Roche limit
/ ROHSH /
Bring a moon closer and closer to its planet and at some point the planet's tides win: they pull the moon apart faster than the moon's own gravity can hold it together. The distance at which this happens is the Roche limit — the danger line inside which a body held together only by its own gravity is torn into rubble.
Closer to the planet, the tidal force (the difference in gravity across the moon) grows steeply, while the moon's self-gravity stays the same. The Roche limit is where they cross. For a body bound only by gravity it lies at roughly 2.4 times the planet's radius, scaled by the cube root of how the two densities compare — so a fluffy, low-density moon is shredded farther out than a dense one. Crucially this applies to bodies held together by gravity; a small rock or a spacecraft is held by its material strength and can survive much closer.
The Roche limit explains one of the Solar System's most beautiful features. Saturn's rings sit inside its Roche limit, which is exactly why they are rings of countless small chunks rather than a single moon: any moon that strayed (or formed) there could never coalesce. It also predicts the fate of objects like a comet that wanders too close to a giant planet — Comet Shoemaker–Levy 9 was tidally torn into a string of fragments before they slammed into Jupiter in 1994.
Saturn's spectacular rings lie within its Roche limit: countless icy chunks that can never gather into a moon, kept forever ground apart by the planet's tides — the Roche limit drawn in light across the sky.
Inside the Roche limit, a moon cannot form — only rings can.
The Roche limit assumes a body held together only by its own gravity. Solid objects with real material strength (a small asteroid, a spacecraft) can orbit well inside it intact, so the limit is about gravitationally bound bodies, not every object.