A comet is a huge object in space that is made of ice and dust. It moves around the sun in a long, oval path. The path of a comet around the sun is shown with a comet a many different points along the path with the tail of the comet always pointing away from the sun The head of a comet is like a dirty snowball. A comet also has one or two tails that appear when it gets near the sun. The sun's heat turns some of the ice into gas. The gas and bits of rock stuck in the ice form the comet's tail. Some comet tails are up to 100 million miles (160 million kilometers) long. Image to right: Comet tails always point away from the sun. Credit: World Book illustration by Rob Wood

You need a telescope to see most comets. Some comets can be seen without a telescope, but only when the comet comes very close to the sun. We can see it because the sunlight makes the dust in the comet shine. Halley's Comet, the most famous comet, passes by Earth about every 76 years. In 1986, the last time it was near Earth, five space rockets flew past the comet. They collected lots of information to help scientists learn more about comets. How to cite this article: To cite this article, World Book recommends the following format: "Comet." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.