Saturn’s mystery: two moons swap places every four years
Saturn has over a hundred moons, but two of them — Janus and Epimetheus — perform something unprecedented in the Solar System. They travel along virtually the same orbit, and once every four years they swap places: the inner one becomes the outer one, and vice versa. This is the only known case of such an “orbital dance” among all planetary moons.
Two moons of Saturn move along the same orbit
Janus and Epimetheus themselves are rather unremarkable irregularly shaped icy chunks. Janus is larger, about 178 km across, while Epimetheus is smaller, roughly 117 km. In terms of size, they rank 10th and 11th among Saturn’s moons and don’t even make the top twenty largest moons in the Solar System. In density and brightness, both resemble porous icy bodies — something like cosmic “rubble piles.”
But here’s what makes this pair unique: the difference between their orbits is only 50 kilometers. For comparison, that’s less than the radius of either moon. You’d think two objects on such close orbits would eventually collide. Yet this doesn’t happen — and here’s why.
In this spacecraft image, Janus (right) and Epimetheus appear to nearly touch due to their almost identical orbits, but the photo was taken when the distance between them was 40,000 km.
How Janus and Epimetheus swap places without colliding
It’s all about gravity. The moon on the inner (closer to Saturn) orbit moves slightly faster — by about 30 seconds per revolution. Gradually, it catches up with the outer moon. When the distance between them shrinks, their mutual gravitational attraction comes into play.
The outer moon pulls the inner one forward, giving it additional energy. Meanwhile, the inner one slows down the outer one. Having received the extra impulse, the inner moon “jumps” to a higher orbit — as if rocket engines were momentarily attached to it. The outer moon, having lost some energy, drops lower. The moons swap places, and the process starts all over again.
Since Janus is roughly four times heavier than Epimetheus, the dance affects them differently. Janus’s orbital radius shifts by only 20 km, while Epimetheus “jumps” a full 80 km with each exchange. At the same time, the moons never approach each other closer than 10,000 km — they simply don’t have time to get close because gravity redirects them to new orbits first.
Diagram of the orbital swap: the inner moon “rises,” the outer one “descends”
What are horseshoe orbits in space
If you observe the motion of Janus and Epimetheus from Saturn’s pole in a reference frame rotating with them, their trajectories resemble horseshoes. That’s why this type of orbit is called a “horseshoe orbit.” Something similar is observed with quasi-satellites of Earth and Venus — for example, the asteroid Kamoʻoalewa or Zoozve. But there’s a fundamental difference: Earth is trillions of times more massive than its quasi-satellites, so nearly all the gravitational effect falls on the smaller body. Earth’s orbit remains virtually unchanged.
But Janus and Epimetheus are comparable in mass, and therefore both noticeably react to the gravitational exchange. This is precisely what makes their system unique — nothing like it has been discovered among the moons of other planets so far.
Where did Janus and Epimetheus come from — Saturn’s dancing moons
Where did this pair come from? There’s no definitive answer yet, but the leading hypothesis suggests that Janus and Epimetheus were once a single body that split apart in a collision. At first glance, it seems that such a delicate dance couldn’t have lasted very long. But the facts say otherwise.
Both moons are covered with numerous impact craters — some exceeding 30 km in diameter, with edges smoothed by dust. This indicates a very respectable surface age. Most likely, the gravitational dance of Janus and Epimetheus has been going on for billions of years — almost since the birth of the Saturnian system itself.
Epimetheus close-up, image from the Cassini probe at close range.
Interestingly, the pair doesn’t exist in isolation. Janus and Epimetheus gravitationally interact with particles in Saturn’s A ring, gradually migrating outward. Over time, this will likely destroy the current dance: Epimetheus will settle into a stable position 60 degrees ahead of or behind Janus, becoming its Trojan satellite. Exactly when this will happen remains unknown.
How scientists discovered two moons instead of one
The discovery story of this pair is almost a detective tale in itself. In December 1966, French astronomer Audouin Dollfus noticed a new moon of Saturn and proposed naming it Janus. Three days later, Richard Walker spotted an object on the same orbit. Everyone assumed it was the same moon.
But the observations didn’t add up: the “single” moon appeared in contradictory positions. That’s why it was named Janus — after the two-faced Roman god. For twelve years, astronomers couldn’t figure out the situation. Only in 1978 did Stephen Larson and John Fountain propose the only explanation: two different moons orbit along the same path. Two years later, Voyager 1 confirmed this seemingly incredible hypothesis.
Janus with craters, image from the Cassini spacecraft.
Later, the Cassini probe not only photographed both moons up close but also recorded orbital exchanges in 2006, 2010, and 2014. The next swap occurred in early 2026 — Epimetheus returned to the inner orbit after four years on the outer one.
Why Janus and Epimetheus matter to science
For scientists, this pair serves as a natural laboratory for studying the three-body problem — one of the fundamental challenges of celestial mechanics. Two bodies of comparable mass, endlessly dancing around a common center, are an extremely rare real-world example of how gravity can create stable yet counterintuitive configurations.
Furthermore, the wave structures that the dancing moons create in the A ring allow scientists to more precisely determine the moons’ masses and even study the internal structure of Saturn itself. And the thin dust ring surrounding their shared orbit is a trace of micrometeorite impacts on their surfaces, discovered thanks to Cassini in 2006.
