951 GASPRA - A PRE-GALILEO ESTIMATE OF ITS SURFACE EVOLUTION

We attempt to quantitatively model the collisional environment and surface evolution of asteroid 951 Gaspra prior to the flyby of the Galileo spacecraft later this year. We compute Gaspra's average intrinsic collisional probability to be 5.4 x 10(-18) km-2 yr-1 with a root mean square impact velocity of 5.0 km sec-1. The projectile population and impact strength for Gaspra are highly uncertain and we consider a range of values. Based on these models, Gaspra is unlikely to be a primordial object surviving from the beginning of the solar system. It is most likely a second or multiple generation fragment forming approximately 10(8) yr ago. For a low impact flux, the crater size distribution directly reveals the size distribution of the impacting population. For a non-equilibrium surface, we present model-dependent isochrons for estimating the age of the surface. Our model sets lower limits for Gaspra's age (approximately 10(9) yr) and flux for the case of a high impact flux producing an equilibrium surface.