GALILEO PHOTOMETRY OF ASTEROID 243-IDA

Galileo imaging observations over phase angles 19.5 degrees to 109.8 d egrees are combined with near-opposition Earth-based data to derive th e photometric properties of Ida. To first order these properties are u niform over the surface and well modeled at lambda = 0.55 mu m by Hapk e parameters <(omega)over tilde>, = 0.22, h = 0.020, B-0 = 1.5, g = -0 .33, and theta = 18 degrees with corresponding geometric albedo p = 0. 21+/-(0.03)(0.01): and Bond albedo A(B) = 0.081+/-(0.017)(0.008). Ida' s photometric properties are more similar to those of ''average S-aste roids'' (P. Helfenstein and J. Veverka 1989, Asteroids II, Univ. of Ar izona Press, Tucson) than are those of 951 Gaspra. Two primary color u nits are identified on Ida: Terrain A exhibits a spectrum with relativ ely shallower 1-mu m absorption and a relatively steeper red spectral slope than average Ida, while Terrain B has a deeper 1-mu m absorption and a less steep red slope. The average photometric properties of Ida and Terrain A are similar while those of Terrain B differ mostly in h aving a slightly higher value of <(omega)over tilde>(0) (0.22 versus 0 .21), suggesting that Terrain B consists of slightly brighter, more tr ansparent regolith particles. Galileo observations of Ida's satellite Dactyl over phase angles 19.5 degrees to 47.6 degrees suggest photomet ric characteristics similar to those of Ida, the major difference bein g Dactyl's slightly lower albedo (0.20 compared to 0.21). (C) 1996 Aca demic Press, Inc.