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What are comets made of?
Submitted by Saavik Ford on Mon, 7/13/2009, 2:00 PM
Comets are made mostly of ice (water ice, as well as carbon dioxide, or "dry" ice, and several other types), and rock. The rock is in the form of small dust or sand grains. In addition, comets possess small amounts of carbon-based or organic compounds. You can think of these as similar to soot. All of this material is contained in a solid object, called the comet nucleus, that is typically only a few miles across (though occasionally they can be much smaller or larger). Comets are often referred to as "dirty snowballs" because of their contents.
Such a small, cold object would be invisible to astronomers, except for one thing: when a comet nucleus comes close enough to the Sun, the ices inside it warm up and vaporize. The vapor, along with dust grains that were embedded in the ice, tends to trail behind the comet on its orbit around the Sun. This leaves a long, bright tail, which we recognize as a comet. It takes only a tiny amount of material from the comet nucleus to leave a very bright and visible tail. Some comets, like Comet Halley, have made many many visible, recorded trips around the Sun without running out of ice!
But comets do occasionally disintegrate under the stress of heating and cooling, or due to other forces (like the tidal force of a nearby planet - see the famous fragmentation of Shoemaker-Levy 9). This usually results in more vaporization and a really spectacular tail.
If you'd like to learn more about the basics of comets, I recommend starting with the Cometography website. Their main page also has information on observing comets yourself. Or build your own at home!
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