What is an asteroid?

Asteroids are rocky objects that orbit around the Sun; most of them are located in a belt between Mars and Jupiter. Scientists believe there may be more than 50,000 asteroids in that belt, and perhaps millions more elsewhere in space. They range in size from nearly 20 feet (6 meters) in diameter to some as large as 600 miles (965 kilometers) across. (While 20 feet seems small compared to 600 miles, the smallest asteroids would still have a strong impact if they hit Earth.) Slight changes in asteroids' orbits occasionally cause them to collide with each other, resulting in small fragments breaking off from the whole. Sometimes these small fragments leave their orbit and fall through Earth's atmosphere as meteors. Some scientists have suggested that it was a huge asteroid's collision with Earth 65 million years ago that caused the massive damage that led to the extinction of dinosaurs.