Prometheus (prōmēˈthēəs) [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn XVI (or S16), Prometheus is an irregularly shaped (nonspherical) body measuring about 90 mi (145 km) by 53 mi (85 km) by 38 mi (62 km); it orbits Saturn at a mean distance of 86,588 mi (139,350 km) and has an orbital period of 0.613 earth days—the rotational period is unknown but is assumed to be the same as the orbital period. It was discovered by a team led by S. Collins in 1980 from an examination of photographs taken by Voyager 1 during its flyby of Saturn. Prometheus has several craters about 12.5 mi (20 km) in diameter and a number of linear ridges and valleys but appears to be less cratered than the neighboring moons Epimetheus, Janus, and Pandora. It is the inner shepherd satellite (a moon that limits the extent of a planetary ring through gravitational forces) of Saturn's F ring.