Space · Moons
Epimetheus
A moon of Saturn — Janus's co-orbital partner — swaps places with Janus every four years.
Quick facts
Parent planet
Diameter (mean)
116 km
Mass
5.27 × 10¹⁷ kg
7.17e-06 Moon masses
Mean orbital radius
151,410 km
Orbital period
0.694 Earth days
Discovery year
1980
Discoverer
John Fountain & Stephen Larson
Naming origin
Titan brother of Prometheus, husband of Pandora
Surface conditions
Epimetheus is Janus's slightly smaller co-orbital partner — both share essentially the same orbit at 151,000 km from Saturn, but with mean orbital radii differing by less than 50 km. Their gravitational interaction causes them to swap inner and outer positions every four years. Epimetheus is irregularly shaped, 130×114×106 km, heavily cratered.
Missions and observations
Every Saturn-system mission has had an opportunity to image or characterize Epimetheus. The list below is the Saturn-system mission catalog; specific Epimetheus encounters are documented in mission archives.
| Mission | Year at Saturn | Status |
|---|---|---|
|
Pioneer 11 NASA |
1979 | Completed |
|
Voyager 1 NASA |
1980 | Completed |
|
Voyager 2 NASA |
1981 | Completed |
|
Cassini-Huygens NASA/ESA/ASI |
2004 | Completed |
|
Dragonfly NASA |
2034 | On the way |
Naming etymology
Epimetheus was the Titan brother of Prometheus and husband of Pandora — 'afterthought' to Prometheus's 'forethought.' The IAU adopted the name in 1983.
Methodology & sources
Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.