HALLEY'S COMET

Perhaps the most famous of all comets is Halley's Comet. It is known as a "short period" comet because it orbits the Sun every 76 years. Halley's comet is like all others. It is a ball of rock and ice (the comet's nucleus), surrounded by clouds of gas and dust (the comet's coma). Usually, the comet makes its journey across the Solar System as an invisible rock. However, when it gets close enough, the Sun's heat causes the comet to burn off some of its dust and ice. Sunlight also reflects the gas particles. This is when the comet's tail becomes visible and appears as it does in the picture on the left. Comets actually have two tails. The tail caused by vapourised dust is usually white, and the tail caused by reflected gas particles is usually a blueish colour. 

The last time Halley's comet was visible from Earth was in 1986. In this year, a spacecraft called Giotto took photographs of the comet closely, revealing a very small rocky nucleus of about 8 by 15 kilometres (5 by 9 miles) and millions of miles of gas spurting out of the comet. In the picture above, you can see the black nucleus surrounded by the blueish coma. 

Halley's Comet orbits the Sun in a tight elliptical orbit. It comes very close to the Sun and is thrown back out into the Solar System to as far out as the orbit of Neptune before returning to orbit the Sun. It is likely that comets such as Halley's comet achieved their tight orbits by being pulled in by the gravity of a large planet, such as Jupiter or Neptune. They are not always pulled in enough to hit the planet (even though this did happen to the Shoemaker-Levy Comet in 1994!). Instead, they use the strong gravity to be forced back outwards in a slingshot effect. This natural occurrence is used in space exploration and was the key to the success of the Voyager mission and missions to the Moon and back. Other comets orbit planets, instead of the Sun, then get flung out into the outer edges of the Solar System, before returning to orbit their planet.