penetration regime, where the maximum penetration depth rarely exceeds the projectile diameter. We designed our experiment to minimize initial projectile spin. Unfortunately projectile spin is not discussed by these early works (though see the discussion on bouncing bombs by @johnson98). Models that predict granular flow above a particular stress level [@bagnold54] might account for the higher lift we infer in our lower velocity experiments that give us ricochets at high impact angles, as the medium could be effectively stiffer at lower impact velocity. Alternatively lower velocity and lower projectile density, giving shallower levels of penetration, may be increasing the likelihood of ricochets in our experiments. Since collisions on asteroids are likely to have similar projectile and substrate densities, our experiments suggest that ricochets could be common in the low velocity regime.

![Classified impacts as a function of impact angle and velocity. Impact angle is measured from horizontal so low $\theta_{impact}$ is a grazing impact. Black circles, denoted ‘Stop’ in the legend, are impacts where the marble stayed within its impact crater. Blue triangles, denoted ‘Roo’ in the legend are roll-outs. Red squares (Ric) are ricochets. Green squares (Roo/i) are on the dividing line of ricochet and roll-out. Brown circles (S/Roo) 