ROSETTA1 11/28/02 3:48 PM Page 10 Rosetta - · PDF filefairing, will be the Rosetta comet chaser, the most ambitious scientific spacecraft ever built in Europe. ... technological research

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Why ‘Rosetta’?The Rosetta spacecraft is named after the famous Rosetta Stone, a slab ofvolcanic basalt now on display in the British Museum in London. French soldiersdiscovered the unique Stone in 1799, as they prepared to demolish a wall nearthe village of Rashid (Rosetta) in Egypt’s Nile delta. The carved inscriptions onthe Stone included hieroglyphics – the written language of ancient Egypt – andGreek, which was readily understood.

By comparing the inscriptions, it eventually became possible to decipher themysterious figures that had been carved several thousand years earlier. Most ofthe pioneering work was carried out by the English physician and physicist,Thomas Young, and French scholar Jean François Champollion. As a result oftheir breakthroughs, scholars were able to piece together the history of a long-lost culture for the first time.

Just as the discovery of the Rosetta Stone eventually led to the unravelling ofthe mysterious hieroglyphics, so Rosetta will help scientists to unravel themysteries of comets. Whereas hieroglyphics were the building blocks of theEgyptian language, comets are considered to be the most primitive objects inthe Solar System, the building blocks from which the planets formed.

Billions of these giant chunks of ice still linger in the depths of space, theremnants of a vast swarm of objects that once surrounded our Sun andeventually came together to form planets. Virtually unchanged after 5 billionyears in the deep freeze of the outer Solar System, they still contain icesand dust from the original solar nebula. They also contain complex organiccompounds which some scientists believe may have provided the rawmaterial from which life on Earth evolved.

Rosetta:Claude Berner, Loic Bourillet, Jan van Casteren, John Ellwood, Michael Kasper, Philippe Kletzkine, Rita Schulz & Gerhard SchwehmESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands

Manfred WarhautESA Directorate of Technical and Operational Support, ESOC, Darmstadt, Germany

Peter BondSpace Consultant, Cranleigh, United Kingdom

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Rosetta

On the night of 12-13 January 2003,one of the most powerful rocketsin the world will blast off from

Kourou spaceport in French Guiana.On top of the giant Ariane-5,cocooned inside a protectivefairing, will be the Rosetta cometchaser, the most ambitiousscientific spacecraft ever built inEurope.

Rosetta’s mission is to complete the most comprehensiveexamination ever made of a piece of primordial cosmicdebris – a comet. After an eight-year trek around the innerSolar System, the spacecraft will home in on its fast-movingtarget, eventually edging to within just a few kilometres of thesolid nucleus, the icy heart of Comet Wirtanen.

By the summer of 2012, the Rosetta Orbiter will be closeenough to map and characterise the nature of the dormantnucleus in unprecedented detail. Once a suitable touchdown siteis identified, a small Lander will descend to the pristine surface,the first object from planet Earth to soft-land on one of theseprimitive worlds. Meanwhile, as the comet inexorably continues onits headlong rush towards the inner Solar System, the RosettaOrbiter will catalogue every eruption of gas and dust as Wirtanen’svolatiles vaporise in the warmth of the Sun.

The final chapter in Rosetta’s decade-long tale of exploration willtake place in July 2013, when the roving explorer returns once again tothe vicinity of Earth’s orbit. However, as with any complex, excitingadventure, it is worth delving into the historical background in order

to understand how it all began. In the case of Rosetta, the tale began 17 years ago in a conference room in Rome.

ESA’s Comet Chaser

The Rosetta Lander is released from its‘mother craft’ as it orbits the nucleus ofComet Wirtanen

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Comet Nucleus Sample ReturnIn January 1985, Ministers responsible forspace matters in ESA’s Member Statescame together to approve an ambitious andfar-seeing programme of scientific andtechnological research. One of their mostsignificant decisions involved approval ofa long-term science plan, which was thennamed ‘Horizon 2000’. A Resolutiondrafted by the Ministers stated, “TheCouncil agrees to reinforce space-scienceactivities in Europe during the next decadewith a view to enabling the scientificcommunity to remain in the vanguard ofspace research.”

Based on inputs from the Europeanspace-science community, therevolutionary plan included ground-breaking missions that would be launchedbetween the mid-1990s and the early yearsof the 21st century. The proposedprogramme was founded on four majorCornerstones, one of which was describedas “a mission to primordial bodiesincluding return of pristine materials”.Even before its Giotto spacecraft reachedComet Halley, ESA was looking forwardto establishing a leading role in theexploration of the smaller bodies of theSolar System by bringing back samples ofmaterial from either a comet or an asteroid.

After Giotto’s remarkably successfulHalley flyby in March 1986, the emphasisswitched to comet sample return, but itsoon became clear that the cost of such amission would be too prohibitive forEurope to carry out alone. As a result, theESA Science Executive began toinvestigate the possibility of conductingthe Planetary Cornerstone as acollaborative venture with NASA, whichwas already pursuing its own CometRendezvous and Asteroid Flyby (CRAF)mission.

At this stage, scientists on both sides ofthe Atlantic were excitedly anticipatingsending a mission to land on a relativelyactive, ‘fresh’ comet that did not approachtoo closely to the Sun. Apart fromcharacterising the surface of the nucleusand obtaining high resolution imagery ofthe landing site, it was hoped to obtainthree types of sample: a ‘core’ drilled to adepth of at least one metre; a sealed sample

of volatile, icy material; and a sample ofnon-volatile surface material. Stored at thesame frigid temperatures experienced onthe comet, the samples would be returnedfor comprehensive analysis in laboratorieson Earth.

By 1991, a joint ESA-NASA RosettaComet Nucleus Sample Return missionhad been defined, with launch anticipatedin December 2002. The spacecraft was tocomprise three modules. A large NASAMariner Mark II Cruiser would provideattitude control, navigation, power,propulsion and communications. Attachedto the main bus would be a Lander, whichwas to carry a drill and surface samplingtool, and an Earth-Return Capsule. Oncethe samples were safely transferred to acontainer in the Capsule, the spacecraftwould lift off, leaving the Lander behindon the comet. Two and a half years later,the Capsule would parachute into theocean with its precious cargo, ready forcollection by helicopter and ship.

A Revised RosettaWithin two years, NASA’s financialdifficulties and resultant cutbacks in itsspace-science programme (notably thecancellation of the CRAF mission) forcedESA to reconsider its options for Rosetta.

The prime consideration was to define acore mission that could be performed byESA alone, using European technology –although the door was left open for otheragencies to participate. The revisedbaseline mission that emerged involved arendezvous with a comet and at least oneasteroid flyby. It was hoped that a smallLander would be added as an additionalexperiment provided by one or morescientific institutions.

To all intents and purposes, that is themission that has survived to the present.The Rosetta that will be launched towardsComet Wirtanen in January 2003comprises two spacecraft: a 3 ton Orbiter,including 165 kg of scientific instruments,and a 100 kg Lander provided by aconsortium including ESA and institutesfrom Austria, Finland, France, Germany,Hungary, Ireland, Italy and the UnitedKingdom, under the leadership of theGerman Space Agency (DLR).

“That such a complex mission can be built inpartnership and delivered on time is a great tribute tothe management and cooperative spirit of industry,the scientists and the many agencies involved, as wellas ESA’s staff,” says John Ellwood, Rosetta ProjectManager.

Artist’s impression of the ESA-NASA Rosetta Comet Nucleus Sample Return spacecraft

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Illustration by AOES Medialab, ©ESA 2001

Low-gain antenna

Solid-state mass memory

On-boardcomputers

Medium-gainantenna

High-gainantenna

Rosetta

Anatomy of a SpacecraftRosetta is truly an international enterprise,involving more than 50 industrialcontractors from 14 European countriesand the United States. The primespacecraft contractor is Astrium Germany,while Astrium UK (spacecraft platform),Astrium France (spacecraft avionics) andAlenia Spazio (assembly, integration andverification) are major subcontractors.

The Rosetta Orbiter resembles a largealuminium box, 2.8 x 2.1 x 2.0 m. The 11scientific instruments are mounted on thePayload Support Module (the ‘top’ of thespacecraft), while the subsystems are onthe ‘base’ or Bus Support Module.Several kilometres of harness –electrical cable – are also built into theheart of each module.

On one side of the Orbiter isthe main communicationsdish – a 2.2 m-diameter,steerable high-gainantenna – while theLander is attached to theopposite face.

Two enormous solar wings extend fromthe other sides. These panels, each 32 m2

in area, have a total span of about 32 m tip-to-tip. Each of them comprises five panels,and both may be rotated through ±180 degto capture the maximum amount ofsunlight.

In the vicinity of Comet Wirtanen, thescientific instruments will almost alwayspoint towards the comet, while theantennas and solar arrays point towards theSun and Earth (at large distances, theyappear fairly close together in the sky).

By contrast, the Orbiter’s side and backpanels are in shade for most of the mission.Since these panels receive little sunlight,they are an ideal location for thespacecraft’s radiators and louvers whichregulate its internal temperature. They will

also face away from the comet, so thatdamage from cometary dust will beminimised.

At the heart of the Orbiter is the mainpropulsion system. Mounted around avertical thrust tube are two large propellanttanks, the upper one containing fuel andthe lower one the oxidiser. The Orbiter alsocarries 24 thrusters for trajectory and

attitude control. Each of these thrusterspushes the spacecraft with a force of 10 Newton, equivalent to that experiencedby someone holding a bag of 10 apples.Over half the launch weight of the entirespacecraft – more than 1.7 tonnes – ismade up of propellant.

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Propellant tanks

Navigationcameras

Pressurant tank

Power supplyunits behindthe panel

Thrusters

Louvers

Exploded view of theRosetta Orbiter

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OSIRIS (Optical, Spectroscopic, and InfraredRemote Imaging System): A Wide AngleCamera and a Narrow Angle Camera toobtain high-resolution images of the comet’snucleus and asteroids Siwa and Otawara.

ALICE (Ultraviolet Imaging Spectrometer):Analyses gases in the coma and tail andmeasures the comet’s production rates ofwater and carbon monoxide/dioxide. Alsoprovides information on the surfacecomposition of the nucleus.

VIRTIS (Visible and Infrared Thermal ImagingSpectrometer): Maps and studies the nature of the solids and the temperature on thesurface of the nucleus. Also identifies cometgases, characterises the physical conditions of the coma and helps to identify the bestlanding sites.

MIRO (Microwave Instrument for the RosettaOrbiter): Used to determine the abundancesof major gases, the surface outgassing rateand the nucleus subsurface temperature. Itwill also measure the sub-surfacetemperatures of Siwa and Otawara, andsearch for gas around them.

ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis): Two sensors willdetermine the composition of the comet’satmosphere and ionosphere, the velocities ofelectrified gas particles, and reactions inwhich they take part. It will also investigatepossible asteroid outgassing.

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The Rosetta PayloadThe Orbiter’s scientific payload includes11 experiments and a small Lander, whichwill conduct its own scientificinvestigations. The instruments on theRosetta Orbiter willexamine every aspect ofthe small cosmic iceberg(see side panels).

Wide and Narrow AngleCameras will image the comet’snucleus and asteroids Siwa andOtawara in order to determine

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their volume, shape, bulk densityand surface properties. Threespectrometers operating atdifferent wavelengths will analysethe gases in the near-nucleusregion, measure the comet’sproduction rates of water and carbonmonoxide/dioxide, and map thetemperature and composition of thenucleus.

Our knowledge of the nucleus should berevolutionised by the CONSERTexperiment, which will probe the comet’sinterior by transmitting and receiving radiowaves that are reflected and scattered asthey pass through the nucleus.

Four more instruments will examine thecomet’s dust and gas environment,measuring the composition and physical

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COSIMA (Cometary Secondary Ion MassAnalyser): Will analyse the characteristics ofdust grains emitted by the comet, includingtheir composition and whether they areorganic or inorganic.

MIDAS (Micro-Imaging Dust AnalysisSystem): Studies the dust environmentaround the asteroids and comet. It providesinformation on particle population, size,volume and shape.

CONSERT (Comet Nucleus SoundingExperiment by Radiowave Transmission):Probes the comet’s interior by studying radiowaves that are reflected and scattered by thenucleus.

GIADA (Grain Impact Analyser and DustAccumulator): Measures the number, mass,momentum and velocity distribution of dustgrains coming from the nucleus and fromother directions (reflected by solar radiationpressure).

RPC (Rosetta Plasma Consortium):Five sensors measure the physical propertiesof the nucleus; examine the structure of theinner coma; monitor cometary activity; andstudy the comet’s interaction with the solarwind.

RSI (Radio Science Investigation): Shifts inthe spacecraft’s radio signals are used tomeasure the mass, density and gravity of thenucleu; define the comet’s orbit; and studythe inner coma. Also be used to measure themass and density of Siwa, and to study thesolar corona during the periods when thespacecraft, as seen from Earth, is passingbehind the Sun.

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characteristics of the particles, e.g.population, size, mass, shape and velocity.

The comet’s plasma environment andinteraction with the electrically chargedparticles of the solar wind will be studiedby the Rosetta Plasma Consortium and theRadio Science Investigation.

The 100 kg Rosetta Lander carries afurther nine experiments, as well as adrilling system to take samples of sub-surface material. The Lander instrumentsare designed to study in situ for the firsttime the composition and structure of thesurface and subsurface material on thenucleus.

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A Space OdysseyAlthough its cometary target has changedsince Rosetta was first envisaged, thelaunch date has altered very little.Rosetta’s 10-year odyssey will begin inJanuary 2003, when an Ariane-5 launcher

boosts the spacecraft into an elliptical(4000 km x 200 km) trajectory around theEarth. After about two hours, Ariane’supper stage re-ignites to send Rosetta onits way towards the asteroid belt.

The hardy spacecraft will then bouncearound the inner Solar System like acosmic billiard ball, circling the Sunalmost four times during its eight-yeartrek to Comet Wirtanen. Along thisroundabout route, Rosetta will enter theasteroid belt twice, enabling it to glimpsethe ancient, battered surfaces of twocontrasting rocky objects, Siwa andOtawara. It will also receive a boost inspeed from gravitational ‘kicks’ providedby close flybys of Mars in 2005 and Earthin 2005 and 2007.

After a large deep-space manoeuvre,Rosetta will break the record for a solarcell-powered spacecraft as its elongatedpath carries it some 800 million km fromthe Sun. It will then be re-activated for themost difficult phase of the mission – thefinal rendezvous with the fast-movingcomet.

Arriving in the comet’s vicinity inNovember 2011, Rosetta’s thrusters willbrake the spacecraft so that it can matchComet Wirtanen’s orbit. Over the next sixmonths, it will edge closer to the black,

The Rosetta LanderThe box-shaped Lander piggybacks on theOrbiter until it arrives in close orbitaround Comet Wirtanen. Once the‘mother craft’ is aligned correctly, theground commands the Lander to push offand unfold its three legs, ready for a gentletouch down at the end of the slow descent.On landing, the legs damp out most of thekinetic energy to reduce the chance ofbouncing, and they can rotate, lift or tilt toreturn the Lander to an upright position.

Immediately after touchdown, aharpoon is fired to anchor the Lander tothe ground and prevent it escaping fromthe comet’s extremely weak gravity. Theminimum mission target is 65 hours, butsurface operations may continue for manymonths.

The Lander structure consists of abaseplate, an instrument platform, and apolygonal sandwich construction, all madeof carbon fibre. Some of the instrumentsand subsystems are beneath a hood, whichis covered with solar cells for powergeneration. An antenna transmits datafrom the surface to Earth via the Orbiter.

Rosetta’s ‘billiard ball’ path to reach Comet Wirtanen

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Illustration by AOES Medialab, ©ESA 2001

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dormant nucleus until it is only a fewdozen kilometres away. The first cameraimages will dramatically improvecalculations of the comet’s position andorbit, as well as its size, shape and rotation.

By the summer of 2012, Rosetta willenter orbit around the comet, sweeping towithin a few kilometres of the coal-blacksurface. However, the almost imperceptiblegravitational pull of the ‘dirty snowball’will mean that Rosetta need only circleWirtanen at a snail’s pace – a fewcentimetres per second. With the alienlandscape now looming large, theOrbiter’s cameras will start to map thenucleus in great detail. Eventually, anumber of potential landing sites will beselected for close observation.

Once a suitable site is chosen, theLander will be released from a height ofabout 1 km. Touching down gently atwalking speed – less than 1 metre persecond – the ambassador from Earth willanchor itself to the nucleus before sendingback high-resolution pictures and otherinformation on the nature of the comet’sices and organic crust. Scientists back onthe distant Earth will eagerly await thetreasure trove of data from the pristinesurface as it is relayed to ground stationsvia the Orbiter.

The way will then be clear for theexciting comet chase towards the Sun.Over a period of 12 months, the Orbiterwill continue to orbit Wirtanen, observingthe dramatic changes that take place as theicy nucleus begins to warm and vaporiseduring the headlong rush towards the Sun.The escort mission will end in July 2013, atthe time of the comet’s closest approach tothe Sun (perihelion). More than 3800 dayswill have elapsed since Rosetta’s dramaticspace odyssey began.

“The Rosetta mission is something thatwe could only dream about 17 years ago,and now it is becoming an exciting reality”,says Gerhard Schwehm, Rosetta ProjectScientist.

Rosetta Lander Scientific Experiments

COSAC (Cometary Sampling and Composition experiment): One of two evolved gasanalysers, it detects and identifies complex organic molecules from their elementaland molecular composition.

MODULUS PTOLEMY: Another evolved gas analyser, which obtains accuratemeasurements of isotopic ratios of light elements.

MUPUS (Multi-Purpose Sensors for Surface and Subsurface Science): Uses sensorson the Lander’s anchor, probe and exterior to measure the thermal and mechanicalproperties of the surface.

ROMAP (Rosetta Lander Magnetometer and Plasma Monitor): A magnetometer andplasma monitor study the local magnetic field and the comet/solar-windinteraction.

SESAME (Surface Electrical, Seismic and Acoustic Monitoring Experiments): Threeinstruments measure properties of the comet’s outer layers. The Cometary AcousticSounding Surface Experiment measures the way in which sound travels through thesurface. The Permittivity Probe investigates its electrical characteristics, and theDust Impact Monitor measures the dust environment to the surface.

APXS (Alpha X-ray Spectrometer): Lowered to within 4 cm of the ground, itdetects back-scattered alpha particles and alpha-induced X-rays, which provideinformation on the elemental composition of the comet’s surface.

CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission):Probes the internal structure of the nucleus. Radio waves from the CONSERTexperiment on the Orbiter travel through the nucleus and are returned by atransponder on the Lander.

ÇIVA: Seven micro-cameras - six mono and one stereo pair - take panoramicpictures of the surface. A visible-light microscope and coupled infrared spectrometerstudies the composition, texture and albedo (reflectivity) of samples collected fromthe surface.

ROLIS (Rosetta Lander Imaging System): A CCD camera to obtain high-resolutionimages during descent and of the nucleus surface below the Lander and of the areassampled by other instruments.

SD2 (Sample and Distribution Device): Drills more than 20 cm into the surface,collects samples and delivers them to different ovens or for microscope inspection.

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“When beggars die, there are no cometsseen:The heavens themselves blaze forth thedeath of princes.”

Shakespeare’s Julius Caesar

For centuries, comets have inspiredawe and wonder. Many ancientcivilisations saw them as portents of

death and disaster, omens of great socialand political upheavals. Shrouded in thin,luminous veils with tails streaming behindthem, these ‘long-haired stars’ were giventhe name ‘comets’ by the ancient Greeks(from the Greek word kome meaning‘hair’).

Apart from their links to soothsayingand astrology, comets – particularly thevery bright objects visible to the naked eye– have always been popular targets for allkinds of observations and speculation.Their sudden appearance and spectacularshape make them very appealing foramateur astronomers and photographerseverywhere, and the 1997 apparition ofComet Hale-Bopp made headlines around

Exploring A CosmicIceberg

Image of Comet Wirtanen taken from Calar Alto Observatoryin Spain on 6 September 2002 in a 3 minute exposure witha broadband red filter (courtesy of K. Birkle & J. Aceituno)

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the world. But why are scientists so keen tostudy these beautiful intruders into theinner Solar System?

Planetary Building BlocksComets are small icy bodies, usually onlya few kilometres across. Their basicingredients are dust and frozen gases thatwere formed and preserved at very lowtemperatures in the vast, rotating cloud ofmaterial which surrounded the young Sun.Since they have not been altered byinternal heating and spend most of theirlives far from the Sun, these primitivebodies have changed little since theircreation. Today we believe that cometsrepresent the oldest building blocks of ourSolar System, cosmic icebergs from whichthe planets were assembled some 4.6billion (4 600 000 000) years ago.

In those violent times, many cometscrashed into the infant planets. Craterscaused by ancient comet and asteroidimpacts can still be seen on the surfaces ofthe Moon, Mercury and many planetarysatellites. Comets may also have providedmuch of the water that now forms Earth’s

oceans, and may even have delivered thecomplex organic chemicals that led to thefirst primitive life forms.

By learning more about individualcomets, scientists also hope to get abroader understanding of the formationand evolution of our Solar System as a

Expiring andexploding stars

Chemical elements

Shed ice near Sun

Rockyplanets Asteroids

Massiveplanets

Surviving comets

Sun

Meteorites

Late cometimpacts

Activecomets

Grains fromcomet trails

Primeval comets

Interstellar gas and dust

Solar nebula

Comet history

Hubble Space Telescope time-lapse compositeimage of Comet Shoemaker-Levy approachingthe planet Jupiter in 1994

History of a comet

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whole. Like detectives trying to gatherclues about a case, they need to obtaindetailed information about the physicaland chemical properties of comets in orderto discover what the interplanetaryenvironment was like when the planetswere born. Unfortunately, this is far fromeasy since most of the comets have beenhurled into the outer parts of the SolarSystem by gravitational interactions withthe giant outer planets. As a result, only the modest number of comets deflectedtowards the Sun become accessible to ourscrutiny.

Dirty SnowballsUntil the 1986 flyby missions to CometHalley, no one knew what a comet nucleuswas really like. The icy heart of a comet isso small that it is almost impossible to seeand analyse from Earth. As soon as thenucleus moves close enough to us fordetailed observation, it is obscured fromview by the coma, an all-envelopingshroud of gas and dust. When it is inactive,and not hidden by the coma, it is too faraway to be resolved by even the besttelescopes and too faint to allow detailedspectroscopic analysis of its surfacematerial.

The most popular theory about thenature of comets was put forward byAmerican astronomer Fred Whipple, oftenknown as the ‘grandfather’ of moderncometary science. Whipple believed theywere like dirty snowballs – large chunks ofwater ice and dust mixed with ammonia,methane and carbon dioxide. As thesnowball approached the Sun, its outer icesbegan to vaporise, releasing large amountsof dust and gas, which spread throughspace to form the characteristic tails.

Today, largely thanks to data from ESA’sGiotto and two Russian Vega spacecraft,along with the recent encounter by NASA’sDeep Space 1, we now know thatWhipple’s model was fairly accurate.Giotto’s pictures provided the final proofthat Comet Halley resembles a lumpy,peanut-shaped body, about 15 km long and7–10 km wide. The ‘dirty snowball’ wasindeed very dark in appearance – blackerthan coal. The images also showed at leastseven jets of vaporised ice and dust

spurting through vents in the surfacematerial that coated the nucleus –convincing evidence to support theconcept that cometary activity is confinedto a small number of localised active areason the rotating nucleus.

Analysis of the material released whenHalley’s nucleus was exposed to the Sunled to major progress in the understandingof the comet’s composition. New insightswere provided into the physical andchemical processes taking place in theinner coma, and the way in which the

comet interacted with the electrifiedparticles of the solar wind.

Simple organic (carbon-rich) molecules,such as formaldehyde and methanol, werealso detected for the first time. A majorsurprise was the discovery of a new classof minuscule particles, each less than amillionth of a metre across – too small tobe detectable by remote-sensing techniques.These were dubbed ‘CHON’ particlesbecause they were rich in the lightelements carbon, hydrogen, oxygen andnitrogen.

Giotto Halley Multicolour Camera (HMC)image of Comet Halley with, inset, a sketch ofits main features (courtesy of MPAe Lindau)

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Breakthroughs with RosettaThe images and other data returned duringthese short-lived flybys have improved ourknowledge of comets tremendously.However, despite such significantadvances, scientists want to learn moreabout these fleeting visitors to the innerSolar System. Further studies areimportant in order to answer these keyquestions. What role, if any, did cometsplay in the evolution of life? How and whydoes a comet change during repeatedapproaches to the Sun? What lies at theheart of the nucleus, beneath its mysteriousblack blanket? Questions such as these canonly be answered by Rosetta, the mostsophisticated spacecraft ever to investigatea ‘long-haired star’.

Rosetta will not just cast a cursoryglance at its target, Comet Wirtanen, but it will provide the first intimate look at one of these intriguing objects. Theinvestigation will begin while the intruderis still in deep space, 650 million km fromthe Sun. As it accompanies the comet onits headlong charge towards the Sun atspeeds of up to 135 000 km/h, instrumentson the Orbiter will determine the basicproperties of the frozen, inactive nucleus,measuring its size, shape, mass anddensity.

area through micropores, or will thesurface be cracked open at certain areas torelease the material underneath? Nobodyyet knows, but Rosetta will be on station tosolve the mystery.

As soon as the nucleus becomes active,the Rosetta Orbiter will start to analyse thematerial that is released. The comet watchwill continue as Wirtanen acceleratestowards the inner Solar System andactivity on the nucleus becomes ever morefrenetic. Instruments will measure theelemental, molecular and isotopiccomposition of the gas and dust, alongwith the dust size distribution. Individualdust particles will also be collected andscanned by an atomic force microscopewith an imaging resolution of a millionthof a millimetre.

From all of these studies, scientists willdiscover how the level of comet activityinfluences the properties of the material itspews into space. The interaction of thecomet with the interplanetary magneticfield and the particles of the solar windwill complete the monitoring of thecomet’s evolution. Only when Wirtanenreaches perihelion, the closest point in itsorbit to the Sun, will Rosetta’s remarkablemission be terminated.

After the Rosetta Orbiter has moved towithin a few kilometres of the pristinesurface, the entire nucleus will be surveyedat wavelengths covering almost the entireelectromagnetic spectrum. Close-rangeimages taken in visible light will becombined to map every bump and surfacecrack in the alien landscape down to aresolution of just a few centimetres.Parallel mapping from infrared tomillimetre wavelengths will measure thecorresponding surface temperatures,identify individual icy and mineralogicalcomponents and determine theirdistribution on the surface.

This will only be the beginning. As thenucleus is heated, the frozen volatiles(gases) start to vaporise and the dust isreleased, Rosetta will become the firstspacecraft to witness how a dormantnucleus begins to stir into activity andevolve.

Over a full year, the Orbiter will observefrom close range the extraordinarymetamorphosis that takes place in thenucleus and its surrounding coma,providing the long-awaited informationneeded to understand the physical andchemical processes causing thisphenomenon. Will the material besmoothly released from the entire heated

Mass spectrum of a cometary ‘CHON’ particlerecorded in 1986 by the Soviet Vega-1 spacecraft’sdust-impact analyzer. The numbers are AtomicMass Units (AMU)

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Rosetta Lander Science

By making the first soft landing on a comet, the Rosetta Lander will be able toconduct unique investigations into what the nucleus of Comet Wirtanen is made of.

Equipped with 9 scientific experiments, the box-shapedspacecraft is dedicated to studies of the physical propertiesand the composition of the nucleus. All of the instrumentswill take at least one contingency measurement immediatelyafter landing, and then continue to follow the evolution ofthe nucleus at it approaches the Sun. These in situinvestigations will provide unprecedented knowledge of thecomet’s icy heart.

Samples of material will be obtained, not only from thesurface but also to a depth of 20 cm, using a special sampledrilling and handling device. The samples will then beimaged in visible and near-infrared light, and analysed indetail to discover their elemental, isotopic, molecular andmineralogical composition.

Panoramic and close-up images of the surface around andbeneath the Lander will give Earthlings their first views ofthe alien, hostile landscape. The physical properties of the

strange, black surface material, the local magnetic field and the comet/solar windinteraction will also be studied. Meanwhile, the CONSERT instrument (half ofwhich is on the Lander and half on the Orbiter) will sound the interior of the 1.1km-wide nucleus, rather like a physician using ultrasound to study an unborn childin its mother’s womb.

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Asteroid Science

On the outward leg of its eight-year trek toComet Wirtanen, Rosetta will make twoexcursions into the main asteroid beltbetween the orbits of Mars and Jupiter.This will enable the spacecraft to encountertwo contrasting asteroids, 4979 Otawara inJuly 2006 and 140 Siwa in July 2008. Theseprimordial, rocky worlds – leftovers fromthe formation of the planets – have beenselected after careful evaluation of thescientific significance of the reachabletargets combined with an assessment of thespacecraft’s fuel budget.

Siwa, a C-type (carbon-rich) asteroid, isparticularly interesting. Unlike the morecommon S-type, the C-type asteroids arebelieved to have undergone little or noheating, so they are considered to be unaltered, volatile-rich bodies, darkened by opaque organic material. Themeteorite analogues of these primitive asteroids are carbonaceous chondrites. Approximately 110 km indiameter, Siwa will be the largest asteroid ever studied during a spacecraft flyby.

Once Siwa was identified as the prime target, subsequent mission analysis showed that Rosetta’s fuel budgetwould also allow a visit to Otawara, a 4-km-wide S-type asteroid. Otawara rotates faster (about once every threehours) than any asteroid so far visited by a spacecraft and so Rosetta should be able to image most of its surfaceduring the fly past.

A multi-wavelength study will be performed of both asteroid targets. Apart from their basic characteristics suchas size, shape and mass, the measurements will allow scientists to discover the mineralogical composition of theirsurfaces and to search for frozen volatiles, particularly water ice. Very sensitive sensors, designed to analyse thegas and dust in the coma of Comet Wirtanen, will also be switched on to search for evidence of any very sparsecomas surrounding the asteroids, particularly the much larger Siwa.

Comet 140 Siwa

Relative sizes of Otawara, Siwa and other known asteroids

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The International Rosetta Missionwas approved as a Cornerstonemission within ESA’s Horizon 2000

Science Programme in November 1993.Even at this early stage, it was envisagedthat the ambitious mission would bescheduled for launch in the 2003timeframe and a number of comet-rendezvous opportunities were identified.Although the original target, CometSchwassman Wachmann 3, has since beensuperseded by another periodic intruderinto the inner Solar System, CometWirtanen, there has been little shift in theoriginal launch schedule. Rosetta is nowset for lift-off from Kourou on the night of12-13 January 2003.

Ever since the mission was accepted andgiven a slot in the long-term Horizons2000 programme, the teams of ESAengineers and scientists have been engagedin a race against time. Once the design andspecifications of the spacecraft and itspayload were fixed in 1998, just four yearsremained for the Assembly, Integration andVerification phase.

Following the conventional spacecraftdevelopment philosophy, the Rosettaproject team and its industrial partnerswere first required to build a Structural andThermal Model in order to evaluate thedesign and thermal characteristics of thesatellite. This was to be followed aboutseven months later by the delivery of anEngineering and Qualification Model thatwould be used to demonstrate thatRosetta’s electrical and other subsystemswould operate correctly in the extremeenvironment of deep space. Only thenwould the Flight Model be assembled andput through a final series of exhaustivetests that would check out overallperformance and flight readiness.

A Race Against Time

Rosetta Structural and Thermal Model undergoing vibrationtesting at ESTEC in January 2001

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Rosetta

Meanwhile, teams from many countrieswere also required to deliver a Structuraland Thermal Model, an Engineering andQualification Model and a Flight Model ofeach of their instruments that would beused to survey the comet. With a three-month launch campaign scheduled tobegin in early September, there was notime to pause for breath and 24-hour shiftsbecame commonplace for the engineersand scientists who endeavoured to ensurethat Rosetta would leave the pad on time.

“The heavens have their own timetable and thecomet won’t wait for us if we’re late,” said John Ellwood,Rosetta Project Manager.

The Flying ItalianPreparing a 3 tonne spacecraft for a seriesof endurance tests is far from easy. Beforethe thermal vacuum checks could takeplace, intrepid Alenia Spazio engineerNatalino Zampirolo was required toimitate an acrobat on a high wire.Suspended from an electric hoist, theengineer was required to ‘fly’ alongside thespacecraft at a height of 5 metres above thefloor of the giant test chamber.

Dangling next to the Rosetta orbiter,Zampirolo gingerly removed the ‘red tag’items – protective covers, arming plugs onthe explosive connectors etc. – that werefitted as a safety precaution during normalwork on the spacecraft. With his tasksuccessfully accomplished, the flyingItalian was relieved to retreat to safety,leaving the spacecraft armed and ready tostart its thermal trial.

Rosetta Runs Hot and ColdOne of the key stages in the testprogramme was to establish whetherRosetta could maintain reasonableworking temperatures throughout itscircuitous trek to the orbit of Jupiter andback. In order to check the efficiency ofRosetta’s thermal-control system, engineersat the European Space Research and

Sensors indicated that the spacecraft’sinsulation and heat control systemsenabled Rosetta to survive these thermaltortures in fine shape, with internaltemperatures restricted to between 40°Cand -10°C. ESTEC engineers confidentlypredicted that, with the aid of its radiatorsand reflective louvers, Rosetta will be the‘coolest spacecraft’ around.

“These tests show that Rosetta can survive thetremendous temperature contrasts it will endure as itflies from the vicinity of the Sun to the orbit of Jupiter,”says Claude Berner, Payload and Operations Manager.“This gives us great confidence that the spacecraft willbe able to survive its long exposure to the harshenvironment of space.”

Technology Centre (ESTEC) in theNetherlands placed the spacecraft in athermal-vacuum chamber where the wildlyfluctuating temperatures that Rosetta willexperience could be replicated.

Imprisoned in the giant airless chamber,the Rosetta Orbiter, the Lander and theircomplement of 20 scientific instrumentswere alternately baked and frozen. In orderto simulate the warmth of the inner SolarSystem, the exterior of the spacecraft washeated to a sizzling 150°C by a solarsimulator comprised of 12 lamps eachradiating 25 kW. During subsequent tests,liquid nitrogen was pumped through pipesin the chamber, causing the temperatureinside to plummet to -180°C.

Rosetta Flight Model in the Large Space Simulator (LSS) at ESTEC in March 2002

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Rosetta Breaks the Sound BarrierEven before Rosetta has left the planet, thespacecraft has managed to break the soundbarrier. In April 2002, the Flight Modelwas removed from the thermal vacuumchamber and prepared for the next stage ofits pre-launch punishment. Once the high-gain antenna and huge solar arrays weremounted, the Orbiter was subjected to aseries of deafening vibration tests in orderto check whether it can survive the stressesit will experience during launch.

“The spacecraft was powered on, while the Lander,the high-gain antenna and the solar arrays were all inlaunch configuration,” explained Claude Berner. “Eventhe propellant tanks were filled with ‘dummy fuel’.”

Placed in a giant acoustic/vibrationchamber, a barrage of sound was directedat Rosetta from a huge amplifier in orderto simulate the noise expected during lift-off. Soaring to a maximum of 135 decibels– ten times louder than Concorde at take-off – the sound levels were so severe thatanyone straying into the chamber wouldhave been killed within seconds.

Following these not-so-good vibrations,Rosetta returned to the clean room tocomplete a rock-and-roll ride on a giantshaker in order to simulate its ride intoorbit aboard an Ariane-5 rocket. Attachedto a table capable of moving the 3 tonnespacecraft from side to side like a metallictoy being mauled by a mastiff, Rosetta wasseverely shaken, first horizontally and then vertically, over a wide range offrequencies. Several hundred accelero-meters on the structure were used tomonitor the spacecraft’s performanceduring each three-minute simulation. Theresults confirmed that the launch by thepowerful Ariane-5 would leave Rosettashaken but not stirred.

Solar WingsOnce the engineers at ESTEC had verifiedthat all of the spacecraft’s electronics hadsurvived intact, it was time to checkwhether the pair of 14 m-long solar arraysand the delicate instrument booms hadsurvived their potentially shattering ordeal.

Most critical of all were the deploymenttests on the two giant ‘wings’ that will

power Rosetta throughout its 10-yearmission to deep space and back. Thesearrays, stretching one and a half times thelength of a tennis court, must gently unfoldto expose the special silicon cells that willgenerate electricity for the spacecraft tosunlight.

The ‘minus-y’ array, located to the left ofthe dish-shaped high-gain antenna, was thefirst to be unfolded. This was followed aday later by deployment of the ‘plus-y’array on the opposite side of thespacecraft. Held in place by six Kevlarcables that will embrace the arrays duringlaunch, each solar array was released aftercommands sent via the spacecraft activatedthe deployment sequence. ‘Thermalknives’ then severed each cable in turn byheating it to a temperature of severalhundred degrees Celsius.

After the sixth cable was cut, the arraybegan to unfold like a giant accordion.Attached to a huge, specially developed,deployment rig, the five panels in eacharray were gradually extended to their full

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equipment that will investigate themagnetic field and particle environmentaround Comet Wirtanen.

The fifth and final deployment testinvolved the release of a wire antenna to beused by the CONSERT experiment. Afteranother explosive charge was fired, thisunusual, H-shaped aerial was gentlyunfolded, suspended beneath five heliumballoons to simulate the weightlessness ofspace.

“Once again, the trial was completed without ahitch,” announced a proud Marc Schwetterle, one of thepayload engineers responsible for the tests.

“All of the deployment tests were very successful,”commented Walter Pinter-Krainer. “These were crucialmoments in our test programme and we were veryhappy to see everything working so well.”

Rosetta Cleared of InterferenceThe hectic schedule continued in Junewhen Rosetta was moved into anotherlarge test chamber at ESTEC, known as theCompact Test Range, where it wassubjected to an extensive electromagnetic-compatibility (EMC) check.

In order to simulate the EMCenvironment during its long trek throughdeep space, Rosetta was placed inside a

length across the clean room. To simulatethe zero-gravity conditions of outer space,the weight of the arrays wascounterbalanced by a mass-compensationdevice equipped with dozens of springs.

“Both tests went very well and there was a big roundof applause when they were successfully completed,”said Walter Pinter-Krainer, Principal AIV SystemsEngineer for Rosetta.

Checking the BoomsConfident that their spacecraft’spowerhouse would deploy properly afterlaunch, the engineers went on to check outRosetta’s other movable parts. First came apartial deployment of the orbiter’s 2.2 m-diameter high-gain antenna, when threepyros (explosive charges) were fired torelease the dish from its stowed launchposition.

The engineers also had to retreat to thesafety of an observation area in the cleanroom for the firing of more pyros duringthe deployment of the upper and lowerexperiment booms on the Orbiter. Each 2 m-long boom carries probes and other

Unfurling of the spacecraft’s solar array

Rosetta Flight Model inthe Acoustic Chamberat ESTEC

The CONSERT experiment unfolded

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chamber lined with cones that absorb radiosignals and prevent reflections. To avoidTV or radio interference, the chamberwalls form a steel ‘Faraday cage’,impenetrable to electromagnetic signalsfrom the outside world. In this radiation-free environment, the ESTEC team wasable to study the radio signals andelectrical noise coming from the varioussystems on the spacecraft and to checkwhether they caused any electromagneticinterference with each other.

“Before a satellite is launched it is essential to ensurethat the electrical and electronic equipment within aspacecraft functions correctly,” explains FlemmingPedersen, a senior AIV engineer for Rosetta. “Forexample, it could be fatal if, when switching on one unit,other instruments or systems such as thetelecommunication link, were disturbed or evendisrupted.”

Like some alien creation, the spacecraftwas cocooned in protective plastic foilwhile the engineers and scientistspainstakingly prepared to switch onRosetta’s systems and payloads. At first,the see-through wrapping proved to be tootight, causing the spacecraft’s temperatureto rise. Once this was remedied and thestaff vacated the chamber, all was set tosimulate the various phases of Rosetta’s10-year mission.

“For some of the time we were measuring the energyemitted by the spacecraft’s high-gain antenna, and thisis hazardous, so the chamber was completely closed andeveryone had to remain outside it whilst themeasurements were made,” said Flemming Pedersen.“It would be like exposing the engineers to the radiationfrom thousands of mobile phones simultaneously.”

The first series of tests studied how thespacecraft behaved in ‘launch mode’. At

this time Rosetta was in its launchconfiguration, with a minimum of systemsactive while awaiting the lift-off of theAriane-5 rocket. This was to ensure thatsignals from the spacecraft would notinterfere with communications betweenthe rocket and ground control during thelaunch phase.

Subsequent EMC tests took place whenthe spacecraft was at various levels ofactivity – from quiet periods when noscience payloads were operating, to spellsof hectic scientific investigation.

“We could switch on each instrument individually andmeasure the electromagnetic waves coming from it,”explains Bodo Gramkow, Principal Payload engineerand EMC expert. “The rest of the instruments were putinto listening mode to see if any of them detected anydisturbance.”

“On other occasions we switched on all of theinstruments, including those on the Lander, in order tosee whether we got any unexpected ‘noise’ orinterference,” he says. “From this we could determinewhether we will need to switch a particular instrumentoff when we are making a very sensitive measurementwith another one.”

“All of the EMC tests proved to be very successful,”says Bodo Gramkow. “This was the last of the three bigsystem-validation tests and Rosetta passed with flyingcolours.”

Ironing-out the ‘Bugs’ (from the Pilot’s Seat)One key question that needs answeringwith confidence is: “Will Rosetta fly?” Ifthe onboard computers are receiving all theright signals to convince them that thespacecraft is flying, will they reactcorrectly? That question was asked andanswered several times throughout 2002 inthe form of an extensive set of MissionSimulation Tests.

It’s 6 a.m. and a new shift starts. Into thecontrol room come the software expertsand they take up their positions in front oflarge, crowded, computer screens. Two setsof computers are synchronised: one set, onthe ground, simulating a ‘real-space’environment for their counterparts

A ‘cocooned’ Rosetta Flight Model in the Compact Test Range facility at ESTEC

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onboard. The ground equipment feeds theSpacecraft Star Tracker with a simulatedstar-field image, and the onboardcomputers react and command a smallattitude change for the spacecraft body.

Standing in the clean room and watchingfrom the outside, nothing changes, but onthe inside, from the perspective of themany onboard computers .... Yes! Rosettais flying! So it goes on. For each missionphase the spacecraft computers are putthrough their paces.

“The functional testing of such a complex spacecraftas Rosetta should not be underestimated,” says MarkNesbit, the ESA engineer responsible for defining muchof the functional test programme. “We test thespacecraft system in many closed-loop simulations, withthe full scientific payload on-line, performing cometobservations just as it will during the actual mission.”

container and purged with nitrogen toprevent contamination, the Orbiter and theattached Lander were loaded onto aRussian Antonov-124 air freighter atAmsterdam airport for the flight toCayenne.

Over the next four months, thespacecraft would be prepared for launch inthe specialist facilities at Kourou. Afterinstallation of the high-gain antenna, thefolded solar wings, the explosive pyros andthe spacecraft batteries, Rosetta is to bemoved to another building for hazardousoperations where its propellant tanks willbe filled and pressurised. By early January2003, it is scheduled for mating with theupper stage of the Ariane-5 launch vehicle.Five days after the fairing installation on 6January, the huge rocket and its preciouspayload will inch along the causeway tothe launch pad.

If all goes well, Rosetta’s long, hard roadfrom initial acceptance to successfullaunch will take place on the night of 12-13 January. Eight years later, the odysseywill be completed when Comet Wirtanensails into view and the expedition toexplore this small, primordial world beginsin earnest. r

“But for Rosetta,” Mark continues, “we have to go afew steps further and we give the onboard computers ahard time. We feed them with unexpected situations,where the onboard autonomy has to take over andautomatically recover the spacecraft into a safe mode.The variety in the mission profile and the long periodswithout ground contact mean that Rosetta has to be ableto take care of itself, and on the ground we have toconvince ourselves that it can.”

The Trip to the TropicsWith the completion of the AIVprogramme, the final leg of Rosetta’s raceto the launch pad could get under way. Itwas time to transport ESA’s comet chaserto the Kourou spaceport on the other sideof the world. First to be packed was theground-support equipment, which wasloaded onto transporters for the short road journey to Rotterdam. There, thecontainers were transferred onto a regularArianespace supply ship for the two-weekvoyage to French Guiana.

This was followed in early September bythe Rosetta spacecraft itself, minus thelarge high-gain antenna and the twin solararrays. Cocooned inside a protective

The Rosetta Lander undergoing testing at ESTEC

The Rosetta Team, with the Flight Model spacecraft in the background

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Few enterprises are more difficult or hazardousthan space travel. Yet, even when compared withthe achievements of its illustrious predecessors,

ESA’s Rosetta mission to orbit Comet Wirtanen anddeploy a lander on its pristine surface must be regardedas one of the most challenging ventures ever undertaken

in more than four decades of space exploration.

The first of the challenges faced by theRosetta project team, the scientific

collaborators and industrialpartners was to design, build andtest the complex comet chaser intime to meet the scheduledlaunch date in January 2003.With less than four years fromthe beginning of thedevelopment phase to launch,it was only possible to meetthe series of tight deadlinesthrough highly efficient andmotivated team work, longshifts and remarkable

dedication by all involved.

Having overcome the timeconstraints associated with the launch,

the hundreds of engineers and scientistsinvolved in Rosetta are now about to face the ultimateassessment of their endeavour – the ability of theircreation to not only survive in deep space for more thana decade, but to successfully operate in the close vicinityof a comet and return a treasure trove of data that willrevolutionise our knowledge of these mysterious worlds.

Rosetta Rises tothe Challenge

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Fortunately, the Ariane-5 operator,Arianespace, has an excellent record inlaunching payloads within such a restrictedtime frame and has expressed itswillingness to do everything possible toensure that the launch will take placewithin the three-week window.

Meanwhile, the project team is alsotaking precautions to ensure that Rosettalaunches on time:

“We do not anticipate any drastic launchpostponement,” says John Ellwood, Rosetta projectmanager, “However, it is always prudent to havecontingency plans. For example, we are able to workthree shifts a day if the spacecraft testing or preparationfall behind schedule. There will be a spare upper stageat the Kourou launch centre if there are problems withAriane-5 close to launch. A spare Ariane-5 main enginewill also be on hand in France ready for shipment within24 hours.”

After the EPS upper stage and itsRosetta payload have been safely placedinto a trajectory around the Earth, the nextchallenge will be to send the spacecraft onits way towards Comet Wirtanen, just twohours after lift-off from Kourou. Prior toreaching perigee (the closest point to theEarth), the upper stage will be re-ignited toinject Rosetta into the required Earth-escape trajectory towards Mars and theasteroid belt. This will be the first time thatan Ariane-5 has boosted a spacecraftbeyond Earth orbit.

Survival in Deep SpaceEnsuring that the spacecraft survives thehazards of travelling through deep spacefor more than 10 years is one of the greatchallenges of the Rosetta mission. Thisprolonged journey will provide theultimate test of the spacecraft’s long-termreliability, robustness and ability to copewith unexpected problems.

Once the spacecraft is safely on its way,


Rosetta

The Launch WindowAs history has frequently shown, the firsthours of a spacecraft’s journey to thedepths of the Solar System can be amongthe most critical. In the case of Rosetta, theodyssey will begin with launch fromKourou spaceport aboard an enhancedversion (P1 Plus) of Ariane-5.

The first obstacle to be overcome is thelimited launch window. In order to meet upwith Comet Wirtanen, Rosetta must belaunched within a period of 19 days,starting on 13 January 2003. On six ofthose days there will be a launch windowof just 20-30 minutes. The remaining daysaccount for a roll back of the rocket forreplenishing of the cryogenic fuel. If thisopportunity is missed, the mission willhave to be postponed while another targetis selected.

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its solar arrays and booms have beendeployed and its systems checkout hasbeen completed, Rosetta will have tosurvive lengthy periods of inactivity,punctuated by relatively short spells ofintense action – the encounters with Mars,Earth and two asteroids.

Apart from the hazards posed by thehostile space environment – dust impacts,energetic solar particles, cosmic rays andextremes of temperature – the spacecraftwill spend roughly two years inhibernation. To limit consumption ofpower and fuel, almost all of Rosetta’selectrical systems will be switched off,with the exception of the radio receivers,command decoders and power supply. Atsuch times, the spacecraft will spin onceper minute while it faces the Sun, so thatits solar panels can receive as muchsunlight as possible.

Onboard AutonomyApart from the necessity to place Rosettain hibernation for half of its interplanetarytrek, the operational situation is furthercomplicated by occasional communicationblackouts (up to four weeks long) due tosolar occultations, when the spacecraftpasses behind the Sun. Even underoptimum conditions, daily communicationopportunities from the Australian groundstation will last a maximum ofabout 12 hours.

So what happens if someunexpected malfunction ordamage occurs during theprolonged interplanetary voyage?There were two options: (i) toprovide Rosetta with enough‘artificial intelligence’ to solvethe problem without humanintervention; or (ii) to analyse theproblem on the ground andtransmit remedial instructions tothe spacecraft.

Unfortunately, although thesecond of these is usuallypreferable, the laws of physicsand the vast distances involvedmean that it is rarely feasible. Formuch of its trek to CometWirtanen, Rosetta will be beyondthe orbit of Mars, eventually

venturing all the way out to the orbit ofJupiter, 780 million kilometres from theSun. Even travelling at the speed of light(300 000 km/s), radio signals will take upto 45 minutes to cross the vast gulf betweenthe distant spacecraft and the Earth. By thetime Rosetta receives a response, at leastone and a half hours will have elapsed.

Since real time command andmonitoring from the ground are out of thequestion, Rosetta has been designed with aconsiderable degree of autonomy. Althoughfour onboard computers are programmedto deal with tasks such as datamanagement and attitude and orbit control,only two are required to be operational atany one time. If the onboard monitoringsystem detects a problem that threatens thehealth of the spacecraft, the computers will take immediate corrective action,switching to a backup system if necessary.

“Rosetta has been designed to carry multiplecomputers that provide it with a sophisticated failurerecognition and recovery capability,” explains Jan vanCasteren, Spacecraft System Manager. “It’s a highlyautonomous system based on two computers, each withtwo separate parts that can be interchanged. We alwayshave the option to upload new, enhanced software overthe 10-year mission. The software for each computercan also be interchanged. This means that both the Data

Management System and the Attitude and Orbit Controlsubsystem can be run on all processors. If the spacecraftis in serious trouble, it automatically goes into safemode – in other words, it goes into hibernation with itssolar arrays pointing at the Sun. These backup systemsshould ensure that the spacecraft will remainoperational during critical mission phases, including thehighly complex scientific observations when Rosetta isorbiting close to the comet's nucleus.”

Solar PowerVenturing far from the Sun puts otherserious constraints on Rosetta’s design andperformance, particularly its electricitysupply and temperature.

Rosetta will set a record as the firstspace mission to journey far beyond themain asteroid belt while relying solely onsolar cells for power generation.Unfortunately, there is a price to pay. WhenRosetta reaches the orbit of Jupiter, wherelevels of sunlight are only 4% those onEarth, the spacecraft will be generatingonly 1/25th of the electricity that it canproduce when in the inner Solar System.

In order to compensate for this drop inpower, Rosetta is equipped with twoenormous, steerable solar arrays that span32 m tip-to-tip (longer than a tennis court)and cover an area of 62 square metres.Each of the ‘wings’ comprises 5 panels and

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is fitted to the spacecraft bodywith a yoke and drive mechanism,allowing 180° rotation in order tocapture the maximum amount ofsunlight. Both sides of the arraysare electrically conductive, toavoid a buildup of electrostaticcharge.

More than 22 000 non-reflectivesilicon cells have been speciallydeveloped for the Rosetta mission.The main challenge was to achievemaximum efficiency by designinga pyramid-shaped, non-reflectiveupper cell surface. Optimised forlow-sunlight (40 W/m2), low-temperature (-130°C) operation,these cells should provide an end-of-life conversion efficiency of15%. They will generate up to8700 W in the vicinity of the Earthand around 400 W (equivalent tothe power consumed by fournormal light bulbs) during thedeep-space comet encounter.

Hot and ColdImagine leaving home for 11 years, toembark on a trek that will take you fromthe frozen wastes of Antarctica to thescorching deserts of Arabia. Working outhow to survive such extremes of hot andcold would be a major headache.

In the same way, dramatic temperaturevariations were a major cause of concernfor Rosetta's designers. When thespacecraft is cruising around the innerSolar System, bathed in the warmth of theSun, its surface temperature may soar to130°C, and even internal equipment mayreach 50°C. However, in order torendezvous with Comet Wirtanen, Rosettawill have to probe the frigid regionsbeyond the asteroid belt, wheretemperatures plummet to -150°C.

Since it is not feasible to wrap aspacecraft in multiple layers of warmclothing for periods of deep freeze, thenstrip these away when sunbathing is theorder of the day, Rosetta has been providedwith alternative ways of regulating itstemperature.

Near the Sun, overheating will beprevented by using radiators to dissipate

CommunicationsCommunications play an even moreimportant role in space missions than theydo in our everyday lives. Whereas it isreasonable to believe that a good friend issafe and sound after several weeks withoutspeaking to them, the same cannot beassumed for a spacecraft millions ofkilometres from home.

The task of keeping in touch with thefar-roaming Rosetta falls on the operationsteam in the Mission Control Centre at theEuropean Space Operations Centre(ESOC) in Darmstadt, Germany. They areresponsible for mission planning,monitoring and controlling the spacecraftthroughout its circuitous voyage to CometWirtanen. The flight-dynamics team atESOC will calculate and predict its attitudeand orbit, prepare orbit manoeuvres, andevaluate spacecraft dynamics andnavigation. Ground controllers will alsoevaluate Rosetta’s performance, preparingand validating modifications to onboardsoftware when necessary.

Planning of the scientific mission andgeneration of commands to experimentswill be conducted either from the Science

surplus heat into space. 14 louvers – high-tech Venetian blinds that control heat lossfrom the spacecraft – are fitted over theradiators on two sides of the spacecraft.Lovingly polished by hand, theseassemblies of thin metal blades must behandled like precious antiques, since anyscratching, contamination or fingerprintswill degrade their heat-reflecting qualities.

The principle behind the louvers is quitesimple. In the balmy regions betweenEarth and Mars, they are left fully open,allowing as much heat as possible toescape into space from Rosetta's radiators.During the prolonged periods ofhibernation and comet rendezvous,however, heat conservation is the order ofthe day. Since the spacecraft’s limitedinternal power supply – equivalent to theoutput from a few light bulbs – thenbecomes the main source of warmth, it isessential to retain as much heat as possible.This means completely closing the louversto prevent any heat from escaping. Heaterslocated at strategic points (e.g. fuel tanks,pipework and thrusters) will also be turnedon and the multi-layered blankets ofinsulating material come into their own.

The Main Control Room at the European SpaceOperations Centre (ESOC) in Darmstadt, Germany

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Operations Centre at ESOC or fromESTEC. During the climax of the mission,ESOC will be responsible for pre-processing, archiving and distributing theunique cometary data to the scientificcommunity. Lander operations will beundertaken from the DLR Lander ControlCentre in Cologne, Germany, with supportfrom the CNES Lander Science Centre inToulouse, France.

In order to keep in touch with theiritinerant explorer, ground control will beable to use radio communications at bothS-band (2 GHz) and X-band (8 GHz)frequencies. Rosetta itself has thecapability to ‘speak’ and ‘listen’ to Earth

via several antennas. Two low-gainantennas with wide-angle coverage willsupport emergency operations in S-band.There are also two medium-gain antennas,one for S-band and one for X-band.However, the primary communicationslink will be the 2.2 m-diameter high-gainantenna, which will be able to send andreceive large amounts of data at bothfrequencies. This lightweight steerabledish is largely made of carbon fibre andtips the scales at just 45 kg, including theelectronics.

The rate at which information can betransmitted from the spacecraft will varyconsiderably with its distance from Earth(up to 930 million km) and its level ofactivity, ranging between 8 bit/s and 64kbit/s. However, during the comet-exploration phase, the minimum telemetry

data rate will be 5 kbit/s. New computersoftware will be used to compress the data,so compensating for the limited downlinkbandwidth and ensuring that as muchinformation as possible will be returned.The unique results from Rosetta’sexperiments can be stored temporarily inthe spacecraft’s 25 Gbit mass memory forrelay to Earth at a convenient time.

To ensure that optimal contact ismaintained with the wandering spacecraft,ESA’s network of ground stations has beenaugmented with a specially-built groundstation at New Norcia (Western Australia).Equipped with a newly constructed 35 mdish and cryogenically cooled, low-noise

amplifiers to receive Rosetta’s weak radiosignal, the state-of-the-art antenna will beremotely controlled from ESOC. NASA’sDeep Space Network will offer back up fortelemetry, telecommand and trackingoperations during critical mission phases,while the Kourou station in French Guianawill provide additional support during thelaunch, early-orbit and near-Earth phasesof the mission.

LongevityOne of the most significant but least-tangible challenges to Rosetta’s success isthe insidious passage of time. Not onlymust the hardware survive for more than10 years in the hostile environment ofspace, but the mission teams must continueto function efficiently throughout theentire voyage.

At the time of the critical near-cometoperations, it is very likely that many ofthe engineers who designed and tested thespacecraft 10 or 15 years earlier will nolonger be available to offer support ifunforeseen circumstances arise. In fact, afair number of them may well have retired.In preparation for this inevitable turnoverof personnel, a number of younger peopleare being drafted into the instrumentteams, including several principalscientific investigators. In order to ensurethat the replacements can slot into theprogramme as easily as possible, theRosetta project is creating a database thatcontains complete information about the

spacecraft and its complex mission.The unusual duration of the mission

means that considerable attention must bepaid to proficiency and cross training ofstaff to guarantee backup support, whilerefreshing skills and motivation. Facilitiesavailable at ESOC to support tests andsimulations will include a spacecraftsimulator, an onboard-software maintenancefacility, and the spacecraft electricalmodel. If spacecraft anomalies arise, theseitems at ESOC will be indispensable forreproducing the problems, developing andtesting solutions prior to implementingcorrective actions on the spacecraft itself.

“The first eight months of the mission will involveintensive activity that will enable everyone to becomefamiliar with the spacecraft’s behaviour,” explainsManfred Warhaut, Rosetta Ground Segment Manager.

The New Norcia ground station in Western Australia

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“There will then be periodic training and communicationactivities – once every 6-12 months – to keep the teamon the ball, as well as for the operations during theEarth and Mars flybys. We will take care to haveadequate training before each manoeuvre.”

NavigationRosetta’s mission resembles a multi-million-kilometre, high-speed chase,hopefully culminating with the spacecraftand comet travelling alongside each otheron parallel paths. Unfortunately, if Rosetta

arrives too early or too late, the comet willnot be there to meet it!

Achieving a long-distance rendezvousand matching orbits requires some highlycomplicated navigational calculations,particularly since not even the powerfulAriane-5 has the capability to send a threetonne spacecraft directly to CometWirtanen. To match the comet’s velocity,Rosetta will have to be accelerated by 7.8 km/s after leaving Earth orbit. Toachieve this, the spacecraft will bouncearound the inner Solar System like acosmic billiard ball, gaining speed from

pull of nearby planets. In the case ofComet Wirtanen, the orbit wassignificantly altered by close encounterswith Jupiter in 1972 and 1984. As a result,the comet’s orbital period was shortenedfrom 6.7 years to 5.5 years. Outgassingfrom the nucleus may also modify acomet’s trajectory, so a continuousobservational programme using ground-based observatories has been put into placeto accurately pin down Wirtanen’s path.

Even if Rosetta follows the optimumroute to its target, the finalrendezvous is complicated bythe fact that the comet’s andthe spacecraft’s orbital planesdo not coincide. Thus, about8.5 years into the mission, thespacecraft’s bi-propellantreaction-control system willhave to be fired to make thelargest manoeuvres of themission in order to phaseRosetta’s orbital plane withthat of the comet. Once theonboard navigation camerashave detected the dark,inactive comet from adistance of some 300 000 km,the task of closing on thenucleus will become morestraightforward.

Remote ScienceRosetta exists for one purpose– to complete humankind’sfirst extended exploration of acomet. Although it has beenequipped with a suite of state-of-the-art instruments, this

alone is insufficient to ensure a successfuloutcome. Different experiments needdifferent conditions to work properly andoptimally. A particular environment,spacecraft orientation or orbit that isperfect for one instrument might fatallycompromise the performance of another.For instance, Rosetta’s dust-collectinginstruments must pass through a dust jetfrom time to time to gather the ejectedpristine material, but care must be taken toensure that too much dust does not collecton optical instruments and other sensitivesurfaces.

three planetary flybys. After an initialboost of 3.4 km/s from Ariane-5’s upperstage, the spacecraft must take advantageof gravity assists from Mars (in 2005) andEarth (in 2005 and 2007).

In order to ensure that Rosetta stays oncourse throughout this circuitous passage,ground controllers will have to carefullymonitor the spacecraft's velocity andposition. This will be done by monitoringthe telemetry and navigation images fromthe spacecraft, and by visual and radar

tracking during the Earth encounters.Studies of the shift in frequency of theradio signals coming from Rosetta – theDoppler shift – will reveal how fast therobotic ambassador is travelling towards oraway from us.

However, this is not sufficient to tiedown Rosetta’s motion. Like a SpaceShuttle pilot trying to rendezvous with thefast-moving International Space Station, itis essential to know the quarry’s position,speed and direction of movement.Unfortunately, comet orbits can beseverely influenced by the gravitational

Artist’s impression of Rosetta’s Mars flyby

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Clearly, proper coordination of thescience operations is very important andevery effort has to be made to coordinatethese operations in order to maximize theoutcome of the mission. This has beendone by defining different ‘missionscenarios’ to meet the needs of thedifferent instruments. In the aboveexample, one mission scenario will bededicated to the collection of dust (andgas) in a very active part of the coma (ajet), during which the cameras will beprotected by leaving their covers on.

Each investigation also presents its ownchallenges. One of the most revolutionaryexperiments is CONSERT, which will sendshort pulses of radio waves through thecometary nucleus. By transmitting andreceiving the pulses on the Orbiter and theLander, the attenuation and the time delayof the radio signal propagating through thecomet are determined. From this soundingdata, information on the interior of thenucleus can be retrieved.

Since the time taken for wavepropagation through different nucleusmaterials varies by tiny amounts, it iscrucial to ensure that the clocks on theOrbiter and Lander are offset by no morethan 5 to 100 nanoseconds (a nanosecondis a billionth of a second).

Imaging with instruments such asOSIRIS presents a different type ofchallenge. During the fast flybys at twoasteroids, scientists want to take as manyimages as possible of the illuminatedareas, as well as numerous high-resolutionimages when Rosetta is at its closest to thesurfaces.

A number of constraints have to beconsidered here. Due to the rotationrequired to follow the asteroid, thespacecraft has times where it is not stableenough to take images. Between images, itmay be necessary to turn the filter wheelvery rapidly. Furthermore, the preciseflyby time is uncertain until a few hoursbeforehand and the number of images hasto be restricted because of the limitedmemory onboard the spacecraft.

Simultaneous imaging of very luminousand very dim objects is a particularheadache close to the comet. One ofRosetta's main objectives is to image the

faint dust jets emanating from the nucleuswhilst the brightly illuminated nucleus is also in the field of view. Suchrequirements were a major design driverfor OSIRIS and could only be achieved bya combination of extremely flat mirroroptics, complex high-resolution readoutelectronics and considerable work by thedevelopers to remove any ‘noise’ from thesystem.

Landing on a Dirty SnowballLandings on other worlds are remarkablydifficult to achieve. During the last 40years, the only objects in the Solar Systemon which robotic spacecraft have soft-landed have been the Moon, Venus, Marsand the near-Earth asteroid Eros. A decadefrom now, it will be the turn of Rosetta tomake history with the first touch down ona comet.

By the summer of 2012, instruments onthe Rosetta Orbiter will have mappedevery square centimetre of the CometWirtanen’s surface, enabling scientists to

select a suitable landing site. The Orbiter'sposition and speed must by that stage havebeen precisely determined to within 10 cmand 1 mm/s, respectively, to ensure thatLander ejection takes place at the correcttime for arrival at the selected site.

Following instructions uploaded frommission control, the 100 kg Lander will bereleased from the Rosetta Orbiter aboutone kilometre above the comet’s nucleus.After a gentle push away from the ‘mothership’, the box-shaped craft will deploy itslanding gear and edge towards its target,prevented from tumbling by an internalflywheel that provides stability as its spins.A single cold-gas thruster will be able toprovide a gradual upward push to improvethe accuracy of the descent.

After a nail-biting 30 minute descent,sensors onboard the Lander will record thehistoric moment of touchdown for thehelpless mission team back on Earth. Sincethe nucleus is so small, its gravitationalpull will be extremely weak – 100 000 to200 000 times lower than on the Earth’s

Rosetta orbiting Comet Wirtanen (artist’s impression)

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the comet. In order to ensure a significantscience return, the most important imagesand measurements will be obtained within60 hours of arrival. The remaining, moredetailed investigations will be conductedas long as the Lander continues tofunction, relying on the remaining batterypower and energy from the solar cells onthe exterior of the Lander.

No one knows what this first softlanding on a cosmic iceberg will reveal.What we can be sure of is that the Orbiterand its little Lander will revolutionise ourknowledge of comets, providing newinsights into the nature and origins of theseprimordial objects. r

surface – causing the Lander to touchdown at no more than walking pace.Nevertheless, a damping system in thelanding gear will be available to reduce theshock of impact and prevent a rebound.

The Lander also carries two harpoons.One of these will be fired at the moment oftouchdown to anchor the spacecraft to thesurface and prevent it from bouncing. Icescrews on each leg will also be rotated tobite into the nucleus and secure the Landerin place. The second harpoon will be heldin reserve for use later in the mission if thefirst one becomes loose.

“Hopefully, gradient will not be a problem, since thespacecraft is designed to stay upright on a slope of up toabout 30 degrees,” says Philippe Kletzkine, RosettaLander Manager.

The operational lifetime of the Lander ishighly uncertain. Its survival will dependon a number of factors such as powersupply, temperature or surface activity on

The Rosetta Lander on Comet Wirtanen’s surface (artist’s impression)

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ROSETTA1 11/28/02 3:48 PM Page 10 Rosetta - · PDF filefairing, will be the Rosetta comet chaser, the most ambitious scientific spacecraft ever built in Europe. ... technological research