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What is a Comet?

A comet is a small icy rocky body that orbits the Sun, some as small as 100 metres in diameter to around 6kms in diameter. Often referred to as a "dirty snowball" comets consist of carbon dioxide, water ice, methane and rock. In the outer reaches of the Solar System they remain inert, it is only when they approach the Sun that wonderful things begin to happen. As they near the Sun the icy matter that makes up the comet becomes energised by the heat of the Sun. The solar winds force the energised matter in the head of the comet backwards forming the tail, that is the reason the tail of a comet always points away from the Sun.

Comets are classified as either Short period comets, or Long period comets.

Short Period Comets

The short period comets have less elliptical orbits taking up to 200 years to complete their orbit around the Sun. Halley's Comet, the most famous short period comet takes 76 years to orbit the Sun. At it's furthest from the Sun it reaches the orbit of Pluto - 35 AU from the Sun.

Short period comets are believed to originate in the Kuiper Belt, a belt of icy rubble, which starts around the orbit of Neptune and extends out to 50 AU. It is believed that Pluto, and Charon originated in the Kuiper Belt.

Long Period Comets

Long period comets take over 200 years to orbit the Sun, many of them having such highly elliptical orbits that they take hundreds of years to complete one solar revolution. One of the best was Comet Bennett that last visited our Sun in 1970, and is due to return in the year 3648. It’s aphelion or the farthest point of it’s orbit is 282 AU from the Sun.

Long period comets are believed to originate in the outer regions of the Solar System, in a region known as the Oort cloud. It is thought that the gravity from passing objects pulls the comets from the cloud sending them on orbits towards the Sun.

Information obtained from the Wikipedia web site. Wikipedia the free encyclopedia.

The first video is not in English, but it is very good, and shows some excellent examples of comets.

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