GALILEO SUPPORT OBSERVATIONS OF ASTEROID 951-GASPRA

We report observations of 951 Gaspra in support of the Galileo spacecraft encounter. Photometric observations yield a synodic rotational period of 7.042 46 +/- 0.000 06 hr (0.293 436 +/- 0.000 003 days), a mean absolute visual magnitude of 11.8026 +/- 0.0025, and a slope parameter of 0.285 +/- 0.005. We review another determination of the slope parameter and suggest that the sub-Earth latitude was changing significantly during the opposition, and so neither we nor other investigators were able to sample uniquely the asteroid's phase function during the 1990 opposition. We suggest adopting a slope parameter of 0.25 from combining available observations, and we also place limits on a possible pole orientation. Our infrared reflectance spectrum (0.8-2.5-mu-m) shows an absorption band near 1-mu-m attributed to olivine and pyroxene, and a weak second pyroxene band centered near 2.0-mu-m. This mineral signature plus the overall redness of the spectrum suggest that Gaspra originated in the lower mantle of a differentiated parent body. Astrometric positions obtained during the course of these observations are also reported.