Space · Moons
Janus
A moon of Saturn — Co-orbital with Epimetheus — the two moons swap orbital positions every four years.
Quick facts
Parent planet
Diameter (mean)
179 km
Mass
1.90 × 10¹⁸ kg
2.58e-05 Moon masses
Mean orbital radius
151,460 km
Orbital period
0.695 Earth days
Discovery year
1966
Discoverer
Audouin Dollfus
Naming origin
Roman god of beginnings and doorways
Surface conditions
Janus shares its orbit with Epimetheus in one of the more unusual orbital configurations in the solar system: every four years the two moons exchange positions, with the inner one moving outward and the outer one moving inward as their gravitational interaction passes them by. Janus is the larger of the pair; both are irregular, heavily cratered, and likely fragments of a single parent body that broke apart.
Missions and observations
Every Saturn-system mission has had an opportunity to image or characterize Janus. The list below is the Saturn-system mission catalog; specific Janus 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
Janus was the Roman god of doorways, beginnings, and transitions — depicted with two faces, one looking forward, one looking back. The naming reflects the moon's co-orbital relationship with Epimetheus. Adopted by the IAU 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.