ANALYSIS OF GASPRA LIGHTCURVES USING GALILEO SHAPE AND PHOTOMETRIC MODELS

Galileo-based models for the shape of 951 Gaspra and the global-averag e photometric behavior of its surface have been used to model a repres entative subset of the asteroid's telescopic lightcurves. Fitting the synthetic lightcurves to the observed timing of lightcurve extrema, an d knowing the orientation of Gaspra's axes at the time of the Galileo flyby, leads to a sidereal rotation period for the asteroid of 7.04202 4 +/- 0.000020 hr, a slight change from the period reported by Magnuss on et al. (1992, Icarus 97, 124-129). Initially, the shapes, amplitude s, and absolute photometry of the synthetic and observed lightcurves a gree with each other to within 0.05-0.1 mag. Small modifications to th e Gaspra shape model on sides of the asteroid poorly imaged by Galileo (changes of 700 m or less in the southern hemisphere at longitudes 90 degrees-270 degrees W) reduce the typical discrepancies to approximat e to 0.05 mag in lightcurve shape and <0.03 mag in absolute photometry . The result demonstrates that Earth-based lightcurves can be used to refine the shape of a spacecraft-imaged irregular object in areas that are poorly constrained by the spacecraft observations. The consistenc y in absolute photometry validates both the absolute brightness and ph ase-angle dependence of the Galileo-based model for Gaspra photometry, supports the accuracy of the absolute calibration of the Galileo SSI camera, and confirms the Earth-based determination of the V-filter geo metric albedo of the asteroid (0.22 +/- 0.03; Tholen et al., submitted for publication). Remaining discrepancies between the synthetic and o bserved lightcurves show no indication of systematic latitudinal varia tions in albedo and also cannot be explained entirely by isolated albe do spots. These discrepancies are most likely caused by (1) small, rem aining, hard-to-constrain errors in the Gaspra shape model and/or (2) moderate variations in macroscopic roughness across the asteroid's sur face, in particular making longitudes 130 degrees-300 degrees W modera tely rougher than the opposite hemisphere. (C) 1995 Academic Press, In c.