The asteroid belt is a doughnut-shaped group of small, rocky objects. Most of these objects orbit the Sun in a belt between Mars and Jupiter, but some are in other orbits. Asteroids that orbit near the Earth are called NEA's (Near Earth Asteroids).
There are about 40,000 asteroids over 0.5 miles (2.4 km) in diameter. The biggest asteroid (called Ceres) is 578 miles (930 kilometers) wide.
An large asteroid may have hit the Earth 65 million years ago causing the extinction of the dinosaurs. The collision would have caused major changes in the Earth's climate, killing the dinosaurs plus many other groups of animals and plants that couldn't adapt to the changes.