Asteroids are some of the many small or minor planets that are members of the solar system.Their movement are in elliptical orbits mostly between the orbits of Mars and Jupiter.The asteroid(IDA) above is a picture from JPL(Jet Propulsion Laboratory.It was taken on August 28,1993 fron the Galileo spacecraft. About 200 asteroids have diameters of about 60 miles or more.Then there are the big ones,Ceres with a diameter of about 640 miles,Pallas and Vesta with diameters of about 340 miles.The total mass of all the asteroids in the solar system is less than the mass of the Moon.The larger asteroids are roughly spherical,but elongated and irregular shapes are common for those with diameters of less than 100 miles.Most asteroids rotate on their axes every 4 to 20 hours.Some asteroids have their own satellites.
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May 8, 2000 -- NASA astronomers have collected the first-ever radar images of a "main belt" asteroid, a metallic, dog bone-shaped rock the size of New Jersey, an apparent leftover from an ancient, violent cosmic collision.
Above: These images show several views from a radar-based computer model of asteroid 216 Kleopatra. The asteroid, named 216 Kleopatra, is a large object in the main asteroid belt between Mars and Jupiter; it measures about 217 kilometers (135 miles) long and about 94 kilometers (58 miles) wide. Kleopatra was discovered in 1880, but until now, its shape was unknown.
With its dog bone shape, Kleopatra is one of the most unusual asteroids we've seen in the Solar System," said Dr. Steven Ostro of NASA's Jet Propulsion Laboratory, Pasadena, CA, who led a team of astronomers observing Kleopatra with the 1,000-foot (305- meter) telescope of the Arecibo Observatory in Puerto Rico. "Kleopatra could be the remnant of an incredibly violent collision between two asteroids that did not completely shatter and disperse all the fragments." The astronomers used the telescope to bounce radar signals off Kleopatra. With sophisticated computer-analysis techniques, they decoded the echoes, transformed them into images, and assembled a computer model of the asteroid's shape. The Arecibo telescope underwent major upgrades in the 1990s, which dramatically improved its sensitivity and made it feasible to image more distant objects. These new radar images were obtained when Kleopatra was about 106 million miles (171 million kilometers) from Earth. Travelling at the speed of light, the transmitted signal took about 19 minutes to make the round trip to Kleopatra and back. "Getting images of Kleopatra from Arecibo was like using a Los Angeles telescope the size of the human eye's lens to image a car in New York," Ostro said.