Roche lobe

The term Roche lobe is used for co-orbiting bodies, particularly interacting binary stars, to describe the area under gravitational influence of one of the bodies, i.e., each has its Roche lobe. In particular, if material from one body "puffs up" (e.g., if the atmosphere expands) enough to exceed its Roche lobe (Roche lobe overflow, RLOF or RLO), into the other's, then mass transfer takes place. The Roche lobes meet at the L1 Lagrangian point. Each can be described as the region in which material stationary in relation to the body is drawn toward that body. I believe the term lobe is to describe its shape, which is not spherical, but has an extension, giving it a water-droplet-like shape, a conical point reaching the L1 point. The lobes are based not merely on gravitation but include the effects of centrifugal force (i.e., inertia) due to the orbital motion.

The following terms also incorporate the name Roche and are somewhat related, having to do with the gravity of orbiting bodies, but do not refer to any aspect of a Roche lobe.

The term Roche limit (aka Roche radius) specifically refers to tidal forces pulling bodies apart, and is affected by the body's radius, to deal with competing forces within the body itself.

A body's Hill sphere (aka Roche sphere) can be thought of as the largest spherical volume within the volume of its influence, and the concept is used in conjunction to orbit stability. Its radius (called the Hill radius, which is not the same as the above-mentioned Roche radius) is easily calculated, whereas the exact boundary of influence (surrounding the Roche lobe) is more difficult to calculate (requiring computation) and for some purposes is approximated by sphere extending to the L1 point.