Comets lie here until the gravity of a nearby star gives one a gentle push and it falls into orbit towards the Sun. About ten comets a year join the inner Solar System, and become visible from Earth.

As the comet gets closer to the Sun, the temperature of its outer layer rises and becomes a gas. This gas forms a cloud around the nucleus, called a coma. As the stream of particles from the Sun, known as the solar wind, hits the comet coma it is blown back to form a tail – this can be many hundreds of millions of kilometres long.

Comets were once seen as omens for good or evil. Their appearance in the night sky might foretell a great battle or the birth of a wise ruler. A comet features in the Bayeux Tapestry of the Norman conquest of England and in paintings of the birth of Jesus Christ.

Although few now believe comets have any mystical properties, they are no less exciting. There is now a consensus among scientists that much of the water on Earth was brought here by comets. It is also likely that the complex organic molecules that formed the basis for life also came from cometary debris. This means we are all mostly made from bits of comets.

Their beautiful appearance alone is enough to interest astronomers but comets can also tell us a lot about the Solar System. By studying comets scientists hope to gain an understanding of what conditions were like as the Solar System formed and gain clues to the origins of life on Earth.

The NASA-led mission Deep Impact smashed an impactor spacecraft into the nucleus of Comet Tempel 1 on the 4 July, 2005. This was the first time scientists were able to ‘see’ into a comet and there has been considerable UK involvement in this mission.

The European Space Agency Rosetta mission, launched in 2004, is one of the most ambitious robotic missions ever conceived. Rosetta is designed to land on a comet and comprises a large orbiter and a small lander. After entering orbit around comet 67P/Churyumov-Gerasimenko in 2014, the spacecraft will release the Philae lander onto the icy nucleus and spend the next two years orbiting the comet as it heads towards the Sun.

On 2 January 2004, NASA’s Stardust mission flew within 236 km of Comet Wild 2 and survived the high-speed impact of millions of dust particles and small rocks. Stardust captured cometary and interstellar particles with an extended, tennis racket shaped collector and brought these back to Earth in a Sample Return Capsule in 2006.

These will be studied to learn about the fundamental nature of interstellar grains (stardust) and other solid materials that assembled to form our Solar System. The UK’s Open University will be involved in analysing the returned particles.