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What are asteroids?

Two asteroids and the distant SunThousands of small lumps of rock or metal orbit the Sun between Mars and Jupiter. These are called asteroids. They form what is called the Asteroid Belt. Asteroids are probably the remains of a planet that was smashed apart when the Solar System was formed billions of years ago. They have kept on colliding with each other ever since, producing smaller fragments called meteoroids.



A diagram of the Asteroid Belt

Shapes and sizes

About 4000 asteroids have been discovered and named so far. Most are small, rocky and irregular in shape, but a few are made of the metals iron and nickel. The largest asteroid, called Ceres, is round. It measures just over 1000 kilometres (600 miles) across. Like Pluto, it is called a dwarf planet.

Close-up view of a typical asteroid

Meteorites

Ceres is the only asteroid that is shaped like a sphere.

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