Asteroids are small Solar System bodies that are not comets.Asteroids are One of thousands of small, rocky members of the solar system that orbit the Sun. The largest asteroids are sometimes called minor planets. The majority of known Asteroids are in the asteroid belt between Mars and Jupiter.Most asteroids are rather small; it is estimated that there are 1 million with diameters greater than 1 km. Some, perhaps 250, have diameters of at least 100 km, while about 30 have diameters of more than 200 km. All of these planets and asteroids are the debris from the formation of the Sun that coalesced slowly through the mutual attraction of gravity.The solar system contains a great many comets and billions of smaller, rock-size meteoroids.Asteroids are composed of stony as well as metallic—mostly iron—materials and are basically tiny planets without atmospheres.It is believed that a few asteroids of more than a half-mile diameter might collide with Earth in the course of a million years. Such impacts would be cataclysmic, each the equivalent of the detonation of several hydrogen bombs. Not only would a great crater, some 8 miles across, be formed, but an Earth-enveloping dust cloud would also darken the skies. Some think the great extinction of the dinosaurs 65 million years ago was due to such an impact. Were the impact to occur in the ocean, tidal waves and massive flooding would result. Earth impacts of smaller objects are not uncommon, but on June 30, 1908, a large object—apparently the icy nucleus of a small comet—fell in the sparsely inhabited Tunguska region of Siberia. The falling object outshone the Sun, and its explosive impact was felt more than 600 miles away. A very wide area of forest was obliterated—quite literally flattened.