CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Moons in Motion

The moon Tethys is upstaged by two smaller moons in this movie from Cassini.

Observations of mutual moon-crossing events, in which one moon passes close to or in front of another, help scientists refine their understanding of the orbits of Saturn's moons. This movie is a concatenation of 15 still images taken over a span of about 33 minutes. The images were reprojected to a uniform view and computer interpolation was used to smooth the moons' motions between the frames.

In the movie, Prometheus (86 kilometers, 53 miles across) enters the frame from the right and passes behind Tethys (1062 kilometers, 660 miles across). Prometheus appears as a tiny bright dot beyond the main rings. Pandora (81 kilometers, 50 miles across) can also be seen at the bottom of the frame. The unlit side of the planet is on the left. Prometheus and Pandora's average speeds are each about 16 kilometers per second (36,000 mph). Tethys travels at an average speed of about 11 kilometers per second (25,000 mph).

(For other movies like this one, see PIA11692 and PIA11693.)

In this view Tethys, at a distance of approximately 1.2 million kilometers (746,000 miles), is closest to Cassini. Prometheus is farthest from the spacecraft at a distance of approximately 1.6 million kilometers (994,000 miles). The part of the rings at the bottom of the image is closer to Cassini than the rings at the top of the image. This view looks toward the northern, sunlit side of the rings from about 1 degree above the ringplane.

The images were taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 16, 2009. The view was obtained at a Sun-Tethys-spacecraft, or phase, angle of 80 degrees. Scale on Tethys is 7 kilometers (4 miles) per pixel.

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: December 23, 2009 (PIA 11691)
Image/Caption Information
  Moons in Motion
PIA 11691

Avg Rating: 9.16/10

Full Size 1479x384:
Flash 2.0 MB
Quicktime 3.1 MB
MP4 movie 72 KB

Half Size 740x192:
Flash 613 KB
Quicktime 1.1 MB
MP4 movie 39 KB


Moons in Motion
PIA 11691

Avg Rating: 9.75/10

Still Image Full Size 1479x384:

Still Image Half Size 740x192:

Alliance Member Comments
Dragon_of_Luck_Mah_Jonng1971 (Dec 31, 2009 at 1:06 PM):
A side note: Two moonlets following their orbits neatly.