Photo taken by Cassini on June 12, 2007, showing Atlas as seen from above its south pole
|Discovered by||Terrile, Voyager 1|
|Discovery date||October, 1980|
|Alternative names||Saturn XV|
|Orbital characteristics |
|Epoch December 31, 2003 (JD 2453005.5)|
|Mean orbit radius||137,670 ± 10 km|
|Orbital period||0.6016947883 d|
|Inclination||0.003 ± 0.004°|
|Dimensions||40.8×35.4×18.8 km |
|Mean radius||15.1 ± 0.9 km |
|Mass||6.60 ± 0.45 ×1015 kg |
|Mean density||0.46 ± 0.11 g/cm³ |
|Equatorial surface gravity||0.0002–0.0020 m/s² |
|Escape velocity||~0.0062 km/s|
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 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 in the Atlantean 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 Roche lobe of the moon. This would mean that for any additional particles impacting the equator, the centrifugal force will nearly overcome the tiny Atlantean gravity, and they will likely 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.
- Green, Daniel W. E. (September 9, 2004). "S/2004 S 3, S/2004 S 4, and R/2004 S 1". IAU Circular 8401. Retrieved 2011-12-30.
- Lakdawalla, Emily (June 13, 2007). "Funny little Atlas". The Planetary Society weblog. Retrieved 2011-12-30.
- Marsden, Brian G. (November 13, 1980). "1980 S 28" (discovery). IAU Circular 3539. Retrieved 2011-12-30.
- Marsden, Brian G. (September 30, 1983). "Satellites of Jupiter and Saturn" (naming the moon). IAU Circular 3872. Retrieved 2011-12-30.
- Spitale, J. N.; Jacobson, R. A.; Porco, C. C.; Owen, W. M., Jr. (2006). "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations". The Astronomical Journal 132 (2): 692–710. Bibcode:2006AJ....132..692S. doi:10.1086/505206.
- Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission". Icarus 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025.
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