The Columbia Encyclopedia, Sixth Edition. 2001-07.
Kirkwood gaps
regions in the asteroid belt within which few asteroids are found. Astronomer Daniel Kirkwood first observed (1886) that few asteroids had an orbital period close to 1/2, 1/3, or 2/5 that of Jupiter. The gaps could have been formed by collisions between asteroids; however, the most widely accepted theory is that the gaps were formed by gravitational interactions with Jupiter, which over time would move any small body into another orbit.
