Janus and Epimetheus, the moons that trade orbits
Janus and Epimetheus are two small moons of Saturn that share nearly the same orbit, about 151,400 km from the planet's center. Their orbits differ by only about 50 km, and every four years, as the inner moon catches up with the outer one, their mutual gravity makes them trade places: the inner moon is lifted outward and the outer one drops inward, without the two ever coming close to colliding.
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A unique co-orbital arrangement
Many objects in the solar system share an orbit with a larger body, such as the Trojan moons of Tethys and Dione, which sit 60° ahead of or behind their host. Janus and Epimetheus are different: they are of comparable size, and neither is a satellite of the other. This makes them the only known pair of co-orbital moons that exchange orbits in this way.
The mechanism follows from simple orbital mechanics. The moon on the slightly lower orbit travels a little faster, so it slowly gains on the other, lapping it roughly once every four years. As it closes in from behind, the leading moon's gravity pulls back on the trailing one. A pull opposed to the direction of motion would normally slow an object, but in orbit it does the reverse of what intuition suggests: the inner moon gains orbital energy and rises to the outer orbit, where it moves more slowly. At the same time, the outer moon is tugged forward, loses energy and drops to the inner orbit, where it moves faster. The gap between them then reopens, and the cycle starts over.
Seen in a frame that rotates with the pair, each moon follows a horseshoe-shaped path. The two never pass each other: their closest approach is around 10,000 km, far more than their own sizes.
Who moves more
Because Janus is several times more massive, it barely shifts during a swap, by roughly 20 km in orbital radius, while Epimetheus moves by around 80 km. The exchanges occurred in January of 2006, 2010, 2014, 2018 and 2022; the next fell in early 2026, and the following one is expected around 2030. Cassini watched several of these switches during its mission from 2004 to 2017.
A tangled discovery
Both moons were found during the ring-plane crossing of 1966, when Saturn's rings were seen edge-on from Earth and their glare faded enough to reveal faint objects close to the planet. Audouin Dollfus, observing from the Pic du Midi observatory in France, reported a new moon on 15 December 1966 and named it Janus. Three days later Richard Walker reported an object in a similar orbit.
For more than a decade, astronomers struggled to reconcile the observations, because they could not fit a single orbit to all of them. In 1978 Stephen Larson and John Fountain showed that the data were best explained by two separate moons on almost identical orbits. Voyager 1 confirmed this in 1980, and the second moon received the name Epimetheus. The confusion is now easy to understand: from Earth, two faint points sharing an orbit look like one body that refuses to stay on schedule.
In mythology, Janus is the two-faced Roman god of doorways and beginnings, linked in Roman tradition to the reign of Saturn in Latium (see the Golden Age). Epimetheus is the Titan brother of Prometheus, whose name is also carried by a nearby shepherd moon.
What the two moons look like
Cassini images show two irregular, heavily cratered bodies. Janus is somewhat rounded, while Epimetheus has a few flat facets that may be the remains of large impacts. Both are covered in fine material, which softens the outlines of craters.
Their densities, around 0.6 g/cm³, are far lower than that of solid water ice (about 0.92 g/cm³). This suggests highly porous interiors, probably loose piles of icy rubble held together mostly by their own weak gravity. A popular hypothesis is that the two moons were once a single body broken apart by a collision, although the evidence is circumstantial.
Influence on the rings
Janus and Epimetheus orbit just outside the main rings, about 15,000 km beyond the outer edge of the A ring and beyond the narrow F ring. Their gravity has a measurable effect on ring structure. The outer edge of the A ring is set by a 7:6 resonance with the pair, which means ring particles there complete seven orbits while the moons complete six. Because the moons' orbits change with each swap, the resonance location shifts slightly, and Cassini detected the A ring edge responding to these four-year changes. This role was once attributed to Atlas, which orbits just beyond the edge.
The pair also has its own faint, dusty ring, discovered in Cassini images in 2006. It is probably made of particles knocked off their surfaces by micrometeorite impacts, a process also seen around the small moons between Mimas and Enceladus.
FAQ
Do Janus and Epimetheus ever collide?
No. As one catches up with the other, their gravitational interaction makes them swap orbits before they get close. Their minimum separation is around 10,000 km.
How often do Janus and Epimetheus switch orbits?
About every four years. Swaps took place in January 2006, 2010, 2014, 2018 and 2022, then again in early 2026; the next is due around 2030.
Which is bigger, Janus or Epimetheus?
Janus is larger, at about 180 km in mean diameter versus about 116 km for Epimetheus, and it is roughly 3.6 times more massive. That is why its orbit changes less during each swap.
Who discovered Janus and Epimetheus?
Audouin Dollfus reported Janus in December 1966 and Richard Walker reported a second object days later. Stephen Larson and John Fountain showed in 1978 that there were two moons, and Voyager 1 confirmed it in 1980.
Sources
- NASA Science: Saturn's moons
- NASA: Cassini mission
- Dermott, S. F. and Murray, C. D., "The dynamics of tadpole and horseshoe orbits", Icarus 48 (1981)
- Thomas, P. C., "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission", Icarus 208 (2010)
- El Moutamid, M. et al., "How Janus' orbital swap affects the edge of Saturn's A ring", Icarus 279 (2016)