This composite image shows Ida as seen from Galileo during its approach on August 28, 1993. The asteroid makes a complete rotation every 4 hours 38 minutes; therefore, this set of images spans about three-quarters of Ida's rotation period and shows most of Ida's surface. The asteroid appears to be about 58 kilometers long and about 23 kilometers wide, with a very irregular shape and volume of some 16,000 cubic kilometers.
Beginning in the upper left, the images are arranged in chronological order from a time 3 hours 51 minutes before closest approach through 33 minutes before closest approach. Ida's rotation axis is roughly vertical in these same-scale images, and the rotation causes the right-hand end of Ida to move toward the viewer as time progresses. The first image was taken from a range of about 171,000 kilometers and provides an image resolution of about 1700 meters per pixel (the highest resolution achieved for Ida is about 25 meters per pixel). The second, taken 70 minutes later, is from 119,000 kilometers, followed by 102,000; 85,000; 50,000; and 25,000 kilometers. The features on Ida are less sharp in the earlier views because of the greater distances.
Prominent in the middle views is a deep depression across the short axis of the asteroid. This feature tends to support the idea that Ida originally may have been formed from two or more separate large objects that collided softly and stuck together. Also visible in the lower left view is an apparent linear albedo or reflectance boundary. Color images yet to be returned from the Galileo spacecraft may help resolve the question of whether or not the two ends of Ida are made of different materials.
Back to Contents Page