Most asteroids fall in the main asteroid belt and cluster together by the immense pull of the Planet Jupiter. As the planet, Jupiter orbits the sun at a distance of roughly 5 AU or about 5 times further than Earth. The main asteroid belt sits with the orbit of Jupiter, closer to the sun. The clusters group together in a ratio of the orbit of Jupiter. Over the billions of years for our solar system, the gravity of Jupiter pulled at the asteroids. The strength of that pull relates to the mass of Jupiter and the mass of the asteroid along with the square of the distance. Voids between the clusters of asteroids occur at the 2:1, 3:1, 5:2, and 7:3 orbital resonances, otherwise known as the Kirkwood Gaps.
The histogram clearly shows the primary Kirkwood gaps in the asteroid main-belt. 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 asteroid’s orbital elements (particularly semi-major axis) sufficient to create these gaps in semi-major axis space.
Kirkwood gaps are shown in this Histogram of the asteroids.
Credit: https://www.physast.uga.edu/~rls/1010/ch12/ch12_04.jpg
Trojan asteroids
