asteroids are sometimes called minor planets. they are lumps of rock orbiting the sun, mostly in the asteroid belt between the orbit of mars and jupiter.
The first and largest asteroid, Ceres, is 936km in diameter and was found on new year's day 1801. Since then thousands have been found. twelve of them are more than 250km wide and 26 are larger thank 200km in diameter. As telescope have improved, more and more small asteroids have been detected. There are probably about 100,000 asteroids larger than 1km in diameter. Some experts think there may be as many as 1.2 million.
Vesta, the fourth asteroid t o be discovered (in 1807), is the only one bright enough to be seen without telescope.
*Astronomers believe that, on average, one asteroid larger than 0.4km strikes earth every 50,000years. Some 65 million years ago a 10 km diameter asteroid crashing to earth may have been responsible for wiping out the dinosaurs. it would have caused a catastrophic explosion. affecting the climate and chemical composition of the atmosphere and destroying the plants and animals on which the dinosaurs fed. As recently as 1991 a small asteroid came within 170,600 km of earth, the closest recorded near miss. on 30 jan 2052 an asteroid is predicted to pass as close 119,678.
*Toutatis (asteroid 4,179) was discovered in 1989. It is name after the celtic god toutatis, whose name is used as an oath by the comic strip character asterix the gaul. toutatis measures 4,6 by 2.4 by 1.9km. it pass earth every four years and is one of the largest space objects to come so close to us. on 29 september 2004 toutatis came within 1.555.818 of earth. Its next visit will be on 9 november 2008, when it will come within 7,524,773km.
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