CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Propeller Swarm

A close-up of Saturn's A ring reveals dozens of small, bright streaks aligned with the orbital direction of the rings. These objects are the propeller-shaped features first captured in Cassini images during the spacecraft's 2004 orbital insertion maneuver, as Cassini skimmed just above the ringplane. The propeller features were announced in 2006 (see PIA07792).

Each propeller is the visible gravitational disturbance created around a small moonlet embedded in the ring. The moonlets are likely between 10 and 100 meters (30 to 300 feet) across. Cassini imaging scientists have previously found that propeller swarms like this occur primarily in three narrow bands in the middle part of the A ring.

This view looks toward the sunlit side of the rings from about 35 degrees below the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 25, 2008. The view was acquired at a distance of approximately 219,000 kilometers (136,000 miles) above the rings and at a Sun-ring-spacecraft, or phase, angle of 127 degrees. Image scale is 1 kilometer (0.6 mile) per pixel in the radial, or outward from Saturn direction and 2 kilometers (1 mile) in the longitudinal, or around Saturn, direction.

The Cassini Equinox Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini Equinox Mission visit, and

Credit: NASA/JPL/Space Science Institute
Released: November 4, 2008 (PIA 10505)
Image/Caption Information
  Propeller Swarm
PIA 10505

Avg Rating: 8.56/10

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Alliance Member Comments
NeKto (Nov 16, 2008 at 5:16 PM):
greetings from the oncology ward of Hartfod Hospital. the Cassini images have been wonderful recreation for me for a long time, but never more than during my stay here.
the dynamics of things going on in this image is mind bogling and facinating. it is also aesthetically pleasing. there are moments when i want to put a phonograph neadle in those ring grooves just to find out what it would sound like. at the moment i would think the "moonlets" that are making the "propellers" are not long lived. from what the CICLOPS and Cassinni scientist have said, they probably lack suficient cohesion to stay together if there are any disruptive forces, tidal, magnetic or otherwise. i guess they are best described as rubble piles. they do make facinating paterns in the ring images!
carolyn (CICLOPS) (Nov 16, 2008 at 3:18 PM):
Harry: The IAU has not tackled this one yet. The ink on the matter of planet/dwarf-planet has not yet dried, so I don't seriously expect this moonlet issue to arise for their consideration any time soon.
Harry (Nov 14, 2008 at 1:13 PM):
Speaking of moonlets, has anyone clarifed a definition of moonlet? With the IAU's penchant for definitions, I would expect there to be a debate about what is a moonlet and what makes a moon different. I don't mean to be divisive, but I assume that it is a topic. I would assume the Cassini team would have an important influence on the IAU's opinion. If it is rule based, is it similar to planet and dwarf planet? A moonlet has not cleared it's orbital path or something along those lines?
Red_dragon (Nov 12, 2008 at 3:47 AM):
Thanks for your input, Carolyn.
carolyn (CICLOPS) (Nov 8, 2008 at 9:49 AM):
The very fine grainy texture is indeed noise. We don't get close enough to the rings to resolve the boulders, the largest being about the size of large houses. But you can see, in this image, propeller-like features created by small moonlets, about a few hundred meters in size. We don't see those moonlets, but we can see the much larger propeller like features they create in the rings.
Red_dragon (Nov 8, 2008 at 4:11 AM):
Great image. I especially like the "grainy" texture of the image; unless it's noise from the CCD, it looks as if the rings were nearly to be resolved into the many boulders that form them.
During any of her orbits, will be Cassini so close to the rings that she will be able to resolve them using their cameras?