Roche limit

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A planet's or star's Roche limit is the smallest distance a satellite (which is held together only by gravitational forces) can approach it without being disintegrated by tidal forces. Generally, there are two forms of the Roche limit: one for a solid satellite, an other, bigger one for a fluid satellite.

According to the NASA/IPAC extragalactic database, for a satellite of negligible mass, zero tensile strength, and the same mean density as its primary, in a circular orbit around its primary, this critical distance is 2.44 times the radius of the primary. (For the Moon, whose density is lower than that of Earth, the Roche limit would be 2.9 Earth radii.)[1]

Thus the Moon couldn't circle the Earth in a distance less than 18,500 km (11,500 miles).

As the Roche lobe and the Roche sphere, the Roche limit is named after the French physicist Édouard Albert Roche (1820 – 1883).



  1. Caltech Astronomical Glossary