Galileo at Gaspra: the first close-up of an asteroid
How a Jupiter-bound spacecraft found a 12-kilometer target with pioneering optical navigation in October 1991.
Published 2026-10-23
Before 1991, every asteroid was just a point of light. That changed on October 29, 1991, when NASA's Galileo spacecraft flew past 951 Gaspra.[3]
A detour on the way to Jupiter
Galileo launched from the cargo bay of Space Shuttle Atlantis on October 18, 1989, bound for Jupiter.[1] Its looping path through the inner Solar System crossed the asteroid belt, giving the team a chance to visit Gaspra. NASA lists the first flyby and imaging of an asteroid — Gaspra, and later Ida — among Galileo's firsts.[1]
Finding a 12 km target
Gaspra's position was known only to within about 200 km, while Galileo's camera saw a field just 5° wide. Closer than about 70,000 km, the team would not know where to point. Their solution was a pioneering optical navigation campaign: images taken during the approach were used to refine Gaspra's position.[3] It worked.
The flyby
Galileo passed within 1,600 km of Gaspra at a relative speed of about 8 km/s.[3] It returned 57 images, the closest taken from 5,300 km, with a best resolution of about 54 meters per pixel. About 80% of the asteroid was seen; the area around the southern pole was not.[3]
What it revealed
- An irregular, elongated body about 18 × 10 × 9 km.[2]
- Relatively few large craters, implying a young surface, perhaps 20–300 million years old.[3]
- Long grooves 100–300 m wide, possibly from impacts that fractured the rock beneath.[3]
- An olivine-rich stony composition.[3]
One thing Galileo could not measure was Gaspra's mass: the asteroid was too small to noticeably tug on the spacecraft at that distance.[3]
A slow trickle of data
A permanent failure of Galileo's high-gain antenna limited how fast data could come home — which is why many images from its next asteroid, Ida, arrived months after that flyby.[5]
Numbered citations link to the sources list on the Gaspra overview page.