A gaggle of moons, featuring Rhea and three other moons, parade around Saturn's rings in this movie from Cassini.
This sight may at first appear confusing, with four moons moving to and fro in their orbits. But imaging sequences like this, in which one moon passes close to or in front or another, actually help scientists refine their understanding of the orbits of Saturn's moons. The movie is a concatenation of 12 still images taken over a span of 19 minutes. The images were reprojected to a uniform view and computer interpolation was used to smooth the moons' motion between the frames.
At the start of the movie, Rhea (1528 kilometers, 949 miles across) is at the lower left and moving to the right as seen by Cassini. Janus (179 kilometers, 111 miles across) is near Rhea in the lower left. Mimas (396 kilometers, 246 miles across) is near the center top and is moving left. About one-third of the way through the movie, Pandora (81 kilometers, 50 miles across) enters the frame on the right, orbiting just beyond Saturn's rings.
Mimas is farthest from Cassini at a distance of approximately 2.5 million kilometers (1.6 million miles). Rhea is closest to the spacecraft at a distance of approximately 1.9 million kilometers (1.2 million miles). Mimas travels at an average speed of 14 kilometers per second (31,000 mph). Janus' and Pandora's average speeds are each about 16 kilometers per second (36,000 mph). Rhea is the slowest of this quartet, traveling at an average speed of about 8 kilometers per second (18,000 mph).
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 105 degrees. Scale on Rhea is 11 kilometers (7 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.