Comets
What are comets?
Comets are lumps of rock, dust and ice, similar to 'dirty
snowballs' that spend most of their lives in the depths of the Solar System.
According to myth they are called "long-haired stars" because they appeared
mysteriously and dramatically in the sky as a sign that evil events are
about to occur. Most of these comets measure a few dozen kilometres across.
They are propelled along a very long path towards the Sun. They stay frozen
and invisible most of the time but begin to shine while they travel towards
the Sun. After it loops around the Sun, it begins its journey back to the
depths of the Solar System. Then it appears smaller and dimmer and loses
its tail as it cool down and freezes. Eventually it disappears from view.
Some comets appear at regular and more frequent intervals, these regular
visitors are called periodic comets.
Why do comets shine?
Comets have set orbits around the Sun. Some pass near
the Sun every few years while others have long orbits and pass close to
the Sun only once. During their journey towards the Sun, solar winds often
'blow' away the dust to give the comet a tail. When they draw closer to
the Sun, heat starts to melt the ice and turn it into gas. Pressure of
the Sun's radiation and the stream of particles it sends out also affects
it. When gas and dust are released and form a long stream of gas, ice and
dust which reflects sunlight thus comets become visible. These comets grow
in size and brightness as more gas and dust are released from its core
or nucleus.
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