Low-velocity microgravity impact experiments into simulated regolith

We describe the Collisions Into Dust Experiment (COLLIDE), a microgravity experiment to study the outcome of very low velocity (1-100 cm/sec) collisions into a simulated regolith, The experiment was flown on the Space Shuttle in April 1998 and included six independent impact experiments into a lunar soil simulant, Results of these impact experiments showed very little dust ejecta was produced in the impacts, which were highly inelastic. We present normal and tangential coefficients of restitution for impacts into simulated regolith at speeds between 15 and 90 cm/sec. We find normal coefficients of restitution of 2 to 3%, significantly lower than is suggested by extrapolation from low-velocity impact experiments with ice, but consistent with higher velocity impacts into powder. Our microgravity experiment confirms the ground-based result of Hartmann (1978, Icarus 33, 50-62) that the presence of a regolith increases the efficiency of planetary accretion. The absence of dust ejecta suggests higher energy collisions are necessary to release the dust observed in planetary rings. (C) 1999 Academic Press.