The International Rosetta Mission

M. VerdantScientific Projects Department, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

G.H. SchwehmSpace Science Department, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

IntroductionDirect evidence of the constitution of cometaryvolatiles is particularly difficult to obtain, as theconstituents observable from Earth and evenduring the flybys of Comet Halley in 1986,

result from physico-chemical processes suchas sublimation and interactions with solarradiation and the solar wind. What we knowtoday about cometary material from thoseearlier missions and ground-basedobservations does, however, demonstrate thelow degree of evolution of cometary materialand hence its tremendous potential forproviding us with unique information about themake up and early evolution of the solarnebula.

The study of cometary material presents amajor challenge due to the very characteristicsthat make it a unique repository of informationabout the formation of the Solar System,namely its high volatiles and organic-materialcontents. A fundamental question that theRosetta mission has to address is, to whatextent can the material accessible for analysisbe considered representative of the bulk of thematerial constituting the comet, and of the earlynebular condensates that constituted thecometesimal 4.57x10

9years ago? This

representativeness issue has to be addressedby first determining the global characteristics ofthe nucleus, namely its mass, density and stateof rotation, which can provide us with clues asto the relationship between the comet’s outerlayers and the underlying material.

The dust and gas activity observed aroundcomets, as well as its rapid response toinsolation, guarantees the presence of volatilesat or very close to the surface in active areas.Analysing material from these areas willtherefore provide information on both thevolatiles and the refractory constituents of thenucleus. The selection of an appropriate site forthe surface-science investigations should berelatively straightforward, given the mission’sextensive remote-sensing observation phaseand the advanced instrumentation, covering abroad range of wavelengths, that is to becarried by the Rosetta Orbiter.

The International Rosetta Mission was approved in November 1993 byESA’s Science Programme Committee as the Planetary CornerstoneMission in ESA’s long-term programme in space science, Horizon2000. The mission’s main goal is a rendezvous with Comet46P/Wirtanen, but it is also intended to study two asteroids duringclose flybys on route to the comet. Rosetta will study the nucleus ofComet Wirtanen and its environment in great detail for a period ofnearly two years, the near-nucleus phase starting at a heliocentricdistance of about 3.25 AU, from the onset of activity through toperihelion, close to 1 AU. On its long journey to the comet, the spacecraftwill pass close to the asteroids Mimistrobell and Siwa or Rodari.

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The dust-emission processes are induced byvery low density gas outflows and shouldpreserve the fragile texture of cometary grains.These grains can be collected at low velocities(a few tens of metres per second) by thespacecraft after short travel times (of the orderof minutes), which will minimise alterationsinduced by any interaction with solar radiation.Similarly, gas analysed in jets or very close tothe surface should yield reliable information onthe volatile content of the cometary material ineach source region.

Comet 46P/WirtanenComet 46P/Wirtanen was discovered on15 January 1948 at Lick Observatory by CarlA. Wirtanen. Its two subsequent closeapproaches to Jupiter, in 1972 (0.28 AU) and1984 (0.46 AU), changed the perihelion of thecomet’s orbit from 1.63 to 1.06 AU and itsperiod from 6.71 to 5.46 years.

46P/ Wirtanen belongs to a large group of theshort-period comets in the Solar System,known as the Jupiter comets. Their orbits withan aphelion around Jupiter’s orbit make themobservable in principle along their entire orbit.Comet Wirtanen has been observed during allbut one (1980) of its apparitions since itsdiscovery. However, only the coordinatedobservation campaign conducted in 1996/1997 in the context of the Rosetta mission haspromoted it to being one of the best-monitored(Fig. 1).

Assuming that it has an albedo of 0.04, a radiusof about 700 m has been derived for thecomet. Given its smallness, the nucleus can berated as fairly active, producing about 10

28

water molecules per second.

The spacecraft and its payloadThe spacecraft design is driven by the keyfeatures of this very complex and challengingmission:

– the fixed and fairly short launch window forComet 46P/Wirtanen: due to the launchcapability required, even with Ariane-5 thereare only very few backup scenarios within areasonable time frame

– the long mission duration of 10.5 years– the critical gravity assists at Mars and the

Earth and the close asteroid flybys, duringwhich both the spacecraft and the payloadwill be active

– the wide variation of spacecraft-Earth-Suncycles and distances, which pose strongthermal-design challenges and require largesolar arrays (approx. 60 m

2) with novel cells

optimised for low-intensity low-temperatureoperation

– the lengthy operations just a few comet-nucleus radii away from 46P/Wirtanen’ssurface, in an environment of nucleus-emitted dust and gas that will not be knownin great detail during the spacecraft’sdevelopment.

The long round-trip light times of up to 90 mincall for a highly autonomous spacecraft thatcan also survive the long hibernation periodswithout any ground contact during the cruisephases.

A preliminary design for the spacecraft isshown in Figure 2, which also gives a goodimpression of the modular approach used.Striking features are the huge solar arrays andthe 2.2 metre diameter High Gain Antenna(HGA), which is steerable in two axes. Thepayload is mounted on one spacecraft wall,which during the close comet approach phasewill be pointed continuously towards thenucleus. All instruments will be body-mountedand the proper attitude will be achieved byrotating the spacecraft with the HGA alwaysEarth-pointing and the solar array pointingtowards the Sun.

Scientific payloadRosetta OrbiterDuring its meeting on 21 February 1996, ESA’sScience Programme Committee (SPC)endorsed the Rosetta Orbiter payload (Table 1),and originally two Surface Science Packages:Champollion to be provided by NASA-JPL/CNES and Roland to be provided by aEuropean consortium led by MPI and DLRfrom Germany. Programmatic difficulties sub-sequently led to NASA’s withdrawal fromChampollion in September 1996. CNES hassince joined the original Roland consortium inan effort to provide a European Lander.

After a one-year ‘science verification phase’intended to lead to a clearer definition ofinterfaces and to identify critical areas wheremore development work was required, allinstruments for the Orbiter’s payload werereconfirmed by the SPC in 1997. During thisstudy phase, the investigator teams wererequired to demonstrate the feasibility ofseveral novel techniques to be applied for thefirst time with Rosetta flight hardware.

The Orbiter payload will have unprecedentedcapabilities for studying the composition ofboth the volatile and refractory materialreleased by the cometary nucleus with veryhigh resolution. The remote-sensing instrumentsuite will allow characterisation of the nucleussurface in a wide range of wavelengths (UV tomm) with high resolution. The OSIRIS Narrow-

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Figure 1. Ground-based observations of Comet 46P/Wirtanen: July – December 1996

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Requirement for 2200 m/s delta V

Growth potential

Total capability

The requirement is based on a 2900kg launch mass and includes residuals. The growth potential represents the available volume in the reference tanks (1108 litre capacity).

1578 kg

20.6%

1903 kg

Propellant

Telecommunications

CDR11100

2.2 metre diameter HGA with 2 axis articulation

2x LGAs for omnidirectional coverage

S band uplink

S/X band downlink

Dual S/X band HGA and MGA

X band TWTA: 28 watts RF

Dual S/X band transponder 5 watts RF

Fully regulated 28V bus

Maximum power point tracking

850 watts peak power for comet operations

Keep alive power for hibernation

4 x 10 Ah NiCd batteries

Separate harnesses for payload and bus modules

Power and Harness

Fixed 0.8 metre diameter MGA

Passive/active to deal with missionextremes

Active louvres

Software and hardware controlled heaters

Passive MLI & radiators

Thermal Control

Modular design

Corrugated Al central cylinder with strengthening rings

Al honeycomb main platform

4 shear panels

Outer panels for equipment/payload mounting

Structure

Z

YX

Figure 2. Exploded view of the Rosetta spacecraft(Courtesy of Matra Marconi Space)

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2 x 1108 litre propellant tanks

4 x 35 litre pressurant tanks

Pressure regulated and blow down operational modes

24 x 10N thrusters

Usable propellant capacity 1571 kg

Propulsion

Notes: Figures represent "typical" mean consumptions. Actual figures vary throughout the mission.

Figures as specified in the ITT

1) AOCS/Propulsion increase by approx. 15W during manoeuvres2) Typical thermal values - will vary according to sun distance and operational scenario3) Represents aphelion worst case hibernation value. This is the solar array sizing case (353W output)4) SADM/E power falls to zero when wings are not rotating5) 54W for S band operations

StructureThermalMechanismsSolar ArrayPowerHarnessPropulsionTelecommunications

Total

TelecommunicationsAOCS/PropulsionDMSSADM/EPowerThermalPayload

Total

All masses include contingencies at equipment level, based on development status

The Solar Arrays and Power Subsystems can satisfy the total power required in all stages of the mission

198.940.738.6

169.790.455.2

171.544.8

809.8

10289

6618

26

100(2)

0

401W

Active Cruise X Band Hibernation Near Comet

8

0290

22190(3)

0

249W

102134661830

40(2)

270

660W

Mass (kg) Power Consumption (W)

2 axis HGA pointing mechanism (±180˚ azimuth 0 to 210˚ elevation)

2 SADMs (-90 to +270˚)

2 payload boom deployment mechanisms

Flexible harness across rotating parts

<0.04˚ control

Rotation rates up to 2.5˚/s

Mechanisms

(5)

(1)

(4)

Payload

Avionics

2 fold out wings

Cant for dihedral spin (TBC)

Low intensity, low temperature (LILT) Si or GaAs cells

68m array area using Si (54m for GaAs)

850 watts at 3.4 AU

Solar Array

*

*

* *

353 watts at 5.2 AU

2 2

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Table 1. The Rosetta Orbiter payload

Acronym Objective Principal Investigator

Remote Sensing

OSIRIS Multi-Colour Imaging H.U. Keller,NAC (Narrow Angle Camera) 2.35°x2.35° MPI für Aeronomie, WAC (Wide Angle Camera) 12°x12° (250 nm-1000 nm) Katlenburg-Lindau, Germany

ALICE UV-Spectroscopy (70 nm-205 nm) A. Stern,Southwest Research Institute, Boulder, CO, USA

VIRTIS VIS and IR Mapping Spectroscopy (0.25 µm-5µm) A. Coradini, IAS-CNR, Rome, Italy

MIRO Microwave Spectroscopy (1.3 mm and 0.5 mm) S. Gulkis, NASA-JPL, Pasadena, CA, USA

Composition Analysis

ROSINA Neutral Gas and Ion Mass Spectroscopy; H. Balsiger, Double-focusing, 12-200 AMU, M/∆M~ 3000 Univ. of Bern, SwitzerlandTime-of-flight, 12-350 AMU, M/∆M~ 500 incl. Neutral Dynamics Monitor

MODULUS Isotopic Ratios of Light Elements by Gas Chromatography C. Pillinger,(D/H; 13C/12C; 18O/16O; 15N/14N) Open University,

Milton Keynes, UKCOSIMA Dust Mass Spectrometer (SIMS, m/∆m ~ 2000) J. Kissel,

MPI für Extraterrestrische Physik,Garching, Germany

MIDAS Grain Morphology (Atomic Force Microscope, nm Resolution) W. Riedler, Univ. of Graz, Austria

Nucleus Large-Scale Structure

CONSERT Radio Sounding, Nucleus Tomography W. Kofman,CEPHAG, Grenoble, France

Dust Flux, Dust Mass Distribution

GIADA Dust Velocity and Impact Momentum Measurement, E. Bussoletti,Contamination Monitor Istituto Univ. Navale, Naples, Italy

Comet Plasma Environment, Solar-Wind Interaction

RPC Langmuir Probe, Ion and Electron Sensor, R. Boström,Flux-Gate Magnetometer, Ion Composition Analyser, Swedish Institute of Space Physics,Mutual Impedance Probe Uppsala, Sweden

J. Burch,Southwest Research Institute, San Antonio, TX, USAK.-H. Glassmeier, TU Braunschweig, GermanyR. Lundin, Swedish Institute for Space Physics,Kiruna, SwedenJ.G. Trotignon,LPCE/CNRS, Orleans, France

RSI Radio-Science Experiment M. Pätzold, Univ. of Cologne, Germany

Figure 3. Ground-activitymission profile

Mission overviewRosetta will by launched in January 2003 by anAriane-5 vehicle from Kourou, in FrenchGuiana. To gain enough orbital energy forRosetta to reach its target, one Mars and twoEarth gravity assists will be required (Figs. 3,4).The long duration of the mission requires theintroduction of extended hibernation periods.The mission can be subdivided into severaldistinct phases (Table 3).

Angle Camera, for example, will achieve aresolution of better than 10 cm on the nucleussurface from the closer orbits.

To complement these instruments and toprovide for proper monitoring of the cometenvironment and its interaction with the solarwind, a Dust Flux Analyser and a PlasmaInstrument Package have been selected.

Rosetta LanderThe Lander science (Table 2) will focus on in-situ study of the composition and structure ofthe material that constitute’s Comet Wirtanen’snucleus. Measurement goals include thedetermination of the elemental, molecular,mineralogical, and isotopic compositions ofboth the cometary surface and subsurfacematerial. The highest priority will be given to theelemental and molecular determinations, as it isbelieved that some mineralogical and isotopicmeasurements can be carried out adequatelyvia the Orbiter science investigations. Inaddition, properties like near-surface strength,density, texture, porosity, ice phases andthermal properties will be derived. Texturecharacterisation will include microscopicstudies of individual grains.

The CONSERT experiment, with hardware onboth the Lander and the Orbiter, will attempt toreveal the coarse structure of the nucleusthrough radio sounding.

Interdisciplinary scientistsFive Interdisciplinary Scientists have beennominated for an initial period of three years tosupport the mission’s implementation:– M. Fulchignoni, DESPA, Observatoire de

Paris, France, to develop physico-chemicalmodels of the possible target asteroids inorder to provide the Rosetta Project and theRosetta Science Working Team with areference data set.

– P. Weissman, NASA-JPL, Pasadena,California, USA, to provide thermophysicalmodelling of the cometary nucleus and ofthe inner coma of comets.

– R. Schulz, MPI für Aeronomie, Katlenburg-Lindau, Germany, now with ESA SpaceScience Department, to liaise with theastronomical community and to derive abasic characterisation of the target cometfrom ground-based observations.

– E. Grün, MPI für Kernphysik, Heidelberg,Germany and M. Fulle, Trieste AstronomicalObservatory, Italy, to provide empirical‘engineering models’ for the dustenvironment of the nucleus of the targetcomet in order to establish a reference dataset for the Rosetta Project and the RosettaScience Teams.

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Table 2. The Lander payload

APXS Alpha-p-X-ray Spectrometer R. Rieder, MPI Chemistry, Mainz, Germany

Sample Acquisition System ASI, Italy

MODULUS Evolved Gas Analyser C. Pillinger, Open University, UK

CIVA Rosetta Lander Imaging System J.P. Bibring, ROLIS IAS, Orsay, France

S. Mottola, DLR Berlin, Germany

SESAME Surface Electrical and Acoustic D. Möhlmann,Monitoring Experiment, DLR Cologne, GermanyDust Impact Monitor H. Laakso,

FMI, FinlandI. Apathy, KFKI, Hungary

MUPUS Multi-Purpose Sensor for Surface and T. Spohn,Sub-Surface Science Univ. of Münster,

Germany

ROMAP RoLand Magnetometer and U. Auster,Plasma Monitor DLR Berlin, Germany

I. Apathy,KFKI, Hungary

CONSERT Comet Nucleus Sounding W. Kofman,CEPHAG, Grenoble, France

Figure 4a,b. Geometric andtemporal schematics of

mission phases

Launch (January 2003)Rosetta will be launched on an Ariane-5. Afterburnout of the lower composite, the upperstage L9.7 will remain together with thespacecraft in an eccentric coast orbit for about2 hours (4000 x 200 km), during which theattitude of the composite can be controlled.Before perigee passage, the upper stage willperform a delayed ignition and inject theRosetta spacecraft onto the required escapehyperbola towards Mars, with an excessvelocity of about 3.4 km/s. The spacecraft willbe separated from the launcher upper stage ina three-axis-stabilised attitude.

Commissioning Phase (3 months)Immediately after separation, the spacecraft willautonomously de-tumble, deploy its solararrays and acquire a coarse three-axis-stabilised Sun-pointing attitude. Groundoperations will acquire the downlink in S-band,using the ESA network, and control thespacecraft to a fine-pointing attitude with theHigh-Gain Antenna (HGA) Earth-pointing usingX-band telemetry. Tracking and orbitdetermination will then be performed and the

departure trajectory verified and corrected ifnecessary. All spacecraft functions requiredduring the cruise to the comet, and in particularthe hibernation functions, will be checked out.The scientific payload will then be commis-sioned, before putting the spacecraft intohibernation mode for the cruise phase to Mars.

Earth-to-Mars Cruise (about 950 days fromlaunch to Mars)The spacecraft will be put into hibernationmode, during which no ground support isbaselined. A long solar conjunction preventsany spacecraft operations being performedfrom the ground between days 440 and 690.The scientific instruments will be switched offduring this cruise.

Mars Gravity Assist (pericentre height about200 km)Daily operations will be resumed three monthsbefore Rosetta’s arrival at Mars. Two orbit-correction manoeuvres are planned at theincoming and outgoing asymptote, which willallow for some science operations during theswingby. During the swingby, there will be anoccultation of the Earth by Mars which will lastabout 37 minutes, causing a communicationsblackout.

Mars-to-Earth Cruise (about 90 days fromMars to Earth)The spacecraft will be kept in active cruisemode for this ‘short’ interplanetary phase.

First Earth Gravity Assist (perigee heightabout 3400 km)Operations will be mainly devoted to trackingand orbit determination and maintenancefrom three months before, until one monthafter swingby. Orbit-correction manoeuvresare to be executed before and after theswingby itself.

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itself will be greater to be commensurate withthe spacecraft angular-rate capabilitiesbecause the Mimistrobell flyby occurs at alower relative velocity.

Asteroid to Comet Cruise (about 1200 dayfrom asteroid to comet rendezvousmanoeuvre)The whole period - apart from an optionaldeep-space manoeuvre - will be spent inhibernation mode. Rosetta will be at its furthestfrom the Sun and from the Earth during thisperiod, i.e. 5.2 AU (aphelion) and 6.2 AU,respectively.

Comet Orbit Matching Manoeuvre (orrendezvous manoeuvre; around day 3140)This is the major orbit manoeuvre that will readythe spacecraft for the rendezvous, by reducingthe spacecraft/comet relative drift rate to about25 m/s. It will be performed before the comet isdetected by Rosetta’s onboard cameras, usingground determination of the comet nucleus’orbit based on a dedicated observationcampaign.

Near-Comet Drift Phase The drift phase will be designed such that,when the spacecraft is less than 4.2 AU fromthe Sun it reaches an appropriate point relativeto the comet for the final approach operationsto begin, such that cometary debris can beavoided and that good comet illuminationconditions are obtained. The final point of thenear-comet drift phase will be the CometAcquisition Point (CAP), where the comet willbe observed for the first time by Rosetta’sonboard navigation camera or by OSIRIS.

Throughout this phase, the spacecraft will be inactive cruise mode.

Far-Approach Trajectory Phase (up to 90 days)The far-approach operations start at the CometAcquisition Phase. After detection, knowledgeof the comet ephemeris will be drasticallyimproved by the processing of the onboardobservations. Image processing on the ground

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Earth-to-Asteroid Cruise (about 300 daysfrom Earth to Mimistrobell)The spacecraft will be put into hibernationmode, during which no ground support isbaselined.

Mimistrobell Flyby (around day 1330)Flyby operations will last from three monthsbefore, until one month after the flyby. Inparallel with the daily tracking, with orbitdetermination and corrections, the scientificpayload will be checked out. The goal is topass within 600 km of an asteroid, on thesunward side. The relative asteroid ephemeriswill be determined with a cross-track accuracyof 20 km by spacecraft optical navigation. Thecameras and scientific payload will be pointingin the direction of the asteroid until after theflyby. Scientific data will be recorded onboardin the mass memory and transmitted after theflyby when the Earth link via the HGA isrecovered.

After the flyby, the necessary orbit correctionwill be performed to put the spacecraft oncourse for the second Earth gravity assist.

Mimistrobell-to-Earth Cruise (about 400 daysfrom asteroid to Earth)The spacecraft will be put into hibernationmode, during which no ground support isbaselined.

Second Earth Gravity Assist (perigee heightabout 2200 km)The operations conducted will be the same asfor the first Earth swing-by. Before the secondEarth gravity assist, however, there is adecision point regarding the further operationsup to comet encounter. The nominal missionincludes a flyby of a second asteroid andRodari is the nominal candidate, but Siwa maybe selected if there is sufficient propellantavailable. The exact rendezvous manoeuvrestrategy will also be selected based on theavailable battery power, solar-arraydegradation and the propellant situation. Thenominal rendezvous manoeuvre is to beexecuted post-aphelion at 4.5 AU to minimisepower demands and allow the solar array to bemade as small as possible.

Earth to Second Asteroid Cruise (about 160days)The spacecraft will be put into hibernationmode as soon as the necessary trackingoperations have been completed and an orbit-correction manoeuvre has been performed.

Rodari Flyby (around day 1930)The operations here will be similar to those forthe Mimistrobell flyby, but the flyby distance

Table 3. Major mission events

Nominal Timing

Launch 21 January 2003Mars gravity assist 26 August 2005Earth gravity assist #1 26 November 2005Mimistrobell flyby 15 September 2006Earth gravity assist #2 26 November 2007Rodari flyby 4 May 2008Rendezvous manoeuvre 24 August 2011Start of near-nucleus operations at 3.25 AU (from Sun) 22 August 2012Perihelion passage (end of mission) 10 July 2013

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will derive a coarse estimation of the comet’ssize, shape and kinematics.

The approach manoeuvre sequence willsuccessively reduce the relative velocitybetween spacecraft and comet, so that it isjust 2 m/s after 90 days. The manoeuvrestrategy will be designed to:– retain an apparent motion of the comet with

respect to the star background– keep the illumination angle (Sun-comet-

spacecraft) below 70 deg– avoid any danger of an impact with the

cometary nucleus in the event ofmanoeuvring difficulties.

The Far-Approach Trajectory ends at theApproach Transition Point (ATP), where a firstestimate of the comet’s attitude and angularvelocity will derived from the analysis of thenavigation-camera or OSIRIS images and

at the end of this phase, which is the OrbitInsertion Point (OIP). At the OIP the spacecraftwill be injected onto a hyperbolic arc to thecomet, at a typical distance of 60 comet radiiand with a velocity of some cm/s relative to thecomet, depending on the comet’s size anddensity.

Transition to Global-Mapping PhaseThe transition to global mapping starts at theOIP. Rosetta will follow a hyperbolic arc until itis about 25 comet radii from the target, wherea capture manoeuvre will close the orbit. Theplane of motion, defined by the spin-axis andSun directions, will be rotated slightly to avoidsolar eclipses and Earth occultations.

Global-Mapping PhaseThis preliminary survey of the comet’s surfaceshould map at least 80% of the sunlit areas,from polar orbits around the comet at heightsof between 5 and 25 nucleus radii. The semi-major axis of the mapping orbit will be chosenas a function of the comet’s gravity and spinrate, taking into account the followingconstraints:– coverage without gaps– safety considerations (no impact on nucleus)– volume of data for real-time transmission– maximum time to complete surface

mapping– optimum resolution and viewing angle to

surface normal, and – continuous communications to Earth.

The orbital period will usually be greater thanthe comet’s spin period and horizontal swathswill cover the nucleus surface as it is presented.Ideally, all of the Orbiter payload instruments willbe operating and the scientific data gatheredwill be buffered in the onboard memory readyfor transmission to the ground station.

During this global-mapping phase, the nucleus’shape, surface properties, kinematics andgravitational characteristics will be derivedusing optical landmark observations. Based onthese mapping and remote-observation data,some five areas (500 m x 500 m) will beselected for close observation.

Close-Observation PhaseManoeuvre strategies will be designed forsequences of close-observation orbits, flyingwithin 1 nucleus radius of selected surfacepoints. Constraints to be taken into accountinclude:– uninterrupted communications– continuous illumination of solar arrays– safety constraints (no nucleus encounter in

case of manoeuvre failure)– avoidance of debris, dust and gas jets

‘landmarks’ will be identified. The ATP is in theSun direction at about 300 comet nucleus radiifrom the nucleus. The spacecraft will be inactive cruise mode during this phase, with thenavigation camera system and some Orbiterpayload items switched on.

Close-Approach Trajectory PhaseThe close-approach trajectory starts at ATP.Lines-of-sight to landmarks and on-groundradiometric measurements will be used toestimate the spacecraft’s relative position andvelocity, and the comet’s absolute position,attitude, angular velocity, and gravitationalconstant.

A very good estimate of the comet’s kinematicsand gravitational constant should be available

Figure 5. The Rosettaground segment

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– adequate illumination of target area, andangle between viewing direction and localsurface normal less than 30 degrees.

This phase is expected to last about 30 daysand, based on the data collected, it will then bedecided to which site the Surface SciencePackage (SSP) will be delivered. A transitionphase to the initial conditions for SSP deliverywill then be implemented.

Surface-Science Package Delivery PhaseSurface-Science Package Delivery will takeplace from an eccentric orbit (pericentrealtitude as low as possible, e.g. 1 km) with apericentre passage near the desired landingsite. The time and direction of SSP separationwill be chosen such that the package arriveswith minimum vertical and horizontal velocitiesrelative to the local (rotating) surface. Anejection mechanism will separate the SSP fromthe spacecraft with a maximum relative velocityof 1.5 m/s.

Relay-Orbit PhaseAfter delivery of the SSP, the spacecraft will beinjected into the most suitable orbit forreceiving the SSP’s data and relaying it toEarth. Commanding of the Package may alsobe required.

Extended-Monitoring Phase (through perihelion)After the completion of the SSP-relatedactivities, the spacecraft will spend at least 200days in orbit in the vicinity of the comet untilperihelion passage. The objective of this phaseis to monitor the nucleus (active regions), dustand gas jets, and to analyse gas, dust andplasma in the inner coma from the onset ofactivity until its peak.

The orbital design and mission planning for thisphase will depend the scientific results alreadyobtained from of the previous observations andsafety considerations associated with theactivity pattern of the comet. Extendedmonitoring of various regions in the vicinity ofthe nucleus could be performed withsuccessive hyperbolic flyby’s following petal-like trajectories.

Run Down (end of mission)The mission nominally ends at the perihelionpass around day 3800, unless a missionextension is agreed if the spacecraftshould survive the cometary environmentunscathed!

Mission operationsThroughout its long and demanding mission,Rosetta will be operated and controlled from

Figure 6. Schematic ofRosetta payload operations

the European Space Operations Centre(ESOC), in Darmstadt, Germany (Fig. 5). Themain ground station for the mission will be thenew 32-metre Deep-Space Antenna to be builtnear Perth, in Western Australia. The NASADeep-Space Network will be used as a back-up during critical mission phases.

During the payload-checkout periods, theasteroid flybys and the near-comet phases ofthe mission, the Rosetta Science OperationsCentre (RSOC) will be co-located with theMission Operations Centre (RMOC) at ESOC inDarmstadt. A schematic of the mission andscience operations facilities is shown in Figure 6.

ConclusionRosetta promises to be one of the mostexciting planetary missions currently inpreparation. It will provide unprecedentedaccess to the original material of the proto-solarnebula and will help us to acquire a realunderstanding of how comets work.

The recent magnificent displays in our skies ofComets Hyakutake and Hale-Bopp havedemonstrated yet again the intriguing mysteriesassociated with these objects, and fuelledenthusiasm in the scientific community for thisnovel mission opportunity.

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The Project successfully passed its firstmilestone on 19 December, with the successfulcompletion of the System RequirementsReview. The Prime Contractor for Rosetta,selected early in 1977, is DASA-DornierSystem of Germany. Responsibilities for thePlatform and Avionics have been assigned toMatra Marconi Space UK and Matra MarconiSpace France, respectively. Alenia Spazio ofItaly is to be responsible for Rosetta’sAssembly, Integration and Testing (AIT). r

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Mission Goals

The prime scientific objective of the mission as defined by the RosettaScience Team is to study the origin of comets, the relationshipbetween cometary and interstellar material, and its implications withregard to the origin of the Solar System. The measurements to bemade in support of this objective are:• Global characterisation of the nucleus, determination of dynamic

properties, surface morphology and composition• Determination of the chemical, mineralogical and isotopic

compositions of volatiles and refractories in a cometary nucleus• Determination of the physical properties and interrelation of volatiles

and refractories in a cometary nucleus• Study of the development of cometary activity and the processes in

the surface layer of the nucleus and the inner coma (dust/gas interaction)

• Global characterisation of asteroids, including the determination ofdynamic properties, surface morphology and composition.