A comet is an object that resembles a fuzzy star; most travel through the solar system on an elliptical (oval or egg shaped) orbit.
The time it takes a comet to make a complete orbit is called its period (Some have short periods of less than seven years, while others travel in such huge orbits that they pass near the sun only once in thousands or even millions of years).
Some of the brightest comets develop a long, shining tail when they come near the sun (the tail of a comet always points away from the sun, however. This tail may stream across space as far as 100 million miles. But when the comet moves away from the sun, its tail leads).
A comet has two distinct parts, the nucleus, (this is its center, that measures less than 10 miles in diameter) and a hazy cloud called a coma that surrounds the nucleus (its diameter may be as large as 1 million miles). The coma and nucleus make up the comet’s head. (As a comet approaches the sun, the evaporation of the nucleus releases dust and gases, which form the coma. And all of a comet’s light comes from the sun.)
All comets seen by astronomers are considered part of the solar system, for none to date seems to have approached the sun from beyond the solar system yet.
Most comets cannot be seen without a telescope. Some are visible to the naked eye, but only when they’re passing closest to the sun for several weeks or months (Halley’s Comet is the best known; it appears every 77 years, but in 1973, Comet Kohoutek became the first comet to be studied by men in space).
Scientists once thought that the comet’s nucleus consisted of tiny solid particles held together by gravity. Today, most astronomers believe that the nucleus (probably consists of frozen gases and frozen water mixed with dust particles. The gases include ammonia, carbon dioxide, carbon monoxide and methane. The gases and water make up from 70 to 80 percent of a comet’s mass. The remaining 20 to 30 percent may resemble meteoric particles-fine grains and larger chunks of metals and rocky material.) is like a dirty snowball that was formed in the cold, outer regions of the solar system.
For centuries, many people believed that the coming of a comet foretold disaster, such as a war or plague (an actual physical collision between the earth and a comet would be very destructive, to say the least).
An asteroid, on the other hand, is an extremely small planet that revolves around the sun between the orbits of Mars and Jupiter. There are thousands of known asteroids and new ones are constantly being discovered.
Asteroids vary greatly in size and most have an irregular shape. Many astronomers estimate the mass to be 1/1000 that of the earth (total mass of all asteroids can’t be determined because many are still undiscovered).
A few small asteroids move in oval-shaped orbits (like a comet) at certain times that bring them near the earth.
The orbits of the asteroids change because of the attraction of Jupiter and other planets. These changes can lead to collisions over a long period of time; the irregularly-shaped fragments that result greatly increase the chance of more collisions. Some of these small fragments reach the earth’s surface as meteorites.
The smaller asteroids were most likely formed in just this way. But the larger ones, such as Ceres (the largest and first asteroid to be discovered – first seen on January 1st, 1801. It’s about 480 miles in diameter) and a few others were probably formed separately at the same time, and in much the same way, as the major planets.
Source- 1. World Book Encyclopedia, World Book-Childcraft International, Inc. 1990.
