History of the Universe

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Comets are lumps of ice made of water, carbon dioxide, methane and ammonia. Frozen in this ice is dust and small lumps of rock. Comets are probably similar to the planetesimals from which the planets formed. Therefore there is great interest in learning more about them. Unfortunately most of them are far away from Earth.

There seem to be two sources of comets. A ring of comets, the Kuiper Belt, lies at around the same orbit as Pluto. Indeed Pluto may be thought of as the largest member of the Kuiper Belt, since it is made of similar icy material. The other source of comets, the Oort cloud, is a spherical shell, like the skin of a football, and lies much further from the Sun. It is thought that originally these outer comets came from the Kuiper Belt and were pulled out into higher orbits later, perhaps as the solar system passed other stars during its journey around the Galaxy.

Sometimes these icy bodies are deflected out of their normal orbits, again perhaps by a passing star, and come close to the Sun. The heat of the Sun melts some of the ice, creating a cloud of dust and gas around the comet. The solar wind then blows this cloud outwards, away from the Sun. As the gas streams out it reflects sunlight. This is the part of the comet which can be seen as the comets tail. The comet itself is only a few kilometers across and so is far too small to see from Earth.

As the ice melts it releases dust particles and small rocks. These are left behind as the comet orbits the Sun, marking its path. When the Earth passes through this path, the small rocks burn up in the atmosphere and form meteor streams

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