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Emily LakdawallaMarch 23, 2015

Prometheus, Pandora, and the braided F ring in motion

On a day when my to-do list is long and overwhelming, I often find myself ignoring the list and turning to the Cassini raw images website. Especially when I know that Cassini has been doing something cool recently, like taking a long, high-resolution movie of the F ring. The F ring is a thin, dusty one with a complex shape that's constantly being disturbed and deranged by the encroachment of Prometheus. Sometimes Prometheus' slightly eccentric orbit takes it right into the F ring and back out again, whereupon it pulls out streamers of material, leaving dark lanes next to the streamers. Prometheus and its streamers orbit Saturn just slightly faster than the stuff located farther from Saturn in the F ring, so the dark lanes stretch out and out and out until they wrap right around Saturn, making alternating light-and-dark spirals. Meanwhile, knots of material in the F ring form and are torn apart again over the course of months. Pandora orbits outside it all, having little effect on the shape of the F ring despite its proximity to it.

98 frames shot between March 15 00:27:00 and March 15 04:53:14 with each frame taken 163 seconds after the last. Some cosmic ray hits have been cleaned up. Video plays once at 20 frames per second, then repeats with annotations at 2 frames per second. Play in HD for best quality.

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NASA / JPL / SSI / Emily Lakdawalla

Here are stills of Prometheus and Pandora with the ring. The phase angle is very high here, so both moonlets and the ring are lit very nearly from behind; the moons appear as very thin crescents, sunlit from the lower right. Each moon is more illuminated by Saturnshine, striking them from the left. In the Prometheus one you can actually see the dark lanes in the inner F ring crossing over at least two other ring waves that have coiled tightly around Saturn by Keplerian shear. I wish I could live on a space station levitating above the rings to watch these dynamics in their natural slow motion!

Prometheus and the F ring

NASA / JPL / SSI / Emily Lakdawalla

Prometheus and the F ring
Prometheus orbits Saturn just inside the skinny F ring. It Saturn's most elongated known moon, being about 148 kilometers long but only 68 kilometers wide. Its periodic encroachment into the F ring creates the dark lanes visible in the inner part of the ring. This photo was taken March 15, 2015 as a part of a high-resolution movie that also included Pandora.
Pandora and the F ring

NASA / JPL / SSI / Emily Lakdawalla

Pandora and the F ring
Pandora orbits Saturn just outside the F ring. It is 110 kilometers long by 62 kilometers wide. This photo was taken March 15, 2015 as a part of a high-resolution movie that also included Prometheus.

Read more: pretty pictures, Cassini, amateur image processing, Saturn's small moons, Saturn's rings

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
Read more articles by Emily Lakdawalla

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