The orbital and absolute magnitude distributions of main belt asteroids

We have developed a model-independent analytical method for debiasing the four-dimensional (a, e, i, H) distribution obtained in any asteroid observation program and have applied the technique to results obtained with the 0.9 m Spacewatch telescope. From 1992 to 1995 Spacewatch observed similar to 3740 deg(2) near the ecliptic and made observations of more than 60,000 asteroids to a limiting magnitude of V similar to 21. The debiased semimajor axis and inclination distributions of main belt asteroids in this sample with 11.5 less than or equal to H < 16 match the distributions of the known asteroids with H < 11.5. The absolute magnitude distribution was studied in the range 8 < H < 17.5. We have found that the set of known asteroids is complete to about absolute magnitudes 12.75, 12.25, and 11.25 in the inner, middle, and outer regions of the belt, respectively. The number distribution as a function of absolute magnitude cannot be represented by a single power law (10(alpha H)) in any region. We were able to define broad ranges in H in each part of the belt where alpha was nearly constant. Within these ranges of H the slope does not correspond to the value of 0.5 expected for an equilibrium cascade in self-similar collisions (Dohnanyi 1971). The value of alpha varies with absolute magnitude and shows a "kink" in all regions of the belt for H similar to 13. This absolute magnitude corresponds to a diameter ranging from about 8.5 to 12.5 km depending on the albedo or region of the belt. (C) 1998 Academic Press.