Planets, stars and galaxies
Comets are balls of ice and dust in orbit around the Sun. The orbits of comets are different from those of planets - they are elliptical. A comet's orbit takes it very close to the Sun and then far away again. The time to complete an orbit varies - some comets take a few years, while others take millions of years to complete an orbit.
Comets are often visible from Earth when they get close to the Sun, because the Sun's heat vaporises material from their surface, and this vapour forms a tail.
Asteroids are rocky objects, smaller than planets. Most of them are found in an 'asteroid belt', in orbit around the Sun between Mars and Jupiter. The minor planet Ceres is found here, too. Asteroids can crash into each other. When they do, they may break apart and their orbit may change.
The orbits of some asteroids cross the Earth's orbit. At various times during the Earth's history, asteroids have hit the Earth. When this happened, a tremendous amount of energy was released, throwing up billions of tonnes of dust. This blocked heat and light from the Sun, making the Earth very cold.
It is thought that it was the collision of an asteroid with the Earth that helped to drive the dinosaurs to extinction. Scientists worry that an asteroid could still hit the Earth and cause a global catastrophe.
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