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The Forefront of Space Science

Thermal Control System of BepiColombo MMO
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MMO's thermal environment and thermal design

Distance between Mercury and Sun is 0.3AU (Astronomical Unit, which is average distance between Sun and Earth, specifically about 150 million km) at perihelion and 0.47AU at aphelion. When Mercury is located at perihelion, the intensity of sunlight is nearly 11 times that of near earth space. Mercury itself is heated by sunlight to up to 430 deg. C at the subsolar point. Since Mercury has no atmosphere and its rotation period is long, its surface temperature is not even like the earth's moon. Moving away from the subsolar point, the surface temperature goes down corresponding to the relation of angles between the Sun and the surface. The temperature becomes lower than E80 deg. C in the shadow areas. The Mercury explorer is designed to endure the fierce environment caused by strong sunlight and strong infrared radiation from Mercury.

MMO's onboard instruments require that it be a spin-stabilized satellite with its spin axis being about 90 deg. against the sunlight. Thus, the shape of the satellite requires axial symmetry. Further, the satellite's concept calls for: solar battery mounted on the side panel; high-gain antenna with despun system (a mechanism to orient the antenna always in the direction of the earth by rotating it against spinning direction) installed on the spin axis; and heat-dissipation plane set on the north/south plane to avoid direct sunlight. Based on this concept, we conducted trade-off studies on various shapes and thermal control policy to design the satellite in more detail.

Fig. 2 shows a conceptual image of MMO in the Mercury orbit. The most desirable shape for the Mercury explorer from the heat aspect is a flat cylinder to minimize the sunlight-receiving area and maximize heat-dissipation area. In the initial design, MMO was cylindrical. Because of ISAS's bitter experience manufacturing its magnetosphere-observation satellite GEOTAIL (the difficulty installing solar cells on the side of a cylinder), however, it was decided the side panels should be flat, and eventually the octagonal cylinder shape was chosen. The size was determined to be 1.8m diameter, taking into account both electric power and weight.

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Figure 2. Artistic image of BepiColombo MMO

Since the side panels receive sunlight directly, they are covered by glass mirrors or Optical Solar Reflectors (OSR), which hardly absorb sunlight and have high heat-dissipation efficiency. Nevertheless, the panel temperature rises to a high of more than 160 deg. C. For this reason, we decided to mount both the bus and observation instruments on so-called decks on the north and south sides, which are thermally insulated from the side panels. Accordingly, the observation instruments are set on the decks with their faces protruding from the side panels. Measures are taken to prevent influx of the solar-light energy into the deck via the side panels and observational instruments. They include Multi Layer Insulation (MLI) and the sun shield to be installed on the sensor exposure areas.

As the solar batteries are mounted on the side panels, they absorb solar energy and rise to high temperatures. For this problem, we will lay OSRs adequately among the rows of the solar batteries to lower the sunlight-absorption rate per unit area. In addition, we will extend the side panels in the north direction, where a high-gain antenna is installed, and put solar cells on the extended area. This allows heat from the rear (spin-axis side) of the side panels to be emitted to outer space, eventually lowering the temperature of the cells. Even so, the temperature reaches up to 230 deg. C

The north deck is not suitable as a heat-dissipation plane of the onboard equipment because it receives the reflected sunlight and infrared radiation from the high-gain antenna, and infrared radiation from the rear sides of the solar batteries. Therefore, we will shut down those heat influxes by covering the surface of the north deck with the MLI. Heat-radiation planes for onboard instruments are thus limited to the south deck only. The side panels are extended to prevent direct sunlight to the south deck. The rear (spin-axis side) of the side panels is thermally insulated from the front side, so as to block the inflow of infrared radiation from the hot-side panels into the heat-dissipation panels of the south deck. The front side surfaces will also incorporate the property of low infrared radiation rate. The outer surface of the south deck is covered by OSRs.

As for the high-gain antenna, the conventional parabola type inevitably has a focusing area, so MMO adopts a planar-type antenna. Since structures called helical arrays are laid in a row on the antenna surface, we cannot attach OSRs on the surface. In addition, since the antenna does not spin, it sometimes faces the sun directly and can become very high in temperature (more than 350 deg. C). To cope with this problem, we developed a special white-color paint with conductivity, which can endure high temperatures. The observation instruments of MMO are required to use these conductive materials on their outer surfaces to avoid as much as possible the generation of electric potential distribution on the surfaces. Accordingly, the white paint is applied to the sunshields for the observation equipment, etc., other than the antenna. Conductive coating is applied to the OSRs and the cover glasses of the solar cells. The outermost layer of the MLI on the north deck surface uses a germanium vapor-deposited black polyimide film with conductive properties.

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