VELOCITY DISTRIBUTIONS AMONG COLLIDING ASTEROIDS

The probability distribution for impact velocities between two given a steroids is wide, non-Gaussian, and often contains spikes according to our new method of analysis in which each possible orbital geometry fo r collision is weighted according to its probability. An average value would give a good representation only if the distribution were smooth and narrow. Therefore, the complete velocity distribution we obtain f or various asteroid populations differs significantly from published h istograms of average velocities. For all pairs among the 682 asteroids in the main-belt with D > 50 km, we find that our computed velocity d istribution is much wider than previously computed histograms of avera ge velocities. In this case, the most probable impact velocity is -4.4 km/sec, compared with the mean impact velocity of 5.3 km/sec. For cas es of a single asteroid (e.g., Gaspra or Ida) relative to an impacting population, the distribution we find yields lower velocities than pre viously reported by others. The width of these velocity distributions implies that mean impact velocities must be used with caution when cal culating asteroid collisional lifetimes or crater-size distributions. Since the most probable impact velocities are lower than the mean, dis ruption events may occur less frequently than previously estimated. Ho wever, this disruption rate may be balanced somewhat by an apparent in crease in the frequency of high-velocity impacts between asteroids. Th ese results have implications for issues such as asteroidal disruption rates, the amount/type of impact ejecta available for meteoritical de livery to the Earth, and the geology and evolution of specific asteroi ds like Gaspra. (C) 1994 Academic Press, Inc.