The specific energy (energy per unit mass) of a space vehicle is composed of two components, the specific potential energy and the specific kinetic energy.
The specific orbital energy of two orbiting bodies is the constant sum of their mutual potential energy and their total kinetic energy, divided by the reduced mass.
Under standard assumptions, specific orbital energy (ϵ) of elliptic orbit is negative and the orbital energy conservation equation, also known as the Vis-viva equation, for this orbit can take the form as,
ϵ = -μ /2a
ϵ = <0
v = Orbital speed of body traveling along an elliptical orbit.
μ = is the standard gravitational parameter
r = the distance between the orbiting bodies
a = the length of the semi-major axis