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Comets are mostly made of ice and dust (like a dirty snowball), with a few chunks of rock and possibly some organic (carbon) compounds. The ices that make the bulk of a comet include water ice, dry ice (solid carbon dioxide), frozen ammonia, solid methane and several common atmospheric gases in solid form such as oxygen.
Where do comets come from?
Comets are thought to be the remnants of the formation of the solar system, and there is a huge cloud of them that surround the Sun in a spherical shell that begins just outside the orbit of Pluto. This cloud of comets is known as the Oort comet cloud, and all the comets we see approaching the Sun are believed to have come from this cloud.
Due to their huge distance from the Sun, the comets in the Oort cloud remain cold and solid at all times. It is only when they come close to the Sun that the ices start to melt and they generate a tail.
Scientists think that gentle gravitational nudges from nearby stars occasionally cause one of the comets to change its orbit so that it approaches the inner solar system. Initially it would have a very stretched elliptical or oval shaped orbit, but this may change as the comet experiences gravitational pulls from the inner planets, particularly Jupiter.
All comets will have a different orbit and therefore take different amounts of time to complete an orbit. Some will end up with orbits of less than a year, but most have orbits between 100 and 100 000 years.
How common are they?
Every year about six new comets are discovered. Some of these are one-off visitors that will never return to the inner solar system. Others may take 10 000 years or more to return and yet others may have a period as short as 2 years. Halley's Comet has a period of 76 years.
Will they hit Earth?
Generally, no. But it has happened and will happen again. It's a bit like the two players serving in a game of tennis at the same time. Mostly the balls would never hit, but just occasionally they might.
Visit NASA’s Jet Propulsion Laboratory for more information on comets, asteroids and meteorites.