We discuss measurements of the properties of ∼10,000 asteroids detected in 500 deg 2 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 ∼5 minutes, resulting in typical velocity errors of ∼3%. Extensive tests show that the sample is at least 98% complete, with the contamination rate of less than 3%.We find that the size distribution of asteroids resembles a broken power-law, independent of the heliocentric distance: D −2.3 for 0.4 km ∼ < D ∼ < 5 km, and D −4 for 5 km ∼ < D ∼ < 40 km. As a consequence of this break, the number of 1 Based on observations obtained with the Sloan Digital Sky Survey.-3asteroids with r * < 21.5 is ten times smaller than predicted by extrapolating the power-law relation observed for brighter asteroids (r * ∼ < 18). The observed counts imply that there are about 530,000 objects with D > 1 km in the asteroid belt, or about four times less than previous estimates. We predict that by its completion SDSS will obtain about 100,000 near simultaneous five-band measurements for a subset drawn from 280,000 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 4-dimensional SDSS color space is bimodal, and the two groups can be associated with S (rocky) and C (carbonaceous) asteroids. A strong bimodality is also seen in the heliocentric distribution of asteroids and suggests the existence of two distinct belts: the inner rocky belt, about 1 AU wide (FWHM) and centered at R ∼2.8 AU, and the outer carbonaceous belt, about 0.5 AU wide and centered at R ∼3.2 AU. The median color of each class becomes bluer by about 0.03 mag AU −1 as the heliocentric distance increases. The observed number ratio of S and C asteroids in a sample with r * < 21.5 is 1.5:1, while in a sample limited by absolute magnitude it changes from 4:1 at 2 AU, to 1:3 at 3.5 AU. In a size-limited sample with D > 1 km, the number ratio of S and C asteroids in the entire main belt is 1:2.3.The colors of Hungarias, Mars crossers, and near-Earth objects, selected by their velocity vectors, are more similar to the C-type than to S-type asteroids, suggesting that they originate in the outer belt. In about 100 deg 2 of sky along the Celestial Equator observed twice two days apart, we find one plausible Kuiper Belt Object (KBO) candidate, in agreement with the expected KBO surface density. The colors of the KBO candidate are significantly redder than the asteroid colors, in agreement with colors of known KBOs. We explore the possibility that SDSS data can be used to search for very red, previously uncatalogued asteroids observed by 2MASS, by extracting objects without SDSS counterparts. We do not find evidence for a significant population of such objects; their contribution is no more than 10% of the asteroid population.
