Janus and Epimetheus

Saturn has two moons that share an orbit. Janus and Epimetheus are considered co-orbital moons. They have a 1:1 resonance with each other. These two moons travel around Saturn in a horseshoe orbit.

Every 3.5 years as one moon almost catches up with the other, they exchange orbital energy. The faster one slows down a little while the slower one speeds up. The slow one becomes the fast one and the fast one becomes the slow one. This prevents them from ever passing each other, as they virtually repel each other. They will meet again on the opposite side of Saturn 3.5 years later only to repeat the process.

The simulation janus.gsim shows Janus and Epimetheus in a rotating frame. The initial starting conditions were obtained using data from JPL's Horizons Ephemeris Computation Service. Be patient, it takes a while for 3.5 years of simulation time to elapse. Watch as the moons approach each other, almost touch each other, and then repel each other.

Download janus.gsim

(You need to have the program Gravity Simulator installed on your computer first. Click Here to download Gravity Simulator.)