asteroid
asteroid Small body in an independent orbit around the Sun. The majority move between the orbits of Mars and Jupiter, in the main asteroid belt. The largest asteroid (and the first to be discovered) was Ceres, with a diameter of 913km (567mi). There are thought to be a million asteroids with a diameter greater than 1km (0.6mi); below this, they decrease in size to dust particles. Some very small objects find their way to Earth as meteorites. Today, nearly 6000 asteroids have been catalogued and have had their orbits calculated. This figure is increasing by several hundred a year. At least 10,000 more have been observed, but not often enough for an orbit to be calculated. Some of the larger asteroids are spherical, but most are irregularly shaped, and a wide variety of compositional types have been identified. Asteroids almost certainly originate from the time of the formation of the Solar System and are not remnants of a large planet that disintegrated, as was once thought.
http://lpl.arizona.edu/nineplanets/nineplanets/asteroids.html; http://jpl.nasa.gov
http://lpl.arizona.edu/nineplanets/nineplanets/asteroids.html; http://jpl.nasa.gov
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asteroid