Atlas (pronounced /ˈætləs/ AT-ləs, or as Greek Άτλας) is an inner satellite of Saturn.
Atlas was discovered by Richard Terrile in 1980 (some time before November 12) from Voyager photos and was designated S/1980 S 28. In 1983 it was officially named after Atlas of Greek mythology, because it "holds the rings on its shoulders" like the Titan Atlas held the sky up above the Earth. It is also designated as Saturn XV.
Atlas is the closest satellite to the sharp outer edge of the A ring, and was long thought to be a shepherd satellite for this ring. However, now it is known that the outer edge of the ring is instead maintained by a 7:6 orbital resonance with the larger but more distant moons Janus and Epimetheus. In 2004 a faint, thin ring, temporarily designated R/2004 S 1, was discovered within Atlas's orbit.
High-resolution images taken in June 2005 by Cassini reveal a saucer-shaped moon with a large smooth equatorial ridge. The most likely explanation for this unusual and prominent structure is that ring material swept up by the moon accumulates on the moon, with a strong preference for the equator due to the ring's thinness. In fact, the size of the equatorial ridge is comparable with the expected size of Atlas's Roche lobe. This would mean that for any further particles attempting to accumulate on the equator, the centrifugal force overcomes Atlas's tiny gravity and they will be lost.
Atlas is significantly perturbed by Prometheus and to a lesser degree by Pandora, leading to excursions in longitude of up to 600 km (~0.25°) away from the precessing Keplerian orbit with a rough period of about 3 years. Since the orbits of Prometheus and Pandora are chaotic, it is suspected that Atlas's may be as well.
Atlas, as imaged by Cassini on June 8, 2005
With A and F rings to scale, Cassini
Near the F ring.
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