Links
- Wikipedia
- Janus on Wikipedia
Janus
Details
Janus is a cratered inner moon of Saturn that shares almost the same orbit as Epimetheus. Roughly every four years, the two moons exchange momentum and swap which one follows the inner path without ever colliding.
Janus is inseparable from Epimetheus, the smaller moon that shares almost the same path around Saturn. Their arrangement was so unexpected that observations made in 1966 were initially treated as sightings of one object; only years later did astronomers recognize that two distinct moons occupied the neighboring orbits, a configuration confirmed by Voyager 1 in 1980.
About every four Earth years, the inner moon catches the outer one. Their mutual gravity then transfers momentum between them: the inner moon moves outward, the outer moon moves inward, and they effectively exchange orbits while remaining thousands of kilometers apart. No other known pair of moons in the Solar System performs this same co-orbital swap.
Janus is also a physical world rather than only an orbital curiosity. It is an oblong, heavily cratered body made largely of water ice, with such a low density that it is probably a loose rubble pile. Impacts eject dust from Janus and Epimetheus into a faint ring along their shared path, linking the moons’ battered surfaces to Saturn’s wider ring system.
88.7km
Key facts
- Category
- Moons
- Object class
- Co-orbital inner moon
- Composition
- Water ice and rock (rubble pile)
- Temperature
- 76 K surface
Scale context
Where Janus sits on the full axis
Shared physical scale
155km
Together, these objects make the size change around Janus easy to compare at a glance.
Sources
References for Janus
Measurements and descriptive context are compiled by the Scale of Space team from the references below. If you find an error, please let us know.
- Janus — Wikipedia
- Visual reference: Wikimedia Commons (NASA / JPL / Space Science Institute, Public domain)
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Scale of Space is a scroll-based journey through the universe, placing objects on a single logarithmic scale so you can compare size across an unbroken range.
Guides turn parts of that scale into curated essays, while focused views let you explore the same range through specific groups of objects.