The proposed asteroid sample return mission MUSES-C calls for a spacecraft to approach an asteroid, touch down on its surface, and collect samples that will be returned to Earth. During the touchdown and sampling phase, the spacecraft will navigate relative to the asteroid surface using optical target markers placed on the asteroid surface before the final approach. By using the target marker as a reference point, navigation during the landing phase will be much more reliable and precise. Because of the microgravity environment on the asteroid surface, the settling time and dynamics of the target markers are items of interest. Thus, it is important to design the target marker with as small a coefficient of restitution as possible to minimize the settling time, which in turn minimizes the time the spacecraft must hover above the asteroid surface. To achieve this small coefficient of restitution, the target marker will be constructed out of a bag with balls stored internally. On impact, the balls will diss ipate energy relative to each other and, hence, will dissipate the total energy of the target marker. To better predict the performance of such a target marker, analytical and numerical investigations are performed that model the motion of a bouncing target marker across the surface of a rotating asteroid. As a result of the analysis, some target limits on the target marker coefficient of restitution are developed. A series of microgravity tests are reported that confirm the basic design and show that the target value of coefficient of restitution can be reached.
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