co-orbital moon · moon of Saturn
Epimetheus
Janus's smaller co-orbital partner, a battered little moon of Saturn that swaps orbits with its companion every few years.
- Distance from Earth today
- 1.28 billion km8.564 AU · light: 1 h 11 min
- Distance from the Sun today
- 1.41 billion km9.425 AU
- As of
- 3 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 116 km
- Mass
- 0.526 × 10¹⁸ kg
- Density
- 638 kg/m³
- Mean distance from the planet
- 151,400 km
- Orbital period
- 16.7 h
- Orbital eccentricity
- 0.020
- Orbital inclination
- 0.3°
Source of the figures: JPL SSD – Planetary Satellites (public domain).
Discovery: 1977 – John Fountain and Stephen Larson
Description
Epimetheus is a small, irregular moon of Saturn that shares its orbit with the slightly larger Janus. Observations in the 1960s probably caught both moons but treated them as one; John Fountain and Stephen Larson showed in 1977 that two separate objects were involved, and Voyager 1 confirmed it. Every few years the two moons approach each other and swap orbits instead of colliding. Cassini images show Epimetheus as a cratered body with smooth, grooved areas, and a faint dusty ring, thought to be fed by impacts on both moons, shares their orbit.