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Comets

Comets are mixtures of ice and dust. They have earned the nickname of "dirty snowballs" by some astronomers. They have many parts, as you can see in the diagram:

Nucleus --Consists of mostly ice and gas, with some dust and other solids.

Coma --A dense water cloud, made up of carbon dioxide and neutral gases, that comes off of the nucleus.

Hydrogen cloud --A huge section of hydrogen.

Dust tail --The most visible part of the comet made of dust particles from the nucleus. Dust tails are very long and smoke-like.

Ion tail --Even longer than the dust tail, consists of plasma. This is the part of the tail that interacts with the solar wind.

Where do they come from?

Comets are actually collections of materials left over from the formation of the solar system. This makes them especially interesting to astronomers because the solar system formed around 4,600 million years ago. Studying the composition of comets can tell us something about the history of the solar system.

What do they have to do with the Sun?

Without the Sun, we wouldn't be able to see comets very easily, maybe at all! This is because they reflect the light from the Sun, rather than having their own source of light. This makes the sun vital to the observation of comets.

The Sun is actually what "makes" the various parts of the comet listed above. As a comet approaches the Sun, the radiation evaporates the ice. This causes gases and dust to drag from the nucleus--the coma. Radiation from the Sun blows the dust away from the head of the comet, creating the dust tail. The solar wind drags ionized gas from the comet, creating the ion tail.

Where do they go?

Comets orbit around the Sun just like planets, but their orbits are more elongated and take longer. Most of them go beyond Pluto! Periodic comets are those that have an orbit of less than 200 years, but they make up the minority of known comets.

How long do they last?

That depends on the comet's orbit and the size it started out as. A comet loses some of its ice and gases every time it goes by the Sun. After about 500 passes, most of the ice and gas is lost, and the comet turns into something close to an asteroid. So the amount of time it takes a comet to orbit the Sun determines how long the comet will last.