On August 28, 1993, the Galileo spacecraft encountered the asteroid Ida. High resolution images revealed that this asteroid has a small satellite. Of 12 asteroids observed by HST ( Storrs et al., 1994), none had detectable companions. Sizes and albedos of these resolved asteroids are comparable with those derived from groundbased infrared radiometry.
Based on Goldstone observations on December 8-13, 1992, when Toutatis passed close to the earth, Ostro et al. (1993) reported that this asteroid is bilobed or is a contact binary and the mean diameters of the two components are 2.4 and 1.5 km. This conclusion requires that a rate of rotation be assumed to interpret the Doppler data as a dimension on the plane of the sky. Simultaneously scheduled broad-band green observations with the PC mode of WFPC-1 revealed an irregular, marginally-resolved object with a maximum dimension of 1.7 km ( Noll et al., Imaging of Asteroid 4179 Toutatis with the Hubble Space Telescope, submitted to Icarus, 1994). Although this indicates that Ostro's analysis yields reasonable dimensions, the resolution was not adequate to confirm Ostro's bilobed model.
A considerable number of other asteroids have been observed and analysis is underway. These two examples illustrate the ability of HST to address the frequency of companion satellites and provide independent verification of the accuracy of infrared radiometric methods and radar inversion techniques.