| Asteroids are chunks of rock that never quite made it to full-fledged planet-hood when our solar system formed. Most of them orbit the sun in a "belt" between Mars (the fourth planet) and Jupiter (the fifth planet). But some asteroids have orbits that cross or come close to Earth's orbit.
You may have heard that it might have been an asteroid slamming into Earth about 65 million years ago that wiped out the dinosaurs. Hmmm. Where are the air traffic controllers when you really need them?
We think there may be about 100,000 asteroids, from 1000 kilometers (600 miles) across down to about 1 kilometer across. We are curious about asteroids because they may tell us interesting things about how our solar system (and maybe others) formed in the first place. Because they are so small, nothing much has happened to change them in all this time. No volcanoes, no water, no weather, not even much gravity to squish things together.
Because asteroids are so small and (we hope) far away, they are very hard to see from Earth. However, the Deep Space 1 spacecraft has come closer to an asteroid than any other spacecraft. It flew within 15 kilometers (about 10 miles) of Braille on July 29, 1999.
 Deep Space 1 took pictures and gathered information about the material making up the asteroid. It observed and measured its size, shape, surface features, and brightness. It also measured what happens to the solar wind when it flows around the asteroid. On Earth, wind is a stream of moving air particles. In space, solar wind is a stream of energetic particles blown off by the sun.
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