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The Forefront of Space Science

To Produce Lunar Gravitational Field Map:Toward Success of SELENE’s Micro Satellites, Rstar and Vstar
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Development of Rstar and Vstar

As mentioned above, the Rstar and Vstar satellites will be exclusively used for gravitational-field observation. They are, therefore, designed as spin satellites, and altitude and attitude control by thrusters, which hinders their detection of gravity anomaly, will not be done. Though, they have nutation dampers for attitude control. In shape they are almost identical: an octagonal- prism main structure equipped with an S/X-band vertical dipole antenna to communicate with the ground station, etc. (upper right in Fig. 1). They are almost 1m wide, relatively large for a micro satellite. This is to allow enough area for solar cells on the side of the structure. The satellite weight is around 45kg with sufficient space inside.

Figure 4
Figure 4. Lightweight release mechanism developed for SELENE’s Rstar and Vstar micro satellites.
In the upper rectangle are released micro satellite components.

Rstar and Vstar employ various innovative ideas to reduce weight. One is the release mechanism (Fig. 4). Since they have no attitude control functions by thrusters or wheels, the attitude and spin at separation are crucial. Therefore, a lightweight but steady mechanism is necessary to ensure stable separation characteristics. Generally, spin satellites are separated using a release mechanism after they have been spun on a turntable. For Rstar and Vstar, we introduced a mechanism to join the two rings with stretching springs and retain the satellites by giving torsion. By releasing the restraint with pyrotechnics placed in three brackets, the extension springs push off the rings to provide separation velocity and, simultaneously, hooks on the bottom of the satellites transfer rotation of the upper rings to give spin. The introduction of this type has reduced the weight of the separation mechanism by one-fourth. To assess performance of the release mechanism, we have to test it under a microgravity environment. However, if conducted on a large scale, it would be difficult to obtain sufficient measuring data by changing conditions or to repeat the test with design feedback from the initial results. Thus, we devised equipment that could simulate a microgravity environment by hanging a dummy satellite with elastic cords and placing the separation mechanism in a position where tension and gravity are balanced. The test was conducted with this equipment. The box in the center of Fig. 5 is the dummy satellite with the same mass characteristics as actual satellites. The elastic cords to hang the dummy satellite are seen above the box. An engineering model was produced based on the data obtained from this test, and its orbital demonstration test was conducted on a piggyback satellite, µ-Lab Sat.

Figure 5
Figure 5. Separation characteristic measurement test on the ground for the release mechanism using dummy satellite and elastic cords.

The Rstar and Vstar are now undergoing a proto flight test. When this article appears on the website, they will probably be under the peak of the thermal vacuum test. The development of the Rstar and Vstar satellite is entering the final stage and attracting worldwide attention as a completion mission that will drastically improve the gravitational-field map of both near and far sides of the Moon, and help elucidate its internal structure.

(Takahiro IWATA)

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