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!