THE SHAPE OF GASPRA

Images of Gaspra obtained during the October 1991 Galileo flyby have been used to derive a shape model of the asteroid. Gaspra is a highly irregular 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 homogeneous, the rotation axis (RA = 9.5-degrees, Dec = + 26.7-degrees) is aligned with the maximum moment of inertia given the estimated uncertainties in the model and pole. The mean radius, axial ratios, and pole orientation derived from the Galileo data agree closely with values inferred from groundbased observations. Gaspra's shape is defined in part by extensive flat areas and large, shallow concavities up to 10 km wide. One of the flat areas is about 6 km across, and defines a plane within +/- 0.2 km. In terms of limb roughness and the size of concavities, Gaspra's shape is moderately more irregular than those of well-imaged small satellites. Patterns of grooves, ridges, and flat surfaces suggest the presence of a global structural grain. Such a structural pattern is consistent with Gaspra being a single object derived from a substantially 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.