Dirty Snowballs in Space

The current theory on the composition of comets came from Fred Whipple in 1950. It is called the dirty snowball theory. At the center of the comet's head is the nucleus, typically several kilometers in diameter. This nucleus is composed of ices of water, ammonia, carbon dioxide and methane with dust mixed in. The dust particles are smaller than the particles in cigarette smoke.

Surrounding the nucleus is the coma which may be over a million km across. The coma is quite bright, both because of reflected sunlight and because some of the freed gasses become ionized and radiate.

Many comets have two tails, a gas tail and a dust tail and they may extend 150 million km. Both always point away from the Sun. The dust tail is caused by dust particles released from the gasses being vaporized. The dust tail usually curves slightly behind the comet. The gas, or ion tail is composed of ionized gasses excited by the ultraviolet radiation from the Sun. This tail is a wispy bluish color and is blown straight back. The gas tail is brighter than the dust tail.

• The Best Way to Watch a Meteor Shower
• Where to Look for the Leonids
• Star Charts
• Leo
• Photographing Meteor Showers
• Other Meteor Showers
• What are Meteors and Meteorites
• Origin of Meteorites
• Thunderstones and Shooting Stars: Meteors and Meteorites in Folklore
• Asteroids and Comets: Proud Parents of Meteors
• Target Earth
• Links to other good sites about the Leonids
• More images

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