The Dancing Moons
May 3, 2006
In their orbital ballet, Janus and Epimetheus swap positions every four years -- one moon moving closer to Saturn, the other moving farther away. The two recently changed positions (the swap occurring on Jan. 21, 2006), and Janus will remain the innermost of the pair until 2010, when they will switch positions again.
Although the moons appear to be close in the image, they are not. Janus (181 kilometers, or 113 miles across at right) is about 40,000 kilometers (25,000 miles) farther away from Cassini than Epimetheus (116 kilometers, or 72 miles across, at left) in this view. In fact, even when they are at their closest, tugging at each other and swapping orbital positions, they are never closer than about 15,000 kilometers (9,000 miles).
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on March 20, 2006 at a distance of approximately 452,000 kilometers (281,000 miles) from Epimetheus and 492,000 kilometers (306,000 miles) from Janus. The image scale is 3 kilometers (2 miles) per pixel on both moons.
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.
Credit: NASA/JPL/Space Science Institute