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Comets are small bodies made out of dust and ices ("dirty snowballs"). The term "comet" derives from the Greek aster kometes, which means "long-haired star"---a reference to the tail. Once thought to be phenomena in our atmosphere, we have known since the observations of Tycho Brahe that they are parts of the Solar System well beyond Earth's atmosphere. Most are on long elliptical orbits (perhaps parabolic in some cases) that take them from the outer reaches of the Solar System to the vicinity of the Sun. If they come near the Sun they are heated and emit gases and dust that are swept by the Solar Wind into the characteristic tail that consequently always points away from the Sun. Historically, comets have been the subject of fear and foreboding because it was once thought that their appearance foretold momentous events. Although a dozen or so comets pass through the inner Solar System each year, comets easily visible to the naked eye occur only every decade of so, on the average. Comets are commonly named after their discoverers, so they are a popular target of amateur and professional observers. The characteristics that distinguish comets observationally are that they move (slowly) with respect to the background stars from night to night, and they have a fuzzy appearance, especially as they near the Sun.
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. Cometary OrbitsComets 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 7 such jets from 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 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 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. The adjacent top-left image shows the coma and beginning of the tail of Halley's Comet in 1986 , 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.
The Tail
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. 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.
Edmund Halley and His Comet |
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A meteor, sometimes called a "shooting star," can be the brightest object in the night sky, yet meteoroids are the smallest bodies in the solar system that can be observed by eye. Wandering through space, perhaps as debris left behind by a comet, meteoroids enter the earth's atmosphere, are heated by friction, and for a few seconds streak across the sky as a meteor with a glowing trail. [an error occurred while processing this directive]
COMETS
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The English astronomer Edmund Halley was a good friend of Isaac Newton. In 1705 he used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits, and when he accounted for the gravitational perturbation on the cometary orbits from Jupiter and Saturn, he concluded that these were different appearances of the same comet. He then used his gravitational calculations to predict the return of this comet in 1758. Halley did not live to see his prediction tested because he died in 1742. But on Christmas night, 1758, the comet destined everafter to bear Halley's name reappeared in a spectacular vindication of his bold conjecture and of Newton's gravitational theory. Tracing back in the historical records for recordings of bright comets and their positions in the sky, it was concluded that Halley had been observed periodically as far back as 240 B.C. The most recent return was in 1986, and the predicted next appearance of Halley in the inner Solar System will be in 2061. 
