Comets are commonly referred to as 'dirty snowballs' because of the knowledge that they contain rock, dust and water ice. It is also widely thought that it may have been comets that brought the building blocks of life to Earth. Conversely, throughout our history, comets have also been thought of and depicted as portents of doom.
It is now known that as well as the rock, dust and water ice, comets contain other frozen gases such as carbon dioxide, carbon monoxide, methane and ammonia, and it has also recently been found that contrary to the dirty snowball description, the water ice of some comets may actually be hidden under an outer 'crust' of rock, dust and other materials.
Comets are also thought to contain a variety of organic materials and compounds, giving much credence to the building blocks of life theory, that may include, amongst others, methanol, formaldehyde, hydrogen cyanide, ethanol and ethane. They may even contain more complex, long-chain hydrocarbons and amino acids. In 2009, NASA's Stardust mission, which collected samples from the coma of comet 'Wild 2', confirmed that the amino acid glycine had been found in dust that was recovered.
The comets themselves, either short-period (those with shorter or more frequent orbits) or long-period (those with longer or less frequent orbits) orginate mostly in either what is known as the Kuiper Belt, an area beyond the orbit of Neptune, or in a still theoretical region known as the Oort Cloud, which is thought to lie in deep space in the outer solar system.
It is believed that these comets are 'nudged' into orbits that plunge them into the inner solar system and their orbits close to the Sun, by the effects of the gravitational pull of the massive outer planets, or even, in the case of the long-period comets originating in the Oort Cloud, by the effects of passing stars.
Known comet nuclei can range from 100m across to more than 40km. It is only as they approach their perihelion however, the closest point in their orbit around the Sun, that the solar wind and solar radiation from the Sun causes the outer crust of the nuclei to 'burn off' releasing the gases that form the coma and tail that have become so recognisable to us.
Comets too, maybe surprisingly, are the cause of many of the meteor 'showers' that are witnessed on Earth. As they travel through their orbit around the Sun, they leave behind them a trail of dust and debris, long after the nucleus has returned to the outer reaches of the solar system. As the Earth passes through these trails, so the many particles of dust and debris get burned up in our atmosphere, creating sometimes hundreds of meteors or shooting stars an hour to be visible over a period days or weeks.