View Full Version : How far?

2010-Jul-04, 04:44 AM
Hi! What is the minimal distance between a moon and its planet for it to not be locked? Most of the major moons in the solar are tidally locked to their primaries. This is because they are relatively close to them. However, Phoebe is far enough away from Saturn that it is not tidally locked because its orbital period is 550 days (from Saturn) while its rotational period is around 9 hours.

2010-Jul-04, 05:55 AM
It depends on lots of variables not only distance but the masses of the moon and the planet, initial rate of rotation, time elapsed and so forth. There is an estimate in Wikepedias article on tidal locking. (http://en.wikipedia.org/wiki/Tidal_locking#Timescale)

2010-Jul-04, 06:09 AM
Indeed, as glappkaeft noted, the time elapsed is important.

The younger the planet/moon is, the less likely it is to be locked. There's a "tidal lock" radius between a planet and a moon of a certain size and density. That radius defines the distance within which we may expect that moon to be tidally locked. It grows outward with time to infinity.