THE SHAPE OF GASPRA

Images of Gaspra obtained during the October 1991 Galileo flyby have b een used to derive a shape model of the asteroid. Gaspra is a highly i rregular object with principal diameters of 18.2, 10.5, and 8.9 km and a mean radius of 6.1 km. Assuming that the mass distribution is homog eneous, the rotation axis (RA = 9.5-degrees, Dec = + 26.7-degrees) is aligned with the maximum moment of inertia given the estimated uncerta inties in the model and pole. The mean radius, axial ratios, and pole orientation derived from the Galileo data agree closely with values in ferred from groundbased observations. Gaspra's shape is defined in par t by extensive flat areas and large, shallow concavities up to 10 km w ide. One of the flat areas is about 6 km across, and defines a plane w ithin +/- 0.2 km. In terms of limb roughness and the size of concaviti es, Gaspra's shape is moderately more irregular than those of well-ima ged small satellites. Patterns of grooves, ridges, and flat surfaces s uggest the presence of a global structural grain. Such a structural pa ttern is consistent with Gaspra being a single object derived from a s ubstantially larger precursor body by collisional fragmentation; it is difficult to reconcile with hypotheses that Gaspra is a binary object or a collection of smaller bodies. (C) 1994 Academic Press, Inc.