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Roche limit
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Distance within which tides tear a satellite apart.
Inside about 2.44 R(ρ_planet/ρ_moon)^{1/3}, tides tear apart a fluid satellite; this likely created Saturn's rings.
For Earth–Moon-like densities, about 18,000 km.
Solid bodies with cohesive strength can survive closer.
In more detail
The Roche limit is the distance within which a moon held together by its own gravity is torn apart by a planet's tidal forces. Rings often lie inside the Roche limit.
Real-world uses
- Saturn's rings lie within its Roche limit, so moons there cannot form.
- Comets that pass close to planets can break apart near the Roche limit.
- Planetary scientists explain why rings form near giant planets, where moons would be torn apart by tidal forces.
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Frequently asked questions
Why do rings form inside the limit?
Because material there cannot coalesce into a moon, so it stays as a ring.
Is the Roche limit fixed?
It depends on the densities of the planet and the satellite.
Is the Roche limit a fixed distance?
No. It depends on the densities of the planet and the satellite, so a denser moon can get closer before breaking up.
Related topics in Gravitation & Orbits
Gravity assist · Tides · Tidal locking · Lagrange points · Two-body problem · Effective potential