Galileo photometry of asteroid 243 Ida

Galileo imaging observations over phase angles 19.5 [degrees] to 109.8 [degrees] are combined with near-opposition Earth-based data to derive the photometric properties of Ida. To first order these properties are uniform over the surface and well modeled at [Lambda] = 0.55 [[micro]meter] by Hapke parameters [Mathematical Expression Omitted], h = 0.020, [B.sub.0] = 1.5, g = -0.33, and [Theta] = 18 [degrees] with corresponding geometric albedo [Mathematical Expression Omitted] and Bond albedo [Mathematical Expression Omitted]. Ida's photometric properties are more similar to those of 'average S-asteroids' (P. Helfenstein and J. Veverka 1989, Asteroids II, Univ. of Arizona Press, Tucson) than are those of 951 Gaspra. Two primary color units are identified on Ida: Terrain A exhibits a spectrum with relatively shallower 1-[[micro]meter] absorption and a relatively steeper red spectral slope than average Ida, while Terrain B has a deeper 1-[[micro]meter] 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 having a slightly higher value of [Mathematical Expression Omitted] (0.22 versus 0.21), suggesting that Terrain B consists of slightly brighter, more transparent regolith particles. Galileo observations of Ida's satellite Dactyl over phase angles 19.5 [degrees] to 47.6 [degrees] suggest photometric characteristics similar to those of Ida, the major difference being Dactyl's slightly lower albedo (0.20 compared to 0.21).