Cassini watches Janus and Epimetheus in their orbital dance as the F ring slides out of view in a movie sequence from Cassini.
These two moons are locked in a gravitational tango that causes them to swap positions about every four years, with one becoming the innermost of the pair and the other becoming the outermost.
The movie was created from 8 original images taken over the course of 11 minutes as the spacecraft's narrow angle camera remained pointed toward Epimetheus. Although Janus (179 kilometers, 111 miles across) moves a greater distance across the field of view, Epimetheus (113 kilometers, 70 miles across) also moved perceptibly during this time. The images were aligned to keep Epimetheus close to the center of the scene. Additional frames were inserted between the 8 Cassini images in order to smooth the appearance of the moons' movement - a scheme called interpolation.
The images were taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 8, 2005 at a distance of approximately 1.8 million kilometers (1.1 million miles) from Janus and 1.7 million kilometers (1 million miles) from Epimetheus. Image scale is 11 kilometers (7 miles) per pixel on Janus and 10 kilometers (6 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 Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. 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.