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PlanckPlanckPlanned achievements: most detailed observations to date of Cosmic Microwave

BackgroundLaunch date: planned for February 2007 (with Herschel)Mission end: after 15 months of observationsLaunch vehicle/site: Ariane-5 from Kourou, French GuianaLaunch mass: about 1430 kg (445 kg science payload)Orbit: planned Lissajous orbit around L2 Lagrangian point, 1.5 million km from

Earth in anti-Sun direction. 4-month transfer from EarthPrincipal contractors: Alcatel Space Industries (prime, Payload Module, AIV), Alenia

Spazio (Service Module). Phase-B April 2001 - June 2002; Phase-C/D July2002 - September 2006, CDR November 2003

Planck is designed to help answer keyquestions: how did the Universe cometo be and how will it evolve? To dothis, it will map with the highestaccuracy yet the first light that filledthe Universe after the Big Bang. Itstelescope will focus radiation from thesky onto two arrays of highlysensitive radio detectors. Together,they will measure the temperature ofthe Cosmic Microwave Background(CMB) radiation over the whole sky,searching for regions slightly warmeror colder than the average 2.73 K.

300 000 years after the Big Bang, theUniverse was 1000 times smallerthan now and had cooled to 3000 K.This was cold enough for hydrogenatoms to form, so light and matternow existed independently and lightcould travel freely for the first time.CMB radiation is that ‘first light’, afossil light carrying information bothabout the past and the future ofthe Universe. This background glowwas discovered in 1964. Athousand million years after theBig Bang, the Universe was a fifthof its present size and stars andgalaxies already existed. Theyformed as matter accreted aroundprimaeval dense ‘clots’ that werepresent in the early Universe andthat left their imprint in theradiation, at the period when lightand matter were still closelycoupled. Today, the fingerprints of

these clots are detected as very slightdifferences – sometimes as small asone in a million – in the apparenttemperature of the CMB. All of thevaluable information lies in theprecise shape and intensity of thesetemperature variations. In 1992,NASA’s COBE satellite made the firstblurry maps of these anisotropies inthe CMB. Planck will map thesefeatures as fully and accurately aspossible.

The anisotropies hold the answers tomany key questions in cosmology.Some refer to the past of theUniverse, such as what triggered theBig Bang, and how long ago ithappened. Others look deep into thefuture. What is the density of matterin the Universe and what is the truenature of this matter? Theseparameters will tell us if the Universe

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will continue its expansion forever orif it will eventually collapse on itself.

Another question is the existence of a‘dark energy’ that might exist in largequantities in our Universe, asindicated by recent experiments. Is itreally there? If so, what are itseffects? Planck will shed light onthese issues, because it will be themost powerful tool yet for analysingthe CMB anisotropies.

Planck’s instruments will focus onmicrowaves with wavelengths of0.3 mm to 1 cm. This wide coveragesolves a major challenge: todistinguish between the actual CMBand the many other undesired signalsthat introduce spurious noise. Manyother objects, such as our ownGalaxy, radiate at the samewavelengths as the CMB itself. These

confusing signals have to be mappedand finally removed from themeasurements. This means thatmany of Planck’s wavelengthchannels are dedicated to measuringsignals other than the CMB. Thesemeasurements in turn will generate awealth of information on the dust andgas in our Galaxy and the propertiesof other galaxies. The Sunyaev-Zeldovich effect (a distortion of theCMB by the hot gas in galacticclusters) will be measured forthousands of clusters.

The detectors must be very cold sothat their own emissions do notswamp the signal from the sky.Some will be cooled down to about20 K and some to 0.1 K. Planck willrotate slowly and sweep a largeswath of the sky each minute. Inabout 15 months, it will havecovered the sky fully, twice over. Itwill operate completelyautonomously and dump the storeddata each day to Earth.

A call for ideas for ESA’s M3 thirdMedium-class science mission for

http://sci.esa.int/planck/

Planck will provideastronomers with a windowback to the early Universe.

Bottom of page: simulations ofobservations of the CMB show thedramatic improvement that can beachieved by increasing the angularresolution from the level of theCOBE satellite (left panel) to the5-10 arcmin of Planck (right panel).

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merged mission with Herschel,operating the instruments in turn; adedicated satellite launched intandem with Herschel. The last,‘carrier’, option was selected by theSPC in May 1998 for a 2007launch.

The Announcement of Opportunityfor the instruments was made inOctober 1997. In September 2000,ESA issued a joint Herschel-PlanckInvitation to Tender to industry forbuilding both spacecraft. Theresponses were submitted by earlyDecember 2000 and Alcatel SpaceIndustries was selected 14 March2001 as the prime contractor for thelargest space science contract yetawarded by ESA: €369 million,signed in June 2001. Phase-Bbegan early April 2001.

Planck’s scientific payload

Low Frequency Instrument (LFI)

LFI’s 56 High Electron Mobility Detectors (HEMTs), fed by a ring of corrugated horns, are cooled to20 K by H2 sorption cooler. PI: Reno Mandolesi, Istituto di Tecnologie e Studio delle RadiazioniExtraterrestri (CNR), Bologna (I).

High Frequency Instrument (HFI)

HFI’s 50 bolometers, fed by corrugated horns and filters, are cooled to 0.1 K by a combination of a20 K sorption cooler, a 4 K Joule-Thompson mechanical cooler and an open-cycle dilutionrefrigerator. PI: Jean-Loup Puget, Institut d’Astrophysique Spatiale (CNRS), Orsay (F).

Telescope

1.5 m-diameter telescope, 8°-FOV, 1.8 m focal length, is offset by 85° from Planck’s spin axis toscan the sky. Both reflectors are CFRP. LFI/HFI occupy the focal plane, with HFI’s detectors in thecentre surrounded by LFI’s in a ring. PI: Hans Ulrik Nørgaard-Nielsen, Danish Space ResearchInstitute. Copenhagen (DK).

Centre Frequency (GHz) 100 143 217 353 545 857Number of Detectors 4 12 12 6 8 6Bandwidth (∆ν/ν) 0.25 0.25 0.25 0.25 0.25 0.25Angular Resolution (arcmin) 10.7 8.0 5.5 5.0 5.0 5.0Average ∆T/T per pixel 1.7 2.0 4.3 14.4 147.0 6670(12 months, 1σ, 10-6 units)Sensitive to linear polarisation? no yes yes no yes no

Centre Frequency (GHz) 30 44 70 100Number of Detectors 4 6 12 34Bandwidth (∆ν/ν) 0.2 0.2 0.2 0.2Angular Resolution (arcmin) 33 23 14 10Average ∆T/T per pixel 1.6 2.4 3.6 4.3(12 months, 1σ, 10-6 units)Sensitive to linear polarisation? yes yes yes yes

launch in 2003 was issued to thescientific community in November1992. Assessment studies began inOctober 1993 on six, including whatwas then called COBRAS/SAMBA(Cosmic Background RadiationAnisotropy Satellite/Satellite forMeasurement of BackgroundAnisotropies, originally two separateproposals). The studies werecompleted in April 1994 for four tocontinue with Phase-A studies untilApril 1996. COBRAS/SAMBA wasselected as M3 by ESA’s ScienceProgramme Committee in June 1996.However, the SPC in February 1996had already ordered a reduction inthe cost of new missions, so startingin 1997 several studies looked at howM3 could be implemented morecheaply. Three scenarios wereconsidered: a dedicated satellite; a

Power system: solar array mountedon Earth/Sun-facing thermal shield,GaAs cells provide 1664 W EOL,supported by 2x36 Ah Li-ionbatteries.

Communications: data rate 100 kbit/s30 W X-band from single 25 Gbitsolid-state recorder to ESA’s Perth(Australia) station; 3-hour data dumpdaily. Controlled from MissionOperations Centre (MOC) at ESOC;science data provided to the two PIData Processing Centres.

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Satellite configuration: octagonalservice module with a sunshieldcarrying solar array. CFRP centralcone, 8 CFRP shear webs, CFRPupper/lower platforms. Payloadmodule houses telescope, twoscience instruments and theircoolers.

Attitude/orbit control: spin-stabilisedat 1 rpm about longitudinal axis toscan telescope across sky. Pointingerror 16.9 arcsec. ERC 32-basedattitude control computer; starmapper, 3x2-axis Sun sensors, 2x3-axis quartz rate sensors. Redundantsets of 6x10 N + 2x1 N thrusters;3x135 kg hydrazine tanks.

Primary Mirror

Service Module

Straylight Shield

Service Module Shield

Solar Array

The COBE satellite in 1992 showed that the CosmicMicrowave Background varies over the sky. Onscales of larger than 10°, the CMB temperaturevaries by about one part in 100 000 from theaverage 2.73 K. Planck will produce a far moredetailed map.

LFI is an array of 56 tunedradio receivers using

High ElectronMobility Transistors(HEMTs) cooled to

20 K to map at fourwavelengths of 3 mm

to 1 cm. HFI is also visible,inserted in the centre of the

LFI ring of horns.

HFI is an array of 48 bolometers cooled to 0.1 K tomap at six wavelengths of 0.3-3 mm.

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HerschelHerschelPlanned achievements: first spaceborne observations at 100-600 µm, largest

telescope ever launched (3.5 m diameter mirror)Launch date: planned for February 2007 (with Planck)Mission end: minimum of 3 years of observationsLaunch vehicle/site: Ariane-5 from Kourou, French GuianaLaunch mass: about 2970 kg (science payload 415 kg)Orbit: planned 350 000 km halo orbit around L2 Lagrangian point, 1.5 million km

from Earth in anti-Sun direction. 4-month transfer from EarthPrincipal contractors: Alcatel Space Industries (prime), Astrium GmbH (AIV,

cryostat), Alenia Spazio (Service Module). Phase-B April 2001 - June 2002;Phase-C/D July 2002 - September 2006, CDR November 2003

ESA’s Herschel Space Observatorywill be the first astronomical satelliteto study the cold Universe at far-infrared and submillimetrewavelengths. Its main goal is to lookat the origins of stars and galaxies,reaching back to when the Universewas only a third of its present age.When and how did galaxies form? Didthey all form at about the same time?Were the first galaxies like those wesee now? Did the stars form first andthen congregate as galaxies? As wellas peering at the distant past,Herschel will also see into the heartsof today’s dust clouds as theycollapse to create stars and planets.

Objects at 5-50 K radiate mostly inHerschel’s wavelength range of 60-670 µm, and gases between 10 K anda few hundred K have their brightestmolecular and emission lines there.Broadband thermal radiation fromsmall dust grains is the commonestcontinuum emission process accrossthis band.

The Universe was probably alreadydusty as the first galaxies formed andother telescopes cannot penetrate theveil. That epoch has therefore so farremained a ‘dark age' forastronomers, although pioneeringinfrared satellites, such as ESA’sInfrared Space Observatory (ISO),have helped to outline a generalscenario. Sometime after the Big

Bang, the first stars formed, possiblyin small clusters. With time, theymerged and grew, and theaccumulation of matter triggered theformation of more stars. These starsproduced dust, which was recycled tomake more stars. By then, the firstgalaxies were already in place, andthey also merged to form largersystems. These galactic collisionstriggered an intense formation ofstars in the Universe. Herschel willsee the emission from dustilluminated by the first big star-bursts in the history of the Universe.

Herschel will also show us new starsforming within their thick cocoons ofdust. Gravity squeezes gas and dusttowards the centre, while cooling

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Infrared-bright regions inthe Trifid Nebula revealdense clouds of cooldust that may harbourforming stars.(ESA/ISOCAM &J. Cernicharo et al.)

mechanisms keep the system at verylow temperatures to avoid a quickcollapse and a premature death ofthe embryonic star. The dust cocoonsand the 13 K temperatures make thepre-star cores invisible to all butradio and infrared telescopes. Theearliest stages of star-birth are thuspoorly known, even though ISOunveiled more than a dozen cocoons.

After starbirth, leftover gas and dustremain swirling around the youngstar, forming a protoplanetary disc.The dust grains are the seeds offuture planets. Once the newplanetary system is formed, only athin ring of debris remains. The discsand debris rings are favourite targetsfor infrared space telescopes. ISO

showed that planets beyond our SolarSystem are common. Almost allyoung stars are surrounded by a thindisc of debris, in which the planet-making process is not completelyfinished, and small bodies likecomets are still very conspicuous.Herschel will shed light on all ofthese theories.

The Solar System was formed 4500million years ago, out of the sameraw material that about 500 millionyears earlier had served to build theSun itself. To help reconstruct thatformation, Herschel will study indetail the chemical composition of theplanets’ atmospheres and surfaces,and especially the chemicalcomposition of comets. Comets are

http://sci.esa.int/herschel/

Herschel: the main science goals

Deep extragalactic broadband photometricsurveys in Herschel’s 100-600 mm primewavelength band for detailed investigation ofthe formation and evolution of galaxy bulgesand elliptical galaxies in the first third of theage of the Universe

Follow-up spectroscopy of interesting objectsdiscovered in the survey. The far-IR/sub-mmband contains the brightest cooling lines ofinterstellar gas, which provides veryimportant information on the physicalprocesses and energy production in galaxies

Detailed studies of the physics and chemistryof the interstellar medium in galaxies, both inour own Galaxy as well as in externalgalaxies, by photometric & spectroscopicsurveys and detailed observations. Includesthe important question of how stars form outof molecular clouds

The chemistry of gas and dust to understandthe stellar/interstellar lifecycle and toinvestigate the physical and chemicalprocesses involved in star formation andearly stellar evolution in our Galaxy. Herschelwill provide unique information on mostphases of this lifecycle

High-resolution spectroscopy of comets andthe atmospheres of the cold outer planetsand their moons

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environments. Most molecules showtheir unmistakable signatures atinfrared and submillimetrewavelengths, which makes Herschelan ideal tool to detect them. It willstudy the chemistry of many regionsin the Universe, from the stars andtheir environments to other galaxies.It will observe objects as chemicallyrich as the molecular clouds in theinterstellar medium, where nearly ahundred different molecules – manyof which were detected in space evenbefore they were ever seen inlaboratories – have been discovered.Herschel will provide a much betterunderstanding of the chemistry of theUniverse.

Herschel’s silicon carbide primarymirror will be the largest ever builtfor a space telescope. It is atechnological challenge: it must bevery light, withstand the extreme coldof space and have a surface accurateto 10-6 m. The infrared detectors ofthe three instruments must be cooler

Herschel’s scientific payload

Photodetector Array Camera & Spectrometer (PACS)

PACS is a camera and low- to medium-resolution spectrometer for 60-210 µm. Two 16×25 Ge:Gaand two bolometer detector arrays cover: 60-90 & 90-130 µm (‘blue’; 3.4 arcsec pixels) and 130-210 µm (‘red’; 6.8 arcsec pixels). As a photometer, images a 1.75×3.5 arcmin FOV simultaneously in‘red’ and one ‘blue’ band. As a spectrometer, covers all three bands with 50×50 arcsec FOV, with150-200 km-1 velocity resolution and instantaneous coverage of 1500 km-1. PI: Albrecht Poglitsch,MPE Garching (D).

Spectral & Photometric Imaging Receiver (SPIRE)

SPIRE is an imaging photometer and low- to medium-resolution imaging Fourier TransformSpectrometer (FTS) for 200-610 µm. Both use bolometer detector arrays: three dedicated tophotometry and two for spectroscopy. As a photometer, it covers a large 4×8 arcmin field of view thatis imaged in three bands (centred on 250, 350, 500 µm) simultaneously. PI: M. Griffin, Queen Mary& Westfield College, London (UK).

Heterodyne Instrument for FIRST

HIFI is a heterodyne spectrometer offering very high velocity-resolution (0.3-300 km-1), combinedwith low-noise detection using superconductor-insulator-superconductor (SIS; bands 1-5 500-1250 GHz) and hot electron bolometer (HEB; bands 6-7, 1410-1910 & 2400-2700 GHz) mixers, for asingle pixel on the sky (imaging by raster mapping or continuous slow scanning). PI: Th. de Graauw,Space Research Organisation Netherlands, Groningen (NL).

the best ‘fossils’ of the earliest SolarSystem. They are made of pristinematerial from that primaeval cloud,including water-ice. They may alsosolve the question of the origin ofEarth’s oceans. Most of Earth’s watermay have come from impactingcomets during the early SolarSystem. Herschel’s spectrographshave unprecedented sensitivity toanalyse the chemical composition ofSolar System bodies, especially withrespect to water. If cometary waterhas the same signature as Earth’s,then the link is confirmed.

Huge amounts of water, and verycomplex molecules of carbon – themost basic building blocks for life –have been detected in the materialsurrounding stars. All living systems,including humans, are literally‘stardust’. Stars are the chemicalfactories of the Universe: mostchemical elements are made in theircores, and many chemicalcompounds are produced in the stars’

cooler for 0.3 K bolometeroperating temperature)and supports thetelescope and somepayload-associatedequipment. The servicemodule (SVM; commonwith Planck) belowprovides theinfrastructure and housesthe ‘warm’ payloadequipment. Ritchey-Chretien telescope with3.5 m-diameter CFRPsegmented primary mirrorwith wavefront error of6 µm, feeding Zerodursecondary. Sunshadeallows mirror to cool to80 K. The Planck matingadapter remains attachedto Herschel.

Attitude/orbit control: 3-axis pointing to2.12 arcsec. ERC 32-based attitude control; 2star trackers, 4-axis gyro,2x2-axis Sun sensors,2x3-axis quartz rate

sensors, 4 skewed RWs. Redundantsets of 6x10 N thrusters; 2x135 kghydrazine tanks.

Power system: solar array mounted onthermal shield, GaAs cells provide1450 W EOL, supported by 2x36 AhLi-ion batteries.

Communications: data rate max.1.5 Mbit/s (25 Gbit solid-statememory) to Perth (Australia).Controlled from Mission OperationsCentre (MOC) at ESOC; science datareturned to Herschel Science Centre(HSC) at Villafranca (Spain) fordistribution to the three InstrumentControl Centres (ICCs). The USHerschel Science Center at JPL servesUS astronomers. The L2 orbit allowscontinuous observations andoperations.

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than the radiation theyare to measure. Thetelescope itself is cooledpassively to 80 K, andparts of all threeinstruments will be keptat 1.65 K in a 2160-litrecryostat filled withsuperfluid helium. TheSPIRE and PACSbolometer detectors willbe cooled to 0.3 K.Herschel’s observationswill end when the cryogenis exhausted.

FIRST was one of theoriginal four Cornerstonemissions of theHorizon 2000 scienceplan; it was selected asCornerstone 4 inNovember 1993 by theAgency’s ScienceProgramme Committee.The Announcement ofOpportunity for thescience instruments wasreleased in October 1997;the SPC made itsselection in May 1998. Almost 40institutes are involved in developingthe three instruments. In September2000, ESA issued a joint Herschel-Planck Invitation to Tender toindustry for building both spacecraft.The responses were submitted byearly December 2000 and AlcatelSpace Industries was selected14 March 2001 as the primecontractor for the largest spacescience contract yet awarded by ESA:€369 million, signed in June 2001.Phase-B began early April 2001.

Satellite configuration: 9 m high,4.5 m wide. Payload module (PLM),based on ISO’s superfluid heliumcryostat technology, houses theoptical bench with the instrumentfocal plane units (SPIRE and PACSeach carry an internal 3He sorption

The observatory’s name,announced in December 2000,

commemmorates Anglo-Germanastronomer William Herschel,

who discovered infrared light in1800. The mission was previously

known as the Far Infrared andSubmillimetre Telescope (FIRST).

Herschel (upper) in launchconfiguration with Planck (lower).(Alcatel Space)

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ADM-AeolusADM-AeolusPlanned achievements: the first direct global wind profile measurements

throughout the atmosphereLaunch date: planned for July 2007Mission end: after 3 years (1-year extension possible)Launch vehicle/site: to be selected (Rockot-class)Launch mass: about 800 kg (300 kg payload)Orbit: planned 408 km circular, 96.99° Sun-synchronous (18:00 local time

ascending node)Principal contractors: spacecraft prime to be selected January 2002; Phase-A June

1998 - June 1999; Phase-B April 2002 - July 2003; Phase-C/D July 2003 -July 2007

The Atmospheric Dynamics Explorer(ADM) will, for the first time, directlymeasure wind speeds throughout thedepth of the atmosphere – a notabledeficiency of current observingsystems. This will improve ourunderstanding of atmosphericprocesses for climate studies,particularly in the tropical regions, aswell as improving the numericalmodels used in weather forecasting.Global atmospheric transport as wellas the global transfer of energy,water, aerosols and chemicals willbecome better understood. This morecomplete 3-D picture of theatmosphere will:

– increase the accuracy ofatmospheric process parameters inmodels;

– advance climate and atmosphere-flow modelling;

– provide better initial conditions forweather forecasting.

ADM was selected in 1999 as thesecond Earth Explorer Core missionin the Living Planet programme. Fourcandidate Core missions wereselected in May 1996 for 12-monthPhase-A studies, completed in June1999. GOCE and ADM were selectedin November 1999 for Phase-B.

The heart of ADM is the ALADIN(Atmospheric Laser DopplerInstrument) lidar emitting ultraviolet

pulses. The backscatter from theatmosphere and the Earth’s surfaceare analysed to measure cross-trackwind speeds in slices up to 30 kmaltitude. The lidar exploits theDoppler shifts from the reflectingaerosols and molecules transportedby the wind. In addition, informationcan be extracted on cloud cover andaerosol content. UV radiation isheavily attenuated by cloud, so acomplete wind profile can be derivedonly in a clear or partly cloud-freeatmosphere through cloud gaps. In

This entry reflects the Phase-A configuration

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an overcast sky, wind profiles can bederived for the layers above theclouds.

Satellite configuration (Phase-A):conventional box-shaped bus withfour side walls attached to centralthrust cone by flat shear walls.Electronics mounted on ±X walls(acting as radiators) that also carrythe solar wings. Lidar mounted oncone by three bipods.

Attitude/orbit control: the dawn-duskorbit means that the +Y axis alwaysfaces away from the Sun, and thesolar wings can be fixed ±Xaft/forward along the line of flight.Attitude control to 0.52 mrad by four10 Nms reaction wheels & three100 Am2 magnetorquers. Attitudeinformation from two star trackers,gyros, GPS receiver & two 3-axismagnetometers. Four 1 N blowdownthrusters for orbit adjust; 60 kghydrazine in central sphere.

Thermal: instrument dissipation of300 W via 3 L-shaped heat pipes to+Y face to deep space. 215 W fromelectronics boxes mounted on ±Xwalls.

Power system: two 3-panel wings(total 8.4 m2) of Si-BSFR cellsproviding 920 W EOL (560 W

required), supported by 18 Ah NiCdbattery. System design driven bylaser pulse power.

Communications: raw datadownlinked at 3.5 Mbit/s on 5.6 WSSPA L-band to at least two groundstations. 2 kbit/s TC & 64 kbit/s TMS-band. Antennas mounted on lidarbaffle. ADM controlled from ESOC viaKiruna. ESA will process & calibratethe data before sending it to adedicated science data centre, whichwill be responsible for quality controland dissemination within 3 h ofobservation to meteorological centresand other users.

http://www.estec.esa.nl/explorer

ADM-Aeolus Payload

ALADIN

The Atmospheric Laser Doppler Instrument(ALADIN) uses a 1 m2 main Cassegrainmirror, surrounded by a 1.2 m-dia baffle.Diode-pumped Nd:YAG laser generates130 mJ (150 mJ goal) 100 Hz pulses for 7 s(+1 s warm-up). ALADIN aims 35° off-nadirand at 90° to the flight direction to avoidDoppler shift from ADM’s own velocity. Ameasurement is made every 200 km over alength of 50 km (integrated from 3.5 kmsteps; 1 km steps possible) in 7 s. A windaccuracy of 2-3 m/s is required, for verticalsteps selectable 0.5-2 km. Raw data aredownlinked for ground processing.

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BepiColomboBepiColomboPlanned achievements: first dual Mercury orbiters; first Mercury lander; third

mission to Mercury; first European deep-space probe using electric propulsionLaunch date: planned August 2009 (arriving October 2012)Mission end: nominally after 1 year in orbit around Mercury (lander 1 week)Launch vehicle/site: two Soyuz-Fregats from Baikonur Cosmodrome, KazakhstanLaunch mass: about 1500 kg (1100 kg delivered to Mercury)Orbit: heliocentric transfer to 400x11800 km (MMO) and 1500 km (MPO) polar

Mercury orbitsPrincipal contractors: definition study Alenia Spazio (I) & Astrium GmbH (D)

As the nearest planet to the Sun,Mercury has an important role inlearning how planets form. Mercury,Venus, Earth and Mars make up thefamily of terrestrial planets, eachcarrying information that is essentialfor tracing the history of the wholegroup. Knowledge about their originand evolution is a key tounderstanding how conditionssupporting life arose in the SolarSystem, and possibly elsewhere. Aslong as Earth-like planets orbitingother stars remain inaccessible toastronomers, the Solar System is theonly laboratory where we can testmodels applicable to other planetarysystems. The exploration of Mercuryis therefore of fundamentalimportance for answering questionsof astrophysical and philosophical

significance, such as ‘Are Earth-like planets common in the

Galaxy?’

A Mercury missionwas proposed in May1993 for the M3selection (eventuallywon by Planck).

Although theassessment studyshowed it to be toocostly for a medium-

class mission, it wasviewed so positively byESA that, when theHorizon 2000 scienceprogramme was

extended in 1994 with

Horizon 2000 Plus, the three newCornerstones included a Mercuryorbiter. GAIA competed in 2000 withBepiColombo for the fifthCornerstone slot. In October 2000,the Science Programme Committeeapproved a package of missions for2008-2013: BepiColombo wasselected as CS-5 (2009) and GAIA asCS-6 (2012). The SPC in September1999 named the mission in honourof Giuseppe (Bepi) Colombo (1920-1984). The Italian scientist explainedMercury’s peculiar rotation - it turnsthree times for every two circuitsaround the Sun – and suggested toNASA that an appropriate orbit forMariner-10 would allow severalflybys in 1974-75.

That US probe remains the onlyvisitor to Mercury, so there aremany questions to be answered,including:

– what will be found on the unseenhemisphere?

– how did the planet evolvegeologically?

– why is Mercury’s density so high?– what is its internal structure and

is there a liquid outer core?– what is the origin of Mercury’s

magnetic field?– what is the chemical composition

of the surface?– is there any water ice in the polar

regions?– which volatile materials form the

vestigial atmosphere (exosphere)?

MMO

MSE

CPM

The mission configuration described herereflects the October 2000 baseline. Alternative

concepts, such as a single spacecraft-composite launch on an Ariane-5, are

possible.

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The pre-definition study design for BepiColombo. In the foreground is MPO, withthe Solar Electric Propulsion Module stillattached via the conical interface that housesthe Chemical Propulsion Module. In thebackground is the MMO/MSE composite.

– how does the planet’smagnetic field interact withthe solar wind?

BepiColombo’s other objectives gobeyond the exploration of the planetand its environment, to takeadvantage of Mercury’s proximity tothe Sun:

– fundamental science: is Einstein’stheory of gravity correct?

– impact threat: what asteroids lurkon the sunward side of the Earth?

A 1-year System and TechnologyStudy completed in April 1999revealed that the best way to fulfil thescientific goals is to fly two Orbitersand a Lander:

– the Mercury Planetary Orbiter(MPO), a 3-axis stabilised andnadir-pointing module in a loworbit for planet-wide remotesensing, radio science and asteroidobservations;

– the Mercury MagnetosphericOrbiter (MMO), a spinner in aneccentric orbit, accommodatingmostly the field, wave and particleinstruments;

– the Mercury Surface Element(MSE) lander, for in situ physical,optical, chemical and mineralogicalobservations that also provideground-truth for the remote-sensing measurements.

How to deliver these elements to their

destinations emerged froma trade-off between mission

cost and launch flexibility. Itcombines electrical propulsion,

chemical propulsion and planetarygravity assists. Interplanetarytransfer is performed by a SolarElectric Propulsion Module (SEPM),jettisoned upon arrival. The orbitinjection manoeuvres then use aChemical Propulsion Module (CPM),which is also jettisoned once thedeployment of the spacecraftelements, MSE included, iscompleted.

A two-launch scenario, with thespacecraft elements divided into twocomposites using near-identicalpropulsion elements, is consideredas the baseline: the MPO and MMO-MSE composites are launched bytwo Soyuz-Fregats to reach Mercuryin 3.5 years. The spacecraft conceptis modular, however, and lends itselfto a variety of schemes compatiblewith the mission objectives.

Two competing industrial definitionstudies are being conducted fromMay 2001 to late 2002 to prepare forthe 1-year Phase-B to begin by mid-2003 and Phase-C/D in 2004.Japan’s ISAS space agency willprovide the MMO element. AnAnnouncement of Opportunity forthe MPO/MSE scientific payloadswill be made by mid-2002. ISAS’own AO for MMO may be slightlylater.

http://sci.esa.int/bepicolombo

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BepiColombo Scientific Instruments

Instrument Measurements Mass Power(kg) (W)

MPONarrow-angle camera NAC imaging, 350-1050 nm, resolution up to 10 m 12 16Wide-angle camera WAC imaging, 350-1050 nm, 200 m-resolution global map

IR spectrometer IRS mineralogical mapping, up to 150 m/pixel, 0.8-2.8 µm, 6 10128 channels

UV spectrometer ALI UV photometry, 70-330 nm, Al, S, Na & OH in exosphere 3.5 3

X-ray spectrometer MXS 0.5-10 keV, surface mineral composition 4.5 8

Gamma-ray spectrometer MGS 0.1-8 MeV, surface elemental composition 7.5 5

Neutron spectrometer MNS 0.01-5 MeV, surface elemental composition 5 3

Radioscience RAD core & mantle structure, Mercury orbit, fundamental science- Transponder KAT 32-34 GHz 3.5 9- Accelerometer ISA 10-4-10 Hz 8 6.3

Laser altimeter TOP 1064±5 nm, topographic mapping 6.5 10

Telescope NET +18 mag, 2º FOV, 880 mm fl search for Near-Earth Objects 8 15

MMOMagnetometer MAG ±4096 nT, two tri-axial fluxgate sensors on radial boom 0.88 0.35

Ion mass spectrometer IMS 50 eV–35 keV, mass, charge & energy of ion species 4.4 4

Electron electrostatic analyser EEA 0-30 keV, 3-D distribution of plasma electrons 1.1 1.2

Cold plasma analyser CPA 0-50 eV, energy & composition of low-energy ions 1.3 1.9

Energetic plasma detector EPD 30-300 keV, energy & composition of high-energy ions 1.2 0.7

Search coil RPW-H 0.1 Hz - 1 MHz, tri-axial magnetometer on boom 5.1 4

Electric antenna RPW-E 0.1 Hz - 16 MHz, two 35 m radial wires

Positive ion emitter PIE 1-100 µA, indium ions emitted to control craft’s potential 2.7 3.8

Camera SCAM surface imaging, 350-1000 nm, 10-20 m/pixel 8 12

MSEHeat flow and physical HP3 thermistor string, accelerometer, radiation densitometer for 1 0.3properties package (Mole) temperature, thermal conductivity, density, hardness

Alpha X-ray spectrometer AXS elemental composition using Cm-244 X-ray fluorescence 0.8 1(Microrover) (Na, Mg, Al, Si, K, Ca, Fe, P, S, Cl, Ti, Mn, Ni)

& α-backscatter (C, O)

Descent camera (on CPM) CLAM-D descent images down to 100 m altitude; 4-position filter wheel 0.5 3

Surface camera CLAM-S panoramic camera to characterise landing site 0.2 3

Magnetometer MLMAG surface magnetic properties 0.5 0.6

Seismometer SEISMO crust/mantle ’quakes, tidal deformations 0.9 0.6

Mole MDD penetrate several m of regolith with HP3 0.4 5

Microrover MMR deploy AXS to several sites; 100 m tether 2 3

Testing a Mole prototype. Microrover will measure rock composition.

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Key technologies arebeing developed 2001-

2004: high-temperature thermalcontrol (materials, louvres, heat pipesembedded in structural panels); high-temperature solar arrays (GaAs cells,arrays, drive mechanisms); high-temperature high-gain antennas(reflector materials, feed andpointing/despin mechanisms);miniaturised integrated avionics;vision-based navigation for landing;robotics and science instruments forsurface operations. SMART-1 willdemonstrate the use of an electricthruster for primary propulsion.

Mission profile: each composite islaunched into Earth orbit with a highapogee (312 500 km), leading to alunar swingby to set up one Earth, 2Venus and 2 Mercury gravityassists during the 3.5-yearcruise. At Mercury, the CPM’s4 kN engine burns to enter a400x11800 km polarorbit. MSE/CPM arereleased here to land.For MPO, CPM reignites toset up a 1500 km circular orbit andis then jettisoned. MPO/MMO areboth designed for 1 year ofobservations (4 Mercury years).

Once MMO is released, CPM burns toreach 10 km perigee in 75 min. Itreignites to kill the descent speed at120 m altitude, where MSE separatesand freefalls to the surface for the30 m/s to be cushioned by airbags.Thermal considerations require alanding site around 85ºN/S. Duringthe planned 1 week of surfaceoperations, MSE transmits its data toMPO or MMO.

MPO: configuration driven by thermalconstraints, requiring high-efficiencyinsulation and 1.5 m2 (200 Wrejection) radiator. Bus is a flat prismwith 3 sides slanting 20º as solararrays, providing 420 W at perihelion(30% cells, 70% optical solarreflectors). 1.5 m dia Ka-band HGAdelivers 1550 Gbit in 1 year. UHFantenna for MSE surface link. 3-axis

control, nadir-pointing. Mass 357 kg(including 60 kgsciencepayload).

MMO: cylindrical bus spinning at15 rpm (axis perpendicular toMercury equator). CFRP thrust conehouses tanks for cold-gas nitrogenthrusters. Hardware mounted onwalls and 2 platforms; top/bottomfaces are radiators. Despun X-band20 W HGA delivers 160 Gbit in1 year; 2 MGAs. UHF patch antennafor MSE surface link. Mass 165 kg(including 24 kg science payload).

MSE: truncated cone 90 cm dia,44 kg (including 7 kg science payload)

delivered to surfaceby CPM + airbags.

8.7 kbit/s to MMO orMPO, 1.7 kWh batterysized for 1-week

operations. If lands insunlight, top cover

jettisoned to expose radiator.Only surface deployables are tetheredMicrorover and Mole.

Solar Electric Propulsion Module:identical for both composites,provides cruise propulsion (jettisonedbefore Mercury insertion). Three200 mN Xe ion thrusters powered by2 wings totalling 33 m2 of GaAs cellsdelivering 5.5 kW at 1 AU. Wings tiltto maintain T < 150ºC. Box-shapedbus, with central thrust cone(housing Xe tanks) as launcherinterface. Mass 600 kg (366 kg dry).

Chemical Propulsion Module: providesMercury capture and orbitmanoeuvres. Acts as structuralinterface between SEPM and sciencecraft (interface cone caps SEPM’sthrust cone and mounts toMMO/MPO), and houses CLAM-Dcamera for MSE descent imaging.4 kN bipropellant engine withredundant 8x20 N attitude thrusters.Propellant load 156/334 kgMPO/MMO-MSE in 4 spheres; drymass 71 kg.

Top left: MPO cutaway. Top right: MMO inoperationalconfiguration. Left: MSEdeployed on thesurface, with thetethered Microrover andthe burrowing Mole.

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NGSTNGSTPlanned achievements: observe the Universe back to the time of the first stars;

extend the NASA/ESA Hubble Space Telescope collaborationLaunch date: about 2009Mission end: after 5 years (consumables sized for 10 years)Launch vehicle/site: to be decided (Atlas/EELV/Ariane-5 class)Launch mass: < 3700 kg Orbit: planned halo orbit around L2 Lagrangian point, 1.5 million km from EarthPrincipal contractors: to be selected. 12-month ESA Phase-A planned start

mid-2001, 18-month Phase-B 2002, 33-month Phase-C/D 2004, for ESAhardware delivery to NASA in 2006

The NASA/ESA Hubble SpaceTelescope (HST) is one of the mostsuccessful astronomical spaceprojects ever undertaken. The equalaccess to the observatory gainedthrough ESA’s active participation inthe mission from the very beginningis hugely beneficial scientifically tothe European astronomicalcommunity.

Since 1996, NASA, ESA and theCanadian Space Agency (CSA) havebeen collaborating on defining aworthy successor to HST – the NextGeneration Space Telescope (NGST).By participating at the financial levelof a Flexi-mission, ESA will gain apartnership of about 15% in theobservatory as well as continuedaccess to HST (qv). In October 2000,ESA’s Science Programme Committeeapproved a package of missions for2008-2013, including NGST as the F2Flexi mission.

NGST is seen as a 6 m-classtelescope, optimised for the near-IR(1-5 µm) region, but with extensionsinto the visible (0.6-1 µm) and mid-IR(5-28 µm). The large aperture andshift to the infrared is driven by thedesire of astronomers to probe theUniverse back in time and redshift tothe epoch of ‘First Light’, when thevery first stars began to shine,perhaps less than 1000 million yearsafter the Big Bang. Nonetheless, likeHST, NGST will be a general-purposeobservatory, with a set of instruments

to address a broad spectrum ofoutstanding problems in galactic andextragalactic astronomy. In contrastwith its predecessor, NGST will beplaced in a halo orbit around the L2Lagrangian point, so will not beaccessible for servicing after launch.

The Design Reference Mission coversthe first 2.5 years of observations,with 23 programmes touching almostall areas of modern astrophysics:

– cosmology and structure of theUniverse,

– origin and evolution of galaxies,– history of the Milky Way and its

neighbours,– the birth and evolution of stars,

Simulated NGST imagewith redshifts marked.

NGST could detectsome 100 galaxies with

redshifts >5 in this smallfraction (< 1%) of thecamera field of view.

(Myungshin Im, SpaceTelescope Science

Institute)

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– origins and evolution of planetarysystems.

NASA has two competing primecontractor teams (TRW and LockheedMartin), one of which will win thecontract to build the observatoryfollowing the Request for Proposals in2001. Detailed assessment studies ofa wide range of instrument conceptsfor NGST have been funded by thethree agencies and carried out bytheir scientific communities andindustries. Based on these studies,the recommended suite consists ofthree core instruments:

– a near-IR Wide-Field Cameracovering 0.6-5 µm;

– a near-IR Multi-ObjectSpectrograph covering 1-5 µm;

– a mid-IR combined Camera/Spectrograph covering 5-28 µm.

Guided by this recommendation, thethree agencies agreed in July 2000on their contributions. ESA’s willclosely follow the HST model, withthree main elements:

Scientific instrumentation: ESA willprocure about half of the corepayload, principally providing thenear-IR Multi-Object Spectrograph. Inaddition, through specialcontributions from its Member States,ESA will provide 50% of the mid-IRCamera/Spectrograph (optics,structure and mechanisms), to bedeveloped jointly by NASA/ESA/CSA.

http://sci.esa.int/ngst

TRW/Ball concept of the NGST observatory

Non-instrument flight hardware:ESA will provide the ServiceModule (assumed to be derivedfrom the Herschel bus) or, ifthat proves impractical, SMsubsystems plus some amountof optical figuring and polishingof the telescope mirrors.

Contributions to operations: ESAwill participate in operations ata similar level to HST.

Through these contributions,ESA will secure for astronomersfrom its Member States fullaccess to the NGST observatoryon identical terms to thoseenjoyed today on HST –representation on all projectadvisory bodies, and observingtime allocated via a joint peerreview process, backed by aguaranteed 15% minimum.

The telescope and instrumentswill be passively cooled in bulkbehind a large deployablesunshade to below 50 K, a leveldetermined by the operatingtemperature of the InSb andHgCdTe 1-5 µm detector arrays.The Si:As detectors to reachbeyond 5 µm require the mid-IRinstrument to be cooled to ~8 Kby a cryostat. The 0.6 µm lowerwavelength limit allows a goldcoating to be used as thereflecting surface in thetelescope and instrument optics.

Science configuration: 3-mirror anastigmattelescope, 6 m-diameter primary (folded forlaunch), f/24 or f/16, diffraction-limited at2 µm. Image stability 10 mas using fast-steering mirror controlled by a fine guidancesensor in the focal plane. Instrument module2.5x2.5x3.0 m, < 1000 kg.

Bus: Pointing accuracy 2 arcsec rms. < 1000 Wrequired from solar array. 1.6 Mbit/s fromscience instruments, stored on 100 Gbit SSR,downlinked at X-band.

NGST Science Goals1 Formation and

evolution ofgalaxies (imaging).

2 Formation andevolution ofgalaxies (spectra).

3 Mapping darkmatter

4 Searching for thereionisation epoch.

5 Measuringcosmologicalparameters.

6 Formation andevolution ofgalaxies –obscured starsand ActiveGalactic Nuclei.

7 Physics of starformation:protostars.

8 Age of the oldeststars.

9 Detection of jovian planets.

10 Evolution ofcircumstellar discs.

11 Measuresupernovae rates.

12 Origins ofsubstellar-massobjects.

13 Formation andevolution ofgalaxies: clusters.

14 Formation andevolution ofgalaxies nearAGN.

15 Cool-field browndwarf neighbours.

16 Survey of trans-neptunian objects.

17 Properties ofKuiper BeltObjects.

18 Evolution oforganic matter inthe interstellarmedium –astrobiology.

19 Microlensing in the Virgo cluster.

20 Ages andchemistry of halopopulations.

21 Cosmic recyclingin the interstellarmedium.

22 IR transients fromgamma-ray bursts and hosts.

23 Initial MassFunction of oldstellar populations.

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Planned achievements: first detection of gravitational waves, first directmeasurements of massive black holes at centres of galaxies

Launch date: planned for August 2011 (science phase begins 2012)Mission end: nominally after 2 years (8-year extension possible)Launch vehicle/site: Delta-II from Complex 17, Cape Canaveral, FloridaLaunch mass: total 1380 kg (on-station 274 kg each, science payload 70 kg each)Orbit: heliocentric, trailing Earth by 20ºPrincipal contractors: to be selected; System & Technology Study June 1999 -

February 2000; LISA Technology Demonstration in Space (part of SMART-2)October 2006 - January 2007; Phase-B June 2006 - October 2007, Phase-C/DNovember 2007 - May 2011

ESA's ambitious Laser InterferometerSpace Antenna (LISA) aims to makethe first detection of gravitationalwaves – ripples in space-time.According to Einstein’s Theory ofGeneral Relativity, these waves aregenerated by exotic objects such asbinary black holes, which distortspace and time as they orbit closely.To detect the elusive gravitationalwaves, the three LISA satellites willcarry state-of-the-art inertial sensors,a laser-interferometry telescopesystem and highly sensitive ionthrusters.

In October 2000, the ScienceProgramme Committee approved apackage of missions for 2008-2013.LISA will fly as Cornerstone 7(Fundamental Physics), but incollaboration with NASA at a cost toESA of a Flexi mission. Thedemanding technologies will be testedby the dedicated SMART-2demonstrator in 2006, during LISA’sPhase-B.

A 4-spacecraft version of LISA wasproposed to ESA in May 1993 for theM3 competition (won by Planck), butthat version clearly exceeded the costlimit for a medium-class mission. A6-spacecraft version was proposed inDecember 1993 as a Cornerstone forthe Horizon 2000 Plus programme,and selected. Being a Cornerstoneimplies the mission is approved, but

the launch date depends on fundinglevels. The 6-spacecraft version wasearmarked for launch after 2017. Byearly 1997, the cost had beendrastically reduced by halving thenumber of satellites and throughcollaboration with NASA.

Massive bodies produce indentationsin the elastic fabric of spacetime, likebilliard balls on a springy surface. If amass distribution moves aspherically,then the spacetime ripples spreadoutwards as gravitational waves. Aperfectly symmetrical collapse of asupernova will produce no waves,while a non-spherical one will emitgravitational radiation. A binarysystem always radiates.

As gravitational waves distortspacetime, they change the distancesbetween free bodies. Gravitationalwaves passing through the SolarSystem changes the distancesbetween all bodies in it. This could bethe distance between a spacecraft andEarth, as in the cases of Ulysses andCassini (attempts continue to measurethese distance fluctuations), or thedistances between shielded proofmasses inside well-spaced satellites,as in LISA. The main problem is thatthe distance changes are exceedinglysmall. For example, the periodicchange between two proof massesowing to a typical white dwarf binaryat a distance of 50 pc is only 10-10 m.

SMARTSMART-2-2LISALISA

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LISA will fly a troika of identical satellites in anequilateral triangle formation to detect

distance changes as they surf gravitationalwaves passing through the Solar System.

Gravitational waves are not weak (asupernova in a not too-distant galaxydrenches every square metre onEarth with kilowatts of gravitationalradiation), but the resulting lengthchanges are small because spacetimeis such an extremely stiff elasticmedium that it takes huge energies toproduce even minute distortions.

If LISA does not detect thegravitational waves from knownbinaries with the intensity andfrequency predicted by Einstein’sGeneral Relativity, it will shake thevery foundations of gravitationalphysics.

LISA’s main objective is to learnabout the formation, growth, spacedensity and surroundings of massive

black holes (MBHs). There is nowcompelling indirect evidence for theexistence of MBHs with masses of106 - 108 Suns in the centres of mostgalaxies, including our own. The mostpowerful sources are the mergers ofMBHs in distant galaxies.Observations of these waves wouldtest General Relativity and,particularly, black-hole theory tounprecedented accuracy. Not much isknown about black holes of massesfrom 100 to 106 Suns. LISA canprovide unique new informationthroughout this mass range.

LISA uses identical satellites5 million km apart in an equilateraltriangle. It is a giant Michelsoninterferometer, with a third armadded for redundancy andindependent information on thewaves’ two polarisations. Theirseparations (the interferometer armlength) determine LISA’s wavefrequency range (10-4 - 10-1 Hz),carefully chosen to cover the mostinteresting sources of gravitationalradiation.

The centre of the triangular formationis in the ecliptic plane 1 AU from the

http://sci.esa.int/lisa/

LISA satellite configuration. The Y-shaped twin-telescope assembly is supported by a carbon-epoxyring. The top solar array is not shown, nor thebottom propulsion module.

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Sun and 20º behind the Earth. Theplane of the triangle is inclined 60º tothe ecliptic. This configuration meansthat the formation is maintainedthroughout the year, and appears torotate around its centre annually.

Each satellite contains two opticalassemblies, each pointing to anidentical assembly on each of theother satellites. A 1 W 1.064 µm IRNd:YAG laser beam is transmitted viaa 30 cm-aperture f/1 Cassegraintelescope. The other satellite’s ownlaser is phase-locked to the incominglight, providing a return beam withfull intensity. The first telescopefocuses the very weak beam (a fewpW) from the distant spacecraft anddirects it to a sensitive photodetector,where it is superimposed with afraction of the original local light,serving as a local oscillator in aheterodyne detection. The distancefluctuations are measured to sub-Åprecision which, combined with the5 million km separations, allows LISAto detect gravitational-wave strainsdown to one part in 10-23 in a year ofobservation with a signal-to-noiseratio of 5.

At the heart of each assembly is avacuum enclosure housing a free-flying polished platinum-gold 4 cmcubic proof mass, which serves as theoptical reference mirror for the lasers.The spacecraft serve mainly to shieldthe proof masses from solar radiation

pressure, and the spacecraft’s actualposition does not directly enter intothe measurement. It is neverthelessnecessary to keep them moderatelycentred (10-8 m/√Hz in themeasurement band) on their proofmasses to reduce spurious local noiseforces. This is done by a drag-freecontrol system consisting of anaccelerometer (or inertial sensor) anda system of µN thrusters. 3-Dcapacitive sensing measures thedisplacements of the proof massesrelative to the spacecraft. Theseposition signals are used to commandField-Emission Electric Propulsion(FEEP) thrusters to enable thespacecraft to follow its proof massesprecisely. The thrusters also controlthe attitude of the spacecraft relativeto the incoming optical wave frontsusing signals derived from quadrant

Each satellite has twoidentical telescopes

pointing 60º apart. The4 cm polished cubes(yellow) are the zeropoints for measuring

distance changesbetween the satellites.

SMART-2

Planned achievements: technologydemonstrator for LISA and Darwin

Launch date: planned for August 2006Launch vehicle/site: Soyuz-Fregat from

Baikonur Cosmodrome, KazakhstanOrbit: heliocentric, trailing Earth by 20ºPrincipal contractors: to be selected

LISA relies on technology validated by theSMART-2 precursor mission, approved bythe SPC in February 2001. FifteenTechnology Development Activities areproving that LISA’s extreme sensitivityvalues can be achieved. These activities willbe completed in 2002 and consolidated intothe LISA Test Package (LTP) for flight onSMART-2. The prime contractor will beselected and Phase-B started by November2002. The flight will provide feedback to thethen-running LISA Phase-B. LTP coversLISA’s proof-mass sensor, FEEP thrusterpackage and the control law software. Themain satellite will carry one or two LTPs(depending on NASA participation). Using asecond satellite, SMART-2 will alsodemonstrate the formation flying and inter-satellite metrology required for IRSI-Darwin.

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photodiodes. As the constellationorbits the Sun in the course of a year,the observed gravitational waves areDoppler-shifted by the orbital motion.For periodic waves with sufficientsignal-to-noise ratio, this allows thedirection of the source to bedetermined to arcminute or degreeprecision, depending on sourcestrength.

LISA is envisaged as a NASA/ESAcollaboration: NASA provides launch,X-band communications, mission andscience operations, and about 50% ofthe payload; ESA provides the threespacecraft, including the ion drives;European institutes, fundednationally, provide the other 50% ofthe payload.

Satellite configuration: total diameter2.2 m across solar array. Mainstructure is a 1.80 m-dia, 48 cm-height ring of graphite-epoxy (lowthermal expansion). Each satellitehouses two identical telescopes andoptical benches in a Y-shapedconfiguration.

Attitude control: drag-free/attitudecontrol maintained by 6 clusters offour FEEP thrusters, controlled byfeedback loop using capacitiveposition sensing of proof masses.Performance: 3x10-15 m/s2 in the band10-4 - 10-3 Hz. Solar-electric propulsionmodule (142 kg, 1.80 m-dia, 40 cm-high, CFRP, redundant 20 mN Xe ionthrusters) jettisoned after 13-monthtransfer from Earth; 4x4.45 N +4x0.9 N hydrazine thrusters provide 3-axis control and orbit adjust. Attitudefrom 4 star trackers, Sun sensors.

Power system: 315 W on-station (72 Wscience), provided from GaAs cells oncircular top face, supported by Li-ionbatteries. 940 W required during ion-powered cruise, from similarpropulsion module array.

Communications: 30 cm-dia X-bandsteerable antenna transmits scienceand engineering data (stored onboardfor 2 days) at 7 kbit/s to the 34 mnetwork of NASA’s Deep SpaceNetwork DSN. Total science rate672 bit/s.

LISA’s sensitivity to binary stars in our Galaxy and black holes in distant galaxies. The heavyblack curve shows LISA’s detection thresholdafter a year’s observations. The vertical scale isthe distance change detected (one part in 1023 isthe goal). At frequencies below 10-3 Hz, binarystars in the Galaxy are so numerous that LISAwill not resolve them (marked ‘Binary confusionnoise’). In lighter green is the region where LISAshould resolve thousands of binaries closer tothe Sun or radiating at higher frequencies. Thesignals expected from two known binaries areindicated by the green triangles. The blue areacovers waves expected from massive blackholes merging in other in galaxies (redshiftz = 1). The red spots mark the mergings of twomillion-solar-mass black holes, and two 10 000-solar-mass black holes. The bottom red spotshows the expected wave from a 10-solar-massblack hole falling into one of a million solarmasses, at a distance of z = 1.

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GAIAGAIAPlanned achievements: map 1000 million stars at 10 microarcsec accuracy to

discover how and when our Galaxy formed, how it evolved and its currentdistribution of dark matter; fundamental importance for all branches ofastronomy; maintain Europe’s lead in astrometry

Launch date: planned for 2012Mission end: 5-year mission planned (4-year observation period); 1-year extension

possibleLaunch vehicle/site: shared Ariane-5 from Kourou, French GuianaLaunch mass: 3137 kg (2267 kg if Ariane-5 provides transfer orbit injection)Orbit: planned Lissajous orbit around L2 point 1.5 million km from EarthPrincipal contractors: to be determined. Phase-B to begin 2005; Phase-C/D 2006

GAIA aims to solve one of the mostdifficult but fundamental challengesin modern astronomy: to determinethe composition, creation andevolution of our Galaxy, in theprocess creating an extraordinarilyprecise 3-D map of more than 1000million stars throughout the Galaxyand beyond.

When ESA’s Horizon 2000 scienceprogramme was extended in 1994with Horizon 2000 Plus, the threenew Cornerstones included aninterferometry mission. Two optionswere proposed. GAIA aimed at10 microarcsec-level astrometry,while Darwin would look for life on

planets discovered around otherstars. Following studies, GAIA’sinterferometric approach wasreplaced in 1997 by measurementsusing simpler monolithic mirrors (andGAIA became a name instead of anacronym for Global AstrometricInteferometer for Astrophysics). GAIAcompeted in 2000 with BepiColombofor the fifth Cornerstone slot. InOctober 2000, the ScienceProgramme Committee approved apackage of missions for 2008-2013:BepiColombo was selected as CS-5(2009) and GAIA as CS-6.

By combining positions with radialvelocities, GAIA will map the stellarmotions that encode the origin andevolution of the Galaxy. It will identifythe detailed physical properties ofeach observed star: brightness,temperature, gravity and elementalcomposition.

GAIA will achieve all this byrepeatedly measuring the positionsand multi-colour brightnesses of allobjects down to magnitude +20.Variable stars, supernovae, transientsources, micro-lensed events andasteroids will be catalogued to thisfaint limit. Final accuracies of10 microarcsec (the diameter of ahuman hair viewed from a distance of1000 km) at magnitude +15, willprovide distances accurate to 10% asfar as the Galactic Centre, 30 000light-years away. Stellar motions will(R. Sword)

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be measured even in the AndromedaGalaxy.

GAIA will provide the raw data to testour theories of how galaxies and starsform and evolve. This is possiblebecause low-mass stars live for muchlonger than the present age of theUniverse, and retain in theiratmospheres a fossil record of thechemical elements in the interstellarmedium when they formed. Theircurrent orbits are the result of theirdynamical histories, so GAIA willidentify where they formed and probethe distribution of dark matter. GAIAwill establish the luminosity functionfor pre-main sequence stars, detectand categorise rapid evolutionarystellar phases, place unprecedentedconstraints on the age, internalstructure and evolution of all startypes, establish a rigorous distancescale framework throughout theGalaxy and beyond, and classify starformation and movements across theLocal Group of galaxies.

GAIA will pinpoint exotic objects incolossal numbers: many thousands ofextrasolar planets will be discovered,and their detailed orbits and massesdetermined; brown dwarfs and whitedwarfs will be identified in their tensof thousands; some 100 000extragalactic supernovae will bediscovered in time for ground-basedfollow-up observations; Solar Systemstudies will receive a massive impetusthrough the discovery of many tens ofthousands of minor planets; innerTrojans and even new trans-Neptunian objects, includingPlutinos, will be discovered.

GAIA will also contribute tofundamental physics. It will quantifythe bending of starlight by the Sunand major planets over the entirecelestial sphere, and thereforedirectly observe the structure ofspace-time. GAIA’s accuracy willallow the long-sought scalar

correction to the tensor form to bedetermined. The Parameterised Post-Newtonian (PPN) parameters γ and ß,and the solar quadrupole moment J2will be determined withunprecedented precision. Newconstraints on the rate of change ofthe gravitational constant, G

., and on

gravitational wave energy over acertain frequency range will beobtained.

Like Hipparcos, GAIA will measurethe angles between targets as itsrotation scans two telescopes(separated by 106º) around the sky.Processing on the ground will linkthese targets in a grid with10 microarcsec accuracy. Distancesand proper motions will ‘fall out’ ofthe processing, as will information ondouble and multiple star systems,photometry, variability and planetarysystems.

http://astro.estec.esa.nl/GAIA

GAIA’sconfigurationidentified fromthe Concept &TechnologyStudy byMatra MarconiSpace,completed2000

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Overview ofGAIA’s

performance,superimposed

on the Lundmap of the

sky

GAIA MeasurementCapabilities

Median parallax errors4 µas at +10 mag 11 µas at +15 mag 160 µas at +20 mag

Distance accuracies2 million better than 1% 50 million better than 2%110 million better than 5%220 million better than 10%

Radial velocity accuracies1-10 km/s to V = +16-17 mag,

depending on spectral type

Catalogue~1000 million stars 26 million to V = +15 mag250 million to V = +18 mag1000 million to V = +20 magcompleteness to about +20 mag

Photometryto V = +20 mag in 4 broad and 11

medium bands

GAIA Science Goals

Galaxyorigin and history of Galaxytests of hierarchical structure

formation theoriesstar-formation historychemical evolutioninner bulge/bar dynamicsdisc/halo interactionsdynamical evolution

nature of the warpstar cluster disruptiondynamics of spiral structuredistribution of dustdistribution of invisible massdetection of tidally-disrupted

debrisGalaxy rotation curvedisc mass profile

Star formation and evolutionin situ luminosity functiondynamics of star-forming regionsluminosity function for pre-main

sequence starsdetection/categorisation of rapid

evolutionary phasescomplete local census to single

brown dwarfsidentification/dating of oldest halo

white dwarfsage censuscensus of binaries/multiple stars

Distance scale and referenceframe

parallax calibration of all distancescale indicators

absolute luminosities of Cepheidsdistance to the Magellanic Cloudsdefinition of the local,

kinematically non-rotating metric

Local Group and beyondrotational parallaxes for Local

Group galaxieskinematical separation of stellar

populationsgalaxy orbits and cosmological

historyzero proper motion quasar survey

cosmological acceleration of SolarSystem

photometry of galaxiesdetection of supernovae

Solar Systemdeep and uniform detection of

minor planetstaxonomy and evolutioninner TrojansKuiper Belt Objectsdisruption of Oort Cloudnear-Earth objects

Extrasolar planetary systemscomplete census of large planets to

200-500 pcorbital characteristics of several

thousand systems

Fundamental physicsγ to ~5x10-7

ß to 3x10-4 - 3x10-5

solar J2 to 10-7 - 10-8

G./G to 10-12 - 10-13 yr-1

constraints on gravitational waveenergy for 10-12 < f < 4x10-9 Hz

constraints on ΩM and ΩΛ fromquasar microlensing

Specific objects106 - 107 resolved galaxies 105 extragalactic supernovae500 000 quasars105 - 106 (new) Solar System

objects50 000 brown dwarfs30 000 extrasolar planets200 000 disc white dwarfs200 microlensed events 107 resolved binaries within 250 pc

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The two moderate-size telescopes uselarge CCD focal plane assemblies,with passive thermal control. GAIA issized specifically to fit on a sharedAriane-5. A Lissajous orbit aroundLagrange point L2 is preferred, fromwhere an average of 1 Mbit/s willreturn to the single ground stationthroughout the 5-year mission(totalling 20 Tbytes of raw data).Every one of the 109 targets will beobserved typically 100 times, eachtime in a complementary set ofphotometric filters, and a largefraction also with the radial velocityspectrograph.

Many analyses will be possible evenduring operations, while some willrequire the whole mission calibrationinformation or even the final datareduction. GAIA will provide excitingscientific data to a very widecommunity, beginning with the firstphotometric observations, and rapidlyincreasing until the fully reduceddata become available 2-3 years aftermission-end. The resulting analyseswill provide a vast scientific legacy,generating a wealth of quantitativedata on which all of astrophysics willbuild.

Satellite configuration: payloadmodule (PLM) 4.2 m diameter, 2.1 mhigh; service module (SVM) 4.2/9.5 mdiameter (stowed/deployed),PLM/SVM mechanically- andthermally-decoupled, separated by9.5 m-diameter solar array/sunshield(the only deployable element), SVMinterfaces with Ariane-5 standardadapter; structure is aluminium withCFRP shear walls. Lateral panelsused as radiators and covered withoptical solar reflectors. SVM is at200ºC, PLM at 200K (stability tens ofµK). 6 solar wings stowed duringlaunch; insulated from PLM with MLIon rear face. Additional insulationsheets, reinforced with kevlar cables,are spread between the wings.

Attitude/orbit control: 7 caesium-FEEP electric thrusters maintain120 arcsec/s spin; 3-axis control by

10 N thrusters during 220-240 daytransfer orbit to L2 point, powered by400 N liquid-propellant thruster (notrequired if Ariane-5 has restartableengine). Lissajous eclipse-free orbitaround L2 provides stable thermalenvironment, high observingefficiency (Sun, Earth and Moonalways out of fields of view) and low-radiation environment.

Power system: 2569 W required,including 1528 W payload, 640 WSVM. Two panels of GaAs cells oneach of 6 CFRP solar wings, totalling24.1 m2.

Communications: 17 W X-band high-gain, electronically-steered phased-array antenna provides 3 Mbit/s(minimum 1 Mbit/s) to ESA’s 35 m-diameter Perth ground station 8 hdaily. SSR ≥200 Gbit, sized to holdfull day’s data. Contolled from ESOC.Low-gain omni antenna for TC andhousekeeping TM.

Spectrometric instrument(primary and tertiary mirrors)

Basic anglemonitoring device

Common optical bench

Wide fieldstar sensor

Platform interface(Titanium bipods)

ASTRO-1 primary mirror

Spectrometric instrument(Secondary Reflector & Focal Plane)

ASTRO-1 secondary mirror

ASTRO-1 focal plane

ASTRO-2 primary mirror

The payload is mounted on a silicon carbide optical bench. Twotelescopes separated by 106º are scanned by GAIA’s 1 rev/hspin rate, for the simultaneous measurement of the angularseparations of thousands of star images as they pass throughthe 1º FOV. The precise angular separation (‘basic angle’) of theprimary 0.7x1.7 m rectangular mirrors (1º FOV, 50 m f.l.), iscontinuously measured. Secondary mirrors are adjustable towithin 1 µm. Each focal plane has an array of 250 CCDs (each2150x2780 pixels) forming a 25x58 mm detector. FOV dividedinto: Astrometric Sky Mapper of 3 CCD strips to identifyentering objects; Astrometric Field (0.5x0.66º) for the actualposition measurements; 4-band photometer. The third telescope(4º FOV, 4.17 m f.l., entrance pupil 75x70 cm) is a spectro-meter, for radial velocity (from Doppler shift) measurementsusing 3 large CCDs and medium-band photometry using 2 largeCCDs with 11 filters.

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Solar OrbiterSolar OrbiterPlanned achievements: closest-ever solar approach; highest-resolution solar

observations; first images of the Sun’s polar regionsLaunch date: planned for February-March 2012 (Venus window every 18 months)Mission end: after 5 years (2-year extension possible)Launch vehicle/site: Soyuz-Fregat from Baikonur Cosmodrome, KazakhstanLaunch mass: about 1300 kg (130 kg science payload)Orbit: planned operational ~0.21x0.9 AU, up to 30º, 149-day heliocentricPrincipal contractors: to be selected. Possible schedule: 18-month Phase-A 2006-

2008, 12-month Phase-B 2008-2009, 36-month Phase-C/D 2009-2012

On 1 October 1999, ESA requestedproposals for the F2 and F3 Flexiscience missions, selecting five of the50 for assessment studies March-May2000: Eddington, Hyper, MASTER,Solar Orbiter and Storms; the NextGeneration Space Telescope wasalready part of the process. InOctober 2000, the ScienceProgramme Committee approved apackage of missions forimplementation in 2008-2013,including Solar Orbiter as a Fleximission.

The Sun’s atmosphere and theheliosphere are unique regions ofspace, where fundamental physicalprocesses common to solar,astrophysical and laboratory plasmascan be studied in detail and underconditions impossible to reproduce onEarth. The results from Soho andUlysses have enormously advancedour understanding of the solarcorona, the associated solar wind andthe 3-D heliosphere. However, wehave reached the point where in situmeasurements much closer in to theSun, combined with high-resolutionimaging and spectroscopy at highlatitudes, promise to bring aboutmajor breakthroughs.

The Solar Orbiter, through a novelorbit design and its state-of-the-artinstruments, will provide exactly theobservations required. For the firsttime, it will:

– explore the uncharted innermostregions of our Solar System,

– study the Sun from close-up: 45solar radii (31.3 million km),

– hover over the surface for long-duration, detailed observations bytuning its orbit for its perihelionspeed to match the Sun’s 27-dayrotation rate;

– provide images of the Sun’s polarregions from latitudes of up to 38º.

The scientific goals of the SolarOrbiter are to:

– determine in situ the propertiesand dynamics of plasma, fields andparticles in the near-Sunheliosphere,

– investigate the fine-scale structureand dynamics of the Sun’smagnetised atmosphere, usingclose-up, high-resolution remotesensing,

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– identify the links between activityon the Sun’s surface and theresulting evolution of the coronaand inner heliosphere, usingsynchronised passes,

– observe and fully characterise theSun’s polar regions and equatorialcorona from high latitudes.

The underlying basic questions thatare relevant to astrophysics ingeneral are:

– why does the Sun vary and howdoes the solar dynamo work?

– what are the fundamental physicalprocesses at work in the solaratmosphere and heliosphere?

– what are the links between themagnetic field-dominated regime inthe solar corona and the particle-dominated regime in theheliosphere?

In particular, Solar Orbiter will allowus to:

– unravel the detailed working of thesolar magnetic field as a key tounderstanding stellar magnetismand variability,

– map and describe the rotation,meridional flows and magnetictopology near the Sun’s poles, inorder to understand the solardynamo,

– investigate the variability of theradiation from the far side of theSun and over the poles,

– reveal the flow of energy throughthe coupled layers of theatmosphere, e.g. to identify thesmall-scale sources of coronalheating and solar-windacceleration,

– analyse fluctuations and wave-particle interactions in the solarwind, in order to understand thefundamental processes related toturbulence at all relevant scales ina tenuous magnetofluid,

– understand the Sun as a prolificand variable particle accelerator,

– study the nature and globaldynamics of solar eruptions such asflares and coronal mass ejections,and their effects on the heliosphere(‘space weather and space climate’).

The near-Sun measurementscombined with simultaneous remote-sensing observations of the Sun itself

http://solarsystem.estec.esa.nl/projects/solar_orbiter.htm

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will allow the spatial and temporalvariations to be untangled during theorbiter’s sychronised passages. Theywill allow us to understand thecharacteristics of the solar wind andenergetic particles in close linkagewith the plasma conditions wherethey are created on the Sun. Byapproaching as close as 45 solarradii, it will view the atmosphere inunprecedented detail (35 km perpixel). Solar Orbiter will returnimages and data from the polarregions and the far side of the Sun –inaccessible to us on Earth.

Solar Orbiter will achieve its wide-ranging aims with a suite ofsophisticated instruments. Owing toits proximity to the Sun, theinstruments can be smaller than onobservatories, such as Soho, orbitingmuch further out.

The spacecraft design will benefitfrom technology developed forBepiColombo (such as Solar Electric

Propulsion), allowing such anambitious project to be carried out asa Flexi mission. SEP in conjunctionwith multiple planetary swingbys willdeliver Solar Orbiter into an orbitwith a perihelion of 45 solar radii andperiod of 149 days after only 2 years.Several Venus swingbys during thenominal 5-year mission will increasethe orbit’s inclination to 30° withrespect to the solar equator. Amission extension of about 2 yearswould allow an increase to 38°.

The spacecraft will be 3-axisstabilised and always Sun-pointed.Given the extreme thermal conditions– 25 times harsher than at Earth’sdistance – the craft’s thermal designwas considered in detail during theassessment study.

Satellite configuration:1.20x1.60x3.00 m box-shaped buswith instruments viewing Sunthrough thermal shield. PayloadModule of CFRP at Sun end; Service

Solar Orbiter Scientific Instruments

Instrument Measurements Specification Mass Power Data(kg) (W) (kbit/s)

SWA Solar Wind Plasma Thermal ions 0-30 kev/Q 6 5 5Analyser and electrons 0-10 keV

RPW Radio & Plasma AC electric and µV/m - V/m 10 7.5 5Analyser magnetic fields 0.1 nT - µT

CRS Radio Sounding Wind density and velocity X-band and Ka-band 0.2 3 0

MAG Magnetometer DC magnetic field to 500 Hz 1 1 0.2

EPD Energetic Particle Solar and cosmic ray particles Ions and electrons 4 3 1.8Detector 0.1-10 MeV

DUD Dust Detector Interplanetary dust particles 10-16 - 10-6 g 1 1 0.05

NPD Neutral Particl Neutral hydrogen and atoms 0.6 - 100 keV 1 2 0.3Detector

NED Neutron Detector Solar neutrons e > 1 MeV 2 1 0.15

VIM Visible-light Imager High-resolution disc intensity Fe 630 line 26 25 20and Magnetograph & velocity; polarimetry

EUS EUV Imager and Imaging and diagnostics of EUV emission lines 22 25 17Spectrometer transition region and corona

EXI EUV Imager Coronal imaging He and Fe ion lines 36 20 20

UVC UV/visible Imaging and diagnostics of Coated-mirror 17 25 5coronagraph the corona coronagraph

RAD Radiometer Solar constant Total solar irradiance 4 6.5 0.5

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Solar Orbiter’s path. The 22-month cruise phaseuses Venus flybys and SEP thruster firings toestablish the 0.2x0.9 AU operational orbit. Duringthe 35-month, 7-orbit operational phase, Venusencounters increase inclination to 30°. The27-month extended mission uses two Venusswingbys over 6 orbits to reach 38°.

Soho observations of the regions where thesolar wind is created illustrate the need for

high-latitude observations and the potential forlinking remote-sensing and in situ

measurements of the surface and solar wind.The green image shows the extreme-UV Sun

(Soho/EIT) with the polar coronal holes clearlyvisible. The inset Soho/SUMER Doppler

images (Ne VIII) show blue- and red-shiftedplasma flows; black lanes mark the

supergranular network boundaries. Theoutflowing solar wind (blue-shifts) clearly

originate in the network cell boundaries andjunctions. This is an intriguing result, but as

Doppler shifts can be measured only close tothe line-of-sight, instruments need to fly wellabove the ecliptic plane to obtain a thorough

understanding of the polar outflow of the high-speed solar wind. In addition, the distribution

of the solar-wind outflow regions should beimprinted on the higher solar wind, which can

be confirmed only by in situ measurements.

Module of aluminium honeycombe atother. Central cylindrical thrust tube.

Thermal control: to cope with 0.21-1.2 AU solar range. Thermal shield atSun-end sized to shadow bus walls,HGA and solar array mechanisms: 3titanium & 15 MLI layers, backed by15 further MLI layers. Surfacefinishes, heaters, MLI, heat-pipes.

Attitude/orbit control: SEP by four0.15 N plasma thrusters; Sun-pointing (stability better than3 arcsec per 15 min) 3-axisstabilisation by four 4 Nms reactionwheels & star tracker. Initial Sunacquisition by gyros, coarse Sunsensors and redundant sets of six 5 Nhydrazine thrusters.

Power system: twin 3-panel solarwings (28 m2 generating 6.2 kW at1 AU) mounted on bus side,jettisoned after SEP thrusting. Tilting10 m2 arrays on bus top providepower for observing mission

Communications: 1.5 m-diameter2-axis steerable Ka-band high-gainantenna on 2.0 m boom to return750 kbit/s at 0.6 AU Earth distance;X-band low-gain antennas forTC/TM. 75 kbit/s science data stored

on 240 Gbit SSR, downlinked onlywhen Sun distance >0.5 AU (thermalrequirements). Controlled from ESOCvia Perth 35 m station.

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DarwinDarwinPlanned achievements: first detection and analysis of Earth-like planets, search

for possible biospheresLaunch date: under development for flight beyond 2013Mission end: nominally after 5 years; 10-year extended mission possibleLaunch vehicle/site: Ariane-5 from Kourou, French GuianaLaunch mass: about 4.2 t for 8 spacecraftOrbit: planned halo orbit around L2 Lagrangian pointPrincipal contractors: to be selected

The Darwin Infrared SpaceInterferometer is being developed tosearch for Earth-like planets orbitingother stars. It is a Cornerstonemission but lies beyond the current2013 horizon of approved missions; itmay be combined with NASA’sTerrestrial Planet Finder.

Darwin’s goal is to detect otherterrestrial worlds, analyse theiratmospheres and determine if theycan support life as we know it. It willalso interferometrically image objectsin the thermal-IR with unprecedentedresolution.

In its current configuration, Darwinconsists of six 1.5 m-dia free-flyinginterferometric telescopestransmitting their light to a centralbeam-combining spacecraft. Thebeam-combiner carries two opticalsystems: one for Michelson imagingat very high spatial resolution, andone for ‘nulling’ interferometry.Nulling uses the wave nature of lightto extinguish the light from a star,revealing any faint close companionsnormally hidden in the glare. Darwincan analyse the light from Earth-likeplanets at interstellar distances of upto 25 pc. The spectrum will show if aplanet is benevolent to life, includingthe detection of a possible biosphere. A separate power and communicationsatellite flying close by allows thetelescope and beam-combinerspacecraft to be passively cooled to< 40 K to increase sensitivity.

Darwin would orbit around the L2Lagrangian point of the Earth-Sunsystem, 1.5 million km from Earth inthe anti-Sun direction. For thermalreasons, the observing zone is a 40ºcone around the anti-Sun direction.Control is by mN and µN thrusters,holding the inter-satellite precisionwith cm accuracy. The control systemrelies on high-precision lasermetrology (~ 8 nm rms), and thetracking of fringes in a separatechannel at ~2 µm. The arraybaselines are 50-250 m for planet-finding and 0.5-1 km for imaging.

The central questions to be addressedby Darwin are:

– how unique is the Earth as aplanet?

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Right: Darwin would use six telescope-spacecraftfeeding their observations to a central combining-

spacecraft. A separate communications satelliteprovides the link with Earth.

Inset: a simulated Darwin observation of our innerSolar System as viewed from a distance of 10 pc.Venus is at the top, Earth to the right and Mars at

the bottom. (B. Mennesson)Left: observing the gap created by a planet in a

star’s accretion disc is a prime target forDarwin. (P. Artymowicz)

– how unique is lifein the Universe?

To answer thesequestions fully, it isnecessary to:

– survey a significant sample ofnearby stars for Earth-size planets

– directly detect planets within the‘habitable zone’, i.e. the orbitalregion around a star where water isliquid, and determine the planets’orbital characteristics by repeatingthe observations several times

– observe the spectrum of the planet(does it have an atmosphere, whatis the effective temperature andtotal flux?) and estimate itsdiameter

– determine the atmosphere’scomposition and search for H2O,CO2 and O3/O2.

Detecting an Earth-size planetcircling a nearby star has beenimpossible so far because of the 109

difference in brightness at visiblewavelengths. Even at IR, where theplanet’s thermal emission increasesand the star’s emission decreases,the contrast is still 106-107. Thethermal-IR flux from an Earth-likeplanet at 10 pc is only 3 or 4 photonsm-2 s-1 in the whole range ofspectroscopic interest (4-20 µm).Nulling interferometry applies phaseshifts between different telescopes inan interferometric array to createdestructive interference on the optical

axis of the combinedbeam, accompanied by

constructive interference a smallangle away. We can search forplanets in the ‘habitable zone’ bypositioning the central star underthis null and the zone where H2Owould be liquid under the peaks.

Many of the fundamental processesin the Universe are best studied at IRwavelengths. The 4-20 µm range isrelatively free of obscuration by colddust, and is instead a very goodprobe of dust warmer than 100 K.Spectroscopy at these wavelengthscan characterise the chemicalcomposition of many objects. Darwinwill be able to resolve objects down tothe milli-arcsecond scale, equivalentto a resolution of 400 000 km at the4.26 light-year distance of the neareststar to our Sun. This means thatDarwin can see the bands sweptclean by condensing planets in theaccretion discs around stars. Adetailed study of the planet-formingprocess is extremely important andwould complement the planet-findingelement of the mission.

SMART-2, planned for 2006, willdemonstrate technologies for LISAand Darwin, including inter-satellitemetrology and control.

http://sci.esa.int/darwin/

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Acronyms & Abbreviations

Ah: amp-hourAIT: Assembly, Integration &

TestAOCS: attitude and orbit

control systemASI: Agenzia Spaziale ItalianaAU: Astronomical Unit

(149.5 million km)

BOL/EOL: Beginning ofLife/End of Life

BSR: Back Scatter Reflector

CCD: Charge Coupled DeviceCCSDS: Consultative

Committee on Space DataSystems

CEA: Commissariat à l'EnergieAtomique (France)

CEN: Centre d’EtudesNucleaires (France)

CESR: Centre d’Etude Spatialedes Rayonnements (France)

CETP: Centre d'étude desEnvironnements Terrestre etPlanétaires (France)

CFD: computational fluiddynamics

CFRP: carbon-fibre reinforcedplastic

CNES: Centre Nationald’Etudes Spatiales (France)

CNRS: Centre National de laRecherche Scientifique(France)

CRPE: Centre de Recherche enPhysique del’Environnement terrestre etplanetaire (France)

CSG: Centre Spatial Guyanais

DLR: DeutscheForschunganstalt für Luft-und Raumfahrt (Germany)

DSP: Digital Signal Processor

EAC: European AstronautCentre (ESA)

ECLSS: Environmental Controland Life Support System

ECU: European Currency UnitEGSE: electrical ground

support equipmentEIRP: equivalent isotropically

radiated powerEOL/BOL: End of

Life/Beginning of LifeESA: European Space AgencyESOC: European Space

Operations Centre (ESA)ESTEC: European Space

Research and TechnologyCentre (ESA)

EUV: Extreme Ultraviolet

FIR: Far-Infrared

f.l.: focal lengthFM: Flight ModelFOV: field of viewFWHM: full width at half

maximum

GaAs: gallium arsenideGEO: geostationaryGN2: gaseous nitrogenGOX: gaseous oxygenGPS: Global Positioning SystemGSOC: German Space

Operations CentreGTO: geostationary transfer

orbit

HGA: High-Gain Antenna

IABG: Industrieanlagen-betriebsgesellschaft GmbH

IAS: Institut d’AstrophysiqueSpatiale (France)

IAS: Istituto di AstrofisicaSpaziale (Italy)

IFOV: Instantanaeous Field ofView

INTA: Instituto Nacional deTecnica Aeroespacial (Spain)

IR: infraredIRFU: Swedish Institute of

Space PhysicsISS: International Space StationIUE: International Ultraviolet

ObservatoryIWF: Institut für

Weltraumforschung (Austria)

JEM: Japanese ExperimentModule (ISS)

LEO: low Earth orbitLHCP: left-hand circular

polarisationLN2: liquid nitrogenLOX: liquid oxygenLPCE: Laboratoire de Physique

et Chimie, del’Environnement (France)

mas: milliarcsecMDM: multiplexer-

demultiplexerMGSE: mechanical ground

support equipmentMMH: monomethyl hydrazineMOS: Metal Oxide

SemiconductorMPAe: Max-Planck-Institut für

Aeronomie (Germany)MPE: Max-Planck-Institut für

Extraterrestrische Physik(Germany)

MPI: Max-Planck-Institut(Germany)

MPK: Max-Planck-Institut fürKernphysik (Germany)

mrad: milliradianMSSL: Mullard Space Science

Laboratory (UK)MW: momentum wheel

N2O4: nitrogen tetroxideNASA: National Aeronautics and

Space Administration (USA)NASDA: National Space

Development Agency ofJapan

NIR: Near-InfraredNOAA: National Oceanographic

& AtmosphericAdministration (USA)

NRL: Naval Research Lab (USA)NTO: nitrogen tetroxide

OBDH: onboard data handlingOSR: optical solar reflector

PI: Principal InvestigatorPMOD: Physikalisch-

MeteorologischesObservatorium Davos(Switzerland)

PMT: photomultiplier tubePN: positive-negative

RAL: Rutherford Appleton Lab(UK)

RCS: reaction control systemRHCP: right-hand circular

polarisationRTG: Radioisotope

Thermoelectric GeneratorRW: reaction wheelRx: receive

SA/CNRS: Service d’Aeronomiedu Centre National de laRecherche Scientifique(France)

SAO: Smithsonian Astro-physical Observatory (USA)

Si: siliconSNG: satellite news gatheringSRON: Space Research

Organisation NetherlandsSSPA: solid-state power

amplifierSSR: solid-state recorderSTM: Structural and Thermal

Model

TC: telecommandTT&C: telemetry, tracking and

controlTWTA: travelling wave tube

ampliferTx: transmit

UV: ultraviolet

VSAT: very small apertureterminal