Asteroids, also called "minor planets," were most likely created when our solar system was formed, approximately 4.6 billion years ago. There is another theory that the asteroids are fragments left by the disintegration of a planet that once existed between Mars and Jupiter. However, it is more likely that they are debris left over from the time of the Solar System's formation. The name "asteroid" means "little star." Asteroids are believed to be relatively small solid objects; some icy, some stony, some metallic.
Most asteroids reside in the region between the orbit of Mars and the orbit of Jupiter. This region is the "asteroid belt." Some asteroids have odd orbits that bring them near the Earth's orbit; two even go inside the orbit of Mercury.
It is unknown how many asteroids there are, but it is thought that they number many thousands. About 10,000 are known at present. Large asteroids now rarely come within a million kilometres of each other, but collisions between asteroids must have occurred in the past, to produce the scattered fragments known as meteoroids.
The largest of the known asteroids is Ceres which has a diameter of 1,003 km; the smallest ones observed to date may be only 300 meters or so across. Not all asteroids are round; many of them have very irregular shapes. Each new asteroid is given a number until its orbit becomes well-known; then the discoverer is allowed to name it. "1 Ceres" was discovered in 1801 by the Sicilian astronomer Piazzi, who named it after the Roman goddess of the harvest.
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.
As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few Astronomical Units (1 a.u. equals approximately 150 million kilometers) of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.
When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 a.u. from the Sun, fluorescence usually becomes more intense than reflected light.
Reproduced from NASA JPL