Cassini eavesdrops on orbit-swapping moons

Contributed by
Apr 8, 2010
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The Cassini spacecraft just had a few close encounters with some of the odder moons in the Saturn system... and given how weird Saturn is, that's saying something. I was particularly enthralled with these two small worlds:


On the left is the moon Janus and on the right is Epimetheus. The scales are not quite the same; Janus is roughly half again as big as Epimetheus' size of 135 x 110 x 105 km (81 x 66 x 63 miles). Cassini was a little over 100,000 km from Epimetheus and 75,000 km from Janus when these images a were taken. These are raw images, so they haven't been processed yet to remove cosmic ray hits, brightness variations, and so on. But they are still fascinating. Epimetheus looks to me exactly how I picture a big asteroid; beaten, battered, looming. The low angle of sunlight on the side accentuates the craters there, making this almost a caricature of what an asteroid looks like. Technically it's not an asteroid; it's a moon. And even if it weren't orbiting Saturn we might not call it an asteroid; it has a high reflectivity indicating a lot of ice on the surface (and a low density consistent with that too). If it orbited the Sun on an elliptical path, we might very well call it a comet!

But there's more to these moons. Amazingly, Janus and Epimetheus are on almost -- but not quite -- the same orbit around Saturn! Currently, Janus is a bit closer to Saturn than Epimetheus.

I say "currently", because every four years these moons swap orbits! Since Janus has an orbit slightly closer to Saturn, it is moving faster around the planet than Epimetheus. It slowly but eventually catches up to the outer moon. As they approach, Janus pulls back slightly on Epimetheus, and Epimetheus pulls Janus forward. In other words, Janus steals orbital energy Epimetheus! This means Epimetheus drops into a slightly lower orbit, and Janus gets boosted into a slightly higher one, effectively swapping the orbits of the two moons. Although the two orbital paths are separated by only about 50 km (30 miles) -- smaller than the radii of either moon -- they never collide. The swap takes place when the moons are still more than 10,000 km apart, so they never get a chance to bump uglies.

How did this weird situation arise? Perhaps, in the distant past, there was one bigger moon orbiting Saturn, and it got whacked by an interloper. The moon disrupted, breaking into two big pieces and lots of littler ones. The debris got cleaned up by the gravity of the two big pieces and other gravitational effects, leaving these two square-dancing satellites on slightly different but still interacting paths.

However, the actual cause of this still isn't known for sure. Cassini observations like this one may help astronomers figure out how it is these two little moons came to be, and why it is that although they can always approach each other, they can never actually touch.