Controlled partial gravity platform for milligravity in drop tower experiments

We detail a platform for partial gravity environment and an experiment for simulated impacts on asteroid surfaces based on it. The partial gravity environment is created by a two stage approach: First, we place the experiment in the microgravity of a free-falling drop tower capsule in the ZARM (German acronym for Center of Applied Space Technology and Microgravity) Bremen drop tower. Second, we convert microgravity to partial gravity by steady acceleration of the experiment volume on a linear drive inside the microgravity environment. The experiment that we conducted on this platform simulates low-velocity impacts into a simulated asteroid surface. To recreate the asteroid environment, in addition to the partial gravity, a vacuum chamber is used. The motivation behind those experiments is to examine granular interactions in the cohesion dominated regime. For example, we show with this new experiment platform that in low velocity impacts, the interparticle cohesion increases the surface elasticity. This increased elasticity in turn constrains the size sorting mechanism suggested by Shinbrot et al. [Phys. Rev. Lett. 118, 111101 (2017)]. We explain requirements, setup, and operation of the partial gravity platform and experiment and discuss its performance. Finally, we are open for requests for external experiments, which might benefit from our platform with 9.3 s of controlled partial gravity down to the mm/s2 range with low g-jitter.