ARTICLE IN PRESS

Planetary and Space Science 58 (2010) 1058–1065

Contents lists available at ScienceDirect

Planetary and Space Science

0032-06

doi:10.1

n Corr

E-m

Kristin.W

Michae

journal homepage: www.elsevier.com/locate/pss

The flyby of Rosetta at asteroid Steins – mission and science operations

Andrea Accomazzo a,n, Kristin R. Wirth b, Sylvain Lodiot a, Michael Kuppers b, Gerhard Schwehm b

a European Space Agency/European Space Operations Center, Darmstadt, Germanyb European Space Agency/European Space Astronomy Center, Villanueva de la Canada (Madrid), Spain

a r t i c l e i n f o

Article history:

Received 13 October 2009

Received in revised form

3 February 2010

Accepted 4 February 2010Available online 10 February 2010

Keywords:

Rosetta

Asteroid

Steins

Flyby

Optical navigation

Autonomous tracking

33/$ - see front matter & 2010 Elsevier Ltd. A

016/j.pss.2010.02.004

esponding author. Tel.: +49 6151902707.

ail addresses: Andrea.Accomazzo@esa.int (A.

irth@esa.int (K.R. Wirth), Sylvain.Lodiot@es

l.Kuppers@esa.int (M. Kuppers), Gerhard.Schw

a b s t r a c t

The international Rosetta mission, a cornerstone mission of the european space agency scientific

Programme, was launched on 2nd March 2004 on its 10 years journey towards a rendezvous with

comet Churyumov-Gerasimenko (Gardini et al., 1999). During its interplanetary flight towards its target

Rosetta crosses the asteroid belt twice with the opportunity to observe at close quarters two asteroids:

(2867)-Steins in 2008 and (21)-Lutetia in 2010. The spacecraft design was such that these opportunities

could be fully exploited to deliver valuable data to the scientific community. The mission trajectory was

controlled such that Rosetta would fly next to asteroid Steins on the 5th of September 2008 with a

relative speed of 8.6 km/s at a minimum distance of 800 km. Mission operations have been carefully

planned to achieve the best possible flyby scenario and scientific outcome. The flyby scenario, the

optical navigation campaign, and the planning of the scientific observations had to be adapted by the

Mission and the Science Operations Centres to the demanding requirements expressed by the scientific

community. The flyby was conducted as planned with a large number of successful observations.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Launched on the 2nd of March 2004 after a launch postpone-ment due to an Ariane 5 problem at the end of 2002, Rosetta isflying towards its target: comet 67 P/Churyumov-Gerasimenko(C-G) (Glassmeir et al., 2007). Upon arrival, the Philae lander,carrying 10 experiments, will be placed on the comet’s surface,and the Rosetta orbiter with 12 experiments will continue to orbitC-G and accompany the comet through perihelion (Schwehm andSchulz, 1998; 1999; Glassmeir et al., 2007). The mission profile issuch that the comet will be reached only in 2014 after a verycomplex interplanetary trajectory, including three Earth swing-bys (2005, 2007, and 2009), one Mars swing-by (2007), and twomajor rendezvous manoeuvres (ca. 800 m/s each) to be executedin January 2011 and May 2014 (Ferri, 2006; Ferri et al., 2007). Atthat stage the spacecraft will be on an approach trajectory to thecomet and will have to be navigated with optical data acquired byits on-board cameras (Fig. 1).

In its revolutions around the Sun Rosetta crosses the asteroidbelt giving the unique opportunity to perform flybys with theseprimordial objects of the solar system. After an almost perfectorbit insertion by Ariane 5, the mission controllers could offer thescientists the opportunity to flyby two asteroids during themission; the choice fell onto asteroid (2867)-Steins (5th Septem-

ll rights reserved.

Accomazzo),

a.int (S. Lodiot),

ehm@esa.int (G. Schwehm).

ber 2008) and asteroid (21)-Lutetia (10th July 2010), and themission profile was adjusted accordingly thus confirming theseevents as secondary mission objectives.

2. Mission scenario

Before reaching its target Rosetta is actually performing fiveorbits around the Sun; it is only during the fourth of theserevolutions that the spacecraft trajectory intercepted for the firsttime the asteroid belt in the second half of 2008. This occurred onthe orbit going from the second to the third Earth swing-by, amission phase otherwise characterised only by the Deep SpaceManoeuvre 4 (ca. 7 m/s) executed in March 2009. In order tocomply with mission needs this cruising phase was organised asfollows:

�

close to Sun cruise from November 2007 to March 2008 � near Sun hibernation phase from April 2008 to June 2008 � asteroid flyby phase from July 2008 to September 2008 � cruise phase from October 2008 to February 2009 � Deep Space Manoeuvre 4 in March 2009 � near Sun hibernation phase from April 2009 to September

2009

At the end of the last hibernation phase the spacecraft wasreactivated to execute the third Earth swing-by phase and therelated activities.

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Fig. 1. Rosetta interplanetary trajectory.

A. Accomazzo et al. / Planetary and Space Science 58 (2010) 1058–1065 1059

The asteroid flyby phase was then concentrated in threemonths from July to September 2008 (Accomazzo et al., 1999).During this phase the spacecraft set its new records in terms ofSun and Earth distances: beyond 2 and 2.6 AU respectively. Thesechallenging environmental conditions had to be taken intoaccount when planning in detail the flyby phase that had toinclude the following activities:

�

spacecraft re-activation after hibernation � payload interactive check-out (4 weeks) � navigation campaign towards the asteroid � asteroid flyby � post asteroid flyby observations

Spacecraft contacts have been scheduled such that the opera-tions teams would have almost daily access to the spacecraft inthe month of July for the check-out activities; operations in themonth of August were designed such that the focus would be onthe navigation campaign leading to a proper setting of thespacecraft trajectory for the flyby. Later in the month coveragewas increased such that almost permanent contact would allow afine control of the trajectory and proper configuration of thespacecraft.

Two aspects have been the driving factors in the design of thefull flyby scenario:

�

the updated scientific requirements (which also affected thesecond one) � the execution of the first optical navigation campaign

When consulted, the scientific community expressed a stronginterest and will in trying to exploit as much as possible this

flyby opportunity and set requirements for the flyby whichwere beyond the ones originally analysed by the spacecraftmanufacturers:

�

good illumination conditions � passage through phase angle zero (in order to observe the

so-called opposition surge, i.e. a non-linear increase of thebrightness of the asteroid surface at zero phase angle.)

� closest possible distance � observation at closest approach (i.e. range rate zero) � continuous observation from before to after closest approach � ‘‘good’’ pointing performance � ‘‘good’’ synchronisation of payload operations with flight events

The challenge for the control team was to fulfil theserequirements with the conditions imposed by the flyby and thespacecraft capabilities:

�

relative velocity at closest approach 8.6 km/s � Sun/spacecraft/asteroid angle at approach 141.5 1

�

Earth/spacecraft/asteroid angle at approach 164.21 � Sun distance 2.14 AU � Earth distance 2.41 AU � constrain the positioning of the solar arrays in regions where a

stuck array would not be too detrimental for the mission

� replica of the kinematics of the future Lutetia asteroid fly-by,

considered scientifically more interesting (to make the Steinsflyby an in-flight test of the Lutetia case)

The last two constraints were posing a significant limitation tothe operations team in designing the flyby scenario; therequirements expressed by the scientists would have not beenmet. After careful analysis and a significant validation campaignthe Mission Operations Center could provide the scientificcommunity, coordinated by the Science Operations Center, ascenario that could meet the requirements deemed necessary toachieve the scientific objectives.

3. Scientific objectives

(2867)-Steins is an E-type asteroid with a diameter ofapproximately 5 km. Fig. 2 shows the important geometryparameters that governed the opportunities for scienceobservations during the flyby.

The angular diameter of Steins was given by the distance ofRosetta to the asteroid, and determined the spatial resolution thatthe remote sensing instruments could achieve. It can be seen thatthe Steins angular diameter was above 0.11 for only 9 min aroundclosest approach (CA), i.e. the flyby happened rather fast.

When Rosetta arrived at Steins, the phase angle or Sun-asteroid-spacecraft angle was below 401, which provided for goodillumination. The phase angle decreased and reached zero 2 minbefore the closest approach to the target so that the remotesensing instruments could observe the opposition effect. After-wards the phase angle increased rapidly, at Steins closestapproach it already was 501 and then went up to 1401. Thereforethe experiments were mainly looking at the night side of theasteroid during departure (note that the dip in the rate of changeof the phase angle at CA – 2 min is an artefact caused by zerophase angle not being exactly sampled).

The solar elongation of Steins as seen from Rosetta was wellapproximated by (1801 – phase angle) because the spacecraft wasmuch closer to the asteroid than to the Sun. Thus the Sun-spacecraft-asteroid angle was 1401 on approach, passed through

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Fig. 2. Steins flyby geometry parameters: (a) the distance of Steins from Rosetta and the angular diameter of Steins as seen from Rosetta are plotted vs. time, (b) the radial

velocity of Steins relative to Rosetta vs. time, (c) the phase angle (i.e. Sun-asteroid-spacecraft angle) and its rate of change vs. time on two different time scales. Note that

the dip in the time derivative of the phase angle at CA – 2 min is an artefact caused by zero phase angle not being exactly sampled.

A. Accomazzo et al. / Planetary and Space Science 58 (2010) 1058–10651060

1801 at phase angle zero and decreased to 401 on the outgoingtrajectory. Therefore straylight disturbance from the Sun was asmall concern only during recession.

The radial velocity between Rosetta and Steins determined theDoppler shift, a very important parameter for the microwavespectrometer MIRO. A high Doppler shift would have shifted thespectral lines of the species of interest (mainly water) out ofthe detectable spectral range of the instrument, but fortunatelythe Doppler shift was low enough to allow MIRO to takemeasurements. MIRO was operating in a dedicated asteroid modefor 21 min centred on the closest approach to the asteroid in orderto make use of the change of sign of the Doppler shift.

As a result of the flyby geometry, the main science acquisitionphase was focused on several hours around Steins closestapproach:

�

The scientific camera system OSIRIS imaged the asteroid forspectrophotometry, surface mapping, and determination of theregolith properties. OSIRIS also looked for possible satellitesand dust in the asteroid environment, and measured a lightcurve. � The visible and infrared mapping spectrometer VIRTIS per-

formed mineralogical mapping and determined a light curve.

� The UV imaging spectrometer ALICE acquired the first

spectrum of an asteroid below 200 nm, and searched for anexosphere or coma around Steins.

� MIRO recorded dual frequency microwave continuum data for

thermal modelling. MIRO also tried to detect spectral lines ofwater.

� The ion and neutral mass spectrometer ROSINA investigated

the outgassing of Steins.

� The plasma environment of the asteroid was studied by the

plasma packages on board both the orbiter and lander, RPC andROMAP, respectively.

�

The radio science investigation RSI attempted to determine themass of Steins. This was mainly an operations rehearsal for theLutetia flyby in 2010 as the Steins mass and geometry were atthe limit of the RSI sensitivity. � The dust detection systems GIADA on the orbiter and SESAME

on the lander tried to observe possible particle impacts.

Many experiments requested calibration, background and darkmeasurements in connection with these scientific observations. Inaddition, OSIRIS recorded an early light curve of Steins alreadytwo weeks before closest approach.

After the Steins flyby OSIRIS continued with a target ofopportunity observation of the galactic bulge over a time periodof four weeks. With 1.6 AU, the projected distance of Rosetta fromEarth was ideal to measure the parallax effect in gravitationalmicrolensing, i.e. differences of light amplification events frommicrolensing observed by OSIRIS and ground-based telescopes.

4. Flyby scenario

One of the challenges posed by such an operation is that theorbit of the asteroid is a-priori known with a very limitedaccuracy. In the case of asteroid Steins this was only based onground observations and affected by a significant uncertainty inthe order of few hundreds km. Analysis of spacecraft capabilitiesshowed that a flyby at 800 km, versus the originally planned1750 km, was possible; this posed additional challenges in termsof navigation that could be performed by means of the opticalinstruments available on the spacecraft, the Navigation Camera(NAVCAM) and the scientific OSIRIS camera.

The flyby phase was mainly structured in the following activities:

�

asteroid detection and preliminary orbit determination � fine trajectory optimisation

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Fig. 3. Rosetta targeting 11-August-08. (For interpretation of the references to

colour in this figure legend, the reader is referred to the web version of this

article.)

Fig. 4. Last Rosetta targeting before flyby.

A. Accomazzo et al. / Planetary and Space Science 58 (2010) 1058–1065 1061

�

flyby observations � post flyby activities

The coarse navigation campaign was structured around threeweeks of two image taking sessions per week and subsequent

orbit determination. The first asteroid detection occurred on the4th of August at a distance of ca. 26 million km. Since the verybeginning the asteroid could be detected both with the spacecraftnavigation camera and with the more powerful scientific OSIRIScamera.

Images taken by these two units were then fed into the groundbased orbit determination process that produced the solutionrequired for the targeting process. This is typically represented asa target (green cross in Fig. 3) in the so-called B-plane; the B-plane(T and R coordinates) is defined as the plane passing through thecenter of the target body (Steins in this case, represented by a redcross) and perpendicular to the hyperbolic velocity of thespacecraft when approaching the body. Fig. 3 reports the resultsobtained on the 11th of August:

�

the black dashed line is the solution using radiometric dataonly (this is affected by the error in the knowledge of theasteroid position) � the orange dashed line is the solution obtained including the

images of the navigation camera of the 4th of August (here theasteroid actual position starts being taken into account)

� the magenta dashed line is the solution obtained with all the

data available on the 7th of August, i.e. including the firstimages from OSIRIS (the significant performance improvementcan be easily seen)

� the blue dashed line is the solution obtained with the data up

to the 11th of August

The quality and consistency of the results were so high thatthe operations team decided for a first trajectory correctionmanoeuvre (TCM) on the 14th of August.

In the last two weeks preceding the closest approach thenavigation campaign switched into fine navigation and dailysessions of image taking were followed by more precise orbitdetermination results.

A small last correction manoeuvre was executed 36 h beforethe closest approach in order to achieve the best possible flybyconditions. The effect of the manoeuvre executed on the 4th ofSeptember (11.8 cm/s) is represented in pink in Fig. 4. A small

violet ellipse shows the last orbit solution obtained before theflyby.

As already mentioned, the requirements set by the scientificcommunity forced the operations team to design a new flybyscenario compared to the one conceived before launch, whichforesaw an interruption of the observations circa 3 min before theclosest approach. This scenario could only partially satisfy therequirements; therefore a different one was designed by pushingthe spacecraft to its limits in terms of performance.

One of the most limiting factors in designing the scenario wasthe spacecraft sensitivity to Sun exposure; the �Z and �X faces ofthe spacecraft are permanently maintained in shadow to preventoverheating of the internal compartments hosting the electroniccomponents. Observing the asteroid throughout the flyby phasemeans turning the spacecraft almost 1801 around the Y axis with awide Sun exposure (from +X to �X passing through �Z). In orderto avoid this a so-called ‘‘flip-manoeuvre’’ was introduced shortlybefore the closest approach; with this 1801 rotation around theZ-axis, the spacecraft, after having ‘‘inverted’’ its attitude, wouldcontinue to expose the same side (+X) to the Sun also whenpassing the asteroid and during the receding phase. Fig. 5 shows aschematic of the manoeuvring when passing by the asteroid andhow the �X face of the spacecraft was exposed to the Sun only fora very limited period of time during the ‘‘flip’’.

Due to the uncertainty of the orbit the spacecraft is equippedwith an autonomous asteroid tracking mode where the attitudeguidance is autonomously determined on-board thanks to theimages of the tracked body as delivered by the on-boardnavigation camera. This mode prevents the attitude off-pointingthat would result from the irresolvable errors in the determina-tion of the along-track component of the relative trajectory.

Due to the flyby scenario this mode could be activated only20 min before the closest approach; before this could be achievedthe ground control team invested a significant amount of real-time fine tuning due to unexpected behaviour of the camera whentracking the object. This behaviour was mainly given by anunexpectedly high number of warm pixels which affected themeasurements delivered to the guidance function (the asteroidposition was distorted by the presence of the warm pixels). Inorder to be able to track the asteroid, parameters of the camera(exposure and gain) had to be adapted and, as Fig. 6 shows,the spacecraft pointing (target being the photometric center ofthe asteroid) was affected during the approach phase when thephase angle with the Sun was below 901. In this phase it can be

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Fig. 5. ‘‘Inverted’’ Steins flyby geometry

Fig. 6. Attitude errors.

A. Accomazzo et al. / Planetary and Space Science 58 (2010) 1058–10651062

seen how the along-track pointing (X component) was biasedthroughout the phase and reached a peak of ca. 0.451 shortlybefore the closest approach (18:38:20 UTC); this is mainly givenby over-exposure caused by the settings required to be able todistinguish the asteroid from the warm pixels at the beginning ofthe tracking phase. The pointing performance in the cross-trackdirection (Y component) was very good as it was the along-trackcomponent during the recessing phase.

The data collected during the flyby both by the navigationcamera in its tracking mode and by the scientific camera in itsscience data collection were used to further resolve the orbitalsolution of the event. As Fig. 7 shows, a slight discrepancy isobservable between the pre (orange) and post (black) flybysolutions: the flyby distance was 802.6 km, ca. 6.6 km away fromthe target point. The distinct separation between the orange andblack error ellipses means that the predicted and final results areinconsistent with each other. This is due to systematic errors inthe direction measurements derived from the optical data during

approach, particularly those at the end of the approach phasesince they have the highest information content. It is interestingto note that, in absence of optical data for navigation purposes,the flyby would have occurred at 916.5 km, 137 km away fromthe target.

5. Scientific observations

The requested payload activities were coordinated in adecentralised planning process involving the Principal Investiga-tor teams, and the Mission and Science Operations Centres(Koschny et al., 2007; Wirth, 2007). Experiment operations weredescribed as observations which were characterised by a scientificor engineering objective, duration, target, pointing mode, requiredpower and generated data volume, and a block of timedtelecommands for the instrument. The operations schedule wasrepresented by a timeline of observations, and a corresponding

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pointing profile and experiment commanding. Conflict resolutionbetween observations was based on scientific value, the con-straints of the spacecraft and payloads, and the availableresources in terms of pointing and data volume (power was ofno concern during the Steins flyby). First the observation timelineand attitude profile were integrated, when the most intenseiterations between the planning entities took place because therequirements of the different experiments and the spacecraftwere closely linked. In the next planning step the experimentcommanding was produced.

When the pointing requested with an observation wasimplemented, this also implied that the corresponding operationswere placed onto the timeline, i.e. the observation timeline andpointing profile were strongly interrelated. The pointing wasmainly driven by the requirement to point the slightly differentviewing directions of the remote sensing instruments to the smallangular size target body. A ‘‘cooperation’’ boresight was foundthat satisfied the needs of all remote sensing instruments so thatthey could together observe the surface of the asteroid aroundclosest approach. Otherwise different boresights were alternatedalong the timeline.

Fig. 8. Overview timeline of th

Fig. 7. Reconstructed Rosetta targeting.

Fig. 8 gives an overview of the operations timeline for thewhole Steins flyby campaign, and Fig. 9 zooms into the 10 daysaround the closest approach (CA) to the target (science slots 2–4in the previous Fig. 8). All scientific observations of the asteroidand supporting experiment operations (described in Section 3)were grouped in four science slots. Slot 1 was used for the earlylight curve measured by OSIRIS, while slots 2–4 comprised theoperations before, around and after Steins closest approach,respectively. The first gravitational microlensing observation ofOSIRIS overlapped with slot 4, and was followed by six moremicrolensing observations spaced over the next four weeks. Inaddition, the navigation slots were used for the image takingsessions of OSIRIS and the navigation camera within theframework of the optical navigation campaign.

The following experiment abbreviations are used in Fig. 9:

SR – OSIRIS Scientific camera systemVR – VIRTIS Visible and IR Thermal Imaging Spectrometer.AL – ALICE UV imaging spectrometer.MR – MIRO Microwave Instrument for the Rosetta Orbiter.RN – ROSINA Rosetta Orbiter Spectrometer for Ion and Neutral

Analysis.RP – RPCRosetta Plasma Consortium.LZ – Lander/ROMAP Rosetta lander Magnetometer and Plasma

monitor.RS – RSI Radio Science Investigation.NAVCAMNavigation Camera system.

Not shown in Fig. 9 are the attempted detection of dustimpacts by GIADA and SESAME, as well as a few tests by landerinstruments that were unrelated to Steins.

Most experiment operations in slot 3 were timed relative to theprecise time of Steins closest approach. On the 12th of August thepredicted flyby time was estimated to be 18:38:16 UTC, and the TCMswere designed not to change this target time. This was necessary inorder to fix the payload and attitude commanding on an absolutetime scale. After the Steins flyby it turned out that the target time wasjust 4 s too early, well within the allocated uncertainty.

The science operations planning also had to make sure thatimages of Steins were available for public relations (PR) purposeson the next morning in Europe. Therefore downlink priority wasgiven to OSIRIS science data during the first 4 h after closest

e Steins flyby operations.

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Fig. 9. Detailed operations timeline around Steins closest approach: the detailed experiment observations in science slots 1–4 are shown, together with the last three slots

for trajectory correction manoeuvres (TCMs) and the four breakpoints (BPs). The experiment abbreviation are explained in the text.

A. Accomazzo et al. / Planetary and Space Science 58 (2010) 1058–10651064

approach, and the internal memory of the OSIRIS instrument wasused to delay the transmission of data not suitable for PR to thesolid state mass memory of the spacecraft.

Furthermore, the planning process included anticipation ofvarious contingency situations. Breakpoints (BPs) were introducedto restart the payload operations in case of a spacecraft anomaly.The breakpoints were linked to the TCM slots, plus a lastbreakpoint was defined 2 h before Steins closest approach.Recovery experiment commanding files were prepared that wouldhave powered on the instruments and merged with the nominaloperations timeline after each breakpoint. A backup slew profile forthe time period around closest approach was designed for theunlikely case that the spacecraft would fail to enter theautonomous asteroid tracking mode. These slews would haverecovered some scientific observations although the size of mostinstrument fields of view was much smaller than the expectedoffpointing given by the limited prior knowledge of the asteroidposition. Fortunately, none of these contingencies occurred.

Almost all payload operations were executed as planned andhave yielded unique scientific results. However, a few problemswere reported:

�

OSIRIS could not acquire the highest resolution images ofSteins as the narrow angle camera went into safe mode about10 min before closest approach. � The phase coverage of the MIRO continuum data was limited

because of pointing inaccuracies during the approach phase.These pointing errors also impaired the scientific output ofVIRTIS and ALICE.

� ROSINA was affected by outgassing of the spacecraft produced

by the ‘‘attitude flip manoeuvre’’.

� The attitude profile was designed to mainly satisfy the

requirements of the priority science observations of theremote sensing instruments, but unfortunately this pointingwas unfavourable for RPC.

� The magnetic field measurements of ROMAP were unexpect-

edly disturbed by the supply current of another instrument onboard the lander that was taking advantage of the ‘‘attitude flipmanoeuvre’’ and the generated temperature changes toimprove its calibration. Similar disturbances were observedby the RPC magnetometer, but two orders of magnitudesmaller. In the future, more attention will be given to such

interferences, in particular for tests that do not contribute tothe scientific objectives.

6. Conclusions

On the 5th of September 2008 Rosetta performed the firstasteroid flyby of a spacecraft operated by ESA. (2867)-Steins, asmall E-type asteroid, also was the first scientific target of theRosetta mission on its way to comet 67 P/Churyumov-Gerasi-menko. Despite some problems observed during the operations,the Steins flyby was a great success, both from the scientific andengineering points of view.

The flyby trajectory was constrained by the fixed velocity vectorof Rosetta relative to Steins, and the requirements of the scientificcommunity governed the selection of the free parameters and thedesign of the overall flyby strategy. The scenario implementedallowed to flyby at the minimum possible distance, to pass throughphase angle zero enabling observation of the opposition effect, andto continuously observe the asteroid from before to after closestapproach with good illumination conditions.

An optical navigation campaign was conducted during themonth before the flyby, another first for Europe. The analysis ofimages of Steins that were regularly acquired by the scientificand navigation camera systems on board Rosetta significantlyimproved the targeting of the spacecraft compared to theaccuracy obtainable only with the asteroid orbit based on groundbased observations. Around Steins closest approach an autono-mous asteroid tracking mode was activated to prevent theoff-pointing that would have been caused by the irresolvableinaccuracy of the along-track component of the relativetrajectory.

Most of the Rosetta payloads performed operations during theSteins flyby phase; scientific observations began two weeks beforeclosest approach with a light curve of the asteroid recorded by thescientific camera system OSIRIS. The remaining experiments werepowered on only a few days before the flyby for a set of calibrationactivities before entering the main science acquisition phase ofseveral hours around closest approach to the target. High resolutionimages and spectra were taken, light curves were determined, andthe exosphere as well as the dust and plasma environment of Steinswere explored. Afterwards OSIRIS continued for about four weekswith the observation of gravitational microlensing events.

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The planning process, having to accommodate the scientificobservations and the operational needs in terms of navigation,revealed to be a rather complex activity. It also includedpreparation for various contingency situations, namely break-points to restart the payload operations in case of a spacecraftanomaly, and a backup slew profile in case of a failure of theautonomous asteroid tracking mode.

During the flyby some problems have been observed both withthe spacecraft systems (camera asteroid tracking mode) and withthe scientific instruments. Actions are being implemented toprevent these problems to affect the operations foreseen in July2010 when Rosetta will fly past asteroid (21)-Lutetia.

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