Round and round they go… the animation above, made from 14 raw images taken by NASA’s Cassini spacecraft on August 23, 2016, shows the moons Prometheus and Atlas orbiting Saturn within the Roche Division gap between its A (top right) and F (center) rings. The gravitational tug of Prometheus (92 miles / 148 km long) is strong enough to pull on the fine, smoke-like icy particles of the F ring, creating streamer and “clump” features that follow it along.
The much smaller Atlas (23 miles / 37 km wide) follows a path around Saturn just past the outer edge of the A ring. It was once thought to be a “shepherd moon” of the A ring, but it’s now known that the pull of the more distant Janus and Epimetheus are responsible for that.
Atlas does have its very own ring, though—a very faint (i.e., not visible above) band of material that runs along its orbit named R/2004 S 1, discovered by the Cassini mission in July 2004.
Image credit: NASA/JPL-Caltech/SSI. Animation by Jason Major.
Prometheus is at it again! On Feb. 5, Cassini acquired a series of images with its narrow-angle camera of Saturn’s reflective and ropy F ring, around the inside of which travels the shepherd moon Prometheus. As it orbits Saturn it regularly arcs outwards toward the inner edge of the F ring and tumbles back inwards again, a scalloping orbit by a potato-shaped moon that yanks at the fine icy particles of the ring with its gravitational tug. However faint they might be, these tidal forces are enough to pull the wispy ring particles into long strands, gaps, and clumps that follow Prometheus’ passage before eventually settling back down again.
Saturn’s F ring is a fascinating structure. Made of fine icy particles — most no larger than the particulates found in cigarette smoke — it orbits Saturn just outside the A ring and is easily perturbed by the gravity of nearby moons and embedded moonlets, which create streamers and clumps that rise up in fanciful shapes.
This brief animation, made from 33 raw images captured by Cassini on December 26 (otherwise known locally as my birthday!) shows the F ring in action as it follows shepherd moons Prometheus and smaller Atlas around Saturn. Some motion is due to the orbits of the rings and moons, and some is due to the spacecraft itself.
You can watch a slower version of the animation below:
Captured on Christmas Day, this is a raw image from Cassini showing Saturn’s F ring buckling inwards at two places due to the gravitational tug of its inner shepherd moon, Prometheus, seen at center.
As the irregularly-shaped moon approaches the ring material in its looping orbit around Saturn it draws material from the ring in towards itself, warping and stretching the fine icy ring particles into waving streamers that eventually settle back into place. It’s a very visual demonstration of gravity at work!
At its widest Prometheus is about 92 miles (148 km) across but only 42 miles (68 km) in width. It orbits Saturn once every 14.7 hours.
Raw image: NASA/JPL/Space Science Institute