Asteroid Main-Belt Distribution
This histogram clearly shows the primary Kirkwood gaps in the
These gaps (labeled "3:1", "5:2", "7:3", "2:1")
are caused by mean-motion resonances between an
asteroid and Jupiter.
For example, the 3:1 Kirkwood gap is located where the ratio of an
asteroid's orbital period to that of Jupiter is 3/1
(the asteroid completes 3 orbits for every 1 orbit of Jupiter).
The effect of these mean-motion resonances is a change in the
orbital elements (particularly semimajor axis)
sufficient to create these gaps in semimajor axis space.
This graph was created in June 2007
using all asteroids
with "well-determined" orbits (specifically, 156929 numbered asteroids).
A Postscript version of this plot is also available.