Asteroids, Comets and Meteors
As the solar system formed, not all the material in the planetary disk was taken up into bodies and moons. Even after hundreds of millions of years of accumulation and bombardment, a lot of debris comes in two main forms--asteroids and comet--which, in a sense, mimic the compositional differences of the inner and outer planets.
Asteroids: are small rocky bodies in orbit around the Sun, like miniature version of the inner terrestrial planets. Most asteroids are found in a broad, circular asteroid belt between Mars and Jupiter-- material though to be collection of planetsimals that never manage to collect into stable planet. The most likely explanation is that the nearby planet Jupiter had a disrupting gravitational effect. In addition, many asteroids posses orbits that cross the Earth's orbit, and they produce occasional large impacts on our planet.
Comets: can be best be though of as “dirty snowballs.” Unlike asteroids, they can consists of chunks, sometimes many miles in diameter, of material such as water ice and methane ice in which a certain amount of solid, rocky material or dirt is embedded. Also, unlike asteroids, most of the comets in the solar system circle the Sun outside the orbit of Pluto. Two main reservoirs of comets are found in the solar system. One of these is a large spherical array called the Oort cloud (named after the Dutch astronomer Jan Oort 1900-1992 who first postulated its existence), located far from the solar system. The other is a disk of comets closer in to the solar system called the Kuiper belt (after Dutch astronomer Gerard Kuiper, who first suggested its existence).
Occasionally, when the orbit of a comet is disturbed, the comet is deflected so that it falls toward the Sun. When it happens, the increasing temperature of the inner solar system begins to boil off materials, and we see a large “tail,” blown away from the Sun by the solar wind, that reflects light to us.
Sometimes a comet is captured and fall into a regular orbit around the Sun. The most famous of these periodic comets is Halley's Comet, which returns to the vicinity of Earth about every 76 years. Halley's return in 1910 was quite spectacular, because the comet passed near the Earth when it was at its highest temperature and therefore had its largest and most spectacular tail. The return in 1986 was much less spectacular because the comet was on the far side of the Sun when it was as its brightest. The next predicted return in 2061, unfortunately will probably be just as unspectacular.
Astronomers have actually detected many object in the inner Kuiper belt with the Hubble Space Telescope. These objects are several hundred miles across. Current thinking is that over the course of time, collisions in the Kuiper belt have gradually removed most of the original comets from it, leaving these objects as the last occupants. In fact, some astronomers have suggested that Pluto, far from being a true planet, is simply a survivor of the process by which the Kuiper belt was emptied early in the life of the solar system.
Comets and Life on Earth: In addition to providing us with one of the first historical tests of the law of universal gravitation, comets may have had an important effect on the evolution of life on Earth. Many scientists suspect that the impacts of comets or asteroids may have drastically altered the Earth's climate and produced mass extinction's, or killings, at various times in the Earth's history. Most scientists, for example, now believe that the dinosaurs and other forms that thrived 65 million years ago were driven into extinction following the impact of a large comet or asteroid that hit he Earth at a site near Mexico's Yucatan Peninsula.
Meteoroids, Meteors, and Meteorites: are small pieces of ancient space debris in orbit around the Sun. Occasionally one of these bits, perhaps the size of a sand grain, will fail into the Earth's atmosphere where it becomes briefly visible as a meteor. Most meteors burn up completely to microscopic particles of ash that slowly, imperceptibly, rain down on Earth. The meteors' bright streaks of light record the path of this burning. Occasionally, if the object is big enough so that the outer surface burns, a piece of rock may actually reach the Earth's surface. Any such rock that fell to Earth from space is a meteorite
Meteor showers are spectacular, regularly occurring events in the night sky. During shower, every minute or so you can see brilliant streaks in the sky, each one caused by the collision of the Earth with swarms of small debris that travel around the orbits of comets. Some of these swarms may be comets that were broken up by gravitational pull of one of the planets.
Meteorites are extremely important in the study of the solar system because they represent the material from which the system was originally made. They are analyzed intensely by scientists, both to get a notion of how and when the Earth was made, and to learn what kinds of materials human beings will find when they leave the Earth to explore the rest of the solar system.
