Rock in space: Encountering asteroid 951 Gaspra.

We don’t think about asteroids very much. When we think about members of our solar family we tend to focus on the big guys, or even the little guys who used to be big guys, like the unfortunate Pluto. But our solar system contains tens of thousands of members, most of them cold jagged rocks tumbling through space in the asteroid belt between Mars and Jupiter. Once upon a time we didn’t know that much about asteroids. That changed, though, on October 29, 1991—thirty-five years ago today—when NASA’s Galileo space probe finally met its first asteroid, called 951 Gaspra.

Galileo was really an amazing machine. It was an unmanned probe designed to study the planet Jupiter, granting humanity its first close look at the largest planet in our solar system. Twelve years in the making, Galileo was finally launched from a space shuttle in October 1989. Two years later it entered the asteroid belt on its way to Jupiter, and NASA scientists pointed its cameras and instruments at Gaspra both as a test of the probe’s systems and also as an object of study in itself. This would be the first asteroid ever glimpsed up-close with manmade instruments.