Two small moons revolve around Saturn on trajectories so close that it has long been difficult to distinguish them. Janus and Epimetheus don’t just share the neighborhood: every four years or so, they exchange their inner and outer positions.
The moon on the inner orbit moves a little faster. She gradually catches up with the other. When their rapprochement becomes sufficient, their gravitational interaction modifies the two trajectories: the one that was inside passes outside, and vice versa.
The exchange is not a collision or a crossing of each other. The bodies remain separate. The initial difference between the distances of the two orbits is about fifty kilometers, a tiny gap on the scale of their path around Saturn.
The 1966 observations had initially suggested a single object. Voyager 1 confirmed in 1980 that it was necessary to recognize two moons to understand their movement.
What we know
Exchange of orbits under gravitational interaction; the moons do not cross and do not collide.
To go to the source
References accessed October 11, 2026. Linked pages may be in another language.
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