Kirkwood gap

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.

[3] More recently, a relatively small number of asteroids have been found to possess high eccentricity orbits which do lie within the Kirkwood gaps.

These orbits slowly increase their eccentricity on a timescale of tens of millions of years, and will eventually break out of the resonance due to close encounters with a major planet.

The actual spatial density of asteroids in these gaps does not differ significantly from the neighboring regions.

Weaker resonances occur at other semi-major axis values, with fewer asteroids found than nearby.

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 2006 May 9, in a manner that exposes the Kirkwood gaps. Similar to the position plot, planets (with trajectories) are orange, Jupiter being the outer most in this view. Various asteroid classes are colour coded: 'generic' main-belt asteroids are white. Inside the main belt, there are the Atens (red), Apollos (green), and Amors (blue). Outside the main belt are the Hildas (blue) and the Trojans (green). 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.