Ricochets on Asteroids

Spin off events and impacts on asteroids can eject boulders from the surface. These processes produce a population of gravitationally bound objects in orbit that can impact an asteroid surface at low velocity and low grazing angles with respect to the surface. We carry out laboratory experiments of low velocity projectiles (100-400 cm/s) at different impact angles into granular media. We use spherical and irregular objects painted with florescent dots and filmed with high-speed cameras in order to utilize tracking software to measure impact velocity, angle, and spin of the impactor. Impacts are classified as ricochets, rolling or stopping, depending upon whether the projectile bounces, rolls out of its crater or stops within its crater. We confirm that ricochets are most likely at low or grazing impact angles. Only nearly normal impacts and those at very low velocities leave projectiles within their crater.

Our impact velocities have Froude numbers 50 to 350. This regime is relevant for 10 m rocks on asteroids 101995 Bennu or 162173 Ryugu impacting the surface at velocities below the escape velocity of 20 cm/s. We propose that boulders perched on the surface of rubble asteroids such as Asteroid Bennu and Asteroid Ryugu could be the result of low velocity and low grazing angle impactors that ricocheted or rolled across the surface, finally coming to rest distant from their initial impact sites.