Anatomy of a Comet

The word comet derives from the Greek word kome, meaning “hair.” The name describes the blurry, diaphanous appearance of a comet’s long tail. But the tail is only part of the anatomy of a comet, and it is not even a permanent part, forming only as the comet nears the sun. For most of the comet’s orbit, only its main, solid body—its nucleus—exists. It is a relatively small (a few miles in diameter) mass of irregular shape made up of ice and something like soot, consisting of the same hydrocarbons and silicates that we find in asteroids.
The orbit of the typical comet is extremely eccentric (elongated), so that most comets (called longperiod comets) travel even beyond Pluto and may take millions of years to complete a single orbit.
So-called “short-period” comets don’t venture beyond Pluto and, therefore, have much shorter orbital periods.
As a comet approaches the sun, the dust on its surface becomes hotter, and the ice below the crusty surface of the nucleus sublimates—immediately changes to a gas without first becoming liquid. The gas leaves the comet, carrying with it some of the dust. The gas molecules absorb solar radiation, then reradiate it at another wavelength while the dust acts to scatter the sunlight. The effect of this is the creation of a coma, a spherical envelope of gas and dust (perhaps 60,000 miles across) surrounding the nucleus and a long tail consisting of gases and more dust particles.