Comets were traditionally defined as solar system bodies that have tails and travel near the Earth. Ideas about comets are changing rapidly as the understanding of the solar system expands, and it is now thought that some comets remain in the outer solar system and never approach the Earth and that some rocky, asteroidal bodies sometimes have icy tails as some of their more volatile material jets out into space. Recent space missions to comets are also changing ideas about what comets are made of.

A decade or so ago, comets were thought to be the exception in the solar system—lonely travelers that flew in immense looping orbits from the outer solar system through to the inner, passing close enough to the Earth to be seen perhaps only once in a lifetime because their orbits took so long to traverse. Improved observational techniques now show that at the orbital distance of Pluto and beyond exist thousands of icy bodies, each of which would form a tail and appear as a comet if its orbit were perturbed into the inner solar system. Comets are in fact the most abundant type of body in the solar system.

The heads of most comets seem to have three parts: the nucleus, the coma (from which the tail extends), and a diffuse hydrogen cloud. The comet's nucleus may be much smaller than its tail, by 80 times or more, but cometary nuclei can still be immense. At least two comets are as large as Pluto! The ices that jet out of the head of the comet consist mainly of fragments of methane (CH4), water (H2O), ammonia (NH3), HCN, and CH CN. Cometary tails seem to form when heat from the Sun cracks their crust and lets out volatiles like C, N, O, H. The nucleus is also thought to contain silicates, oxides, and sulfides, all rock-forming materials.

When all that could be seen of comets was their long tail, they were thought to consist mainly of ice, with some dust, and so were described as dirty iceballs. When it became clear that volatiles jetted out of cracks in the material, it was thought that they were more dust and rock and less ice, and one scientist said they were icy dirtballs, not dirty iceballs. Results from three space missions to comets indicate that cometary nuclei may be much stronger and rockier than previously thought. This further blurs the distinction between asteroids and comets: There clearly is a continuum of compositions among solar system bodies.

The coma appears when the comet is within 3 or 4 AU of the Sun because it is only within that distance of the Sun that there is enough energy from the Sun to cause the ices of the comet to sublimate (move directly from an ice state to a gas state). As shown in the lower color insert on page C-7, cometary tails can be up to 1 AU in length (about 150,000,000 kilometers, the distance from the Sun to the Earth), and there are three distinct types of tails on each comet. One is a blue plasma tail from ionized CO, and the next is a dust tail, yellow from reflected sunlight. Dust tails are usually sweeping arcs, while plasma tails are straight but nonuniform, from the patterns in the solar wind that cause them. The third is a sodium tail, found recently on comet Hale-Bopp.

Many comets have been observed over at least several return trips to the inner solar system. Some of the most famous are listed in the accompanying table. Comets are divided into short period, those with orbits taking 200 years or less, and long period, which may have periods of tens of thousands of years or even more. Only in the last 50 years have these two categories come to be understood in terms of their source populations: Comets have given us the clues necessary to predict and then find the Kuiper belt surrounding Pluto, and the far more distant Oort cloud, which reaches a great fraction of the distance to the nearest stars.

This image of comet Hale-Bopp (see image on page 126) was taken in 1997 from an island off Florida. Hale-Bopp was discovered on July 23, 1995, independently by two amateur astronomers, Alan Hale in New Mexico and Thomas Bopp in Arizona. Hale-Bopp was unusually bright and was sighted while still outside Jupiter's orbit, at 7.15 AU.

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