Anatomy of a Comet OK, so comets aren't tools of the devil. But what are they? Comets are fuzzy, luminous objects that orbit the sun. Far more comets travel through Earth's neighborhood than most people realize (one little speedster, Encke's Comet, orbits the sun in just 3.3 years). So why do we hear so little about comets? Usually, only easily seen comets, like the new-found Hyakutake, or famous comets, like Halley (rhymes with alley), get all the ink. They can be visible for months on end and their trademark tails can stretch across vast portions of the sky -- if we're lucky enough to have the right conditions. Astronomers think comets were created about 4.6 billion years ago -- at the time our solar system formed, along with the planets, moons and random chunks of rock we call meteors (defined). The most popular astronomical theory (defined) to explain comets was advanced in 1950 by Fred L. Whipple of the Harvard-Smithsonian Center for Astrophysics. This so-called "dirty snowball" theory asserts that the comet nucleus -- (it's usually tens of kilometers across -- or no bigger than Manhattan), is composed mostly of various kinds of ices -- frozen (pdb) water (pdb), carbon dioxide (pdb), ammonia (pdb) and methane (pdb) [ viewing pdb with RasMol, we recommend that you Display these as "Ball & Stick" ] -- with a little dust thrown in for good measure. Want to concoct a comet in your classroom or kitchen? While the comet nucleus is so small that it's hard to see from Earth, the coma (defined), the other part of the comet head, is humungous (defined). This halo, composed of gas and dust, can be more than a million kilometers across, and shines because it reflects sunlight and fluorescence (defined).

Bringing Up the Rear...in Style Then there's the famous comet's tail. This, the most distinctive feature of these speeding cosmic bullets, gave rise to the very name ("comet" comes from the Greek "kometes," meaning "long-haired"). Comets often have two kinds of tails: The dust tail is composed of dust blown from the comet as it nears the sun. This dust "shines "by reflecting sunlight. The gas or plasma tail, is composed of ions (defined). The gas tail is straighter and narrower than the billowing dust tail. Forced away by the solar wind (the energetic stream of particles moving away from the sun), it is virtually straight and shines, like the gas in the coma, by fluorescence. Although comet tails contain just a little matter, they can stretch as far as 150 million kilometers, making comets the largest inhabitants of our solar system. So where do you keep something that big?

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