Wednesday, May 03, 2006
Saturnian Two-StepOne of the remarkable discoveries by Voyager at Saturn was that some moons share the same orbit, or nearly the same orbit around the planet. Janus and Epimetheus, above, switch orbits every four years. Janus (right) currently orbits a little closer to Saturn than Epimetheus, taking four years to lap its brother. Then something rather strange happens. As Janus approaches the other moon from behind, gravity speeds it a little bit to the "outside track," and a bit of pull from Janus tugs Epimetheus to the inside. In 2010, these moons will switch positions, then Epimetheus will take the next four years to gradually catch up with Janus, and the process will repeat. The Cassini site gives a bit of info today but mainly you need to know the moons in this picture are only lined by along the camera angle. In fact, in this image, the two are 25,000 miles apart. They never pass closer than 9,000 miles during their orbit switch.