Stories We Like: A Guide to the Comma
Any of numerous small, often irregularly shaped rocky bodies that orbit the Sun primarily in the asteroid belt, a region between the orbits of Mars and Jupiter. Asteroids are intermediate in size between planets and meteoroids; the largest asteroid in the asteroid belt is Ceres, approximately 1,000 km (620 mi) in diameter, while the lower limit is variously given in the tens or hundreds of meters. While more than 1,800 asteroids have been cataloged, and as many as a million or more smaller ones may exist, their total mass has been estimated to be less than three percent of the Moon's. Asteroids are thought to be left over from the early formation of the solar system, when planetesimals in a protoplanetary disk were scattered after coming under Jupiter's gravitational influence. The continuing collision of planetesimals that remained between Jupiter and Mars caused many of them to fragment, creating the asteroids that exist today. Also called minor planet, planetoid.