Rochelle

Meet Rochelle, a moon who keeps a careful distance from her planet, because too close and tides pull her apart.

A moon is pulled harder on its near side than its far side. That difference, the tidal force, tries to stretch it. A small moon held together only by its own gravity survives only if its self-gravity beats the stretch. Édouard Roche worked out the critical distance in 1848. For a fluid satellite it is about 2.44 times the planet's radius times the cube root of the planet-to-moon density ratio.

If the two densities are equal, the limit sits at 2.44 planet radii. A rigid moon, which resists stretching, can come closer, to roughly 1.26 planet radii by the simplest estimate.

Planetary rings, including Saturn's main rings, lie mostly inside the Roche limit, where material cannot gather into a large moon. Comet Shoemaker–Levy 9 broke into fragments after passing close to Jupiter in 1992.

Real moons are held together partly by rock strength, not just gravity, so small solid bodies can survive inside the limit. The 2.44 and 1.26 coefficients bracket idealised cases.