The Size Distribution of 10 km-scale Main-belt Asteroids: Spacewatch Observations and Collisional Models

D. D. Durda, R. Jedicke, R. Greenberg (LPL, U. Arizona)

Recent analysis of Spacewatch data on over 60,000 main-belt asteroids obtained from 1992 to 1995 has produced a determination of the debiased absolute magnitude distribution of small main-belt asteroids of unprecedented detail (Jedicke and Metcalfe 1997, submitted to Icarus). The distribution is complete to tex2html_wrap_inline12 for the entire main-belt, and cannot be represented by a simple power-law distribution with a single slope index in any region of the belt. Instead, the absolute magnitude distribution shows a `kink', or small hump, at tex2html_wrap_inline14 , corresponding to a diameter of approximately 10 km for a mean albedo of 0.1. Our numerical models of the collisional evolution of main-belt asteroids, which allows for size-dependent impact strengths and stochastic collisions, reproduces just such a `kink'. The hump is imposed on the size distribution as a result of the transition between rare, stochastic collisions between large asteroids and the more uniform, statistical nature of frequent collisions between smaller bodies. The observed position and magnitude of the hump confirm the theoretical results of Durda (1993, Ph.D. Thesis, U. Florida). By providing an observational constraint on the evolved size distribution output by continually improving collisional models (a function of the size dependence of the asteroid impact strengths), the Spacewatch data will provide knowledge of the internal structures of 10 km-scale asteroids.