Well, my recent post on resonance in beam dynamics got me thinking about one of my other favorite resonances: Kirkwood gaps. If you look at the semimajor axes (that's half the sum of closest and furthest distances from the Sun throughout the asteroid's orbit) of all the known asteroids and put them in a histogram of some sort, there's a very peculiar structure. There are noticeable gaps in the distribution, like there are some distances asteroids just don't want to live. Take a look:
What could cause this? The answer, it turns out, is our solar system's most massive planet, Jupiter, whose mass is within an order of magnitude of becoming an actual star. You see, the big gaps correspond to asteroid orbital periods that are rational number multiples of Jupiter's orbital period. Take the giant 3:1 gap around 2.5 AU. If some asteroid occupied that particular region, for every three of its turns around the Sun, Jupiter would go around once, which means that every three asteroid-years, Jupiter would be in the same relative position. This produces a resonance, and Jupiter, over the course of many, many asteroid-years, either slings the asteroid out of the solar system or adjusts its orbit so that its semimajor axis is no longer in that 'danger zone.' Similar stories apply to the three other prominent resonances.
These resonant regions are called Kirkwood gaps. It's just one of many really interesting phenomena resulting from resonances in physics.
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