By assumption, the perturbing potential h (r) is a conservative central force, which implies that the total energy E and angular momentum vector L are conserved. Thus, the motion still lies in a plane perpendicular to L and the magnitude A is conserved, from the equation A = m k + 2 mEL. The perturbation potential h (r) may be any sort of function, but should be significantly weaker than the main inverse-square force between the two bodies.