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The Forefront of Space Science

Thermal Control System of BepiColombo MMO
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BepiColombo is the name of a Mercury exploration mission that is being advanced jointly by the European Space Agency (ESA) and JAXA. Mercury has some peculiar features, one of which is its 2:3 ratio of rotation to revolution periods. This fact was first pointed out by Giuseppe Colombo (1920-1984), the Italian applied mathematician. Since Mercury is the planet nearest to the Sun, observation of it from the earth is difficult. Observation by explorer has been conducted only twice to date, by the U.S's Mariner 10 (1974 to 1975) and MESSENGER (from 2011). Mariner 10 approached Mercury three times using Venus swingby. This technique was offered by Dr. Colombo to NASA.

BepiColombo (named for Dr. Giuseppe "Bepi" Colombo) was officially approved as the fifth cornerstone mission of ESA in October 2000. Originally an ESA program, the possibility of ISAS's cooperation was first broached by ESA at the ESA-ISAS meeting in the autumn of 1999. Discussion about ISAS joining the mission followed. ESA made the offer because a Japanese-original mission had been studied by ISAS's Mercury exploration working group since before 1998, when the author joined ISAS. As a result of the discussion, an organizational framework was set up to proceed with the BepiColombo mission by international cooperation, mainly Japan and Europe. ISAS is responsible for the design, manufacture, and operation of the Mercury Magnetospheric Orbiter (MMO) and scientific support to the Mercury Planetary Orbiter (MPO) provided by ESA.

ESA is, except for MMO, responsible for:
(1) Overall mission design of BepiColombo
(2) Design, manufacture, and operation of MPO, Mercury Transfer Module (MTM), and MMO Sunshield and Interface structure (MOSIF)
(3) Assembly, testing, and launch of the Mercury Composite Spacecraft (MCS)

BepiColombo's mission is a full-scale attempt to elucidate Mercury's mysteries Efrom its solid inside to its surface and magnetosphere Eusing two satellites, MMO and MPO. Its main objectives are as follows:
•Study of the origin and evolution of a planet near to a star
•Study of Mercury as a planet; its configuration, internal structure, geology, composition, craters
•Study of the Mercurial exosphere atmosphere: its structure and dynamics
•Study of the Mercurial magnetosphere: its structure and dynamics
•Study of the origin of the Mercurial magnetic field
•Study of the materials in Mercurial poles: their composition and origin
•Demonstrate Einstein's general theory of relativity
MMO will carry equipment to measure the electric and magnetic fields, equipment to monitor plasma and neutral particles, an atmospheric camera, and a dust monitor to explore the Mercurial magnetosphere in particular.

MMO will be transported to Mercury as part of the MCS together with MPO (Fig 1). MCS consists of an electrical propulsion module (or MTM), MPO, MOSIF and MMO. MCS will be operated by 3-axis stabilized attitude control. Meanwhile, MMO is a spin-stabilized satellite as explained later in detail. A 3-axis stabilized attitude control satellite without spinning cannot endure high temperatures when it comes close to the Sun. For this reason, MMO will be protected from sunlight with MOSIF until its arrival at Mercury.

Figure 1
Figure 1. Breakdown figure of BepiColombo Mercury Composite Spacecraft (MCS)
From left, MMO, MMO Sunshield and Interface structure (MOSIF), Mercury Planetary Orbiter (MPO), and Mercury Transfer Module (MTM). (Courtesy of ESA)

MSC will be launched by Arian 5 rocket from French Guiana in 2014. It will be propelled by electric propulsion system and arrive at Mercury six years later, 2020, after swingbys of the earth, Venus and Mercury. The MTM will be released first after arrival at Mercury. The remainder will be inserted into Mercurial orbit by MPO's two-component type chemical propulsion system. MMO will be released into its orbit (polar orbit, 400km perihermion, 11,824km apohermion). After that, MOSIF will be released from MPO. Then, MPO will gradually lower its apohermion altitude to enter its orbit (polar orbit, 400km perihermion, 1,508km apohermion). From launch to just before release, MMO will be operated in dormant mode except for the periods just after the launch, and periodic checks several times per year.

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