Most meteorites originate in the asteroid belt, situated between the orbits of Mars and Jupiter. At least 1 million asteroids reside in this belt with diameters greater than 0.6 mile (1 km), 1,000 with diameters greater than 18 miles (30 km), and 200 with diameters greater than 60 miles (100 km). Asteroids and meteoroids are distinguished only by their size, asteroids being greater than 328 feet (100 m) in diameter. Meteoroids are referred to as meteors only after they enter the Earth's atmosphere, and as meteorites after they land on the surface. These are thought to be either remnants of a small planet that was destroyed by a large impact event, or perhaps fragments of rocky material that failed to coalesce into a planet, probably due to the gravitational effects of the nearby massive planet of Jupiter. Most scientists favor the second hypothesis, but recognize that collisions between asteroids have fragmented a large body to expose a planetlike core and mantle now preserved in the asteroid belt.
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