# Epimetheus

Saturn XI - 1980S3

Epimetheus [ep-eh-MEE-thee-us] is the fifth satellite of Saturn. It was discovered by R. Walker in 1980. Epimetheus was the son of Iapetus and brother to Prometheus and Atlas. Epimetheus means hindsight in Greek. It has an irregular shape with a size of 144x108x98 kilometers (89x67x61 miles) in diameter. It is traversed by several large and small grooves, valleys and ridges. Several craters larger than 30 km can be seen on its surface. The pervasive cratering indicates that its surface must be several billion years old. The above image shows the shadow of Saturn's F Ring crossing its surface.

Epimetheus and Janus share the same orbit of 151,472 kilometers from Saturn's center or 91,000 kilometers above the cloud tops. They are only separated by about 50 kilometers. As these two satellites approach each other they exchange a little momentum and trade orbits; the inner satellite becomes the outer and the outer moves to the inner position. This exchange happens about once every four years. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites. If this is the case, the disruption must have happened early in the history of the satellite system.

## Epimetheus Statistics

    Discovered by ..................................... R. Walker
    Date of discovery ...................................... 1980

    Mass (kg) .......................................... 5.6e+17*
    Mass (Earth = 1) ................................. 9.3708e-08
    Radius (km) ........................................ 72x54x49
    Radius (Earth = 1) ............................... 1.1289e-02
    Mean density (gm/cm^3) ................................. 0.7*

    Mean distance from Saturn (km) ...................... 151,422
    Rotational period (days) ............................. 0.6942
    Orbital period (days) ................................ 0.6942
    Mean orbital velocity (km/sec) ........................ 15.87

    Orbital eccentricity .................................. 0.009
    Orbital inclination .................................... 0.34°

    Escape velocity (km/sec) ............................. 0.0322

    Visual geometric albedo ................................. 0.8
    Magnitude (Vo) ......................................... 15.7

## Views of Epimetheus

Epimetheus (GIF, 43K)
This image of Epimetheus was acquired by the Voyager 1 spacecraft on November 11, 1980. (Credit: Calvin J. Hamilton)

Conformal Projection of Epimetheus (GIF, 91K; caption)
This image is a shaded relief map of Epimetheus, the smaller co-orbital satellite of Saturn. As with all maps, it is the cartographer's interpretation and not all features are necessarily certain given the limited data available. This interpretation stretches the data as far as is feasible. This image shows two different views of Epimetheus in a Morphographic Conformal Projection. One view shows the leading side and the other the trailing side. (Courtesy Phil Stooke, NSSDC, and NASA)

## References

Stooke, P. J., "The Topography of Epimetheus," Earth, Moon and Planets, 63 (1993), 67-83.

Saturn Pandora Janus

##### Author: Calvin J. Hamilton.