On the asteroid belt's orbital and size distribution

Abstract For absolute magnitudes greater than the current completeness limit of H -magnitude ∼15 the main asteroid belt's size distribution is imperfectly known. We have acquired good-quality orbital and absolute H -magnitude determinations for a sample of small main-belt asteroids in order to study the orbital and size distribution beyond H = 15 , down to sub-kilometer sizes ( H > 18 ). Based on six observing nights over a 11-night baseline we have detected, measured photometry for, and linked observations of 1087 asteroids which have one-week time baselines or more. The linkages allow the computation of full heliocentric orbits (as opposed to statistical distances determined by some past surveys). Judged by known asteroids in the field the typical uncertainty in the ( a / e / i ) orbital elements is less than 0.03 AU/0.03/0.5°. The distances to the objects are sufficiently well known that photometric uncertainties (of 0.3 magnitudes or better) dominate the error budget of their derived H -magnitudes. The detected asteroids range from H R = 12 – 22 and provide a set of objects down to sizes below 1 km in diameter. We find an on-sky surface density of 210 asteroids per square degree in the ecliptic with opposition magnitudes brighter than m R = 23 , with the cumulative number of asteroids increasing by a factor of 10 0.27 /mag from m R = 18 down to the m R ≃ 23.5 limit of our survey. In terms of absolute H magnitudes, we find that beyond H = 15 the belt exhibits a constant power-law slope with the number increasing proportional to 10 0.30 H from H ≃ 15 to 18 , after which incompleteness begins in the survey. Examining only the subset of detections inside 2.5 AU, we find weak evidence for a mildly shallower slope for H = 15 – 19.5 . We provide the information necessary such that anyone wishing to model the main asteroid belt can compare a detailed model to our detected sample.