Any of the many small rocky objects found mainly in a flat ring, or belt, between the orbits of Mars and Jupiter.
It is thought that the gravitational influence of what became Jupiter kept the asteroids from aggregating into a single planet while the solar system was forming. Also called minor planet s, asteroids are smaller than any of the nine major planets; only about 30 are more than 125 mi (200 km) across. Ceres is the largest known asteroid. Millions of boulder-sized asteroidal fragments are thought to exist in the solar system. Asteroids or their fragments regularly strike Earth, plunging through the atmosphere as meteor s to reach its surface (see meteorite ). Asteroids appear to be composed of carbonaceous, stony, and metallic (mainly iron) materials. See also Earth-crossing asteroid ; Trojan asteroids .