Comets

A comet is a small icy celestial body that orbits around the sun. It is made up of four parts. At the centre is the nucleus of the comet. This is the solid object upon which the sun's force of gravity acts, creating the highly elliptical orbit of a comet. The nucleus is made up of rock, ice and dust. It is the nucleus of a comet that interests scientists. The material which makes up the nucleus is "left over" from the big bang and the creation of the planets. It is the excess that was not needed to make stars and planets. Now this excess matter orbits the stars of the universe. The nucleus contains most of the mass of the comet, but is small in comparison to the other parts of the comet, ranging from about one to ten kilometres across.

The next part of the comet is the coma, a roughly spherical ball of gas that surrounds the comet's nucleus. The coma is comprised of water vapour, carbon dioxide gas, ammonia, dust and neutral gases that have sublimed from the solid nucleus.

The tail of a comet is in fact made up of two different tails. The first is known as the dust tail and is a stream of dust particles knocked off of the comet. This tail flows behind the comet, along the path of motion. The second tail is known as the ion tail. An ion is a charged particle. It is created by solar wind, which is a stream of ions flowing from the sun. These ions ionise parts of the coma, pushing them away from the comet, forming a long tail that always points away from the sun. This tail fades the further away from the sun the comet gets. This is why astronomers can have no idea how many comets there are that pass through our solar system. The tail caused by the sun is the main detectable characteristic of a comet, but once the comet has passed the portion of its orbit close to the sun it becomes fairly undetectable as the tail disappears.