A comet is a small body of rock, ice, and gas orbiting the sun. A comet's orbit is usually much more elongated than that of a planet. The time required for an orbit varies from a few years to a few hundred thousand years, depending on the comet. Before the scientific nature of comets was discovered, they were believed to be an evil influence on the affairs of people. Comets may be the leftover bits of matter from the formation of the planets. Scientists believe that comets begin as part of the Oort Cloud, a region of matter that surrounds the solar system, far beyond the planets. At times the gravitational force of a nearby star may pull a comet out of the Oort Cloud. It may then enter the solar system as a new comet. Almost all such comets decay and disappear completely in less than 1 million years. But new comets are forming all the time. Parts of a cometThe solid center of a comet is called the nucleus. A typical nucleus is about 25 percent dust and 75 percent frozen water and other compounds. The largest known nucleus of a comet measures about 42 miles (68 kilometers) across. The nucleus of the well-known Halley's Comet is irregular in shape, about 9 by 5 miles (15 by 8 kilometers) in size, and very black due to the presence of carbon and other sooty materials. When the nucleus approaches the sun, it begins to shed gas and dust. This creates a coma, or a ball of matter surrounding the nucleus. The coma gives the head of a comet its blurry appearance. The coma of a comet is sometimes larger than the Earth. Sunlight reflecting off the coma causes it to glow, making the comet more easily seen from the Earth. Some of the gases are pushed away from the coma by the solar wind, the stream of particles coming from the sun. These particles create a tail that usually appears on the side of the comet away from the sun. The tail of a comet can be very long—as long as the distance from the Earth to the sun. Comets and the EarthComets that struck the Earth while the planet was forming helped shape the planet. They probably provided some of the water and chemicals needed to form an atmosphere and oceans. When the Earth passes through the orbit of a comet, dust and particles left behind by the comet enter the Earth's atmosphere. The friction with the atmosphere causes these particles to burn up, creating a meteor shower. Sometimes comets collide with the Earth. Scientists believe that a comet may have disintegrated in the atmosphere over Siberia, in eastern Russia, on June 30, 1908, producing a huge explosion and fireball. The blast, known as the Tunguska event, flattened 770 square miles (2,000 square kilometers) of forest but left no crater. Halley's CometIn the 1700s an English astronomer named Edmond Halley noted that bright comets had appeared in the sky about 76 years apart for centuries. He suspected that these events were all caused by the same comet as it came back again and again. He predicted that the next appearance would be in 1758, and he was right. Halley showed that comets were in orbits rather than just objects passing through the solar system. Halley's Comet (called Comet P/Halley by astronomers) is the best-known comet for two reasons. It is bright enough to be seen without special equipment, and it appears often enough that many people can see it at least once in their lives. |