Ricochets And Stranding Boulders On Rubble Pile Asteroids

"Many asteroids are 'rubble piles', conglomerations of weakly gravitationally bounded material. Impacts and spin-out events can produce a population of gravitational bound objects in orbit that can impact the surface at low velocity and a distribution of impact angles. For the first study, we present low velocity, oblique impacts of spherical projectiles into fine sand. With high speed video and fluorescent markers on the projectiles we track projectile trajectory and spin during impact. We delineate impact velocities and angles where projectiles ricochet, roll-out from their impact crater, or stop within their impact crater. We estimate constraints on projectile radius, velocity, and impact angle that would allow projectiles on asteroids to ricochet or roll away from initial impact site. The critical grazing impact angle dividing ricochets from roll-outs increases with impact velocity and is matched by an empirical model that balances a lift force against gravity during the rebound phase. The required friction coefficient during the penetration phase is 4-5 times lower than for sand suggesting it is fluidized near the projectile surface during penetration. In a second study, we compared ricocheting impactors with the same impact velocity and angle into granular media with different mean grain size. The ratio of projectile diameter to mean grain length ranges from 4 in our coarsest medium to 50 in our finest sand. We measure coefficients of restitutions, the ratio of pre- to post-impact horizontal and vertical velocity components, and the angle of deflection caused by the impact in the horizontal plane. Coefficients of restitution are sensitive to mean grain size with the horizontal coefficient about twice as large for our coarsest gravel compared to our finest sand. This implies that coefficients for hydro-static-like, drag-like and lift-like forces, used in empirical force laws are sensitive to mean grain size, with the lift coefficient being the most sensitive. The deflection angles are largest in the coarser media and their size approximately depends on grain size to the 3/2 power. The dependence of impact dynamics on substrate size distribution should be considered in future models for impacts on granular asteroid surfaces. For the third study, we consider how a subsurface seismic pulse affects the surface of an asteroid distant from the impact site. Using dry polydisperse gravel mixture, we film and track the ejecta launched by a single upward propagating pressure pulse. We find ejecta trajectories are independent of particle size, and collisions primarily take place upon landing, where smaller particles are scattered off larger particles, a process dubbed 'ballistic sorting'. A single strong pulse can strand previously buried boulders on the surface and is an additional mechanism for large boulders present on the surface of rubble asteroids such as 162173 Ryugu, 101955 Bennu and 25143 Itokawa"--Pages viii-ix.