12th Largest Moon of Saturn - Prometheus (4th Moon outwards from Saturn)

Prometheus is an inner satellite of Saturn.


Prometheus is the 12th largest moon of Saturn, and the 49th largest moon in the Solar System currently known.


Discovery

Prometheus was discovered in 1980 (some time before October 25) from photos taken by the Voyager 1 probe.



Naming

Prometheus was provisionally designated S/1980 S27. In late 1985, the moon was officially named after Prometheus, a Titan in Greek mythology.

In Greek mythology, Prometheus is a Titan, the son of Iapetus and Clymene, and brother to Atlas, Epimetheus and Menoetius. He was a champion of mankind, known for his wily intelligence, who stole fire from Zeus and gave it to mortals. Zeus then punished him for his crime by having him bound to a rock while a great eagle ate his liver every day only to have it grow back to be eaten again the next day. His myth has been treated by a number of ancient sources, in which Prometheus is credited with – or blamed for – playing a pivotal role in the early history of mankind.

Stats

Diameter (mean): 86.2 ± 5.4 km

Semi-major axis: 139,380 km

Orbital Period: 0.613 days

Orbit

Prometheus rotates synchronously with its orbital period, keeping one face pointed toward Saturn.

Prometheus acts as a shepherd satellite for the inner edge of Saturn's F Ring. Recent images from the Cassini probe show that the Promethean gravitational field creates kinks and knots in the F Ring as the moon 'steals' material from it.

The orbit of Prometheus appears to be chaotic, as a consequence of a series of four 121:118 mean motion resonances with Pandora. The most appreciable changes in their orbits occur approximately every 6.2 years, when the periapsis of Pandora lines up with the apoapsis of Prometheus and the moons approach to within about 1400 km.

Prometheus is itself a significant perturber of Atlas, with which it is in a 53:54 mean longitude resonance.

Physical Characteristics

Prometheus is extremely elongated, measuring about 136 by 79 by 59 km. It has several ridges and valleys and a number of impact craters of about 20 km diameter are visible, but it is less cratered than nearby Pandora, Epimetheus and Janus.

From its very low density and relatively high albedo, it seems likely that Prometheus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.