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 ∼3740 deg2near the ecliptic and made observations of more than 60,000 asteroids to a limiting magnitude of V ∼ 21. The debiased semimajor axis and inclination distributions of main belt asteroids in this sample with 11.5 ≤H< 16 match the distributions of the known asteroids withH< 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 (10αH) in any region. We were able to define broad ranges inHin each part of the belt where α was nearly constant. Within these ranges ofHthe slope does not correspond to the value of 0.5 expected for an equilibrium cascade in self-similar collisions (Dohnanyi 1971). The value of α varies with absolute magnitude and shows a “kink” in all regions of the belt forH∼ 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.

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