ASTEROID TRAILS IN HUBBLE-SPACE-TELESCOPE WFPC2 IMAGES - FIRST RESULTS

Careful examination of 28,460 selected Wide Field and Planetary Camera 2 (WFPC2) long exposures from 1994, 1995, and early 1996 has revealed trails of 96 distinct moving objects. They have been reported to the International Astronomical Union's (IAU) Minor Planet Center for their asteroid database and a few have been identified with known asteroids and used to update their orbits. Most of the objects are new, as they are too faint to show up on ground-based surveys. The trails often sh ow a characteristic curvature due to the parallax induced by HST's orb ital motion during the exposures. Using ephemerides for HST, the dista nce to each object can be directly determined from the parallax contri bution to the trail shapes, Based on these distances, constraints on t he orbits, and photometry of the trails (16 < V < 24), most of the mov ing objects appear to be small, main-belt asteroids a few km in diamet er, A few are known objects-three are potential Mars crossers. Modern wide-field CCD surveys detect asteroids nearly as faint as these(V < 2 1), but the corresponding absolute magnitudes are uncertain unless the ir orbits have been established. The detected objects span the absolut e magnitude range 13.6 < H < 19.3 (H is the symbol for absolute magnit ude, not H-band). Statistics of the detections imply a reservoir of (3 .1 +/- 0.6) x 10(5) such asteroids within 25 degrees of the ecliptic. We find that the slope of the cumulative distribution of absolute magn itudes follows a power law N proportional to H-0.2 to N proportional t o: H-0.3 over this absolute magnitude range in the three distance rang es defined by the Palomar-Leiden survey. These are significantly shall ower slopes than those inferred by the Palomar-Leiden survey or extrap olated from population studies of larger asteroids. (C) 1998 Academic Press.