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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.