Al Petterson (eyelessgame) wrote,
Al Petterson
eyelessgame

Saturn's Coolest #5: Epimetheus and Janus

The most common reaction I've heard when these two moons are explained to people (*) is "you have got to be shitting me."

(* People, as opposed to physicists.)

Like Prometheus and Pandora, these two aren't worlds (they aren't spheres); they're just rocks, and in fact are not far from that pair. Epimetheus and Janus orbit inside of Mimas, very close to the rings, but outside of the F ring that Prometheus and Pandora make.

Rocks aren't, by and large, all that interesting. But these two are not just interesting but dumbfounding, not because of what they are but because of what they do.

I'll let you figure out the problem. They're each about a hunded miles in diameter (on average; they're irregular rocks). Their orbits are perfectly circular, and they both orbit exactly in the plane of the rings. And the radii of their orbits - the paths traveled by the two moons' centers - differ by only thirty miles.

Think about that for a minute. I'll wait.

Their orbits are thirty miles apart. They're each a hundred miles in diameter. That means they each extend fifty miles in each direction from the paths of their orbits. Their orbits have to be a hundred miles apart, or they'll collide. Right?

Wrong.

When the inside one approaches the outside one (happens once every four years) and they are just about to collide, they swap orbits.

I feel compelled to assure you I am not making this up.

(There's a small subset of people - physicists - who at this point will roll their eyes at me and think I'm acting like an exciteable moron - "Well of course they swap orbits. What did you think they would do?" The explanation that follows is for everybody else.)

Higher orbits (more distant from the thing you're orbiting, in this case Saturn) have more energy... but the thing in the higher-energy orbit moves more slowly. Yeah, that's weird, but it's how orbits are. Pretend you're Epimetheus, and you're orbiting just "below and behind" Janus - catching up to it, in a lower orbit (closer to Saturn). Just as you catch up, the gravitational pull between you and Janus causes you to steal some of its kinetic energy, pulling back on it, as it pulls you forward. Since you get more energy from being pulled forward, you climb into a higher orbit and, since Janus lost energy from being pulled back, this drops Janus into a lower one.

Now that you're in a higher orbit, you move slower, and Janus - now in a lower orbit - moves faster. So Janus pulls away from you, like a car that downshifted. Then four years pass, while you each orbit Saturn two thousand times, Janus going ever so slightly faster. After these four years, Janus catches up to you from behind, pulls on you, and reverses the process; you (Epimetheus) are the one dropping into the lower orbit, and Janus climbing up to the higher one. And you now move faster, pull away from Janus, and then take another four years to make up the lap, and then the swap happens again.

And again. A billion times, since the system formed, unless something happened substantially more recently to push these moons into this orbit... that's something we'd like to find out someday.

You can almost think of the two moons orbiting each other. They whip around Saturn once every seventeen hours, and they orbit each other with a period of eight years, during most of which they have no gravitational effect on each other whatsoever.

Saturn's moons are weird.

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