The small moon Janus overtakes the larger moon Rhea in a dance played out before Saturn and its rings.
Observations of mutual moon-crossing events like this one, 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 12 still images taken over a span of 24 minutes. The images were reprojected to a uniform view and computer interpolation was used to smooth the moons' motions between the frames.
At the start of the movie, Janus (179 kilometers, 111 miles across) appears as a small, bright dot just below the rings on the left of the frame. Rhea (1528 kilometers, 949 miles across) also starts on the left but can be seen moving more slowly across the frame. Janus is traveling almost twice as fast as Rhea with an average speed of about 16 kilometers per second (36,000 mph) compared to Rhea's average speed of roughly 8 kilometers per second (18,000 mph).
In this view, Janus is approximately 2.3 million kilometers (1.4 million miles) away from Cassini. Rhea is closer to the spacecraft at a distance of approximately 1.9 million kilometers (1.2 million miles). In the original images, the scale of Rhea was 11 kilometers (7 miles) per pixel. The images have been scaled down by a factor of two to reduce the movie's dimensions to a viewable size.
This view looks toward the northern, sunlit side of the rings from just above the ringplane.
The images were taken in visible light with the Cassini spacecraft narrow-angle camera on Nov. 8, 2009. The view was obtained at a Sun-Rhea-spacecraft, or phase, angle of 103 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.