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 km³(mean radius of 15.7 ± 0.6 km); the surface area is 3900 ± 300 km². 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° and 260°E longitudes give the appearance of a "waist." A sharp ridge about 40 km in length spans nearly 150° 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°; 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/cm³), and rapid spin (P= 4.63 hr) of Ida mean that surface accelerations vary greatly (0.3 to 1.1 cm/sec²); 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°, although very little of the surface area has slopes in excess of 35°.