Comets

Comets are small planet-like bodies that orbit the Sun much as the regular planets do. They are much smaller than planets, though, and they have very long, elliptical orbits that take them very close to the Sun and then out to the frozen outer reaches of the solar system. Most of their lives are spent far beyond the planets, where they cannot be seen from Earth.

Comets are sometimes called “dirty snowballs,” because they consist of a mixture of rock and frozen gases. When a comet approaches the inner solar system, the Sun’s heat begins to thaw the comet’s volatile gases. They erupt off the surface of the comet, carrying particles of dust with them. The dust and gas form a cloud, called the coma, around the nucleus of the comet. As the comet gets closer to the Sun, this material streams off in a long tail. A comet’s tail always points away from the sun.

Some well-known comets and their orbital periods:

Encke: 3.3 years
Kohoutek: 6.23 years
Halley: 77 years
Swift-Tuttle: 130 years
West: 500,000 years

At one time scientists were concerned that Comet Swift-Tuttle might be on a collision course with the Earth. However, refined calculations show that the comet will not be a danger to our planet for several hundred thousand years.

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