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–twenty-three 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.

Lest you think that navigating the asteroid belt is a tricky business, it really isn’t. It’s not like Han Solo’s treacherous zipping around in the Millennium Falcon in The Empire Strikes Back with space rocks flying everywhere; in reality, while they do collide sometimes, asteroids are pretty far from one another and spacecraft navigating through the asteroid belt don’t really have that much to worry about. Galileo flew past Gaspra without incident, coming as close as 1,600 km to the rock. This is actually pretty impressive, as Gaspra is very small: its total surface area would fit comfortably in Hong Kong with lots of room left over.


After surveying Gaspra, the little Galileo spacecraft went on to even greater glories exploring Jupiter and its moons. In terms of return on investment, Galileo is one of the most cost-effective voyages of discovery of all time.

As you might expect from an asteroid, Gaspra is not a very hospitable place. It has no atmosphere, virtually no gravity and totally incapable of supporting life. It’s what Galileo’s photos suggested: a rock in space. There’s no reason to believe Gaspra is different than many other asteroids. In fact it’s perfectly representative of most of them, which in a curious roundabout way is an excellent reason to study it. Though we had not seen it up close before 1991, the asteroid was actually discovered in 1916 via telescope, by a Russian astronomer; “Gaspra” is the name of a resort on the Black Sea.

Despite their apparently uninteresting nature, asteroids could actually be fabulously useful to the human race. The idea of mining asteroids for metallic ores or other minerals–they contain many of them–first floated in science fiction decades ago, but there’s reason to believe it’s inching closer to probability. Many asteroids are only loosely held together by gravity, so breaking them apart would take much less effort than large-scale mining operations on Earth, and you wouldn’t have the environmental damage. Asteroids could also be ideal places to grow crystals for use in high-tech industries. Science and SF writer Isaac Asimov even suggested hollowing one out and building cities inside of it.

After passing by Gaspra, Galileo went on to a fascinating and storied career of exploring Jupiter and its environs. It arrived at the gas giant planet in 1995 and spent two years measuring, photographing and observing. The climax of the mission was an atmospheric probe that the spacecraft released which penetrated the thick atmosphere of Jupiter, sending back data that scientists could only dream of. Galileo itself later suffered radiation damage and was mostly deactivated. Its orbit decayed and it fell into Jupiter in September 2003.

The Jupiter study was clearly Galileo’s primary mission, and its main contribution to science and space exploration. But let’s not forget the fascinating glimpse it also gave us of this much smaller member of our solar system, which has its own interesting story to tell.