Comets travel in elliptical
(oval-shaped) orbits around the sun. Comets are very fast. They move
at speeds of 40 to 60 kilometers per second, which is about 230,000 to
300,000 miles per hour! Comets appear to move slowly in the sky only
because they are so far away from the Earth.
When a comet is far away from the
Sun, it is very cold. The comet consists of a solid lump of dust and
frozen gas. This lump is the nucleus of the comet. It may be up to 10
kilometers in width. The comet changes as it swings in closer to the
Sun. It becomes active. Energy from the Sun heats the comet. This
heating causes some of its gasses to vaporize, or change from a solid
to gaseous state. This creates a glowing head, or coma, around the
nucleus of a comet. The head may be over 100 kilometers wide. Heating
also causes jets of gas to erupt on the cometí surface. These form
the cometís long, streaming tails. These changes make the comet
visible from the Earth.