In astrodynamics, the vis viva equation, also referred to as orbital energy conservation equation, is one of the fundamental equations that govern the motion of orbiting bodies. It is the direct result of the law of conservation of energy, which requires that the sum of kinetic and potential energy is constant at all points along the orbit.
Vis viva (Latin for “live force”) is a term from the history of mechanics, and it survives in this sole context. It represents the principle that the difference between the aggregate work of the accelerating forces of a system and that of the retarding forces is equal to one half the vis viva accumulated or lost in the system while the work is being done.
|v||relative speed of the two bodies (m/s)|
|G||Newtonian constant of gravitation|
|M||mass of the central body (kg)|
|r||radial distance (m)|
|a||semi-major axis (m)|