esa bulletin 103 - bepicolombo

BepiColombo – A MultidisciplinaryMission to a Hot Planet

R. GrardSpace Science Department, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands

M. Novara and G. ScoonScience Future Projects and Technology Office, ESA Directorate of ScientificProgrammes, ESTEC, Noordwijk, The Netherlands

means that the same side of the planet facesthe Sun every two hermean years. Its orbit isvery eccentric and its distance to the Sun variesbetween 0.308 and 0.466 AU*.

Mercury was already known to the ancientEgyptians (Fig. 1), but is still largely unexplored.Its proximity to the Sun makes it a difficult targetfor ground-based observations and spacemissions (Fig. 2). Seen from Earth, Mercury’smaximum elongation from the Sun is 28°. It isvisible for just two hours before sunrise or aftersunset, so that Earth-based observations have normally to be performed in front of astrong sky background. Earth-orbiting opticaltelescopes, such as the Hubble SpaceTelescope, usually cannot target Mercury either,because of the high potential risk to instrumentswhen pointed so close to the Sun. On the otherhand, putting a spacecraft into orbit aroundMercury is not a trivial task, because of the

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Figure 1. Mercury was already known in ancient Egypt (after an engraving of RégnierBarbant in G. Flammarion, Astronomie Populaire, 1881)

Figure 2. The terrestrial planets: the cradle of life

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As the inner end-member of the planetary system, Mercury plays animportant role in constraining and testing dynamical andcompositional theories of planetary formation. With its companionsVenus, Earth and Mars, it forms the family of terrestrial planets, acategory of celestial object where each member holds informationessential for retracing the history of the whole group. For example,knowledge about the origin and evolution of these planets is one ofthe keys to understanding how conditions to support life have beenmet in the Solar System and, possibly, elsewhere. This quest is all themore important as terrestrial-like objects orbiting other stars are notaccessible; our own environment remains the only laboratory wherewe can test models that are also applicable to other planetarysystems. The exploration of Mercury is therefore of fundamentalimportance for answering questions of astrophysical andphilosophical significance, such as: ‘Are terrestrial bodies a commonfeature of most planetary systems in the Galaxy?’.

* 1 Astronomical Unit is the average distance from theEarth to the Sun, which is about 150 million km.

IntroductionThe planet Mercury has a radius of 2440 kmand is slightly larger than our Moon. It revolvesaround the Sun in approximately 88 days androtates around itself in two-thirds of that time,i.e. 58.6 days. Quite surprisingly, this resonance

BepiColombo 21/8/00 2:08 pm Page 3

Figure 3. Advance ofMercury’s perihelion

(schematic only)

large difference in the gravitational potentials ofthe Sun at the orbits of Earth and Mercury. Thesolar irradiation is about 10 times larger atMercury and the heat flux is further increasedabove the dayside because of reflected sunlightand infrared emission, which puts enormousthermal constraints on any orbiter.

Missions to the giant planets and to the smallundifferentiated bodies, such as comets andasteroids, provide information on the coldregions of the Solar System. With the Rosettamission to comet P/Wirtanen, to be launched in2003 as Cornerstone-3, ESA is conducting aprogramme that will investigate some of thepristine material found in the outer regions ofthe heliosphere. Mercury represents the otherchallenge, since this small and important bodywill yield complementary data about planetaryformation in the hottest part of the proto-solarnebula. Consequently, the Cornerstone missionto Mercury, BepiColombo, appears the logicalnext step for the Agency’s planetary explorationprogramme.

Einstein explained the advance of Mercury’sperihelion (43 arcsec per century) in terms ofspace-time curvature (Fig. 3). Owing to theproximity of the Sun, a mission to Mercuryoffers, in addition, unique possibilities fortesting general relativity and exploring the limitsof other metric theories of gravitation withunprecedented accuracy. The discovery of anyviolation of general relativity would haveprofound consequences in theoretical physicsand cosmology.

Mercury is also an unrivalled vantage point fromwhich to observe minor bodies with semi-majoraxes of less than 1 AU, the so-called Atens andInner-Earth Objects which might possiblyimpact our planet.

In summary, BepiColombo not only coversobjectives related to the exploration of theplanet and its environment, but also addressesfundamental science and minor-body issues.Some of the questions that form the rationalebehind this mission are:– What is on the unimaged hemisphere of

Mercury?– How did the planet evolve geologically?– What is the chemical composition of the

surface?– Why is Mercury’s density so high?– What is Mercury’s internal structure and is

there a liquid outer core?– What is the origin of the magnetic field?– How does the planetary magnetic field

interact with the solar wind in the absence ofany ionosphere?

– Is there any water ice in the polar regions?– Which volatiles compose the exosphere?– What new constraints can we set on general

relativity and gravitational theories?– Is the Earth threatened by cosmic impactors?

The space segment of the BepiColombomission consists of two orbiters and one lander,to fulfil the scientific goals in an optimum way:– The Mercury Planetary Orbiter (MPO), a three-

axis-stabilised and nadir-pointing module,revolves around the planet at a relatively lowaltitude and is dedicated to planet-wideremote sensing and radio science.

– The Mercury Magnetospheric Orbiter (MMO),a spinner in a relatively eccentric orbit,accommodates mostly the field, wave andparticle instruments.

– The Mercury Surface Element (MSE), a landermodule, performs in-situ ground-truth physical,optical, chemical and mineralogicalobservations, which serve as references forthe remote-sensing measurements.

The method selected for transporting thespacecraft elements to their destinations is theresult of a trade-off between mission cost andlaunch flexibility. It combines electrical propulsion,chemical propulsion and gravity assists. Theinterplanetary transfer is performed by a SolarElectric Propulsion Module (SEPM), which isjettisoned upon arrival. The orbit injectionmanoeuvres are then realised with a ChemicalPropulsion Module (CPM), which is alsojettisoned once deployment of the spacecraftelements is completed. The spacecraft conceptis modular and lends itself to a large variety ofschemes. Two specific scenarios have beenstudied:– a single-launch scenario, in which the three

spacecraft elements and the two propulsionmodules are injected together into an inter-planetary orbit with a large rocket, such asan Ariane-5

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Figure 4. Hemisphere ofMercury imaged by Mariner-10 (courtesy of NASA: nodata available in blankareas)

Figure 5. Caloris basin (1300 km in diameter), thecurved features of whichwere formed when a giantprojectile hit Mercury 3.9 Gaago. Many small craters aresuperimposed on the sunlitpart; the rest of the basin isin shadow. Strangelineaments extendingoutwards can be traced tothe opposite side of theplanet (courtesy of NASA)

energies were relatively more important onMercury than on any other terrestrial planet,because of the lack of an atmosphere and thelarger relative velocities between impactor and target (Fig. 5). Inter-crater plains have been formed before the end of the heavybombardment, 4 Ga ago, but it is not knownwhether these features are associated withvolcanic activity or widespread basin ejecta.Mercury may still be tectonically active now; the relaxation of the equatorial bulge, thecontraction due to the cooling of the mantleand the tidal stresses caused by a highlyeccentric orbit, have induced scarps, faults andlineaments, which bear evidence of theseprocesses. A systematic investigation of the

– a dual-launch scenario, in which thespacecraft is divided into two compositeswith nearly identical propulsion elements,which are launched separately with smallerrockets, such as a Soyuz-Fregat.

The two approaches have been shown to befeasible and compatible with the given missionobjectives and scientific instrumentation. Theyalso provide flexibility and offer alternativeroutes with different schedules and fundingscenarios.

RationaleWhat is on the unimaged hemisphere ofMercury? Mariner-10 returned images of less than half ofthe planet (Fig. 4). This first question is thereforepragmatic and reflects the curiosity of both thelayman and the scientist. Our knowledge of thetopography of Mercury, in terms of globalcoverage and spatial resolution, reminds us of that of the Moon in the Sixties, which was derived from Earth-based telescopicobservations. The images of Mariner-10 show acratered and lunar-like landscape, but withmany different characteristics, indicating thedifferent evolutions of the two bodies. As for theMoon, the unknown hemisphere might provequite different from the known side; forexample, ground-based radar observationssuggest the presence of a gigantic dome onthe unseen hemisphere.

How did the planet evolve geologically?The surface of Mercury has been shaped byvarious exogenic (bombardment) andendogenic (volcanic) processes. The majorimpacts occurred before the end of theaccretionary period and the age of the surfacegenerally exceeds 3.5 Ga*. The collisional

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* One giga-annum (Ga) is equivalent to one thousandmillion years


Figure 6. Absolute densitiesof the terrestrial planets and

the Moon

geologic evolution of Mercury will require theglobal imaging of the surface, as well as dataon topography, core and crust densities,mascons and gravity anomalies.

What is the chemical composition of thesurface?The mineralogical and elemental compositionmapping of the surface provides the means ofdistinguishing between various models of theorigin and evolution of the planet. The iron-oxide content of silicates, for example, is oneindicator of the condensation temperature ofthe solar nebula during the accretion of theplanet. The concentration ratio of key elementssuch as potassium, uranium and thorium alsoreflects the temperature of the feeding zonewhere the body was accreted.

Why is Mercury’s density so high?The density of Mercury does not line up withthose of the other terrestrial planets, includingthe Moon (Fig. 6); when corrected forcompression due to size, it is the largest of all.Several scenarios have been proposed toexplain this anomaly:(a) The iron concentration was larger in the

feeding zone where the planet accreted.(b)Oxides were reduced to metallic form due to

the proximity of the Sun.(c) The temperature of the young Sun was

sufficient to sublimate and blow off silicates,thereby leaving only materials with highercondensation temperatures.

(d) The initial composition of the planet hasbeen significantly altered by gigantic impacts,which may have removed a substantial partof the mantle.

What is Mercury’s internal structure and isthere a liquid outer core?The high density also suggests a relatively largeiron core in which 70 to 80% of the planetarymass is concentrated, and implies a low

moment of inertia factor. The very existence ofa molten outer core is a challenge becausesuch a small planet should have frozen outearly in its history. A small concentration ofsulphur (1 to 5%) could, however, account forthe molten shell, because this element woulddepress the freezing point of the core alloy.

Knowledge of global shape, gravity field androtational state are required to estimate theradius and the mass of the core. For example,the amplitude of the 88-day libration in longitude,which is influenced by the orbit eccentricity, issmall for a rigid body and increases significantlywhen the surface layer (crust and mantle) isdecoupled from the solid inner core by a moltenshell.

What is the origin of the magnetic field?The previous issue is all the more important asit is directly related to the existence of themagnetic field, one of the most remarkablediscoveries of Mariner-10 (Fig. 7). The field isweak, a few 100 nT at the equator equivalent toabout one hundredth of that of the Earth, andcould be generated by an internal hydromagneticdynamo driven by a liquid shell, perhaps 500 km thick, in the outer core. While it ispossible to produce thermal and compositionalmodels compatible with a planetary dynamo,we must also account for the absence ofsubstantial magnetic fields at Venus and Mars.

A detailed mapping of the magnetic field willprovide the necessary constraints on thestructure and mechanism of the internal dynamo.

How does the planetary magnetic fieldinteract with the solar wind in the absence ofany ionosphere?Much can be learned from a comparative studyof the magnetospheres of Earth and Mercury,due to their vastly different volumes andboundary conditions. The size of the hermeanmagnetosphere is only 5% of that of the Earth,although the planetary radii differ by less than afactor of 3 (Fig. 8). The magnetosphere ofMercury is exposed to a solar-wind density andan interplanetary magnetic field (IMF) which are4 to 9 times larger than at 1 AU. The absenceof an ionosphere and the massive emission ofphotoelectrons on the dayside posesinteresting problems regarding the closure ofthe magnetospheric currents, the topology ofwhich might differ significantly from thatobserved at the Earth.

If magnetospheric substorms occur, are theytriggered by IMF reversals or internalinstabilities? Are they waves at the electrongyro frequency similar to the auroral kilometricradiation emitted from the Earth? Is the


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Figure 7. Modulus ofMercury’s magnetic field (innanotesla) measured duringthe third flyby of Mariner-10(after Ness et al., J. Geophys.Res.. 80: 2708, 1975)

Figure 8. The magnetosphereof Mercury (from Slavin etal., Planet. Space Sci. 45:133, 1997)

and ion sputtering, and impact vaporisation byin-falling micrometeorites. Study of theexosphere will therefore provide another clueas to the chemical composition of the surface.

Can we take advantage of the proximity of theSun to test general relativity with improvedaccuracy?A Mercury orbiter offers a unique opportunity totest general relativity and alternative theories ofgravity. Classical tests can be repeated withimproved accuracy and new experimentsbased upon different observable quantities canbe performed due to the proximity of the Sun,the high eccentricity of Mercury’s orbit andfrequent solar occultations. The classical testsrely upon the precession of the perihelion ofMercury, the deflection of radio waves by theSun, and the time delay of radio signals. Theaccurate orbital determination required by

planetary magnetic field perturbed by a ringcurrent associated with possible radiationbelts? How are field-line resonances, if theyoccur, affected by the reflection properties ofthe surface? Magnetic-field, wave and particleobservations will tell us whether phenomenareminiscent of the Earth’s environment alsotake place in the magnetosphere of Mercury.

Is there any water ice in the polar regions?Mercury is a world of extremes. The surfacetemperature at the sub-solar point reaches 700K (427°C), 100°C above the melting point oflead, but it can be as low as 100 K (-173°C) inshadowed areas. New observations from theground have added new questions to the longlist left open by Mariner-10. A major discoverywas made by radar observations in 1992. Thepossibility that water ice or, more prosaically,sulphur may be present in permanentlyshadowed craters near the poles, depositedthere by meteorites or diffused and trappedfrom the planet’s crust, is potentially importantfor the study of surface processes.

Which volatiles compose the exosphere ofMercury?Mercury has no stable atmosphere; the gaseousenvironment of the planet is best described asan exosphere, i.e. a medium so rarefied that itsneutral constituents never collide. Theexistence of five elements – O, H, Ne, Na andK – has been established by Mariner-10 and byground-based observations. Other elements,contributed by the regolith, and possible icesnear the poles may be detected using UVspectroscopic observations of the limb.Production mechanisms include solar photo-

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these measurements also yields thequadrupole moment of the Sun and the timederivative of the gravitational ‘constant’. All ofthese experiments require precision spacecrafttracking, a good solution for the gravity field ofMercury, and accurate measurement of non-gravitational accelerations, in particular theradiation pressure.

The importance of the tests of general relativitycould indeed justify a fully dedicated Mercuryorbiter, but BepiColombo combines theseobjectives with others pertinent to planetaryand magnetospheric physics in a truly multi-disciplinary mission.

Is the Earth threatened by cosmic impactors?A mere 65 million years ago,an impact createdthe Chicxulub crater in Mexico and wiped out70% of the Earth’s living species, including thedinosaurs. It is believed that there are manyNear-Earth Objects (NEOs) with small aphelia,or whose orbits lie entirely within that of theEarth (IEOs), which have never been detected.BepiColombo has the potential to observesuch objects at distances from the Sun assmall as 0.4 AU.

Launch configuration and mission designThe scientific payload is a combination of high-priority instruments and forms a representativemodel that addresses the scientific objectivesof BepiColombo. These instruments do notnecessarily constitute the final payload, butthey provide a set of realistic requirements forthe system design, mission analysis, data linksand flight operations.

The planetary and magnetospheric instrumentshave very specific requirements in terms oforbit, attitude and electromagnetic cleanliness.They are therefore carried by two differentspacecraft elements: MPO (Mercury PlanetaryOrbiter) and MMO (Mercury MagnetosphericOrbiter).

The main requirements of the orbiters andthose of MSE (Mercury Surface Element) arecompared in Table 1. The orbits are polar inorder to ensure global coverage of the planet.The data volume of MPO is about 10 times that

of MMO because Ka-band rather than X-bandtelemetry is required to fulfil the imagingrequirements. The MSE data are relayed by oneof the two orbiters; the largest data volume isachieved with MPO, which has an orbital periodfour times smaller than that of MMO andtherefore offers more opportunities fortelemetry links with MSE.

A mass of 1000-1200 kg must be placed inorbit around Mercury to fulfil the missionobjectives. The elements of the spacecraftcomposite can either be launched together onone large rocket (Ariane-5) from Kourou, orseparately on several smaller rockets (Soyuz-Starsem) from Baikonur.

In the single-launch approach, the spacecraftcomposite consists of MPO, MMO and MSE;the wet mass of the total system (SEPM andCPM included) is 2500-2800 kg at launch. Inthe dual-mission scenario, MPO and the MMO-MSE composite are launched separately withtheir own electric and chemical propulsionmodules, the overall system masses at launchboth being close to 1500 kg.

An artist’s impression of the single-launchcruise configuration (height 5.1 m; wing spanup to 32.8 m) is shown in the frontispiece. Thesplit configurations are illustrated in Figures9a,b, which show MMO on top of MSE andMPO, respectively. The split-spacecraft elementsare designed for a dual launch on a Starsem-Soyuz, but are also compatible with a singleAriane-5 launch, using the Speltra adapter.

Combining electrical propulsion with Venus,Mercury and even Moon gravity assistsprovides mission flexibility and short cruisetimes of 2.6 to 3.6 years, against 6 years ormore for entirely ballistic flights, whichconstitute back-up options. Electrical propulsionis therefore considered as a baseline (Table 2);launch windows at intervals of 1.6 years, thesynodic period of Venus, offer optimalconditions for the first gravity assist from thisplanet.

Depending upon the size of the spacecraftcomposite, SEPM is equipped with a solararray delivering 6 to 10 kW of power at 1 AUand 3 or 5 engines having individual nominalthrusts of 0.2 N, which provides for recoverystrategies in the event of single, or even double,thruster failures. NASA has successfully testedelectrical propulsion with the DS1 mission and ESA will launch SMART-1 in late 2002 tovalidate all aspects of a mission associating thistechnique with gravity assists.

The Mercury capture manoeuvres are executed


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Table 1. Requirements summary

Spacecraft Element MPO MMO MSE

Stabilisation 3-axis Spin NAAltitude/latitude 400 km x 1500 km 400 km x 12000 km ± 85 °Payload mass > 50 kg > 25 kg ~ 6 kgData volume 1550 Gb/yr 160 Gb/yr 75-138 Mb/weekEquivalent bit rate 50 kb/s 5 kb/s 128-228 b/sNominal lifetime > 1 yr > 1 yr > 1 week


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Table 2. Mission opportunities with Ariane-5 and Starsem-Soyuz

Date Launcher Mass margin* Cruise time (yr)

2009/01 Ariane-5 41 % 2.62008/01 Starsem-Soyuz** 24 % 3.62009/07 Starsem-Soyuz** 22 % 3.3

* with ion thrusters

** with lunar flyby

with CPM, which can also be used earlierduring the transit for a partial recovery of themission in the event of total SEPM failure. Thechemical module is a bi-propellant system witha 400 N main engine and eight 20 N thrustersfor attitude control during the cruise.

Figure 10 shows one of many possibleinterplanetary trajectories. In the single-launchscenario, MMO is first placed in its Mercuryorbit with CPM; a second burn lowers theapoapsis to an altitude of 1500 km as requiredfor MPO. In the split-launch scenario, a similarapproach is used to insert MMO and MPOindependently into their nominal orbits (Fig. 11).The pericentre of the two orbits is in theantisolar direction when Mercury is at perihelionto minimise thermal constraints; the ratio of theorbital periods is an integer (4:1), so that aback-up telemetry relay configuration can beimplemented around periapsis if the twospacecraft are operated simultaneously. MSE isdelivered from the MPO or MMO orbit to itsdestination on the surface of the planet, at alatitude of ± 85° near the terminator, where theenvironmental conditions are less severe.

Spacecraft compositePlanetary OrbiterThe Planetary Orbiter configuration is driven byscientific requirements as well as thermalconstraints (Fig. 12). It has the shape of atruncated pyramid; the apertures of theremote-sensing instruments are located on thebase and point constantly along the nadirdirection; the antenna is mounted on anarticulated arm attached to the opposite sideand has a diameter of 1.5 m. The radiator isnever illuminated by the Sun and is protectedfrom the planet IR flux by a shield; the threeother sides are covered with solar cells, whichdeliver 420 W. The mass of MPO is 360 kg.

An imager system performs a global mappingof the surface at better than 200 m resolutionand explores selected areas (up to 5% of thetotal surface) at better than 20 m resolution; theorbital period of 2.3 h provides for a suitableshift in ground track between successive orbits.An IR spectrometer has a range that covers theabsorption bands of most minerals, and itsspatial resolution varies from 150 m to 1.25 km.A UV spectrometer observes the limb airglowby means of an articulated mirror and identifiesthe constituents of the exosphere through theiremission lines. A geochemistry package yieldsthe surface concentrations of various elementsand searches for polar water deposits.

A radio-science experiment (RAD) investigatesthe rotation state (libration), global gravity fieldand gravity anomalies (mascons) of the planet

Figure 9a. Split-spacecraftcruise configuration: MMOand MSE

Figure 10. Ecliptic projection of the trajectory from Earth toMercury for a launch in 2009 (the SEPM thrust is parallel orantiparallel to the direction of motion along the green and red arcs,respectively; planetary flybys are indicated by stars)

Figure 9b. Split-spacecraftcruise configuration: MPO



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Figure 12. The MercuryPlanetary Orbiter (MPO)

Figure 13. The Mercury Magnetospheric Orbiter (MMO)

Directionof theSun atPerihelion

Figure 11. The nominalorbits of MMO and MPOaround Mercury

to constrain its internal structure and thephysical state of its core; RAD observes themotion of Mercury around the Sun and studiesthe propagation of electromagnetic wavesbetween Mercury and the Earth to solve forfundamental quantities such as the oblatenessof the Sun, J2, the general relativity parameters,β, γ and η, and the time derivative of thegravitational ‘constant’ G, with unprecedentedaccuracy. RAD is a complex experiment whichcombines the measurements performed with adedicated radio transponder, an accelerometer,a high-resolution imager and a star tracker.

A laser altimeter is also considered as adesirable addition to the payload, becausetopographic measurements with a resolution ofa few 10 m are required for the evaluation of thegravimetry data.

A small telescope with an aperture of 20 cmcan be dedicated to the observation of NEOswith few additional constraints on spacecraftresources and operation. Owing to the uniquelocation of Mercury, it is believed that, in orderto fulfil similar objectives from an Earth orbit,one would require an instrument with thecapability of detecting objects with magnitudesof the order of 20-21 and pointing at angles ofless than 20 deg from the Sun.

Magnetospheric OrbiterThe Magnetospheric Orbiter is spin-stabilisedat 15 rpm about an axis perpendicular toMercury’s equator, which facilitates thedeployment of a wire antenna and the azimuthalscan of the particle detector fields of views. Theline of apsides of the orbit lies in the equatorialplane, which makes it possible to explore themagnetotail up to planetocentric distances ofalmost 6 Mercury radii.

MMO is cylindrical in shape (Fig. 13); the topand bottom are used as radiators and the sidewall carries solar cells, which deliver 185 W. A despun 1m-diameter antenna is used tocommunicate with Earth. The overall mass ofMMO is 160 kg.

A magnetometer is essential since it addressesboth the planetary and magnetosphericobjectives. A set of charged-particle detectorscovers a combined energy range of several 100keV. The spectrum of electromagnetic waves ismeasured with a search coil and a 70m-longelectric antenna. The mass of the wave andparticle instruments is minimised by including acommon central processor, which ensuressingle interfaces for telecommands, telemetryand power. MMO is electrostatically andelectromagnetically clean. The surface of thespacecraft is conductive and an ion emitter is


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included to prevent its potential from reachingseveral 10 V and invalidating the low-energyparticle measurements.

A multicolour camera is considered as abackup for the imagers carried by MPO; itsresolution varies from a few 10 m to a few 100 mfrom periapsis to apoapsis. This payloadcomplement is certainly not exhaustive andadditional, or alternative, instruments such asan energetic neutral imager are also desirable.

Surface ElementTwo versions of the Surface Element have beenstudied: a hard-lander consisting of a penetratorlinked to a surface station by means of anumbilical, and a soft-lander equipped with aself-penetrating device (Fig. 14). The hard-landerrelies on a solid-propellant descent motor andcrushable material to limit the impactdeceleration. The soft-lander makes use of aliquid propellant motor and airbags to furtherconstrain the impact shock below 250 g (1 g =9.81 ms-2). The dry mass of MSE is of the orderof 50-70 kg. The hard-lander version isassumed in the cruise configurations illustratedin the frontispiece and in Figure 9a.

A heat flow and physical properties instrumentperforms measurements which can only beachieved in-situ; it can be integrated in the fore-body, or penetrator, of a hard lander or in a self-penetrating device, or mole, in the case of asoft-lander. A alpha X-ray spectrometer istransported to selected surface areas by amicro-rover and provides measurements thatserve as ground-truth for the MPO observations.

Two cameras record images before and afterlanding. A magnetometer characterises themagnetic properties of the surface and yieldsthe electrical conductivity of the ground by

recording simultaneously, both on MMO andMSE, the magnetic-field fluctuations inducedby the solar wind. A seismometer enhances thescientific return provided that MSE’s lifetime issignificantly longer than one week.

ConclusionThe potential scientific return from theBepiColombo mission is both significant andnovel; it addresses the planet’s internalstructure and magnetic field, the surfacefeatures and composition, the planetaryenvironment, as well as important fundamentalscience issues and Near-Earth Object (NEO)observations.

The study has demonstrated that an attractivestrategy exists for interplanetary transfer toMercury, combining gravity assists and electricpropulsion. The requirements of the electric-propulsion elements (thrusters, solar array) arecompatible with current technologies. Theproposed concept is modular, and lends itselfto reconfiguration depending on the futureevolution in terms of mission goals, fundingscenarios and international cooperation.

AcknowledgementsThis article is based on contributions providedby the members of, and consultants to theBepiColombo Scientific Advisory Group – A. Balogh, A. Boattini, L. Blomberg, J. Brückner, A. Carusi, L. Iess, Y. Langevin, A. Milani, S. Mottola, T. Spohn, N. Thomas andP. Wurz – and by the industrial manager of theSystem and Technology Study, A. Anselmi.Thanks are also due to M. Coradini from theESA Science Mission Coordination Office.

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Figure 14. The hard and softlander concepts (left andright)


esa bulletin 103 - bepicolombo

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