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The Solar System

What is a Comet?

Comets are thought to be frozen balls
of ice, carbon dioxide, methane and ammonia.
More complex molecules may also exist.
Comets have been described as dirty snowballs of ice and dust.

As the comet approaches the Sun,
part of the surface material starts to melt or sublime
and particles stream away from the comet into space,
giving it its characteristic long tail.
The tail always points away from the Sun,
even when the comet is moving away from the Sun.
This is because the Sun emits a very fast stream of
charged particles called the "Solar Wind".
The Solar wind blows the tail away from the comet.

When the comet is far from the Sun
the tail disappears because it is too cold
for the surface material to melt or sublime.

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