The shape of Ida

Ida's shape is more irregular than that of any solar system object previously encountered by spacecraft, Approximately 95% of the surface was imaged by the Galileo spacecraft at better than 1 km/pixel and 30% at better than 40 m/pixel, Ida's volume is 16,100 +/- 1900 km(3) (mean radius of 15.7 +/- 0.6 km); the surface area is 3900 +/- 300 km(2), The maximum and minimum dimensions are 55.8 and 14.8 km. Viewed from the poles Ida has a somewhat crescent shape. Smaller scale overlapping depressions along much of the 110 degrees and 260 degrees E longitudes give the appearance of a ''waist.'' A sharp ridge about 40 km in length spans nearly 150 degrees of longitude. The distinctive topography on either side of the ''waist'' suggests a difference of mechanical properties along the length of the asteroid. These differences might arise from structures inherited in place from a larger precursor object, or might indicate a two component object, The spin pole aligns with the model maximum principal moment of inertia axis to within 2 degrees; this relationship rules out extreme density asymmetries inside Ida, but does not limit modest changes in porosity or composition across the asteroid, The irregular shape, low mean density (2.6 +/- 0.5 gm/cm(3)), and rapid spin (P = 4.63 hr) of Ida mean that surface accelerations vary greatly (0.3 to 1.1 cm/sec(2)); gravity is lowest at the greatest and minimum radii due to the effects of rotation and the amount of mass interior to surface location, Maximum slopes are about 50 degrees, although very little of the surface area has slopes in excess of 35 degrees. (C) 1996 Academic Press, Inc.