If you’ve ever wondered what it would be like to hang out near the Cassini Saturn spacecraft and get the same view it does, then put on your red/green glasses and check out this anaglyph of the moon Prometheus:
Mmmm, threedeealicious. Click to enjovianate.
Prometheus is a bit weird. OK, it’s a lot weird. It’s an irregularly-shaped elongated spud of a moon, measuring about 119 x 87 x 61 km (71 x 52 x 37 miles) in size. The long axis always points toward Saturn due to tides; basically the change in Saturn’s gravity from the front end of Prometheus to the back end acts like a stretching force on the moon, keeping it aligned. The tip on the right always points toward Saturn, and the long side we see in the image is the leading half of the moon, always facing ahead into the direction it orbits. Think of it as facing into the wind if that helps any.
Prometheus is a shepherd satellite, meaning its orbit gets it near Saturn’s F ring, where it helps keep the ring particles in place. It does this along with Pandora, another smallish moon. Prometheus orbits Saturn inside the F ring. When it gets close to the ring, it gives a little bit of its orbital energy to any ring particles that are on the inside edge of the ring, which boosts them to a slightly higher, slower orbit. Pandora does the opposite; it orbits outside the ring, and it steals energy from ring particles on the outside edge, which drops them into a slightly lower, faster orbit. Together, the two moons shepherd the F ring particles, corralling them and keeping the ring narrow. The animation shows the effects of Prometheus on the inner edge of the ring.
Just so you know, I think this is one of the coolest things ever. Shepherding moons were theoretically discussed for a long time, but we didn’t have any evidence of them until Voyager swept past Saturn a few decades ago, and now Cassini has the chance to study them in detail. It’s such a weird thing, and there it is playing out in the solar system for us to examine! It’s a good reminder that Nature is sneaky, and a lot more clever than we are. I’m glad we’re clever enough to catch up with it, too.
