Orbital resonance is aresonance of twoorbiting bodies exerting a regular, periodicgravitational effect on each other. Theirorbital periods may be related by a ratio of two smallintegers. It is caused by the changinggravitational forces of bodies which go round each other. Thestability of theSolar System was first investigated byLaplace, and there is still much that is not known about it.[1]
As asatellite goes around aplanet or twostars go around each other, the gravitational forces can change, sometimes by a lot. This is partly because orbits are usuallyellipses, not circular, and so the forces change accordingly. Also, the planets and stars are usually notspherical. They spin, and vary in their degree of compression along their axis of rotation (oblateness). This also changes the forces on an orbiting body.
In particular, the forces may beunstable, so the smaller partner may change until the forces are stable (do not change with time).[2] Satellites often end up with one face towards their planet, because that is the most stable position (tidal locking).
There are other stability effects. Gaps inSaturn's rings are caused by the particles shifting into more stable positions. In the rings ofSaturn, theCassini Division is a gap between the inner B Ring and the outer A Ring. It was cleared by a 2:1 resonance with the moonMimas. Jupiter makes similarKirkwood gaps in theasteroid belt.
There is a stability ratio forNeptune andPluto: the 2:3 ratio means Pluto completes two orbits in the time it takes Neptune to complete three.[3]
The area ofmechanics which is used for these studies is calledcelestial mechanics.
