Roche Limit

The Roche limit is the distance from a planet within which tidal forces would tear apart a moon held together only by its own gravity.

The Roche limit is the minimum distance at which a moon held together only by its own gravity can orbit a planet without being pulled apart by tidal forces. It is named after the French astronomer Édouard Roche, who calculated it in 1848.

Inside this limit, loose material cannot clump into a moon, so it tends to spread into rings. That is why Saturn's rings lie close to the planet. Solid objects held together by their own strength, like spacecraft or small rocky moons, can survive inside it.

The exact distance depends on the densities of the planet and the moon; for icy material around Earth, it is roughly three Earth radii from the centre.