Astrodude Space

Janus and Epimetheus: Saturn’s Orbit-Swapping Twin Moons

Janus and Epimetheus are two of Saturn’s moons that share the same orbit, separated by only about 50 kilometers (31 miles). When they approach each other, they exchange momentum and switch orbits—the inner moon becomes the outer, and vice versa. This orbital swap occurs roughly every four years.

Janus was discovered by Audouin Dollfus in 1966, although it had been photographed earlier by Jean Texereau. The unique co-orbital relationship with Epimetheus was confirmed by the Voyager 1 spacecraft in 1980.

Janus has a potato-like shape and a surface covered in impact craters, indicating that it is geologically old. Its low density and high albedo (reflectivity) suggest that it is composed primarily of porous ice, resembling a “rubble pile” in structure.

Janus orbits Saturn at a distance of approximately 151,472 kilometers (94,125 miles) from the planet’s center, situated between Saturn’s F and G rings.

 

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