More than 5,000 minor planets, or orbiting asteroids have been cataloged since 1801, when the largest, numbered 1 and named Ceres (1 Ceres), was sighted by Italian astronomer Giuseppe Piazzi.
Some 200 more are discovered every year.
Astronomers estimate that 50 percent of all asteroids that are 6 miles or larger in diameter have been identified, including about 99 percent of all large asteroids, which are over 60 miles across.
The main asteroid belt, filled with about 5,000 asteroids, lies between the orbits of Jupiter and Mars.
The combined mass of all the asteroids is less than one-thousandth that of Earth.
That fact, and the gravitational battle between Jupiter and Mars, may help to explain why the asteroids never formed a planet.