the received beam and transmit-ter stray light in the heterodyne signal,and, in combination with a laser switching

scheme during acquisition and polariza-tion routing, a mitigation of the detrimen-tal effects. A detailed modeling to validate

the robustness of the scheme is presentlybeing developed.

Astrium has evolved the payload architec-ture considerably during the first part ofthe study in order to cope with these chal-lenges and to achieve a robust architectureof the payload with optimized mass and

power budgets and to define sound as-sembly and verification procedures. In thepresent baseline architecture, each space-craft carries a Y-shaped arrangement,

comprising two optical payload assembliesserving the two adjacent interferometerarms, respectively. The two optical assem-blies can be actuated as a whole with re-gard to their relative pointing within theplane of the constellation in order to copewith the breathing angle. The two adja-

cent laser assemblies are phase correlatedvia a backside fiber link and also servemutually on the other optical bench aslocal oscillators for the heterodyne phase

detection ofthe picowatti n c o m i n g beams. Payloadelectronics and

laser assembliesare distributedam o n g t h es p a c e c r a f tcompartments.

A 40-cm-aper-ture, baffled

Cassegrain tele-scope is iso-s t a t i c a l l y mounted to atitanium framethat carries on its back side an opticalbench made of ultra-low-expansion(ULE) glass and a gravitational referencesensor. All elements are thermally isolatedin order to secure thermoelastic stabilityand thermalization. The optical bench

oriented perpendicular to the telescopeaxis carries the transmitter and receiverinterferometer optics in polarization mul-tiplexing, the transmitter laser fiberlauncher, the acquisition sensor, the point-ahead actuator, the differential wavefrontsensing phase-meter head, and a local laser

phase correlator. Differently from all pre- vious architectures, it carries also a dedi-cated laser interferometer for precisionsensing of proof-mass axial position andlateral attitude.In summary, new payload features pro-posed and introduced in the study include: Two-step interferometry (strap-down

architecture) decoupling technically andfunctionally the gravitational-reference

optical readout (ORO) and the inter-spacecraft interferometry.

On-bench variable point-ahead actuatormechanism.

Frequency swap for both ORO andinter-spacecraft interferometer (usingsame laser sources).

Optical bench oriented perpendicularto telescope line of sight.

Open vacuum system throughout with

venting into space. Optomechanical configuration opti-

mized for actuation accommodation,coping with launch loads and providingthermoelastic stability.

In addition to the definition of the base-line payload architecture, Astrium has alsoproposed and preliminarily assessed some

innovative and exotic payload architec-tures, with the goal to further reduceoverall complexity and to improve budgets

and costs. One of these advanced concepts is characterized by a single opticabench and active gravitational referenceserving both adjacent interferometer armsvia two rigidly connected off-axis tele

scopes. The breathing-angle compensa

tion is accomplished by in-field-of-viewpointing actuation of the lasers lines ofsight. Therefore a dedicated actuationmechanism located on the optical bench isemployed in addition to the requiredpoint-ahead actuators. Technical chal

lenges here include pointing jitter of theactuation mechanism and monitoring andcalibration of the laser phase walk thaoccurs while changing the optical pathinside the optical assembly during re-pointing. Presumably, an internal lasermetrology truss derived from the existinginterferometry is required to accomplish

this task. The scheme is exploiting thetwo-step interferometry and employs a

dedicated full laser interferometer opticareadout of critical degrees of freedom ofthe proof mass. The single proof mass isstill cubic, but in free-fall in the lateraldegrees of freedom within the constellation plane.

Also, the option of a completely frespherical proof mass with full laser (opti

cal) readout has been investigated concep-tually. The spherical proof mass wouldrotate slowly and would not be spin con-

trolled, but allowed to tumble. Imperfec-tions in sphericity and density would becoped with by providing attitude informa-tion via a grid of tick marks etched onto

the surface and monitored by the lasereadout. Imperfections could be calibratedduring commissioning by mapping in ahigh-spin mode. However, at the presenstage of investigations, these novel payload concepts are not mature enough tochallenge the baseline configuration.

9

theLISAnew

sletter2006May1

LISA payload optical assembly configuration.


10/12

Measuring gravitational waves requires adrag-free environment with stringent,

high-resolution requirements on both thepointing and the translation of the space-

craft. Since the stability of the spacecraft

relates directly to the quality of the sci-ence measurements, the propulsion system

is a critical component. Keeping thespacecraft centered on the proof masses

requires microthrusters capable of balanc-ing the solar radiation pressure and other

disturbances, including small variations. With three sets of two operational thrus-ters distributed equally around the space-

craft, each LISA spacecraft requires thrustlevels between 530 micronewton (N) with a resolution of 0.1 N and a thrustnoise < 0.1 N/Hz in the LISA meas-

urement bandwidth (0.03mHz10Hz). Two microthruster technologies that canmeet the LISA requirements will be dem-

onstrated on LISA Pathfinder (LPF) andST7-DRS (the specific U.S. experiment on

LPF). The Colloid Micro-Newton Thrus-ter (CMNT) is being developed by BusekCo. in the U.S. for ST7-DRS, and the Ce-

sium Slit Field Emission Electric Propul-sion (FEEP) thruster is being developed

by ALTA in Italy for the LPF spacecraft.Although LPF and ST7 have similar per-

formance requirements, the LISA missionhas critical differences: a 25-fold increasein the lifetime and a tenfold increase in the

requirements for total impulse per thrus-ter. No single microthruster technology

has, to date, demon-strated all of these

requirements, espe-cially the long life-time, which is the

most challenging forLISA. Although

thrust range, noise,

and precision havebeen measured di-rectly for the CMNTand inferred from

current and voltagemeasurements of the

FEEP thrusters inthe laboratory, the

LISA requirement for55,000 hours of op-

eration (5 years

+25%) with 8.5 years of expendable pro-pellant has proven difficult to demon-

strate. In fact, a lifetime of greater than4000 hours has not yet been demonstrated

by any of the thruster technologies being

considered by NASA or ESA.Currently NASA is responsible for devel-

oping U.S. microthruster technologies, andESA is responsible for developing Euro-

pean microthruster technologies for LISA.In the US, we are focusing on further development of the Busek CMNT to meet

the LISA lifetime requirements. Thrusterlife will be determined by physics-based

models validated by laboratory experi-ments and short-term wear testing. This

methodology must be employed because itis impossible to demonstrate a 55,000-

hour lifetime by the end of the LISAtechnology development program withground tests alone. Multiple short-

duration (10004000 hr) tests are used toidentify failure mechanisms, and physics

models are developed for each failuremode. One long-duration lifetest (> 8000hr) will be conducted on a prototype

thruster prior to the LISA PreliminaryDesign Review (PDR) (late 2009) in order

to verify the models; this test might becontinued through to the Critical Design

Review (CDR). A second priority for U.S.efforts is to measure and understandproperties of the exhaust beam, including

contamination concerns and thrust noiseat low frequencies.

Development of the CMNT is currentlyfocused on long-duration testing and

flight hardware delivery by the end of thesummer in 2006. Three separate single-

emitter tests have all demonstrated the

required lifetime for ST7 (3300 hours ocontinuous operation) with no evidence o

significant emitter erosion, plugging, operformance degradation. Developmenta

multiple-emitter long-duration tests haveaccumulated over 1500 hours of total

operation, in the course of which we havesolved problems related to propellant contamination and cleanliness, bubble forma-

tion and blockage, and equal distributionof propellant between the emitters. A new

test is about to begin that will demonstratethe 3300-hour lifetime requirement for

ST7 using a complete single thruster system.

For LISA, we are focused on two critica

issues to extend the lifetime of theCMNT: emitter blockage or clogging, and

overspray of the exhaust beam onto theaccelerating electrodes. Although both othese issues appear to have been solved

for the ST7 lifetime requirements, theymay still be issues for LISA lifetime re

quirements. In 2006 our technology de-velopment program is focused on resolv

ing both these issues through minimachanges to the flight ST7 configurationUsing as much ST7 heritage as possible

this summer we will begin tests of sixdifferent emitter and electrode design

10

theLISA

newsletterMay1 An update on LISA micronewton thruster technology John Zieme


Left panel: a Field Emission Electric Propulsion Thruster (FEEP). Right panel: a Colloid Micro-

Newton Thruster (CMNT).


11/12

that could relieve the lifetime issues forLISA. These component-level tests will

last for at least 3000 hours to verify mod-els of emitter clogging and the exhaust-

beam particle trajectories. As describedearlier, the results and models will used todesign an engineering model of a thruster

head that will go into a system-level longduration test that will last for at least 8000

hours before the LISA PDR.Performance of the CMNT has beenmeasured on a thrust stand and shown to

meet thrust range, resolution, and noiserequirements down to 7 mHz. Below this

frequency, thrust-stand noise and driftdominate the measurement. However,

verified thrust models based on voltageand current measurements have shownthat the noise requirements can be met

down to 0.3 mHz. We continue to workon thrust measurements and control algo-

rithms for ST7 and expect that the LISArequirements will be demonstrated and

verified on orbit by the ST7 mission.Long-duration thrust noise measurements will be part of the LISA microthruster

technology development program.

The plume of the CMNT has been inves-

tigated using various plasma diagnosticsincluding measurements of current den-sity, plasma potential, and mass deposi-

tion, as well as droplet energy, mass, andcharge throughout the exhaust beam. All

of these results have been used to gener-ate verified models of droplet and ion

formation at the tip of the emitter andpropagation downstream. The models willbe used to design new electrode geome-

tries and predict beam spread as a func-tion of current and thrust, to insure that

the plume does not impinge on the space-craft. We are also examining interactions

of the positively charged beam with theelectrons from the cathode neutralizer. The measurements have shown that no

charged particles exit the thruster beyonda 35-degree half-angle at the maximum-

divergence operating condition set by elec-trode geometry. Measurements have

shown that no measurable mass deposi-tion occurs outside of a 45-degree half-angle, and more detailed measurements at

various angles will begin soon. Busek hadeveloped a carbon nano-tube field

emission cathode neutralizer that hasdemonstrated over 13,000 hours of con

tinuous operation at higher-than-requiredcurrent levels. This neutralizer may noeven be required for the colloid mi

crothruster technology, since electronphotoemission from the spacecrafts solar

panels might provide enough low-energyelectrons to maintain the spacecraft poten

tial at a low level.

For the LISA microthruster technologydevelopment program, our focus is on

thruster lifetime, performance, and spacecraft interactions. We will continue to

work closely with the ST7 CMNT development, relying on the flight heritage andexperience to reduce risk and cost for

LISA. Our efforts in the next two yearwill focus on extending the lifetime of the

ST7 design through better understandingthe of the thruster operation, verified

models of thruster performance and lifetime limiting mechanisms, and multiplelong-duration tests.

Forthcoming LISA meetings

48 June, 2006 Calgary, Alberta

208th Meeting of the American Astronomical Society www.aas.org/meetings

516 June 2006 South Padre Island, Texas

3rd GW Astronomy Summer School cgwa.phys.utb.edu/Events/SummerSchool.php

1718 June 2006 Goddard Space Flight CenterLIST Working Groups, LIST Meeting www.srl.caltech.edu/lisa

1925 June 2006 Goddard Space Flight Center6th International LISA Symposium (LISA 6) lisa6.gsfc.nasa.gov

914 July 2006 Santa Fe, New Mexico

Physics and Astrophysics of Supermassive BHs qso.lanl.gov/meetings/meet2006

1623 July 2006 Beijing, China

36th COSPAR Scientific Assembly meetings.copernicus.org/cospar2006

2329 July 2006 Berlin, Germany

11th Marcel Grossman Meeting www.icra.it/MG/mg11

47 October 2006 San Francisco, California

HEAD 2006 www.confcon.com/head2006

2125 August 2006 Prague, Czechoslovakia

IAU Symposium 238: Black Holes from Stars to Galaxies astro.cas.cz/iaus238

1115 December 2006 Melbourne, Australia

23rd Texas Symposium on Relativistic Astrophysics www.texas06.com

1821 December 2006 Albert Einstein Institute, Golm, Germany

GWDAW-11 gwdaw11.aei.mpg.de

Fore more LISA information:

lisa.nasa.gov

lisa.esa.int

For technical and scientific details:

www.srl.caltech.edu/lisa

This Newsletter was published by theU.S. LISA Mission Science Office at

the Jet Propulsion Laboratory, undercontract with the National Aeronautics

and Space Administration.

Issue 2006-1 was edited by Bonny

Schumaker, Michele Vallisneri, KarenWillacy, and Teviet Creighton (JPL).

Figure credits:

NASA (cover, this page);

Teviet Creighton, JPL (p. 3);

Stefano Vitale, Univ. of Trento, Italy(figs. 1, 2, 3 on pp. 4 and 5);

Carlo Gavazzi Space (fig. 4, p. 5);

Albert Einstein Institute & Eads

Astrium GmbH (fig. 5, p. 5);

Frans Pretorius, University of Alberta

(fig. 1, p. 6);

Manuela Campanelli, Univ. of Texas,

Brownsville (fig. 2, p. 6);

John Baker, GSFC (p. 7);

Astrium GmbH (p. 9);

John Ziemer, JPL (p. 10).

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theLISAnew

sletter2006May1
http://cgwa.phys.utb.edu/Events/SummerSchool.phphttp://cgwa.phys.utb.edu/Events/SummerSchool.phphttp://gwdaw11.aei.mpg.de/http://www.texas06.com/http://astro.cas.cz/iaus238http://astro.cas.cz/iaus238http://www.confcon.com/head2006http://www.confcon.com/head2006http://www.icra.it/MG/mg11http://meetings.copernicus.org/cospar2006http://meetings.copernicus.org/cospar2006http://qso.lanl.gov/meetings/meet2006http://qso.lanl.gov/meetings/meet2006http://lisa6.gsfc.nasa.gov/http://lisa6.gsfc.nasa.gov/http://www.srl.caltech.edu/lisahttp://cgwa.phys.utb.edu/Events/SummerSchool.phphttp://cgwa.phys.utb.edu/Events/SummerSchool.phphttp://www.aas.org/meetingshttp://www.srl.caltech.edu/lisahttp://lisa.esa.int/http://lisa.nasa.gov/http://www.srl.caltech.edu/lisahttp://www.srl.caltech.edu/lisahttp://lisa.esa.int/http://lisa.esa.int/http://lisa.nasa.gov/http://lisa.nasa.gov/http://gwdaw11.aei.mpg.de/http://gwdaw11.aei.mpg.de/http://www.texas06.com/http://www.texas06.com/http://astro.cas.cz/iaus238http://astro.cas.cz/iaus238http://www.confcon.com/head2006http://www.confcon.com/head2006http://www.icra.it/MG/mg11http://www.icra.it/MG/mg11http://meetings.copernicus.org/cospar2006http://meetings.copernicus.org/cospar2006http://qso.lanl.gov/meetings/meet2006http://qso.lanl.gov/meetings/meet2006http://lisa6.gsfc.nasa.gov/http://lisa6.gsfc.nasa.gov/http://www.srl.caltech.edu/lisahttp://www.srl.caltech.edu/lisahttp://cgwa.phys.utb.edu/Events/SummerSchool.phphttp://cgwa.phys.utb.edu/Events/SummerSchool.phphttp://www.aas.org/meetingshttp://www.aas.org/meetings


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National Aeronautics and Space Administration

Jet Propulsion Laboratory Goddard Space Flight CenterCalifornia Institute of Technology 8800 Greenbelt Rd

4800 Oak Grove Dr Greenbelt MD 20771

Pasadena CA 91106

www.nasa.gov

in partnership with

European Space Agency

European Space Research & Technology Center

Keplerlaan 1, Postbus 299

2200 AB Noordwijk, The Netherlands

www.esa.int

JPL 410-70 Issue 1 05/06

the LISAnewsletter2006 May 1Issue 2006-1

theLISAnewsletter

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4800OakGroveDr.

PasadenaCA91106
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