Asteroids are chunks of rock and metal. The solar system has thousands of asteroids that are in orbit around the Sun but are too small to be planets. Most of these asteroids are found in the asteroid belt between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and have hit Earth.
Asteroids are material left over from the formation of the solar system.
Much of what we know about asteroids comes from pieces that fall to the surface of Earth.