People now look forward with interest to sighting a comet, but for many centuries comets were believed to have an evil influence on human affairs; in particular, they were thought to foretell plagues, wars, and death. It was once thought that comets were made of vapor and had risen from the Earth. It was not until the 17th century that they began to be properly understood. In 1682 a comet appeared and was observed by the astronomer Edmond Halley. He studied the written accounts of 24 comets that had been seen from time to time since 1337 and calculated their orbits. He found that the comets of 1531, 1607, and 1682 moved in almost the same paths, and he concluded they were all the same one and that it would return in about 1758.

   His forecast was correct, for the comet did appear in that year, though Halley himself did not see it; he had died in 1742. For the first time scientists realized that comets can be regular visitors, and the great comet was named after Halley. It appeared in 1910, and in late 1982 it was spotted as a very faint dot, close to its expected position, on its way to swing around the sun in 1986.

   The nucleus, or core, of a comet resembles a large, dirty snowball. It is thought to consist of about 25 percent dust and chunks of rocky or metallic material and about 75 percent ice. The ice is mainly frozen water, with a mixture of compounds containing methane, ammonia, and carbon dioxide radicals, or subunits of molecules. Of the approximately 650 comets that have so far been scientifically recorded, the largest has a nucleus measuring about 42 miles (68 kilometers) in diameter and the smallest about 0.3 mile (0.5 kilometer). Smaller comets undoubtedly exist, but they do not become bright enough to be detected by light telescopes. When a comet passes close to the sun it loses some of its matter, usually about 0.3 percent of its mass per orbit. The nuclei of comets are slightly less dense than water and are covered by a thin crust of dust. They are fragile and have been observed breaking up into fragments.

   As a comet nears the sun, its ice begins to sublimate that is, to pass directly from a solid to a gas form. The gas carries with it some of the loosely bound dust particles. The gases spread out around the nucleus, forming a large, thin atmosphere called the coma. Sunlight causes the atoms in the coma to glow as a pearly, spherical patch of light up to 62,000 miles (100,000 kilometers) in diameter. If the supply of gases from the nucleus changes, a comet can brighten or fade unexpectedly, so astronomers cannot predict how bright a comet will become.

   As the comet approaches the sun the solar wind, which consists of high-speed atomic nuclei, protons, and electrons, sweeps cometary gases away from the sun, producing a straight tail of up to 93 million miles (150 million kilometers) in length. A second tail consisting of dust particles may also appear. This dust tail is shorter and more curved than the gas tail. Comet tails always point away from the sun because of the force exerted by solar wind and radiation on the cometary material . When comets travel away from the sun, therefore, their tail or tails are always in front of them. Some comets have been observed to develop as many as nine tails.

   Comets can be divided into two groups: short-period comets, with orbital periods of less than 200 years, and long-period comets, with periods on the order of millions of years. Short-period comets are members of the inner solar system. When farthest away from the sun, long-period comets can be halfway to the nearest stars. Comets can hit Earth; for example, the Tunguska explosion that occurred in 1908 in central Siberia is thought to have resulted from such a collision.

   Several theories for the origin of comets have been proposed. According to one widely accepted view, comets are as old as the solar system and are the remnants of the building blocks that produced Uranus and Neptune. They were tugged out of their nearly circular orbits into extremely elongated orbits by the gravitational fields of the outer planets, which were still forming. Hundreds of millions of comet nuclei exist in a region called the Oort Cloud that surrounds the solar system like a bubble. Such nuclei may travel out of the cloud when disturbed by the gravitational force of a nearby star and enter the inner solar system .

   On the average five new comets are discovered each year. Most of these are faint and can only be observed with the aid of a telescope. The few exceptionally bright comets seen from the Earth since the early years of the 20th century include Halley's comet, Arend-Roland comet, and Ikeya-Seki comet.

   Many amateur astronomers watch the sky closely for new comets. Newly discovered comets are first given a numerical designation made up of the year of the discovery followed by a small letter showing the order of the discovery among those of that year. For example, 1990e would be the fifth new comet spotted in the year 1990. Later, when the exact time of its reaching perihelion, or its closest approach to the sun, has been determined, the numerical designation will be changed to a date followed by a Roman numeral. If 1990e is the third comet to reach perihelion in 1991, it will become known as 1991 III. At that time, it will also be officially named with the names of up to three discoverers.

Image of a comet