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| Asteroids, Meteoroids and Comets |
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| Asteroids are very small planets of rock and metal, which revolve round the sun mainly between the orbits of Mars and Jupiter. This area is known as the asteroid belt. Most of the asteroids have a diameter of only 1 kilometer. But some of them have diameters of more than 100 kilometers. |
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| Asteroids |
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| Some have elliptical orbits that pass inside the orbit of earth or even that of mercury. |
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| There are different theories regarding the asteroids. According to some astronomers, they are the remnants of former comets. Earlier they were thought to be the fragments of a shattered planet. Most astronomers however now believe that they are lumps of material left over from the solar system's early history. |
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| Comets are like dirty snowballs from space. They are made up of rocks, ice and dust. They are basically aggregates of dust and frozen gases. The head of a comet is only a few kilometers wide, but its tail can be millions of kilometers long. |
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| Comet |
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| Comets orbit the sun in long, looping
orbits. Most of the time we cannot see them. But when comets come close to
the sun, its heat turns their ice into gas. The gas and dust spread out into
space in the form of a glowing tail that always points away from the sun. |
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| Many comets orbit the sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system.
The most famous is the Halley's comet, which returns to the inner solar
system once in 76-years. Its most recent return was in 1986. In July 1994 fragments of comet Shoemaker-Levy 9 bombarded Jupiter's dense atmosphere. |
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| Meteors are popularly known as "falling stars" or "shooting stars".
They are often seen as a streak of light flashing across the night sky. They are really not stars. A meteor is caused by a grain of dust from space. As it enters the blanket of gases that surround the earth, the dust starts to burn up, leaving a glowing trail behind it. This is because, when a meteor enters into the atmosphere of earth with high speed, a lot of heat is produced due to the resistance of air, which burns the meteor. |
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| Meteors |
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| A meteor, which does not burn completely on entering the earth's atmosphere and lands on earth, is known as a meteorite. Meteorites are stone-like bodies some of which are composed of rock; others of iron and the rest are a mixture of both. Many meteorites hit the earth each year but as three-quarters of the surface of the earth is covered with water, most of them fall in the oceans. They may even fall in remote areas where they are not discovered. Meteorites give us valuable information and evidence about the nature of the solar system. |
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| When a huge meteorite falls on the earth,
it forms a big crater on the surface. The most famous of these kinds of
craters is the one in Arizona in the US. The number of meteorites striking
the moon's surface is quite large, as the moon has no atmosphere to burn the
falling meteors. |
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