NASA’s Galileo spacecraft flew past asteroid Gaspra on Oct. 29, 1991, making it the first asteroid ever visited by a spacecraft. The encounter took place during Galileo’s journey to Jupiter, roughly two months after the probe entered the asteroid belt.
Galileo passed about 990 miles (1,593 kilometers) from Gaspra at a relative speed of approximately 5 miles (8 kilometers) per second. It photographed the asteroid and used its Near-Infrared Mapping Spectrometer to examine its composition and physical properties.
The observations showed that Gaspra is elongated and irregular, measuring about 12 miles long and 7 miles wide, and that its surface is marked by craters. Galileo was too far away for the asteroid’s gravity to affect the spacecraft, so it could not determine Gaspra’s mass.
The flyby was a milestone during Galileo’s six-year trip to Jupiter. Launched in October 1989, the spacecraft had previously passed Venus and Earth. It later flew by Earth again in 1992, encountered asteroid Ida in 1993 and reached Jupiter in 1995. During its journey, Galileo also observed Comet Shoemaker-Levy 9 breaking apart and striking Jupiter in July 1994.
Sources: Space.com.