A Kirkwood gap is a gap or dip in the distribution of main belt asteroids with semi-major axis (or equivalently their orbital period), as seen in the histogram below. They correspond to the location of orbital resonances with Jupiter.
For example, there are very few asteroids with semimajor axis near 2.50 AU, period 3.95 years, which would make three orbits for each orbit of Jupiter (hence, called the 3:1 orbital resonance). Other orbital resonances correspond to orbital periods whose lengths are simple fractions of Jupiter's. The weaker resonances lead only to a depletion of asteroids, while spikes in the histogram are often due to the presence of a prominent asteroid family.
The gaps were first noticed in 1857 by Daniel Kirkwood, who also correctly explained their origin in the orbital resonances with Jupiter while a professor at Jefferson College in Canonsburg, Pennsylvania.
More recently, a relatively small number of asteroids have been found to possess high eccentricity orbits which do lie within the Kirkwood gaps. Examples include the Alinda family and the Griqua family. 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 most prominent Kirkwood gaps (see diagram) are located at mean orbital radii of:
- 2.06 AU (4:1 resonance)
- 2.5 AU (3:1 resonance), home to the Alinda family of asteroids
- 2.82 AU (5:2 resonance)
- 2.95 AU (7:3 resonance)
- 3.27 AU (2:1 resonance), home to the Griqua family of asteroids
Weaker and/or narrower gaps are also found at:
- 1.9 AU (9:2 resonance)
- 2.25 AU (7:2 resonance)
- 2.33 AU (10:3 resonance)
- 2.71 AU (8:3 resonance)
- 3.03 AU (9:4 resonance)
- 3.075 AU (11:5 resonance)
- 3.47 AU (11:6 resonance)
- 3.7 AU (5:3 resonance)
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