Comets consist essentially of lumps just a few kilometres across that are made of a mixture of ice, carbonaceous material and rock dust. A comet can develop a spectacular million kilometre-long tail of gas and dust when its elongated orbit brings it sufficiently close to the Sun for the temperature to become high enough to vaporise the water, carbon monoxide and other volatile substances that are otherwise held as ice. At such times jets of gas escape from the solid part of the comet (its nucleus) to feed the growing tail. However, for most of the time a comet is too far from the Sun, and it is simply a dark, dusty potato shaped object too faint to detect even using the largest telescopes.
Two classes of comets are distinguished by their orbital characteristics. Long period comets spend most of their time in a diffuse cometary halo called the Oort Cloud which begins more than 10,000 times further out from the Sun than the Earth’s orbit, and 300 times further away than Neptune’s orbit. We can see one of these when it appears unexpectedly (and from any direction) as a result of having been dislodged, causing it to shoot inwards towards the Sun. Such a comet will swing past the Sun before receding into the distance, and is likely to make only one passage through the inner Solar System during human history.
On the other hand, short period comets have much smaller (and therefore faster) orbits. They appear to have been scattered inwards from the Kuiper Belt, which is the home of icy debris left over from the birth of the Solar System just beyond the orbit of Neptune. Because gas is lost each time a comet’s orbit brings it close to the Sun, short period comets tend to become less spectacular as time goes by. Many short period comets take only a few years to complete an orbit. The famous Halley’s comet is a short period comet, but has a relatively large orbit that takes 76 years to complete.
Impacts by comets are responsible for maybe one crater in ten on the Moon’s surface (the others being formed by asteroid impacts), and comets also hit the Earth from time to time. For example, in 1908 an explosion in Siberia that was heard 1000 km away was found to have flattened trees up to 30 km from a central point in the Tunguska valley. This was probably the result of the nucleus of a small comet entering the Earth’s atmosphere at more than 30 km per second, which failed to produce a crater because it was vaporised by friction just before it reached the ground. In June 1994 fragments of the comet Shoemaker Levy 9 hit the planet Jupiter in spectacular fashion. Large cometary impacts such as these towards the end of the Earth’s formation 4.5 billion years ago may have delivered much of the water now found in the oceans.
Unlike all the other bodies in the Solar System, comets are traditionally named after their discoverers. These days comets are often discovered by teams, or by several observers simultaneously, and so have quite complicated names. Halley’s comet is an exception, Edmond Halley did not discover it, but he was the first to realise that a comet that he had observed in 1682 was the same one that had appeared in 1531 and 1607. In 1705, using his friend Isaac Newton’s recently formulated laws of motion, he correctly predicted the comet’s return in 1758, but he died in 1742 and so did not live to see it again. Halley’s comet has in fact been seen many times, and its 1066 appearance is even recorded on the Bayeux tapestry.