Gaps in the Asteroid Belt

A few weeks back, I wrote a Bitesized page about gaps in the rings of Saturn. These gaps are caused by resonances with Saturn's moons: if a bit of ice in the ring orbits twice around Saturn every time a moon farther out orbits once, then the ice gets tugged out of that orbit. This happens with many simple fractions of moons' orbits (like 2:3, or 5:7).

Well, the same thing happens in the asteroid belt. In 1866, Daniel Kirkwood realized that gaps in the distributions of asteroids around the Sun were simple fractions of Jupiter's orbital period. For example, a big gap in the asteroid belt is located about three and a quarter times the Earth's distance to the Sun (that distance is sometimes called an "astronomical unit, or AU for short). An asteroid in this orbit would be in a 1:2 ratio of Jupiter's orbit (Jupiter takes 12 years or so to revolve around the Sun, and an asteroid 3.28 AU from the Sun orbits in about 6 years). Many other such gaps exist as well. Honoring the man that figured it out, these gaps are known generically as "Kirkwood gaps" (or sometimes "Kirkwood resonance").

Of what use are Kirkwood gaps? None that I am aware of, except as a reminder that sometimes nature has a wonderful subtleness about it, and given enough time it can make its voice heard.