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 comet’ surface. These form the comet’s long, streaming tails. These changes make the comet visible from the Earth.