CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Tiny Moonlet Within G Ring Arc

This sequence of three images, obtained by NASA's Cassini spacecraft over the course of about 10 minutes, shows the path of a newly found moonlet in a bright arc of Saturn's faint G ring.

In each image, a small streak of light within the ring is visible. Unlike the streaks in the background, which are distant stars smeared by the camera's long exposure time of 46 seconds, this streak is aligned with the G ring and moves along the ring as expected for an object embedded in the ring.

Cassini scientists interpret the moving streak to be reflected light from a tiny moon half a kilometer (a third of mile) wide that is likely a major source of material in the arc and the rest of the G ring. Debris knocked off this moon forms a relatively bright arc of material near the inner edge of the G ring, the most visible part of the ring in these images. That arc, in turn, leaks material to form the entire ring.

These images were captured by Cassini's narrow-angle camera on Oct. 27, 2008. The first image (left) was taken in visible light, the second image (middle) was taken in red light, and the third image (right) in near-infrared light centered at a wavelength of 750 nanometers. Image scale for the first image is 7 kilometers (4 miles) per pixel. The second and third images were taken at reduced resolution. These spatially compressed images were captured at 14 kilometers (9 miles) per pixel and then displayed at a size equal to the first image. This view looks toward the un-illuminated side of the rings from about 5 degrees above the ringplane. The view was acquired at a distance of approximately 1.2 million kilometers (751,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 23 degrees.

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: March 3, 2009 (PIA 11148)
Image/Caption Information
  Tiny Moonlet Within G Ring Arc
PIA 11148

Avg Rating: 8.59/10

Full Size 1132x695:
PNG 266 KB


Tiny Moonlet Within G Ring Arc
PIA 11148

Avg Rating: 9.02/10

Full Size (labeled) 1132x695:
PNG 266 KB

Alliance Member Comments
Jay55 (Mar 7, 2009 at 11:11 PM):
Yes well said Harry. We can all appreciate the amazing diversity now evident within our own Solar System. Gone are the days where everything was nice and neat. Jupiter had 12 moons, Saturn had 9, Uranus 5 etc. No such thing as moonlets or ringlets. My purpose was to generate discussion and find out what other people thought. It is in our nature to categorize though, especially in astonomy, they just love it. Stars, nebulae, planets, galaxies have all been categorized extensively (eg G type star, planetary nebulae, barred spiral). So whether we like it or not, I think there will be an attempt by some to define 'moon'. As we find out more with the Keppler telescope then they may have to rewrite everything anyway. Like the issues surrounding the brown dwarf CFBDS J005910.83-011401.3. They don't seem sure to what to call it. Its possible that it more closely resembles a large planet? All I could say about defining a moon though is that could it be as simple as size eg. >100km radius and orbiting parent body = moon. Everything smaller than this is minor moon/moonlet/ringlet. These definitions or new ones can come later as we find out more. I am appreciating the fact though that we live in a time where these kind of discussions are even possible. It is very important not to get bogged down in definitions and appreciate the discoveries for what they are. I hear you loud and clear in tis regard.
Red_dragon (Mar 7, 2009 at 9:23 AM):
Another option could be to consider "moonlets" those bodies that have cleared (or mostly cleared) their orbits and "ringlets" those embedded within the rings -I believe that has been suggested before-. But still so, the variety of celestial bodies defies any attempt of classifing them, and that without accounting for what may exist beyond our solar system -some of that hopefully will be find by the Kepler mission-
Harry (Mar 6, 2009 at 2:02 PM):
As the computer geeks say "There are 10 types of people in the world, those that understand binary and those that don't." :^)) A corollary implies that some people, astronomers too, must categorize/organize "things" and others that are happy with multiple dimensions of diversity. Evidently since 1919, the IAU has the authority to name everything in space. In 2006, they did a poor job of gerrymandering the planetary definition and Pluto's status. The continuing debate and frequent acrimony has made a mockery of the process. I really would hate to see them try defining "moon". By just looking at the diversity of only Saturn's "moons", how could anyone generate a concise clean definition and/or list of object classes? Obviously a "body" such as S/2008-S1 and Charon are different. But, are Charon, Titan, Triton, Phobos and the Moon going to be "moons"? Will Charon get axed because it orbits a dwarf whatever? In my humble view, mankind is barely scratching the surface of planetary exploration. We are still cataloging objects in our own planetary system. It has been only the last decade when we discovered the existence of objects around other stars. Isn't it a little early to be forcing categories? I am sure there is value in being the discoverer of a moon over a moonlet; planet over dwarf planet. Being a layman, I don't have "a dog in that hunt". History books aside, clinging to hard definitions and the insuing debates do not add to the body of scientific knowledge. I think it is wonderful every time we find something new and different. Why can't we spend our time and resources discovering the nature of these objects; not forcing them into "boxes". Is S1/2008-S1 a rubble pile? I would be excited to find out what is inside that caused the agglomeration. Could it be a NiFe "rock" that was captured by Saturn eons ago? Is S1/2008-S1 eroding to form the G-ring or is it causing the G-ring to diffuse? I hope I have not offended anyone. Sorry for the diatribe.
Red_dragon (Mar 6, 2009 at 8:20 AM):
I think if CICLOPS had to name *all* the moonlets that orbit Saturn, there would not be enough mythological names for them. Perhaps one way to name them could be to do that just to those that seem to be durable over largue timescales (let's say centuries) and the others using just numbers or alike.
The problem is, of course, how to define what's a moon and a moonlet, and I suspect it's not so "easy" as was to name a planet.
Jay55 (Mar 5, 2009 at 1:27 AM):
To find such a small body within the ring is an amazing feat. Congratulations to the Ciclops team. How small do we go though before we dismiss these bodies as moons or moonlets and consider them just part of the ring? 500m,300m,50m . 500m is a very small moon. After defining a planet maybe we should consider redefining our definition of a moon/moonlet. Have they suggested any names?
Jay55 (Mar 5, 2009 at 1:25 AM):
To find such a small body within the ring is an amazing feat. Congratulations to the Ciclops team. How small do we go though before we dismiss this bodies as moons or moonlets and consider them just part of the ring? 500m,300m,50m . 500m is a very small moon. After defining a planet maybe we should consider redefining our definition of a moon/moonlet. Have they suggested any names?
Dragon_of_Luck_Mah_Jonng1971 (Mar 4, 2009 at 2:31 PM):
By their intelligent ways of observations the Cassini Team was even able to find a tiny moonlet that is only 500 m across ! Once again, Cassini sent us image data that were many better by resolution and by size than the early 80s' Voyagers that had been the last mission to Saturn before Cassini.
Red_dragon (Mar 4, 2009 at 5:04 AM):
Excellent work again, CICLOPS!