for ricochet. However, these early models assume low angles of impact and nearly horizontal and constant velocity during the encounter between projectile and substrate and neglect spin. The lift was based on a hydrodynamic pressure exerted on the submerged projectile surface and may not be consistent with more recent empirical force models or experimental measurements. Models consistent with a broader range of phenomena, including ricochets and oblique impacts, would be helpful for understanding processes taking place in low gravity environments.

Laboratory Experiments of Projectiles Impacting Similar Density Granular Material {#sec:exp}
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We carry out experiments of spherical projectiles into sand. We restrict the density of our projectiles to similar materials and densities as the solids in our substrate material so as to be similar to natural low velocity impacts on asteroids. Prior to this study we had explored non-spherical projectiles (pebbles) into coarser substrates (gravel), however we found that experiments were often not easily reproducible. This was likely due to the complex projectile shapes, their spin and phase of impact with respect to rotation, and irregularities in the substrate. We have reduced the degrees of freedom so as to try to understand simpler systems as a first approximation. The projectiles 