Kirkwood gaps
Narrow zones in the main asteroid belt with very few asteroids, cleared by orbital resonances with Jupiter.
Plotting the number of main-belt asteroids against orbital semi-major axis shows sharp dips. The American astronomer Daniel Kirkwood drew attention to them in the 1860s and explained them as places where an asteroid's orbital period would be a simple fraction of Jupiter's (about 11.86 years). The main Kirkwood gaps lie near 2.50 astronomical units (3:1 mean-motion resonance), 2.82 au (5:2), 2.95 au (7:3) and 3.27 au (2:1).
In these resonances Jupiter's tugs repeat at the same points of the orbit, so their effects add up instead of averaging out. Eccentricity grows chaotically, typically over a million years or less, until the orbit crosses Mars or Earth. The gaps are therefore not just empty zones; they are escape hatches.
That is why they matter for meteoritics. Together with the ν6 secular resonance at the inner edge of the belt, the 3:1 resonance is thought to be a major route that sends fragments from inner-belt collisions towards Earth. Orbits reconstructed from camera networks for many recovered falls, ordinary chondrites included, trace back to the inner belt in a way consistent with delivery through these channels. The gaps are seen only statistically, in histograms of asteroid orbits, never as visible empty lanes.
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