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Asteroids: Earth Asteroids
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Asteroids are thought to be the remnants of a stillborn planet. According to this idea, the newborn Jupiter gravitationally scattered nearby large planetesimals – accreting lumps of matter in the embryonic stage of planet-formation – some of which may have been as massive as Earth is today. Some of these big planetesimals strongly perturbed the orbits of the planetesimals in the region of the asteroid belt, raising their mutual velocities to the average 5 km/s seen today. As a result, what had been mild accretionary collisions in the future belt region became catastrophic disruptions. Only objects larger than about 500 km in diameter could have survived 5 km/s collisions with objects of comparable size. Ever since, the asteroids have been collisionally evolving so that, with the exception of the largest, most present-day asteroids are either remnants or fragments of past impacts.
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Asteroids are minor planets, most of which orbit the Sun between Mars and Jupiter. Their orbits are typically more eccentric than those of the planets. The largest asteroid, Ceres, is 940 kilometers in diameter and has about 1/10000 the mass of the Earth. With over 7000 asteroids catalogued, the total number may exceed 100,000. However, their combined mass is less than 1/10 that of the Earth's Moon.
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Asteroids have a confusing system of nomenclature, especially when they are on Earth. While still in orbit, they are asteroids. Once they enter the atmosphere, they are called meteors, and once they land, they are termed meteorites.
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Asteroids are small, have very little gravity and are hard to see in space because their rocky surfaces don't reflect a lot of light. Nevertheless, NASA's Near Earth Asteroid Rendezvous spacecraft achieved a tricky landing on asteroid Eros in February 2001.
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Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most... are contained within a main belt that exists between the orbits of Mars and Jupiter.
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Asteroids range in size from dust particles to many miles across. Most current theories hold that asteroids are bits and pieces left over from the formation of the solar system. They are ... formed from other asteroids as they collide and break apart, as comets disintegrate, or even when the outer moons of the larger planets collide. Past theories have suggested that the asteroids are remnants of a planet that was destroyed early in the solar system's history. However, that theory is no longer held in much regard, for if all of the asteroids in the belt were combined, they would form a body less than 1500 km (932 miles) in diameter -- less than half the size of Earth's moon, and so there is not enough material to make a planet.
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