Moons in Motion

December 23, 2009
PIA NumberPIA11691
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The moon Tethys is joined by two smaller moons in this movie from NASA's Cassini spacecraft.

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 obtained over a span of about 33 minutes. The images were re-projected 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 (1,062 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 Catching Big Sister and Moon Jumble.)

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 ring plane.

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

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. 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 and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit . The Cassini imaging team homepage is at .

Credit: NASA/JPL/Space Science Institute