(FOR THE SDSS COLLABORATION)

We discuss measurements of the properties of D13,000 asteroids detected in 500 deg2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data. The moving objects are detected in the magnitude range 14 \ r * \ 21.5, with a baseline of D5 minutes, resulting in typical velocity errors of D3%. Exten-sive tests show that the sample is at least 98 % complete, with a contamination rate of less than 3%. We Ðnd that the size distribution of asteroids resembles a broken power law, independent of the heliocentric distance: D~2.3 for 0.4 km, and D~4 for 5 km. As a consequence of thiskm [D [ 5 km[D [ 40 break, the number of asteroids with r * \ 21.5 is 10 times smaller than predicted by extrapolating the power-law relation observed for brighter asteroids The observed counts imply that there are(r * [ 18). about 670,000 objects with D [ 1 km in the asteroid belt, or up to 3 times less than previous estimates. The revised best estimate for the impact rate of the so-called killer ÏÏ asteroids (D [ 1 km) is about 1 every 500,000 yr, uncertain to within a factor of 2. We predict that by its completion SDSS will obtain about 100,000 near simultaneous Ðve-band measurements for a subset drawn from 340,000 asteroids brighter than r * \ 21.5 at opposition. Only about a third of these asteroids have been previously observed, and usually in just one band. The distribution of main-belt asteroids in the four-dimensiona