A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles.Comets are distinguished from asteroids by the presence of a coma or a tail.
The word comet derives from the Greek word kome, meaning “hair.” The name describes the blurry, diaphanous appearance of a comet’s long tail. most comets (called long-period comets) travel even beyond Pluto and might 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—that is, 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. This process creates 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. Both the ion and dust tails point away from the Sun, regardless of the direction of the comet’s travel. This is because a comet tail is “blown” like a wind sock by the solar wind, a constant stream of matter and radiation that escapes from the Sun.
The solar system has two cometary reservoirs. The nearer reservoir is called the Kuiper Belt,the short-period comets, those with orbital periods less than 200 years. gravitational influence sends one into an eccentric orbit that takes it outside of the belt. Long-period comets,originate in the Oort Cloud a vast area surrounding the solar system.The gravitational field of a passing star from time to time deflects a comet out of its orbit within the Oort Cloud, sending it on a path to the inner solar system.
After a short-period or long-period comet is kicked out of its Kuiper Belt or Oort Cloud home, it assumes its eccentric orbit indefinitely. It can’t go home again. Each time a comet passes close to the Sun, a bit of its mass is boiled away—about 1⁄1,000 of its mass with each pass. After some 100 passages, a comet typically fragments and continues to orbit as a collection of debris or coalesces with the Sun. As Earth passes through the orbital paths of such debris, we experience meteor showers. Whenever a comet makes its nearest approach to the Sun, some pieces break off from its nucleus. The larger fragments take up orbits near the parent comet, but some fall behind, so that the comet’s path is eventually filled with these tiny micrometeoroids. Periodically, Earth’s orbit intersects with a cluster of such micrometeoroids, resulting in a meteor shower as the fragments burn up in our upper atmosphere.
Most comets actually have two tails. The dust tail is usually broader and more diffuse than the ion tail, which is more linear. The ion tail is made up of ionized atoms—that is, atoms that are electrically charged. Both the dust tail and the ion tail point away from the Sun, but the dust tail is usually seen to have a curved shape that trails the
direction of motion of the comet.