Comets are small, irregularly shaped bodies that consist of a mixture of grains and frozen gases. They move in elliptical orbits that will bring them very close to the sun and then deeply into space, many times beyond the orbit of Pluto.

They all have a surrounding cloud of diffuse material, called a coma, which usually grows in size and brightness the closer it gets to the sun.
Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.

The further away from the sun it gets the colder the coma gets and inside the coma it freezes solid. When it is frozen it can only be seen by reflected sunlight. However, when the coma develops, the dust reflects more sunlight. As the comet absorbs ultraviolet light a chemical process is started and forms a hydrogen envelope.

The blue tail is made up of gases and the broad white tail of microscopic dust particles. A comet is said to be short-lived on a cosmological time scale and most scientist believe that asteroids are extinct comets.

The most famous comet must be Halleys comet and it has been known since at least 240 BC and possibly since 1059 BC. Its most famous appearance was in 1066 AD when it was seen right before the battle of Hastings. It was named after Edmund Halley who calculated its orbit. It has a 76-year orbit and its next appearance for us on earth will be in 2062.