Roche Limit

Synonyms

Roche radius

Definition

The Roche limit is the orbital distance below which a satellite is tidally destroyed by the body around which it is orbiting. Édouard Roche is the French astronomer who first calculated this theoretical limit in 1848.

Overview

Tidal forces are simply the difference in gravitational forces felt by different parts of an extended body. The force of gravity felt by a satellite orbiting a primary body (e.g., by a moon orbiting a planet or a planet orbiting a star) decreases as the inverse square of the orbital distance such that different parts of the satellite feel different gravitational forces. From the point of view of the satellite, this results in a stretching because the side that is closest to the primary body is pulled more strongly than the side farthest from the primary (see Fig. 1).

Roche Limit, Fig. 1

Tidal forces, as seen in the absolute reference frame (top) and in the body’s reference frame (bottom)