How often do comets come near the Earth?
Comets are like dirty snowballs, lumps of rock, ice and gas, that travel around the Solar System, guided in their orbit by the gravity of the Sun. They reappear in the night sky at regular intervals, at anything between every six years for some and every 2000 years for others, depending on the size of their orbits. Some comets shine spectacularly as they approach the Sun and ice burns off as gas vapour, forming a blazing tail up to 300 million km(200 million miles) long. Halley's comet, which reappears every 76 years, was first spotted in 239 BC, it was first illustrated in the Bayeux tapestry, which commemorated the Norman invasion of England, and was photographed most recently on March 13, 1986, by the space probe Giotto.

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