Effects of Particle Shape and Size on Granular Mechanics in Vacuum and Microgravity Environments

Abstract Rubble pile asteroids and regolith-covered surfaces on small planetary bodies undergo evolution that is likely driven by particle–particle granular mechanics interactions. Interparticle interactions and porosity may regulate physical interactions as a result of shaking, impacts, and other disruptive behavior. The Strata-1 experiment, flown on the International Space Station (ISS), included four simulant tubes to characterize granular material behavior in microgravity and act as a bridge between simple spherical simulations and complex rubble pile asteroid observations. We analyze the behavior of the more idealized simulants, glass shards and spheres, in an effort to isolate the role of particle shape (spherical versus irregular) in the behavior of granular material sorting in microgravity. Additionally, ISS accelerometers are used to quantify experiment-level transient events (ISS SAMS) and the ISS quasi-steady acceleration environment (ISS MAMS). Long-duration forcing events such as reboosts, thruster events, and inversions are also identified. The median acceleration is small (<1 μ g, MAMS), indicating that forcing events dominate the acceleration environment. The shards were more active (showed higher variance) than the spheres, and the smallest particles were more active than the larger particles. We also find that areas of the tube with higher macroporosity show more activity. These results are consistent with the void-filling Brazil Nut Effect mechanism. Most, but not all, changes in particle packing can be explained by at least one concurrent ISS event, but not all events induce the same magnitude of change, indicating that granular bodies do have some capacity to resist disruption by external events.