In the gravitational two-body problem, the specific orbital energy ε ε (or vis-viva energy) of two orbiting bodies is the constant sum of their mutual potential energy ( ε ε p ) and their total kinetic energy ( ε ε k ), divided by the reduced mass. According to the orbital energy conservation equation (also referred to as vis-viva equation), it does not vary with time: ε ε = ε ε k + ε ε p = v 2 2 − − μ μ r = − − 1 2 μ μ 2 h 2 ( 1 − − e 2 ) = − − μ μ 2 a where