Bob the Alien's Tour of the Solar System


	INTRODUCTION	HALLEY'S COMET
	COMET HALE-BOPP	SHOEMAKER-LEVY COMET
Comets are often described as dirty snowballs. They are balls of rock and ice which formed in the outer edges of the Solar System, beyond the orbit of Pluto in a cloud of condensation called the Oort Cloud. A typical comet is about 20 kilometres in diameter. Comets orbit the Sun in elliptical orbits, travelling from a great distance to become very close to the Sun. They spin around the Sun and are then flung back outwards. When a comet is close enough to the Sun to be affected by its heat and radiation, it begins to burn off gas and dust and forms a bright tail. This tail can often appear spectacular from Earth. Some comets take thousands of years to orbit the Sun, some take a few years and some are thrown out of the Solar System for good after they spin around the Sun!