Launched on its historic voyage to Jupiter on October 18, 1989, NASA’s Galileo spacecraft also got some good looks at several members of our solar system before it reached the giant planet — and one of them was the 12-mile-long asteroid Gaspra, of which it made its closest pass on October 29, 1991.
The image above is a high resolution enhanced-color mosaic of Gaspra made from images Galileo acquired from about 3,300 miles away. Ten minutes later Galileo passed Gaspra at 995 miles — it was the first close pass of an asteroid by a manmade spacecraft!
First discovered in 1916, Gaspra is an irregular body with dimensions about 19 x 12 x 11 kilometers (12 x 7.5 x 7 miles). The portion illuminated in the view above is about 18 kilometers (11 miles) from lower left to upper right. The north pole is located at upper left; Gaspra rotates counterclockwise every 7 hours.
A striking feature of Gaspra’s surface is the abundance of small craters. More than 600 craters, 100-500 meters (330-1650 feet) in diameter are visible here. The number of such small craters compared to larger ones is much greater for Gaspra than for previously studied bodies of comparable size such as the satellites of Mars.
The fact that Gaspra is irregular in shape and lacks any large craters suggests that it has a comparatively recent origin, most likely from the collisional breakup of a larger body. Gaspra has probably been in its present state for the last 300 to 500 million years.
Source: CICLOPS site
Lear more about the Galileo mission and its many discoveries here, and see more images from Galileo here.