comets and asteroids

Comets

Comets are balls of ice and dust in orbit around the Sun. (They are sometimes referred to as dirty snowballs). The orbits of comets are different from those of planets: they are elliptical. (An ellipse is a squashed circle.) 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.

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 that points away from the sun and which is quite visible.

Diagram of the orbit of a comet crossing the orbits of planets	Schwassmann-Wachmann 3 comet (NAOJ photo)	Comet Wild 2 (NASA photo)

Asteroids

	Diagram of the asteroid belt

Asteroids are rocky objects, smaller than planets, orbiting the sun. Most are found in the asteroid belt, located between Mars and Jupiter.

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.

A common theory is that it was the collision of an asteroid with the Earth that helped to drive the dinosaurs to extinction, though there are other theories too.

An asteroid could still hit the Earth in the near or distant future and cause a global catastrophe.

By far the largest and most massive asteroid is Ceres, which is also classed as a dwarf planet.

NASA photo of asteroid 951 Gaspra, which looks like a battered potato	NASA photo of the asteroid Eros	A NASA artist's impression of an asteroid colliding with the Earth	The dwarf planet Ceres is in the main asteroid belt.

The size of Ceres, the dwarf planet which is also the largest asteroid, shown to scale together with the Moon, Pluto and the UK.

[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]