M e a n   M o t i o n   R e s o n a n c e

A mean motion resonance is a dynamic relationship between two objects orbiting the same primary, such that the ratio of their orbital periods can be expressed by two small integers; e.g., 2:1, 3:1, 4:1, 3:2, or 4:3.

In the Solar System, the classic instance of mean motion resonance is the system of the four Galilean moons of Jupiter. The orbital periods of the three inner Galileans (Io, Europa, and Ganymede) exist in the ratio of 1:2:4. This is the only known instance of a three-way mean motion resonance among the planets and moons of our system. The orbit of the fourth Galilean moon, Callisto, falls just outside a 2:1 resonance with the third moon, Ganymede.

Other mean motion resonances in our system include a 3:2 relationship between the orbits of Neptune and Pluto and a series of resonances among the larger moons of Saturn: 2:1 for Dione/Enceladus and Tethys/Mimas, and 4:3 for Hyperion/Titan.