Kirkwood gap

Histogram showing the four most prominent Kirkwood gaps and a possible division into inner, middle and outer main-belt asteroids:   inner main-belt (a < 2.5 AU)  intermediate main-belt (2.5 AU < a < 2.82 AU)  outer main-belt (a > 2.82 AU) A plot of inner solar system asteroids and planets as of 2026 May 15, in a manner that exposes the Kirkwood gaps. Similar to the position plot, planets (with trajectories) are gold, Jupiter being the outer most in this view. Various asteroid classes are colour coded: 'generic' main-belt asteroids are white. Inside the main belt, the near-Earth asteroids are shown in yellow and the Hungarias in orange. Outside the main belt are the Cybeles (blue), Hildas (purple), and the Trojans (pink). All object position vectors have been normalized to the length of the object's semi-major axis. The Kirkwood gaps are visible in the main belt.

A Kirkwood gap is a gap or dip in the distribution of the semi-major axes (or equivalently of the orbital periods) of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter. The gaps were first noticed in 1866 by Daniel Kirkwood, who also correctly explained their origin in the orbital resonances with Jupiter while a professor at Jefferson College in Canonsburg, Pennsylvania.