A comet is an icy space body releasing gas and dust as its orbits the Sun. Astronomers have called them "dirty snowballs" or "snowy dirtballs" as they resemble piles of dust and rock rubble held together by ice. A comet's orbit around the Sun is elliptical. Comets have a low mass and are usually irregularly shaped.
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A belt of icy comets, the Kuiper Belt, exists just past the orbit of Neptune. It is located at a distance of between 30 astronomical units (AU), or 4.5 billion kilometres and 50 AU, or 7.5 billion kilometres, from the Sun. These have formed from space debris left over from the creation of the Solar System some 4.5 billion years ago.
These comets orbit the Sun over periods of 200 years or less, which is why they are called short-period comets. They have been observed from Earth over thousands of years. Astronomers are able to predict their nearest approach to the Sun (the perihelion), their furthest distance from the Sun (the aphelion) and the time they will pass closest to Earth or other planets.
Long-period comets are those that arrive from the Oort Cloud, some 100,000 AU or 15,000 billion kilometres from Earth. These comets may take up to 30 million years to complete an orbit around the Sun.
The core of a comet is the nucleus. Its size is usually less than 16 kilometres in diameter. Water ice is the main frozen substance but it may also include frozen carbon dioxide, methane and ammonia. Sometimes hydrogen cyanide and ethane are present. The organic substances include methanol and formaldehyde and occasionally long-chain organic compounds such as amino acids, the building blocks of proteins.
The coma is an atmosphere of dust and gas that surrounds the nucleus, while the tail is the dust and ionised gas trail emitted behind it during its orbit. This is formed by the Sun’s radiation, which pushes the dust behind the comet. When illuminated by the Sun, the coma and tail are visible from Earth as the comet passes through the Solar System. Meteor showers are created when the comet’s path passes through the Earth’s orbit and debris from the tail enters the Earth’s atmosphere.
Some comets, known as sun grazers, orbit very close to the Sun or collide with it, smashing into pieces or evaporating. They can also collide with other planets. Fragments of comets are believed to have delivered water and organic compounds to Earth.