The asteroid Gaspra looms in the sights of the Galileo spacecraft in this 1991 image. Gaspra was the first asteroid ever visited by a spacecraft, with Galileo passing just a few thousand miles from the rocky body en route to Jupiter. Gaspra has an average diameter of about seven miles, and is marked by impact craters and long cracks that suggest Gaspra was once part of a larger body. [NASA/JPL]
Galileo at Gaspra
Over the last decade or so, spacecraft have studied more than a half-dozen asteroids from close range. One craft even brought back a sample of one asteroid, and another is orbiting the large asteroid Vesta right now.
The first asteroid encounter came 20 years ago today. The Galileo spacecraft was headed for Jupiter. As it traversed the asteroid belt, between the orbits of Jupiter and Mars, it flew within a thousand miles of Gaspra.
It was a tricky encounter, because Gaspra’s position was a little uncertain. Seen from Earth, the asteroid is no more than a bright point of light. The light is smeared by the atmosphere, so it’s impossible to pinpoint the asteroid’s location to less than about a hundred miles. So on the way in, Galileo used its own cameras to plot a more precise location for Gaspra.
With that help, Galileo was able to snap dozens of pictures of the asteroid. They showed that it’s shaped like a large, wide spearhead, with an average diameter of about seven miles. It’s covered with impact craters, and criss-crossed by long, deep cracks. The cracks suggest that Gaspra was once part of a larger body. But a giant impact split it apart, creating a family of asteroids and leaving Gaspra with deep scars.
Two years later, Galileo flew by a second asteroid, Ida. And it made a bonus discovery: a small moon orbits Ida — the first asteroid moon ever found.
Script by Damond Benningfield, Copyright 2011
For more skywatching tips, astronomy news, and much more, read StarDate magazine.