(gaps in asteroid orbital radii corresponding to Jupiter resonances)
A Kirkwood gap is an area within the solar system
with few asteroids,
that corresponds to a location where a body would
be in orbital resonance with Jupiter.
For example, at the location where an object would have
a third the orbital period of Jupiter's, there are few asteroids.
The most prominent gaps are at the 4:1, 3:1, 5:2, 7:3, and
2:1 orbital resonances. The gaps form because asteroids in
such resonances gain eccentricity, the evolving
orbits eventually losing the resonance.
Daniel Kirkwood first discovered the phenomena in 1866.