GALILEO ENCOUNTER WITH 951-GASPRA - OVERVIEW

On 29 October 1991, the Galileo spacecraft carried out the first ever encounter of a minor planet, flying by Asteroid 951 Gaspra at a distance of 1600 km. We summarize findings derived from the 57 images obtained by Galileo's SSI camera which cover about 80% of the surface of this S-asteroid. The highest resolution achieved was 54 m/pixel. The analysis of the imaging data is described in the series of papers making up this special issue of Icarus. Gaspra is a highly irregular body with principal diameters of 18.2, 10.5, and 8.9 km (average radius = 6.1 +/- 0.4 km). Gaspra's irregular shape and the prominence of grooves, linear depressions 100-300 m wide and tens of meters deep, suggest that the asteroid was derived from a larger body by catastrophic collision. Features that appear to reflect structural grain, including ridges, grooves, and flat surfaces, suggest that Gaspra is a single coherent body and not a binary or a ''rubble pile.'' Craters on Gaspra can be divided into two categories: a very subdued, apparently older, population and a population of much crisper (younger?) craters. The latter is characterized by an index (exponent of the power law describing the cumulative distribution of craters as a function of diameter) of - 3.1 +/- 0.3, definitely steeper than the theoretically derived value of - 2.5 expected for collisional equilibrium. Estimates of the cratering age of Gaspra's surface give very young values ranging from 20 to 300 myr. Estimated characteristic times for the collisional disruption of Gaspra-sized bodies at 2.2 AU range from 200 to 1000 myr. Analysis of spectral imaging data (0.40 to 1.10 mum) reveals small but significant color variations over the asteroid's surface. The spectrally most distinct materials on Gaspra are distinguished by a more prominent 1-mum absorption band and tend to be slightly brighter and bluer than average Gaspra. Often such materials are associated with small, fresh-appearing craters along ridges. A strong correlation exists between the infrared/violet color ratio and elevations on Gaspra, a correlation which can be explained in terms of downhill migration of a regolith. Gaspra's photometric properties are lunar-like, but with higher values of omega0BAR and thetaBAR (0.36 and 29-degrees, respectively). The average geometric albedo is 0.23 near 0. 56 mum. An important result is the confirmation that pre-Galileo inferences from telescopic observations about Gaspra's size, shape, albedo, and rotation state are accurate. (C) 1994 Academic Press, Inc.