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In the gravitational two-body problem, the specific orbital energy ε {\displaystyle \varepsilon } of two orbiting bodies is the constant sum of their mutual potential energy and their total kinetic energy , divided by the reduced mass. According to the orbital energy conservation equation , it does not vary with time:
It is expressed in MJ or k g ⋅ k m 2 s 2 {\displaystyle {\frac {kg\cdot km^{2}}{s^{2}}}}. For an elliptic orbit the specific orbital energy is the negative of the additional energy required to accelerate a mass of one kilogram to escape velocity. For a hyperbolic orbit, it is equal to the excess energy compared to that of a parabolic orbit. In this case the specific orbital energy is also referred to as characteristic energy.