Asteroid 243 Ida: Groundbased Photometry and a Pre-Galileo Physical Model

Groundbased photometric observations of asteroid 243 Ida over seven apparitions from 1980 to 1993 allow us to derive a model for its spin vector and shape prior to the August 1993 encounter by the Galileo spacecraft. Two equally likely solutions for the spin vector are at ecliptic coordinates (1950) λp1 = 73, βp1 = -53 or λp2 = 254, βp2 = -55 with an uncertainty of ± 10° in each coordinate. Ida's low orbital inclination restricts the available aspect angles for Earth-based observers so that the spin vector ambiguity cannot be resolved. We offer predictions for the flyby geometry for both solutions, where the Galileo spacecraft images will resolve this ambiguity easily. Ida's rotation is retrograde with a sidereal period of 4.633632 ± 0.000007 hr. Assuming a triaxial ellipsoid model, we estimate axial ratios of a/b = 1.82 ± 0.10 and b/c = 1.15 ± 0.10. The morphology of the observed lightcurves suggest that Ida has a nonuniform albedo distribution, but we cannot completely rule out observational uncertainties or simplifying model assumptions (e.g., convex shape, geometric scattering) as sources for this suggestion. It is not possible to predict uniquely the distribution of any such albedo variegation.