Part of a series on
Astrodynamics
Orbital mechanics
Orbital elements Apsis Argument of periapsis Eccentricity Inclination Mean anomaly Orbital nodes Semi-major axis True anomaly
Types of two-body orbits by eccentricity Circular orbit Elliptic orbit Transfer orbit (Hohmann transfer orbit Bi-elliptic transfer orbit) Parabolic orbit Hyperbolic orbit Radial orbit Decaying orbit
Equations Dynamical friction Escape velocity Kepler's equation Kepler's laws of planetary motion Orbital period Orbital velocity Surface gravity Specific orbital energy Vis-viva equation
Celestial mechanics
Gravitational influences Barycenter Hill sphere Perturbations Sphere of influence
N-body orbits Lagrangian points (Halo orbits) Lissajous orbits Lyapunov orbits
Engineering and efficiency
Preflight engineering Mass ratio Payload fraction Propellant mass fraction Tsiolkovsky rocket equation
Efficiency measures Gravity assist Oberth effect

In the gravitational two-body problem, the specific orbital energy ${\displaystyle \varepsilon }$ (or vis-viva energy) of two orbiting bodies is the constant sum of their mutual potential energy (${\displaystyle \varepsilon _{p}}$) and their total kinetic energy (${\displaystyle \varepsilon _{k}}$), divided by the reduced mass.^[1] According to the orbital energy conservation equation (also referred to as vis-viva equation), it does not vary with time:

$${\displaystyle {\begin{aligned}\varepsilon &=\varepsilon _{k}+\varepsilon _{p}\\&={\frac {v^{2}}{2}}-{\frac {\mu }{r}}=-{\frac {1}{2}}{\frac {\mu ^{2}}{h^{2}}}\left(1-e^{2}\right)=-{\frac {\mu }{2a}}\end{aligned}}}$$

where

• ${\displaystyle v}$ is the relative orbital speed;
• ${\displaystyle r}$ is the orbital distance between the bodies;
• ${\displaystyle \mu ={G}(m_{1}+m_{2})}$ is the sum of the standard gravitational parameters of the bodies;
• ${\displaystyle h}$ is the specific relative angular momentum in the sense of relative angular momentum divided by the reduced mass;
• ${\displaystyle e}$ is the orbital eccentricity;
• ${\displaystyle a}$ is the semi-major axis.

It is typically expressed in ${\displaystyle {\frac {\text{MJ}}{\text{kg}}}}$ (megajoule per kilogram) or ${\displaystyle {\frac {{\text{km}}^{2}}{{\text{s}}^{2}}}}$ (squared kilometer per squared second). 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 (parabolic orbit). 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.

Equation forms for different orbits

For an elliptic orbit, the specific orbital energy equation, when combined with conservation of specific angular momentum at one of the orbit's apsides, simplifies to:^[2]

$${\displaystyle \varepsilon =-{\frac {\mu }{2a}}}$$

where

• ${\displaystyle \mu =G\left(m_{1}+m_{2}\right)}$ is the standard gravitational parameter;
• ${\displaystyle a}$ is semi-major axis of the orbit.

Proof

For an elliptic orbit with specific angular momentum h given by

$${\displaystyle h^{2}=\mu p=\mu a\left(1-e^{2}\right)}$$

we use the general form of the specific orbital energy equation,

$${\displaystyle \varepsilon ={\frac {v^{2}}{2}}-{\frac {\mu }{r}}}$$

with the relation that the relative velocity at periapsis is

$${\displaystyle v_{p}^{2}={h^{2} \over r_{p}^{2}}={h^{2} \over a^{2}(1-e)^{2}}={\mu a\left(1-e^{2}\right) \over a^{2}(1-e)^{2}}={\mu \left(1-e^{2}\right) \over a(1-e)^{2}}}$$

Thus our specific orbital energy equation becomes

$${\displaystyle \varepsilon ={\frac {\mu }{a}}{\left[{1-e^{2} \over 2(1-e)^{2}}-{1 \over 1-e}\right]}={\frac {\mu }{a}}{\left[{(1-e)(1+e) \over 2(1-e)^{2}}-{1 \over 1-e}\right]}={\frac {\mu }{a}}{\left[{1+e \over 2(1-e)}-{2 \over 2(1-e)}\right]}={\frac {\mu }{a}}{\left[{e-1 \over 2(1-e)}\right]}}$$

and finally with the last simplification we obtain:

$${\displaystyle \varepsilon =-{\mu \over 2a}}$$

For a parabolic orbit this equation simplifies to

$${\displaystyle \varepsilon =0.}$$

For a hyperbolic trajectory this specific orbital energy is either given by

$${\displaystyle \varepsilon ={\mu \over 2a}.}$$

or the same as for an ellipse, depending on the convention for the sign of a.

In this case the specific orbital energy is also referred to as characteristic energy (or ${\displaystyle C_{3}}$) and is equal to the excess specific energy compared to that for a parabolic orbit.

It is related to the hyperbolic excess velocity ${\displaystyle v_{\infty }}$ (the orbital velocity at infinity) by

$${\displaystyle 2\varepsilon =C_{3}=v_{\infty }^{2}.}$$

It is relevant for interplanetary missions.

Thus, if orbital position vector (${\displaystyle \mathbf {r} }$) and orbital velocity vector (${\displaystyle \mathbf {v} }$) are known at one position, and ${\displaystyle \mu }$ is known, then the energy can be computed and from that, for any other position, the orbital speed.

Rate of change

For an elliptic orbit the rate of change of the specific orbital energy with respect to a change in the semi-major axis is

$${\displaystyle {\frac {\mu }{2a^{2}}}}$$

where

• ${\displaystyle \mu ={G}(m_{1}+m_{2})}$ is the standard gravitational parameter;
• ${\displaystyle a\,\!}$ is semi-major axis of the orbit.

In the case of circular orbits, this rate is one half of the gravitation at the orbit. This corresponds to the fact that for such orbits the total energy is one half of the potential energy, because the kinetic energy is minus one half of the potential energy.

Additional energy

If the central body has radius R, then the additional specific energy of an elliptic orbit compared to being stationary at the surface is

$${\displaystyle -{\frac {\mu }{2a}}+{\frac {\mu }{R}}={\frac {\mu (2a-R)}{2aR}}}$$

The quantity ${\displaystyle 2a-R}$ is the height the ellipse extends above the surface, plus the periapsis distance (the distance the ellipse extends beyond the center of the Earth). For the Earth and ${\displaystyle a}$ just little more than ${\displaystyle R}$ the additional specific energy is ${\displaystyle (gR/2)}$; which is the kinetic energy of the horizontal component of the velocity, i.e. ${\textstyle {\frac {1}{2}}V^{2}={\frac {1}{2}}gR}$, ${\displaystyle V={\sqrt {gR}}}$.

Examples

Voyager 1

For Voyager 1, with respect to the Sun:

• ${\displaystyle \mu =GM}$ = 132,712,440,018 km³⋅s⁻² is the standard gravitational parameter of the Sun
• r = 17 billion kilometers
• v = 17.1 km/s

Hence:

$${\displaystyle \varepsilon =\varepsilon _{k}+\varepsilon _{p}={\frac {v^{2}}{2}}-{\frac {\mu }{r}}=\mathrm {146\,km^{2}s^{-2}} -\mathrm {8\,km^{2}s^{-2}} =\mathrm {138\,km^{2}s^{-2}} }$$

Thus the hyperbolic excess velocity (the theoretical orbital velocity at infinity) is given by

$${\displaystyle v_{\infty }=\mathrm {16.6\,km/s} }$$

However, Voyager 1 does not have enough velocity to leave the Milky Way. The computed speed applies far away from the Sun, but at such a position that the potential energy with respect to the Milky Way as a whole has changed negligibly, and only if there is no strong interaction with celestial bodies other than the Sun.

Applying thrust

Assume:

• a is the acceleration due to thrust (the time-rate at which delta-v is spent)
• g is the gravitational field strength
• v is the velocity of the rocket

Then the time-rate of change of the specific energy of the rocket is ${\displaystyle \mathbf {v} \cdot \mathbf {a} }$: an amount ${\displaystyle \mathbf {v} \cdot (\mathbf {a} -\mathbf {g} )}$ for the kinetic energy and an amount ${\displaystyle \mathbf {v} \cdot \mathbf {g} }$ for the potential energy.

The change of the specific energy of the rocket per unit change of delta-v is

$${\displaystyle {\frac {\mathbf {v\cdot a} }{|\mathbf {a} |}}}$$

which is |v| times the cosine of the angle between v and a.

Thus, when applying delta-v to increase specific orbital energy, this is done most efficiently if a is applied in the direction of v, and when |v| is large. If the angle between v and g is obtuse, for example in a launch and in a transfer to a higher orbit, this means applying the delta-v as early as possible and at full capacity. See also gravity drag. When passing by a celestial body it means applying thrust when nearest to the body. When gradually making an elliptic orbit larger, it means applying thrust each time when near the periapsis.

When applying delta-v to decrease specific orbital energy, this is done most efficiently if a is applied in the direction opposite to that of v, and again when |v| is large. If the angle between v and g is acute, for example in a landing (on a celestial body without atmosphere) and in a transfer to a circular orbit around a celestial body when arriving from outside, this means applying the delta-v as late as possible. When passing by a planet it means applying thrust when nearest to the planet. When gradually making an elliptic orbit smaller, it means applying thrust each time when near the periapsis.

If a is in the direction of v:

$${\displaystyle \Delta \varepsilon =\int v\,d(\Delta v)=\int v\,adt}$$

Tangential velocities at altitude

The lower axis gives orbital speeds of some orbits

See also

• Specific energy change of rockets
• Characteristic energy C3 (Double the specific orbital energy)

References