IMPACT CRATERS ON ASTEROIDS - DOES GRAVITY OR STRENGTH CONTROL THEIR SIZE

The formation of kilometer-size craters on asteroids is qualitatively different from the formation of meter-size (laboratory- and weapons-sc ale) craters on Earth. A numerical hydrocode model is used to examine the outcomes of various-size cratering impacts into spheres and half-s paces. A shock wave fractures the target in advance of the crater exca vation flow; thus, for impactors larger than 100 m, impacting at typic al asteroid impact velocities, target tensile strength is irrelevant t o the impact outcome. This result holds whether the target is initiall y intact or a ''rubble pile,'' even ignoring the effects of gravity. B ecause of the shock-induced fracture, crater excavation is controlled by gravity at smaller sizes than would otherwise be predicted. Determi ning the strength-gravity transition by comparing the physical strengt h of the material to the force of gravity will not work, because stren gth is eliminated by the shock wave. (C) 1996 Academic Press, Inc.