Epimetheus

[mmcirvin]

This shot from a few weeks ago is a really nice picture of Epimetheus--the best one ever taken, I think. I missed it in the raw images archive.

As with many small, irregular bodies, you can see what looks like some sort of loose material softening the contours of the moon in the craters and hollows, and rougher material sticking out above that. On asteroids the loose stuff is usually some sort of gravel or dust--as the caption points out, though, here it's probably mostly made of water ice.

Epimetheus orbits just outside of Saturn's main rings and is famous for its peculiar co-orbital relation with Janus. The orbits are so close together that they are separated by less than the moons' widths, but they never collide--instead, every few years the difference in orbital velocities brings them close together, and they gravitationally swap orbits and start to move apart again. It's somewhat similar to the relationship between 3753 Cruithne and Earth, though that situation's more complex, and there the difference in mass means that Cruithne's orbit does most of the changing.

Comments

[muffyjo]

I learn so much from you and these posts. Thank you.

[mmcirvin.livejournal.com]

The history of Janus (http://en.wikipedia.org/wiki/Janus_%28moon%29) and Epimetheus's discovery is kind of interesting, since they were seen prior to space probes arriving at Saturn, but only just barely. In the sixties there were various observations of a satellite just outside the rings that was referred to as Janus, but the observations weren't entirely consistent with each other and not everyone saw it. In 1978, other astronomers figured out that there were actually two different objects.

Then in 1979, Pioneer 11 (which was mainly intended as a Jupiter probe) flew past Saturn in an encounter primarily designed to help planning for the subsequent Voyager flybys. By sheer coincidence, it nearly collided with either Janus or Epimetheus--I think that which one is actually unclear, because of the uncertainty in their precise orbits at that point, but the moon's shadow was observed by Pioneer 11's particle detectors.