Prometheus and Pandora are a tag-team of moons shaping Saturn's F-ring. As Prometheus passes through the ring, it gouges a gore in the ring, drawing the material out as a streamer as the moon exits the ring.

Saturn's sunlit rings as observed by the Cassini spacecraft in visible light on February 11th, 2014. Scale is 13 kilometers per pixel. Image credit: NASA/JPL-Caltech/Space Science Institute


Prometheus is a small, lumpy moon just 86 kilometers (53 miles) in diameter. Its orbit regularly brings it through the F ring. When it does, it clear a channel, or gore, in the ring, and then draws the material behind it to leave a wake of streamers.

The process of passing through the ring is a soft collision. The dust-sized material pulls a streamer pointing ahead of the moon. The moon orbits faster than the rim, so Prometheus quickly outdistances its streamers.


The movie covers half a streamer-channel cycle, with orbital motion to the right. The dark frames are when the moon and ring are hidden in Saturn's shadow.

Each orbit, Prometheus gets a little further ahead, drawing a new streamer just slightly ahead of the last (3.2 degrees ahead, to be exact). Eventually, shear forces disperse the dust particles, dissipating the streamer-channel.

Prometheus is nearly-invisible to the right of the lowest streamer. Image credit: NASA/JPL/Space Science Institute

Prometheus tugging a streamer out of the F-ring. Image credit: NASA/JPL/SSI

Daphnis performs a similar function as a shepherd-moon for the A-ring. It may be more accurate to call these shaper-moons, not shepherd-moons, as they have a strong disruptive element by yanking and tugging at the rings.