Roche limit

The Roche Limit was a scientific principle.

The idea behind the limit was for determining the minimum distance from a planet's center that a "fluid satellite can orbit and remain stable against tidal disruption." If the planet and satellite densities matched equally, the limit was 2,456 x planet's radius.

Solid satellites could exist within the Roche Limit as rock's tensile strength (of which natual, solid satellites were made) held the satellite together.