Epimetheus (pronounced ep-eh-MEE-thee-us; adjective: Epimethean) and the neighboring moon Janus have been referred to as the Siamese twins of Saturn because they orbit Saturn in nearly the same orbit. This co-orbital condition (also called 1:1 resonance) confused astronomers, who at first could not believe that two moons could share nearly identical orbits without colliding.
These two moons lie amongst Saturn's rings and have orbital radial distances from Saturn of roughly 151,500 km (94,100 miles). One moon orbits 50 km (31 miles) higher (farther away from the planet) and consequently moves slightly slower than the other. The slight velocity difference means the inner satellite catches up to the other in approximately four Earth years. Then, the gravity interaction between the two pulls the inner moon faster moving it to a higher orbit. At the same time, the catching-up inner moon drags the leading outer moon backward so that it drops into a lower orbit. The result is that the two exchange places, and the nearest they approach is within 15,000 km (6,200 miles). During the 2010 trade off the Epimethean orbital radius dropped by approximately 80 km (50 miles), while Janus increased by only approximately 20 km (12.4 miles). The Janus orbit changes only a quarter of the Epimetheus change because Janus is four times more massive than Epimetheus.
Epimetheus and Janus may have formed by the break-up of one moon. If so, it would have happened early in the life of the Saturn system since both moons have ancient cratered surfaces, many with soft edges because of dust. They also have some grooves (similar to grooves on the Martian moon Phobos) suggesting some glancing blows from other bodies. Together, the moons trail enough particles to generate a faint ring. However, except for very powerful telescopes, the region of their common orbit (and the faint ring) appears as a gap between Saturn's more prominent F and G rings.
Epimetheus and Janus are the fifth and sixth moons in distance from Saturn. Both are phase locked with their parent; one side always faces toward Saturn. Being so close, they orbit in less than 17 hours. They are both thought to be composed largely of water ice, but their density of less than 0.7 is much less than that of water. Thus, they are probably "rubble piles" -- each a collection of numerous pieces held loosely together by gravity. Each moon has dark, smoother areas, along with brighter areas of terrain. One interpretation of this is that the darker material evidently moves down slopes, leaving shinier material such as water ice on the walls of fractures. Their temperature is approximately -319 degrees Fahrenheit (-195 degrees Celsius). Their reflectivity (or albedo) of 0.7 to 0.8 in the visual range again suggests a composition largely of water ice.
The Epimethean mean diameter of 113 km (70 miles) comes from its potato-shaped dimensions of 135 x 108 x 105 km (84 x 67 x 65 miles, respectively). These numbers reflect pronounced flattening at the Epimethean south pole associated with the remains of a large crater. Epimetheus has several craters larger than 30 km, including Hilairea and Pollux.
Audouin Dollfus observed a moon on 15 December 1966, for which he proposed the name "Janus." On 18 December of the same year, Richard Walker made a similar observation, now credited as the discovery of Epimetheus. At the time, astronomers believed that there was only one moon, unofficially known as "Janus," in the given orbit. Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realized that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Voyager I confirmed this in 1980, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.
The Cassini spacecraft has made several close approaches and provided detailed images of the moon since it achieved orbit around Saturn in 2004.
How Epimetheus Got its Name:
John Herschel suggested that the moons of Saturn be associated with the mythical brothers and sisters of Kronus. (Kronus is the equivalent of the Roman god Saturn in Greek mythology.) The International Astronomical Union now controls the official naming of astronomical bodies.
The name Epimetheus comes from the Greek god (or Titan) Epimetheus (or hindsight), who was the brother of Prometheus (foresight). Together, they represented humanity. Epimetheus and Prometheus' father is Iapetus (who is one of Kronus' brothers).
The craters on Epimetheus include Hilaeira (who was a priestess of Artemis and Athena) and Pollux (who was a warrior in "The Iliad" and who carried off Hilaeira).
Astronomers also refer to Epimetheus as Saturn XI and as S/1980 S3.