What are Kirkwood gaps? What causes them?

Kirkwood gaps are regions in the main asteroid belt between Mars and Jupiter where relatively few asteroids are found. They were discovered by and named after Daniel Kirkwood, an American astronomer, in the mid-19th century. The gaps are not empty but have a clearly diminished population of asteroids.

The gaps are caused by gravitational perturbations from Jupiter. They correspond to certain orbital resonances with the planet. Orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other due to their orbital periods being related by a ratio of two small integers. In the case of the asteroid belt, these resonances occur with the orbital period of Jupiter. When an asteroid's orbit around the Sun has a period that is a simple fraction of Jupiter's, it is in an orbit that can be disturbed by Jupiter's gravity over time, possibly throwing it into a different, non-resonant orbit, or even out of the asteroid belt altogether. For example, an asteroid with an orbital period one third that of Jupiter (a 3:1 resonance) won't remain there for long on a cosmic timescale, hence it's a Kirkwood gap.

The Kirkwood gaps are important in understanding the history of our solar system. By studying the distribution of asteroids and the gaps, scientists can learn about the past interactions between asteroids and Jupiter and the origins of some comets and meteorites. The gaps help in sorting the asteroid population into different regions and provide evidence supporting the idea that Jupiter's gravity has a strong influence over the structure of the asteroid belt.