Characteristic energy
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In astrodynamics a characteristic energy (
) is a measure of the energy required for an interplanetary mission that requires attaining an excess orbital velocity over an escape velocity required for additional orbital maneuvers. The unit of the characteristic energy is km2s-2.
Characteristic energy can be computed as:
where
is the orbital velocity when the orbital distance tends to infinity. Note that, since the kinetic energy is one half mv2, C3 is in fact equal to twice the specific energy.
[edit] Parabolic trajectory
For a spacecraft that is leaving the central body (e.g. earth) on a parabolic trajectory:
[edit] Hyperbolic trajectory
For a spacecraft that is leaving the central body on a hyperbolic trajectory:
where:
is 
is length of semi-major axis of orbit's hyperbola.
