The most concise description of comets is that they are dirty snowballs. They are a few kilometers across, and appear to mostly be composed of water, carbon dioxide, ammonia, and methane ices, with dust mixed in. We believe that this composition represents a sampling of primordial material from which the Solar System was made. Therefore, they are of considerable scientific interest for the information that they may carry concerning the early history of the Solar System.
Comets interact gravitationally with the Sun (and other objects in the Solar System). Their motion is also influenced to some degree by gases jetting out of them, so their orbits are primarily but not completely determined by gravity. Here is an image showing Hale-Bopp, a recently-discovered, long-period comet that was very bright comet in the Spring of 1997.
Most cometary orbits appear to be elliptical, or in some cases parabolic. The most common comets belong to a
population called the short-period comets that have only mildly elliptical orbits that carry them out to a region lying from
Jupiter to beyond the orbit of Neptune. A dozen or so of these comets pass through the inner Solar System each year, but
they usually are only seen in telescopes.
The Head and Coma
The center of a comet's head is called its nucleus. The nucleus is a few kilometers across and is surrounded by a diffuse, bright region called the coma that may be a million kilometers in diameter; the coma is formed from gas and dust ejected from the nucleus as it is heated by the Sun.
The coma is bright both because it reflects sunlight and because its gases are excited by sunlight and emit electromagnetic radiation.
Coma and beginning of the tail of Halley's Comet in 1986
Closeup view of Halley's nucleus as photographed by the Giotto Spacecraft
The tails of bright comets can be 150 million kilometers (1 A.U.) in length, making them the "largest" objects in the Solar System. However, the tail is composed of gas and dust emitted from the nucleus and is very diffuse. The vacuum in the tail is much better than any vacuum we can produce on Earth.
Many comets have two tails, a gas tail (also called the ion tail) composed of ions blown out of the comet away from the Sun by the solar wind, and a dust tail composed of dust particles liberated from the nucleus as the ices are vaporized. The dust particles are left behind in the comet's orbit, and blown slightly away from the Sun by the pressure of the light from the Sun. Thus, they tend to curve relative to the straight ion tail. The ion tail often shows structure associated with variations in the ejection rate from the nucleus over time.
This image of Comet West (1976) illustrates distinct ion and dust tails. As seen in this example, the ion tail and the dust tail usually have different appearances. The ion tail is typically bluer in color, narrow, and straight; the dust tail is more diffuse, often looks curved, and is more white in color. These differences in appearance are directly correlated with the different sources and compositions of the two tails.
More information on Comets and other solar system small bodies may be found by clicking here.