r 
. If we assume a fixed
albedo of 0.15, then this is for a size range 0.3 km < r 
< 4 km.
This is consistant with other surveys that have approached this size
range, in particular the Palomar-Leiden survey.
Serendipitous Asteroid Trails in Archival
Hubble Space Telescope WFPC2 Images
Robin W. Evans (JPL), Karl R. Stapelfeldt (JPL), Daniel P. Peters (JPL), John T. Trauger (JPL), WFPC2 IDT
Examination of 7642 observations (30,568 frames) of selected long exposures from archival Wide Field and Planetary Camera 2 (WFPC2) images has revealed trails from 96 distinct moving objects. They have been reported to the International Astronomical Union's (IAU) Minor Planet Center for their asteroid database. Seven have been tentatively identified with known asteroids and in 3 cases used to update their orbits. The other objects are new, in that they are fainter than typical detection limits of ground-based surveys. Three of these objects are potential Mars crossers.
HST's orbital motion induces a time-varying parallax during the exposures,
usually resulting in curved asteroid trails. Fitting of the trail shapes
yields accurate distances to approximatly half of these asteroids, thus
permitting their absolute magnitude to be deduced independent of an
object's phase angle. We find the slope of the cumulative size distribution
follows the power law N r . If we assume a fixed
albedo of 0.15, then this is for a size range 0.3 km < r < 4 km.
This is consistant with other surveys that have approached this size
range, in particular the Palomar-Leiden survey.
Statistics of these finds imply a reservoir of 4x10 
7x10 
asteroids in this size range within 25 
of the ecliptic.