Initial function confirmation test conducted for the solar array panels (SAP)

Article by Imada Takane, MMX Project Lead Engineer

From July to September 2023, an initial function test of the solar array panel (SAP) that will be installed on the Martian Moons eXploration (MMX) spacecraft was conducted at Mitsubishi Electric Kamakura Works. The purpose of this test was to confirm the integrity of the manufactured solar panels, and also to allow a comparative check of their performance after a variety of environmental tests that will be performed in the future.

Figure 1: The solar panel array (before deployment). The silver sheet on the reverse side of the panel is visible.

The image in Figure 1 shows a deployment test, which is one of the initial function confirmation checks. The solar panels are initially folded and then deployed one by one. Figure 2 shows how the solar panels look after deployment. Black solar cells are attached to the surface of each panel (Figure 2), which receive sunlight and generate electricity. The reverse side of the panel is covered with a silver sheet to control temperature (Figure 1).  

Solar panels are familiar on Earth for use in solar power generation. In space, solar panels are an essential part of satellites and spacecraft. Various improvements have been made to the design of our solar panels so that they can fulfil their task in outer space.

One of these adjustments is the weight of the solar panels. Mars is more than 1.5 times further from the Sun than the Earth, and spacecraft in the vicinity of Mars only receive about 40% of the sunlight compared to those orbiting the Earth or Moon. This means that large solar panels are required. However, in order to travel to Mars and return with a limited mass available for launch, the spacecraft mass much be reduced to the absolute minimum.

For this reason, the MMX spacecraft is equipped with the latest high-efficiency, thin-film, lightweight solar cells, and the panels themselves also have a special lightweight structure. These special solar cells can be seen on the front side of the panel in Figure 2.

A second factor is “rigidity”. The MMX spacecraft will free-fall from a point approximately 10m above the surface of the Martian moon Phobos during landing. The impact of landing is about 0.3 ~ 0.4 times the force of the Earth’s gravity, but even so, the solar panels on MMX must be able to withstand this landing unlike normal spacecraft flying through space. Also, if the solar panels bend due to the landing impact, the edges of the solar panels will hit the surface of Phobos. Therefore, a solar panel design with high rigidity (resistance of deformation), coupled with a narrow solar paddle width is needed.

Figure 2: The solar array panel (after deployment). The black area is the surface of the panel, and the thin-film lightweight solar cells are visible.

The two-step deployment mechanism, vertical and horizontal, is a characteristic of the MMX solar panels and is due to the unique requirements for MMX during the landing.

The solar panels are essential for the long journey to and from Mars, the landing on Phobos, and the sample return. We will be keeping an eye on their performance even after launch.

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