Current status of the BepiColombo/MMO spacecraft design

Advances in Space Research 33 (2004) 2133–2141

www.elsevier.com/locate/asr

Current status of the BepiColombo/MMO spacecraft design

Hiroshi Yamakawa a,*, Hiroyuki Ogawa a, Yasumasa Kasaba a, Hajime Hayakawa a,Toshifumi Mukai a, Masaki Adachi b

a Institute of Space and Astronautical Science (ISAS), 3-1-1, Yoshinodai Sagamihara, Kanagawa 229-8510, Japanb NEC-TOSHIBA Space Systems, Ltd., 4035 Ikebecho, Tuzuki-ku, Yokohama, Kanagawa 224-8555, Japan

Received 19 October 2002; received in revised form 31 January 2003; accepted 4 February 2003

Abstract

This paper shows the current baseline of the conceptual design of the BepiColombo/MMO (mercury magnetospheric orbiter)

spacecraft, which is conducted by the ISAS Mercury Exploration Working Group. The MMO is a spinning spacecraft of 200 kg

mass whose spin axis is nearly perpendicular to the Mercury orbital plane. The current status of the overall MMO system and

subsystems such as thermal control, communication, power, etc. are described. The latest status of the development of critical

technologies for the MMO and the outline of the international cooperation between ESA and ISAS are also presented.

� 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Mercury; BepiColombo mission; MMO spacecraft

1. Introduction

The BepiColombo mission to Mercury was selected asESA�s Fifth Cornerstone Mission in September of 2000.

The BepiColombo mission comprises three science ele-

ments, themercury planetary orbiter (MPO), themercury

magnetospheric orbiter (MMO) and the mercury surface

element (MSE). The MPO carries remote sensing, ra-

dioscience and potentially, Near-Earth Object instru-

mentation. The MMO carries fields and particle science

instrumentation, while the MSE will carry out in situmeasurements of the chemical and physical properties of

the Mercury surface. The system baseline assumes the

launch of the MPO and of an MMO–MSE composite

spacecraft on 2 Soyuz/Fregat vehicles during the same

launch window in the 2010–2011 time frame. The SEPM

(solar electric propulsion module) and the CPM (chemi-

cal propulsion module) are utilized in interplanetary

cruise, the Mercury orbit insertion and the MSE landingonMercury. The configuration of the two mercury cruise

composite spacecraft (MCCS) is as follows:MPO–CPM–

SEPM and MMO–SM–MSE–CPM–SEPM. SM stands

for Service Module which mechanically connects the

* Corresponding author.

E-mail address: [email protected] (H. Yamakawa).

0273-1177/$30 � 2003 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/S0273-1177(03)00437-X

MMO and CPM and is responsible for the composite

operation during interplanetary cruise phase.

ISAS is expected to provide the MMO, and ESA willprovide the launch, MPO, MSE and the propulsion

modules. ISAS has proposed the project budget and is

waiting for the budgetary approval from the Japanese

Government. Below is the summary of the recent ISAS

BepiColombo activities.

The overall mission summary as well as the current

June 1997 ISAS Mercury Exploration

Working Group formed

September 2000 ISAS Intent of Participation inBepiColombo

October 2000 BepiColombo Selected as the

ESA Cornerstone 5

September 2001 Proposal submission to ISAS

Steering Committee for Space

Science (SCSS)

September 2001 BepiColombo Science Workshop

(ISAS)January 2002 The proposal was approved by

ISAS SCSS

status of the MPO and MSE are described in Novara

(2001), while the paper of Yamakawa et al. (2002) fo-cuses on the MMO system and subsystems such as

the thermal control, communication, power, attitude

ved.

mail to: [email protected]

2134 H. Yamakawa et al. / Advances in Space Research 33 (2004) 2133–2141

control, etc. This paper is considered as an update of

Yamakawa et al. (2002).

2. MMO system description

The MMO�s launch configuration in Soyuz/Fregat

fairing is shown in Fig. 1. Here, the Sun shield for the

MMO and MSE are not depicted. The dimension of

MMO configuration is shown in Fig. 2 (side view).

(1) The MMO is a spin-stabliized spacecraft after it is

separated from the BepiColombo cruise composite

following the Mercury orbit insertion. The nominal

spin rate is 15 rpm (spin period of 4 s) due to the sci-entific data frequency requirements. The spin axis is

pointed nearly perpendicular to the Mercury equa-

tor. The MMO has attitude control capability while

orbit control function is not required. When the

MMO is attached to the cruise composite before

Mercury arrival, the MMO works as a subsystem

or as a dormant payload of the three-axis-stabilized

cruise composite.

Fig. 1. Launch configuration in Soyuz-Fregat fairing.

Fig. 2. MMO configuration (side view).

(2) The MMO main structure consists of two decks

(upper and lower), a central cylinder (thrust tube)

and four bulkheads. The external appearance has

an octagonal shape, which can be surrounded by a

1.8 diameter circle. The height of the side panel is0.9 m, whose upper portion is covered by 54% solar

cells and 46% SSM (second surface mirror) and low-

er portion is covered by only SSM.

(3) The instruments are located on the upper and lower

decks whose interval is 40 cm. The external surface

of the upper deck is covered by MLI (multi-layer in-

sulator) for thermal isolation, while the external sur-

face of the lower deck works as a heat radiator andcovered by the SSM.

(4) Inside the central cylinder are located the batteries,

nutation damper, UHF antenna for MMO–MSE

communication relay and a tank for the cold gas

jet system.

(5) For the HGA (high gain antenna), a helical array

anntena of 80 cm diameter excited by the radial line

is assumed. The, HGA is pointed toward the Earthby the ADM (antenna despun motor) and an eleva-

tion control mechanism, the APM (antenna pointing

mechanism). As for the MGA (medium gain an-

tenna), a bi-reflector type antenna is mounted on

the lower surface with an extendible mechanism.

(6) Most of the scientific instruments (particle sensors,

etc.) are allocated on the side panel, while two pairs

of probe antennas for plasma wave instruments andone pair of extendible booms for magnetometers

and search coils are installed.

Table 1 gives a summary of the weight of each sub-

system, which includes the equipment-level margin. The

total mass with equipment-level margin but without

system level margin is 215.8 kg. Equipment-level is de-

fined as follows: >5%: Off-The-Shelf items with no

modifications, >10%: Off-The-Shelf items with minormodifications, >20% few-design items or items with

major re-design, Fuel: >100%: Attitude maintenance

fuel. The mass due to the changes of the interface be-

Table 1

MMO mass budget (in kg)

Science instruments 38.0

Including probe antenna 3.0

Boom 3.4� 2

Common instruments 162.8

Power 30.8

Communication 36.1

MSE interface 5.5

Command and data handling 7.3

Attitude control 22.2

Wire harness 16.0

Structure 34.5

Thermal 10.5

Total MMO (after separation) 200.8

Separation mechanism 15.0

MMO mass 215.8

H. Yamakawa et al. / Advances in Space Research 33 (2004) 2133–2141 2135

tween MMO and the external systems (i.e., SM, CPM,

SEPM, MSE and MPO) is included in the system level

margin but not in the equipment-level margin.

Fig. 4. Mercury orbit and MMO orbital plane direction.

3. Operation

3.1. Launch and cruise phase operation

The current assumption is that the MMO is in dor-

mant mode at launch. During the interplanetary cruise

phase, the MMO is located behind the Sun shade at-

tached to the SM during the interplanetary cruise orbit,

and its power is supplied from the MCCS. Nominal

mission operations will be pre-scheduled for one-weekcycles. The contacts between the Mission Operations

Center (MOC) at ESOC (European Space Operation

Center) and the MCCS serves for collecting science data

and housekeeping telemetry, and for pre-programming

the autonomous operations functions of the spacecraft.

3.2. Mercury orbit insertion and MMO/MSE deployment

The tentative MCCS Mercury orbit insertion and the

MMO deployment sequence is as follows: (a) SEPM

jettison, (b) CPM burn for Mercury orbit insertion, (c)

MMO separation with spin and ejection device (SED),

(d) SM jettison and (e) CPM burn for MSE landing.

As for the MMO after its separation, the following

sequence is assumed, (a) MMO separation, (b) MMO

spin-up by MMO thrusters (above 20 rpm), (c) MMOprobe antennae deployment, (d) MMO boom deploy-

ment and (e) MMO spin-up by MMO thrusters (to 15

rpm).

3.3. Mercury observation phase

The MMO will be delivered into an orbit around

Mercury having the following nominal parameters (seeFigs. 3 and 4). As for the MMO spin axis, it is nearly

Fig. 3. MPO, MMO orbit and MSE landing point.

perpendicular to the Mercury equator taking into ac-count of the scientific observation requirement as well as

the communication with the Earth. Precisely speaking,

the MMO�s attitude would be controlled to satisfy the

sun angle of 92� in order to prevent the shadow on the

probe antennas due to the spacecraft body, which de-

grades the science return.

3.4. Ground segment

Until its release into its operational orbit, the MMOis operated by ESOC via the MCCS system. After re-

lease into its operational orbit, the MMO is operated by

ISAS at the Sagamihara Space Operations Center

(SSOC). The 64 m ISAS station at Usuda will be used

for contact with the MMO spacecraft.

� Apocentre height (BOL): 11,817 km

� Pericentre height (BOL): 400 km� Inclination: polar (�90�)� Argument of pericentre: 180�� Period: 9.3 h

� Inertial direction of

pericenter:

at local noon when

Mercury at aphelion

� Eclipse duration: <2 h

4. Structure

The main structure of the MMO consists of upper

and lower decks for instrument arrangements, a central

cylinder and four bulkheads in order to satisfy the 40 Hz

stiffness requirement from the BepiColombo system

(Fig. 5(a)). An octagonal prism consisting of eight solar

cell panels covers the main structure. A pedestal for the

HGA is mounted on the upper deck (Fig. 5(b)). A GN2tank for cold gas jet system is installed inside the central

cylinder, while the battery is allocated near the lower

deck. The material selection for each component should

be carefully done from the viewpoint of thermal design,

Fig. 6. Helical spring ejection device for MUSES-C capsule (Manufacturer: NIPPI Corporation).

Fig. 5. (a) MMO structure (upper). (b) MMO structure (side view).


allowable temperature and weight reduction. The

brackets of solar cell panels (upper prism) are required

to have high thermal isolation.ESA provides the SM, and ISAS provides the sepa-

ration mechanism between the SM and MMO. Fig. 6 is

a picture of the helical SED for the ISAS MUSES-C

capsule ejection, which is a candidate mechanism for the

MMO–SM SED. The temperature of the pyrotechnics

for separation mechanism should be kept less than the

storage temperature during cruise phase.

5. Thermal control

The harsh environment near Mercury (0.31 AU from

the Sun) imposes 11 solar intensities on the MMO

spacecraft, while its thermal control system is required

to maintain the onboard equipment and the spacecraft

structure in proper temperature range during the entiremission phases (Ogawa et al., 2002). The MMO is

controlled by means of passive thermal design technique

and some components are controlled by means of

combined method of passive and active techniques. The

thermal control configuration of the spacecraft is shown

in Figs. 7(a) and (b) and 8. The passive control elements

are the SSM, thermal shield, paints, films and multi-

layer insulation blankets (MLI). All external surfaces

have electrical conductivity.

The internal surfaces of the upper and lower deck havehigh emissivity surfaces (black paint) to equalize internal

temperature. The external surface of the upper deck is

covered by MLI for isolation from the external thermal

environment. The external surface of lower deck has low

absorptivity and high emissivity; SSMs. The GN2 tank

and batteries are mounted inside the central cylinder, and

they are covered with MLI as well as the central cylinder.

The ADM and its pedestal are surrounded by thermalshield. The thermal shield is covered with MLI.

The octagonal prism is divided into three parts; up-

per, middle and lower prism. The solar cells and SSMs

are put on the external surface of upper prism in the

ratio of 54:46, and SSMs are put on the internal surface

of the upper prism, to reduce the cell temperature. The

external surface of the middle prism is affixed by SSMs,

while its internal surface is covered by MLls for isolationfrom the external thermal environment. The external

and internal surfaces of lower prism are affixed by SSMs

to reflect the direct solar flux. The octagonal prism

(substrate) is isolated from the upper and lower decks

with thermal standoffs.

Most of the internal components have a surface of

high emissivity (black paint) to equalize the internal

Fig. 8. Thermal design (internal).

Fig. 7. (a) Thermal design (upper). (b) Thermal design (lower).

Fig. 9. Temperature history (Mercury orbiting phase).


temperature. The batteries are controlled independently

with 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

structure by MLI and a thermal standoffs. The radiator

has a surface of high emissivity (SSMs). The ADM and

GN2 tank are covered with MLI for isolation from the

external thermal environment. The HGA disks are

painted white. Fig. 9 elucidates the feasibility of the

thermal design concept by showing that the upper andlower deck temperature is controlled within the allow-

able limits.

The MMO is thermally designed as a spin-stabilized

spacecraft focusing on the Mercury observation phase.

Therefore, this design would require some limitation of

sunlit during cruise phase. Since the main radiation

surface and battery are located in the lower surface, the

solar input by reflection from the SM to the MMO�slower surface should be minimized. Moreover, the atti-

tude of the MCCS during the interplanetary cruise

phase should be kept without solar input to the MMO�supper surface, even if it is an attitude for safe hold mode

of the MCCS.

In the cruise phase the total power of about 350 W is

required to keep the upper and lower decks be in the

temperature range. This power is estimated on the as-

sumption that (a) MMO is thermally isolated from the

SM, the MSE, the CPM and the SEPM, (b) a sun shield

blocks off the sun light, and the MMO is not exposed tothe direct solar flux, (c) the IR radiation from the sun

shield is small, (d) the upper and lower deck tempera-

tures are higher than )20 �C and (e) the radiator shield

of 1800 mm diameter is assumed to be located below

MMO at a distance of 300 mm (This radiator is a part of

the SM structure).


6. Communication

6.1. Communication system

A HGA of 80 cm diameter is used for the high speed

X-band telemetry (TLM)/command (CM) and ranginglink, with the use of a 20 W power amplifier (Fig. 10).

The Ka-band is not taken into account for the MMO

due to the limited resource, while it is nominally used for

the MPO communication system. The MMO HGA is

pointed towards the Earth with the ADM and the an-

tenna pointing mechanism (APM) for the elevation an-

gle control of at least 22� (zero margin) determined by

the geometry of the planets (see Fig. 11). A medium gainantenna (MGA) is accommodated for emergency TLM

(4 bp)/CM link. The MGA is installed during the cruise

phase at the lower surface of the MMO and extended

after the MMO separation. At the bottom panel, an

Fig. 10. MMO antenna coverage.

Fig. 11. Relation between Earth and Mercury distance, and the angle

between MMO spin-axis and the Earth direction.

UHF patch antenna provided by ESA for MMO–MSE

communication is attached. The MMO–MSE link

function is considered as the backup for the MPO–MSE

link. Fig. 10 summarizes the antenna coverage of these

three antennas. At Mercury�s orbit, the MMO�s telem-

etry rate would be changed as elucidated in Fig. 12 as afunction of the range from the Earth. The average bit

rate is 16 kbp, which is in turn translates into 40 MB/day

assuming a 6-h consecutive pass.

6.2. High gain antenna

The helical array antenna excited by a radial-line is

adopted for theHGA system (Nakano et al., 1992). There

are mainly four reasons: (1) high efficiency (and lightweight) due to the low propagation loss in the radial-line

wave-guide, which can be achieved by the phase tuning of

the helical elements, (2) wide frequency band suitable for

uplink and downlink owing to the helical antennas as

emission elements, (3) simple flat structure compared to a

parabolic shape, which yields insensitivity to the thermal

expansion and (4) avoidance of solar flux concentration

as predicted for a parabola type antenna. Fig. 13 is anengineering model of the half-size HGA dish and Fig. 14

depicts the shape of helical elements.

Fig. 13. Engineering model of the half-size MMO HGA dish.

Fig. 12. Telemetry rate after Mercury orbit insertion.

Fig. 14. Cross-section of HGA.

Fig. 16. Flow diagram of MMO propulsion system.


6.3. Telemetry/command and data handling

Each instrument is connected to the serial data bus

with peripheral interface module (PIM). Both telemetry

and command are based on the CCSDS Telecommand

Recommendation packet type, As for the data recorder,0.5–2.0 GB volume is assumed for MMO housekeeping

and science data.

7. Attitude control system

The spin-stabilized MMO spacecraft attitude would

be determined by a pair of sun sensors on the side panel,and a star scanner attached at the bottom surface. The

attitude is controlled by the propulsion system with cold

gas jet. A nutation dumper installed inside the central

cylinder is used for passive nutation dumping. Fig. 15

depicts the field of view of the sun sensors and star

scanner.

The MMO propulsion system adopts cold gas jet

system, 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,

Fig. 15. Field of view of attitude sensors.

valves, piping and thermal control equipment (heaters

and sensors). The flow diagram is shown in Fig. 16. The

four tangential thrusters for roll control are allocated on

the side panel, while the two axial thrusters are mounted

at the bottom of the spacecraft body. The GN2 tank

consists of titanium alloy liner and carbon fiber shell.

The tank volume is 14.7 1iter and MDP (maximumdesigned pressure) is 27.6 MPa. About 4.25 kg of GN2 is

loaded including unexpelled propellant of 0.25 kg.

The valve module consists of one filter, three high-

pressure latch valves, two regulators, one low-pressure

latch valve and pressure transducers. The three high-

pressure latch valves are installed for safety during the

ground operation and launch phase. HLV-2 and HLV-3

are closed until the MMO reaches Mercury, in order toprevent the overpressure of secondary pressure line

caused by the internal leakage of the regulator. HLV-1 is

installed for back up of HLV-2 and HLV-3. If internal

leakage occurs at HLV-2 or HLV-3. HLV-1 shall be

closed and shut off the primary pressure. The regulator

is installed to regulate the primary pressure to the sec-

ondary pressure. Two similar regulators are installed for

redundancy, and each regulator can supply the propel-lant for all thrusters. The low-pressure latch valve would

be opened in normal condition. It would be closed in the

contingency or degradation mode caused by malfunc-

tion of the thruster(s) in one of the thruster modules.

8. Power and heater system

During the interplanetary cruise phase, the composite

provides the heater power directly to the MMO bus,

since the MMO spacecraft is not exposed to the direct

solar flux owing to the MMO Sun shield (see Fig. 17).

The MMO is equipped with the primary heaters con-

trolled by the MMO heater control electronics as well as

the survival heater with thermostats.

Fig. 17. MMO–SM power and telecommand interface.


After MMO separation, the temperature of the solar

cell suffers wide variation due to the variation ofMercury�s distance from the Sun (0.31–0.47 AU). This is

the motivation why the MMO is taking the peak power-

tracking configuration with the series-switching regula-

tor. The current assumed bus voltage is 28 V. The solar

cell assumes the multi-junction cell with conductive

coating type cover glass. Li-Ion secondary type is as-

sumed for the battery in order to cope with the 2-h

maximum eclipse condition around Mercury.

9. Probe antenna and extendible boom

Two pairs of probe antennas for plasma wave in-

struments are equipped with MMO, whose tip-to-tip

length is tentatively 30 m. And a single pair of extendible

threefold booms for magnetometers are installed whoselength is respectively around 3 m (see Fig. 18).

Fig. 18. MMO configuration on Mercury orbit.

10. Concluding remarks

A feasibility study of the MMO (mercury magneto-

spheric orbiter) is performed in order to satisfy the sci-

ence requirements as well as the BepiColombo system

requirements, under the harsh environment near Mer-

cury. It shows a feasible solution on the whole, but more

detailed thermal and structural system study considering

the detailed instrument allocation and thermal analysistaking microscopic structure into account is required.

Figs. 1–4, 5(a), 7(a) and (b), 8–10, 12, 14–16 and 18

were reprinted from Yamakawa et al. (2002) with per-

mission from Elsevier Science.

Acknowledgements

The authors express their sincere gratitude to Prof. H.

Nakano of Hosei University, Prof. Y. Kogo of the To-

kyo University of Science (Material), Mr. M. Nakano of

Astro Research (Thermal Control), NTSpace engineers

(Systems), NIPPI Corporation engineers (Mechanism)

and the following ISAS staff for their contribution to-

wards the MMO system design: Dr. Z. Yamamoto, Dr.

T. Toda (Communication), Mr. Y. Kamata,, Dr. T.Mizuno (High Gain Antenna), Dr. T. Hashimoto, Mr.

E. Hirokawa (Attitude Control & Sensors), Mr. M.

Shida, Dr. S. Sawai (Propulsion), Dr. M. Tajima, Dr. K.

Hirose, Mr. K. Takahashi (Power), Dr. A. Ohnishi, Mr.

S. Tachikawa, Dr. Y. Kobayashi (Thermal Control),

Dr. K. Minesugi (Strutcure), Dr. Y. Morita (Mecha-

nism), 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).


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Documents

Current status of the BepiColombo/MMO spacecraft design