What are asteroids?

Asteroids are small rocky bodies that orbit the sun. Most asteroids orbit in a belt between Mars and Jupiter called the asteroid belt. They are sometimes called planetoids or minor planets. There are an estimated 400,000 asteroids. However, the large asteroids of about 100 km diameter are few. Some large asteroids have their own moon, such as the asteroid Ida, which has a tiny moon named Dactyl.