Asteroids are pieces of metallic rock floating in orbit around the sun.

Inside the asteroid belt © Getty Images

Inside the asteroid belt © Getty Images

Scientists believe that they are pieces left over when the solar system was formed.

Most of the asteroids in space are found in the the space between the orbits of the planets Mars and Jupiter. This space is known as the ‘asteroid belt’. However some asteroids are found outside the ‘asteroid belt’.

More than 7000 asteroids have been discovered, but there are probably hundreds of thousands of asteroids in space. The largest one that scientists know about is called Ceres. It has a diameter of 933 kilometres. Ceres was discovered on January 1, 1801, by Giuseppe Piazzi. Ceres the largest asteroid is a dwarf planet, just like Pluto. 

One kind, called Apollo asteroids, cross the orbit of the earth as they move around the sun. Scientists think that an Apollo asteroid struck the Earth about 90 million years ago in the time of the dinosaurs.

Asteroids are also known as planetoids or minor planets. Some have moons that orbit around them. Pieces of asteroids that collide with the Earth are called meteorites.

A meteorite collides with Earth © Getty images

A meteorite collides with Earth © Getty images

You can watch a video about asteroids here.