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.

Prometheus Teases the F-Ring of Saturn

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.

Prometheus Teases the F-Ring of Saturn

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 Teases the F-Ring of Saturn

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

Prometheus Teases the F-Ring of Saturn

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.