As we have noted in the introduction, 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.
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 comets that are more likely to be easily visible are much rarer, and are thought to come from a great spherical cloud of cometary material surrounding the Solar System called the Oort Cloud. This sphere is a light year (50,000 A. U.) in radius, so it is enormous, but the total mass of cometary material in this cloud is probably less than that of the Earth. Occasionally a comet in this cloud is disturbed gravitationally, for example by a passing star, and started on a long elliptical or parabolic orbit toward the Sun. These long-period comets are primarily responsible for the brighter comets observed historically. The orbits of both the long-period and short-period comets may be strongly influenced if they pass near the Jovian planets, particularly Jupiter itself.
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. The adjacent top-left image shows the coma and beginning of the tail of Halley's Comet in 1986 (Ref), while the image on the lower left shows a closeup view of Halley's nucleus as photographed by the Giotto Spacecraft. The remaining image shows an artist's conception of Halley's nucleus based on the Giotto data.
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. The adjacent image of Comet West (1976) illustrates distinct ion and dust tails (Ref). 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.