Acta Astronautica 62 (2008) 699–705www.elsevier.com/locate/actaastro

BepiColombo Mercury magnetospheric orbiter designHiroshi Yamakawaa,∗, Hiroyuki Ogawaa, Yoshitsugu Sonea, Hajime Hayakawaa,Yasumasa Kasabaa, Takeshi Takashimaa, Toshifumi Mukaia, Takahiko Tanakab,

Masaki Adachib

aBepiColombo Project, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara,Kanagawa 229-8510, Japan

bNEC-TOSHIBA Space Systems Ltd., 1-10 Nisshin, Fuchu, Tokyo 183-8551, Japan

Received 15 February 2006; received in revised form 15 February 2007; accepted 17 January 2008Available online 16 May 2008

Abstract

This paper summarizes the current status of the BepiColombo Mercury magnetospheric orbiter (MMO) spacecraft design.The MMO is a spinning spacecraft of 223 kg mass whose spin axis is nearly perpendicular to the Mercury orbital plane. Thecurrent status of the overall MMO system and subsystems such as thermal control, communication, power, etc., are described.The critical technologies are also outlined. Furthermore, the outline of the international cooperation between Japan AerospaceExploration Agency and European Space Agency is also presented.© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The BepiColombo mission to Mercury was selectedas European Space Agency (ESA)’s Fifth CornerstoneMission in September of 2000 [1]. The BepiColombomission comprises two science elements, the Mercuryplanetary orbiter (MPO) and the Mercury magne-tospheric orbiter (MMO). The MPO mainly carriesremote sensing and radioscience instrumentation. TheMMO mainly carries fields and particle science instru-mentation. The system baseline assumes the launchof the MPO and MMO spacecraft on a single launchvehicle during the launch window in the 2013 timeframe. The solar electrical propulsion module (SEPM)

∗ Corresponding author. Present address: Research Institute forSustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto,611-0011 Japan.

E-mail address: yamakawa@rish.kyoto-u.ac.jp (H. Yamakawa).

0094-5765/$ - see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.actaastro.2008.01.040

equipped with solar electric propulsion is utilizedin interplanetary cruise, and the chemical propul-sion module (CPM) with chemical propulsion systemfor the Mercury orbit insertion. The configuration ofthe Mercury cruise spacecraft (MCS) is as follows:MMO–MPO–CPM–SEPM. SEPM is interfaced withthe launch vehicle. JAXA will provide the MMO, andESA will provide the launch, the MPO, the SEPMand the CPM. Below is the summary of the recentInstitute of Space and Astronautical Science (ISAS)and the Japan Aerospace Exploration Agency (JAXA)BepiColombo activities:

June 1997 [ISAS] Formation of Mercury Explo-ration Working Group

September 2000 [ISAS] Intent of Participation to theESA BepiColombo project

October 2000 [ESA] BepiColombo selected as theCornerstone 5

700 H. Yamakawa et al. / Acta Astronautica 62 (2008) 699–705

September 2001 [ISAS] Proposal submission to Steer-ing Committee for Space Science(SCSS)

September 2001 International Mercury Science Work-shop held at ISAS

January 2002 [ISAS] The proposal was approved bythe ISAS SCSS

June 2003 [ISAS] Evaluation of the phase-up ofthe BepiColombo by Space ActivitiesCommission of the Japanese Govern-ment: concluded to be reasonable

October 2003 [JAXA] Unification of ISAS to JAXANovember 2003 [ESA] Science Programme Commit-

tee approved BepiColombo MPOand MMO complement with launchin 2012, and cancelled the Mercurysurface element.

November 2004 MMO and MPO payloads finally se-lected by MMO and MPO PayloadReview Committee after Announce-ment of Opportunity.

November 2004 Letter of Agreement between ESAand JAXA signed.

April 2005 [JAXA] Phase B start of the JAXABepiColombo Project.

Previous papers [1,2] focused on the MMO systemand subsystems such as the thermal control, communi-cation, power, attitude control, etc. The thermal designaspects were also summarized in Refs. [3,4] in detailand science payloads were noted in Refs. [5,6]. Thispaper is considered as an update of Ref. [2].

2. MMO system description

The artistic view of the MMO in Mercury orbit isshown in Fig. 1. The MMO configuration is shown inFig. 2.

(1) The MMO is a spin-stabilized spacecraft after it isseparated from the MCS following the Mercury or-bit insertion. The nominal spin rate is 15 rpm (spinperiod of 4 s) due to the data acquisition frequencyrequirements for particle sensors. The spin axis ispointed nearly perpendicular to the Mercury equa-tor and the sun angle is controlled to 92 ± 1◦. TheMMO has attitude control capability while orbitcontrol function is not required. When the MMOis attached to the MCS before Mercury arrival, theMMO is mainly in a dormant mode.

(2) The MMO main structure consists of two decks(upper and lower), a central cylinder (thrust tube)

Fig. 1. Artistic image of the MMO around Mercury (Courtesy:Research Institute for Sustainable Humanosphere Kyoto University).

and four bulkheads. The external appearance hasan octagonal shape, which can be surrounded bya 1.8 diameter circle. The height of the side panelis 0.9 m, whose upper portion is covered by solarcells and second surface mirror (SSM) and lowerportion is covered by only SSM.

(3) The instruments are located on the upper and lowerdecks whose interval is 40 cm. The external sur-face of the upper deck is covered by multi-layerinsulator (MLI) for thermal isolation, while the ex-ternal surface of the lower deck works as a heatradiator and covered by the SSM.

(4) Inside the central cylinder are located the batteries,nutation damper and a tank for the cold nitrogengas jet system.

(5) For the high gain antenna (HGA), a helical ar-ray antenna of 80 cm diameter excited by the ra-dial line is assumed. The HGA is pointed towardthe Earth by the antenna despun motor (ADM)and an elevation control mechanism, the antennapointing mechanism (APM). As for the mediumgain antenna (MGA), a bi-reflector type antenna ismounted on the lower surface with an extendiblemechanism.

(6) Most of the scientific instruments (particle sensors,etc.) are allocated on the side panel, and two pairsof probe antennas for plasma wave instruments andone pair of extendible masts for magnetometersand search coils are installed at the lower deck.

Table 1 gives a summary of the mass of each subsys-tem, which includes the equipment-level margin. On topof it, 20% system-level margin is allocated to cope withdrastic system design change. Equipment-level margin

H. Yamakawa et al. / Acta Astronautica 62 (2008) 699–705 701

Fig. 2. MMO configuration.

Table 1MMO mass budget

Science (including probe antenna and mast) 41.0 kgPower 34.4 kgCommunication 37.3 kgCommand and data handling 6.7 kgAttitude control 23.7 kgWire harness 16.0 kgStructure 34.8 kgThermal 10.9 kg

Total MMO (after separation) 204.7 kgSpin and ejection device 18.8 mkg

MMO total 222.6 kg

is defined as follows:

>5%: off-the-shelf items with no modifications,>10%: off-the-shelf items with minor modifications,>20%: new-design items or items with major re-

design,>100%: attitude maintenance fuel.

3. Operation

Fig. 3 depicts the mission sequence and architectureof the BepiColombo mission.

3.1. Launch and cruise phase operation

The current assumption is that the MMO is in dor-mant mode at launch. During the interplanetary cruisephase, the MMO is located behind the Sun shade at-

9.3h400km x 12,000km

2.3h400km x 1500km

Observation: 1 Earth year

Fig. 3. BepiColombo mission operation.

tached to the MPO during the interplanetary cruise or-bit, and its power is supplied from the MPO. Nominalmission operations will be pre-scheduled for one-weekcycles. The contacts between the Mission OperationsCenter at European Space Operation Center (ESOC) andthe MCS serves for collecting science data and house-keeping telemetry (TLM), and for pre-programming theautonomous operations functions of the spacecraft.

3.2. Mercury orbit insertion and MMO/MPOdeployment

The tentative MCS Mercury orbit insertion and theMMO/MPO deployment sequence is as follows: (a)solar electric propulsion module jettison, (b) chemi-cal propulsion burn for the MMO Mercury orbit inser-tion, (c) MMO separation with spin and ejection device(SED), (d) CPM burn for MPO Mercury orbit insertion,(e) MPO separation (see Fig. 3).

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Table 2MMO orbital parameters

Apocenter height 11 817 kmPericenter height 400 kmInclination polar (∼ 90◦)Argument of pericenter 180◦Period 9.3 hInertial directionof pericenter

at local noon whenmercury at aphelion

Eclipse duration < 2 h

As for the MMO after its separation, the following se-quence is assumed: (a) MMO separation with SED at aspin rate of about 6 rpm with its spin axis perpendicularto the Sun, (b) MMO spin-up to several tens of rpm byMMO thrusters, (c) MMO probe antennae deployment,(d) MMO mast deployment resulting in spin-down toseveral rpm, (e) MMO spin-up by MMO thrusters to15 rpm which is the nominal rate for scientific obser-vation, (f) spin axis control to attain nominal sun angle(i.e. 92◦).

3.3. Mercury observation phase

The MMO will be delivered into an orbit around mer-cury having the nominal parameters listed in Table 2.As for the MMO spin axis, it is nearly perpendicularto the Mercury equator taking account of the scientificobservation requirement as well as the communicationwith the Earth. Precisely speaking, the MMO’s attitudewould be controlled to satisfy the sun angle of 92 ± 1◦in order to prevent the shadow on the probe antennasdue to the spacecraft body, which degrades the sciencereturn

3.4. Ground segment

Until its release into its operational orbit, the MMOis operated by ESOC via the MCS system. After releaseinto its operational orbit, the MMO is operated by JAXAat the Sagamihara Space Operations Center. The 64 mJAXA station at Usuda will be used for contact with theMMO spacecraft.

4. Structure

The main structure of the MMO consists of upperand lower decks for instrument arrangements, a cen-tral cylinder and four bulkheads in order to satisfy thestiffness requirement from the BepiColombo system(Fig. 4). An octagonal prism consisting of eight solarcell panels covers the main structure. A pedestal for

Upper deck

Lower deck

Upper prism

Lower prism

Middle prism

BatteryTank

Fig. 4. MMO structure (side view) [2].

the HGA is mounted on the upper deck. A nitrogengas tank for cold gas jet system is installed inside thecentral cylinder, while the battery is allocated near thelower deck. The material selection for each componentis carefully done from the viewpoint of thermal design,allowable temperature and mass reduction. The brack-ets of solar cell panels (upper prism) are required tohave high thermal isolation.

5. Thermal control

The harsh environment near Mercury (0.31 AU fromthe Sun) imposes 11 solar intensities on the MMOspacecraft, while its thermal control system is requiredto maintain the onboard equipment and the spacecraftstructure in proper temperature range during the entiremission phases [3,4]. The MMO is controlled by meansof passive thermal design techniques and some compo-nents are controlled by means of combined passive andactive techniques. The thermal control configuration ofthe spacecraft is shown in Figs. 5 and 6. The passivecontrol elements are the SSM, thermal shield, paints,films and MLI blankets. All external surfaces have elec-trical conductivity.

The internal surfaces of the upper and lower deckshave high emissivity surfaces (black paint) to equal-ize internal temperature. The external surface of theupper deck is covered by MLI for isolation fromthe external thermal environment. The external sur-face of the lower deck has low absorptivity and highemissivity SSMs. The nitrogen gas tank and batter-ies are mounted inside the central cylinder, and theyare covered with MLI as well as the central cylin-der. The ADM and its pedestal are surrounded bythermal shield. The thermal shield is covered withMLI.

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White Paint

SSM MLI

SSM

Cel l 50%SSM 50%

Fig. 5. Thermal design (upper) [2].

SSM

White Paint

SSM or variableemissivity device

Fig. 6. Thermal design (lower) [2].

The octagonal prism is divided into three parts: up-per, middle and lower prisms. The solar cells and SSMsare put on the external surface of the upper prism in theratio of 50:50, and SSMs are put on the internal sur-face of the upper prism to reduce the cell temperature.SSMs are affixed to the external surface of the middleprism, while its internal surface is covered by MLI forisolation from the external thermal environment. SSMsare affixed to both the external and internal surfaces ofthe lower prism to reflect the direct solar flux. The oc-tagonal prism (substrate) is isolated from the upper andlower decks with thermal standoffs.

Most of the internal components have a surface ofhigh emissivity (black paint) to equalize the internaltemperature. The batteries are controlled independentlywith the aid of radiators and heaters, which are installedon a battery panel. This panel is attached to the bottom

of the central cylinder and isolated from the main struc-ture by MLI and thermal standoffs. The radiator has asurface covered by high emissivity SSMs. The ADMand nitrogen gas tank are covered with MLI for isola-tion from the external thermal environment. The HGAdisks are painted white.

The MMO is thermally designed as a spin-stabilizedspacecraft focusing on the Mercury observation phase.Therefore, this design would require some limitation ofsunlit during cruise phase. Since the main radiation sur-face and battery are located in the lower surface, thesolar input by reflection from the MPO to the MMO’slower surface should be minimized. Moreover, the atti-tude of the MCS during the interplanetary cruise phaseis kept without solar input to the MMO’s upper surface,even if it is an attitude for safe hold mode of the MCS.

6. Communication

(1) Communication system: An HGA of 80 cmdiameter is used for the high speed X-bandTLM/Command (CM) and ranging link, with theuse of a 20 W power amplifier. The Ka-band is nottaken into account for the MMO due to the limitedmass budget, while it is nominally used for theMPO communication system. The MMO HGA ispointed toward the Earth with the ADM and theAPM for the elevation angle control of at least 22◦degree (zero margin) determined by the geome-try of the planets. An MGA is accommodated foremergency TLM (4 bps)/CM link. The MGA is in-stalled during the cruise phase at the lower surfaceof the MMO, and extended after the MMO sepa-ration. At Mercury’s orbit, the MMO’s TLM ratewould be changed as a function of the range fromthe Earth. The average bit rate is 16 kbps, which inturn translates into ∼ 40 Mbyte/day assuming a 6 hconsecutive pass in average.

(2) HGA: The helical array antenna excited by a radialline is adopted for the HGA system [7]. There aremainly four reasons: (1) high efficiency (and lightweight) due to the low propagation loss in the ra-dial line wave guide, which can be achieved by thephase tuning of the helical elements, (2) wide fre-quency band suitable for uplink and downlink ow-ing to the helical antennas as emission elements,(3) simple flat structure compared to a parabolicshape, which yields insensitivity to the thermal ex-pansion, (4) avoidance of solar flux concentrationas predicted for a parabola type antenna. Fig. 7 isan engineering model of the half-size HGA dish.

704 H. Yamakawa et al. / Acta Astronautica 62 (2008) 699–705

Fig. 7. Engineering model of the half-size MMO HGA dish [2].

(3) TLM/CM and data handling: Each instrument isconnected to the serial data bus by low-voltagedifferential signaling with SpaceWire protocol [8],which will keep some compatibilities to peripheralinterface module, classical protocol used in the pastISAS missions. Both TLM and CM are based onthe Consultative Committee for Space Data Sys-tems Telecommand Recommendation packet type.As for the data recorder, 2.0 GByte volume is as-sumed for the storage of MMO housekeeping andscience data before the transmission to the Earth.

7. Attitude control system

The spin-stabilized MMO spacecraft attitude wouldbe determined by a pair of sun sensors on the side panel,and a star scanner attached at the bottom surface. Theattitude is controlled by the propulsion system with acold gas jet. A nutation dumper installed inside the cen-tral cylinder is used for passive nutation dumping.

The MMO propulsion system adopts cold gas jet sys-tem, since only attitude control capability is required(i.e. no orbit control function). It consists of one pro-pellant tank, six 0.2 N class nitrogen gas jet thrusters,valves, piping and thermal control equipment (heatersand sensors). The four tangential thrusters for roll con-trol are allocated on the side panel, while the two axialthrusters are mounted at the bottom of the spacecraftbody. The nitrogen gas tank consists of titanium alloyliner and carbon fiber shell. The tank volume is 14.7 land the maximum designed pressure is 27.6 MPa. Anamount of 4.25 kg of nitrogen gas is loaded includingunexpelled propellant of 0.25 kg.

8. Power and heater system

During the interplanetary cruise phase, the compositeprovides the heater power directly to the MMO bus,since the MMO spacecraft is not exposed to the directsolar flux owing to the MMO Sun shield. The MMOis equipped with the primary heaters controlled by theMMO heater control electronics as well as the survivalheater with thermostats.

After MMO separation, the temperature of the solarcell suffers wide variation due to the variation of Mer-cury’s distance from the Sun (0.31–0.47 AU). The cur-rent assumed bus voltage is 50 V (non-regulated). Thesolar cell assumes the multi-junction cell with conduc-tive coating type cover glass. Li-ion secondary type isassumed for the battery in order to cope with the 2 hmaximum eclipse condition around the Mercury.

9. Wire antenna and extendible mast

Two pairs of wire probe antennas for plasma waveinstrument are equipped with MMO, whose tip-to-tiplength is tentatively 32 m, located at the outer side of thelower deck. A single pair of extendible masts for mag-netometers (magnetic field analyzers and search coils)is installed whose length is 5 m. Extension of those willbe the initial phase of the MMO operation just after theseparation of the MMO from the MCS at the Mercuryorbit.

10. Concluding remarks

A definition study of the MMO was performed in or-der to satisfy the science requirements as well as theBepiColombo system requirements, under the harsh en-vironment near Mercury. It shows a feasible solution onthe whole, but more detailed thermal and structural sys-tem study considering the detailed instrument allocationand thermal analysis taking microscopic structure intoaccount is under way.

Acknowledgments

The authors wish to express their sincere gratitude toDr. J. v Casteren, Dr. M. Novara, Dr. R. Carli, Dr. R.Schulz of ESA BepiColombo Project, Prof. H. Nakanoof Hosei University, Prof. Y. Kogo of the Tokyo Univer-sity of Science (Material), NTSpace, NIPPI, MHI, SHI,MEC and Astro Research engineers and the followingISAS staff for their contribution toward the MMO sys-tem design: Dr. Z. Yamamoto Dr. T. Toda (Commu-nication), Mr. Y. Kamata„ Dr. T. Mizuno (High Gain

H. Yamakawa et al. / Acta Astronautica 62 (2008) 699–705 705

Antenna), Dr. T. Hashimoto, Mr. E. Hirokawa (Atti-tude Control & Sensors), Mr. M. Shida Dr. S. Sawai(Propulsion), Dr. M. Tajima, Dr. K. Hirose (Power), Dr.A. Ohnishi, Mr. S. Tachikawa (Thermal Control), Dr.K. Minesugi (Structure), Dr. Y. Morita (Mechanism),Dr. K. Goto, Dr. R. Yokota, Dr. H. Hatta, Dr. K. Hori(Material), Dr. T. Yamada (Telemetry & Command), Dr.H. Saito (Electrical Subsystems) and Dr. M. Yoshikawa(Orbit Determination).

References

[1] H. Yamakawa, H. Ogawa, Y. Kasaba, H. Hayakawa, T. Mukai,M. Adachi, ISAS feasibility study on the bepiColombo/MMOspacecraft design, Acta Astronautica 51 (1–9) (2002) 397–404.

[2] H. Yamakawa, H. Ogawa, Y. Kasaba, H. Hayakawa, T. Mukai,M. Adachi, Current status of the BepiColombo/MMO spacecraftdesign, Advances in Space Research 33 (2004) 2133–2141.

[3] H. Ogawa, H. Yamakawa, Y. Kobayashi, M. Nakano, Thermalfeasibility study on the BepiColombo/MMO spacecraft, Paper02ICES-236 32nd International Conference on EnvironmentalSystems (ICES), San Antonio, TX, USA, July 15–18, 2002.

[4] H. Ogawa, H. Yamakawa, K. Goto, Thermal protection systemfor BepiColombo/Mercury magnetospheric orbiter, ESA-SpecialPublications-521 (2002) pp. 109–115.

[5] H. Hayakawa, Y. Kasaba, H. Yamakawa, H. Ogawa, T. Mukai,The BepiColombo/MMO model payload and operation plan,Advances in Space Research 33 (2004) 2142–2146.

[6] T. Mukai, H. Yamakawa, H. Hayakawa, Y. Kasaba, H. Ogawa,Present status of the BepiColombo/Mercury magnetosphericorbiter, Advances in Space Research 38 (2006) 578–582.

[7] H. Nakano, Y. Takeda, H. Kitamura, H. Mimaki, J. Yamauchi,Low-profile helical array antenna fed from a radial waveguide,IEEE Transactions on Antennas and Propagation 4 (3) (1992)279–284.

[8] 〈http://www.spacewire.esa.int/tech/spacewire〉 cited November2006.