Vis-viva equation explained

In astrodynamics, the vis-viva equation, also referred to as orbital-energy-invariance law or Burgas formula^[1] , is one of the equations that model the motion of orbiting bodies. It is the direct result of the principle of conservation of mechanical energy which applies when the only force acting on an object is its own weight which is the gravitational force determined by the product of the mass of the object and the strength of the surrounding gravitational field.

Vis viva (Latin for "living 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 total 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.

Equation

For any Keplerian orbit (elliptic, parabolic, hyperbolic, or radial), the vis-viva equation^[2] is as follows:^[3] $$v^2\; =\; GM\; \backslash left(-\; \backslash right)$$where:

• is the relative speed of the two bodies
• is the distance between the two bodies' centers of mass
• is the length of the semi-major axis (for ellipses, or for parabolas, and for hyperbolas)
• is the gravitational constant
• is the mass of the central body

The product of can also be expressed as the standard gravitational parameter using the Greek letter .

Derivation for elliptic orbits (0 ≤ eccentricity < 1)

In the vis-viva equation the mass of the orbiting body (e.g., a spacecraft) is taken to be negligible in comparison to the mass of the central body (e.g., the Earth). The central body and orbiting body are also often referred to as the primary and a particle respectively. In the specific cases of an elliptical or circular orbit, the vis-viva equation may be readily derived from conservation of energy and momentum.

Specific total energy is constant throughout the orbit. Thus, using the subscripts and to denote apoapsis (apogee) and periapsis (perigee), respectively,$$\backslash varepsilon\; =\; \backslash frac\; -\; \backslash frac\; =\; \backslash frac\; -\; \backslash frac$$

Rearranging,$$\backslash frac\; -\; \backslash frac\; =\; \backslash frac\; -\; \backslash frac$$

Recalling that for an elliptical orbit (and hence also a circular orbit) the velocity and radius vectors are perpendicular at apoapsis and periapsis, conservation of angular momentum requires specific angular momentum

h = r _{pv} _{p} = r _{av} _{a} = constant , thus v _{p} = (r _{a})/(r _{p}) v _{a} :$$\backslash frac\; \backslash left(1-\backslash frac\; \backslash right)\; v\_a^2\; =\; \backslash frac\; -\; \backslash frac$$$$\backslash frac\; \backslash left(\backslash frac\; \backslash right)\; v\_a^2\; =\; \backslash frac\; -\; \backslash frac$$

Isolating the kinetic energy at apoapsis and simplifying,

From the geometry of an ellipse,

2 a = r _{p+r} _{a} where a is the length of the semimajor axis. Thus,$$\backslash frac\; v\_a^2\; =\; GM\; \backslash frac\; =\; GM\; \backslash left(\backslash frac\; -\; \backslash frac\; \backslash right)\; =\; \backslash frac\; -\; \backslash frac$$

Substituting this into our original expression for specific orbital energy,$$\backslash varepsilon\; =\; \backslash frac\; -\; \backslash frac\; =\; \backslash frac\; -\; \backslash frac\; =\; \backslash frac\; -\; \backslash frac\; =\; -\; \backslash frac$$

Thus,

\varepsilon = - (GM)/(2 a) and the vis-viva equation may be written$$\backslash frac\; -\; \backslash frac\; =\; -\backslash frac$$or$$v^2\; =\; GM\; \backslash left(\backslash frac\; -\; \backslash frac\; \backslash right)$$

Therefore, the conserved angular momentum can be derived using

r _{a} + r _{p} = 2 a and r _{a} r _{p} = b ^{2} , where is semi-major axis and is semi-minor axis of the elliptical orbit, as follows:$$v\_a^2\; =\; GM\; \backslash left(\backslash frac\; -\; \backslash frac\; \backslash right)\; =\; \backslash frac\; \backslash left(\backslash frac\; \backslash right)\; =\; \backslash frac\; \backslash left(\backslash frac\; \backslash right)\; =\; \backslash frac\; \backslash left(\backslash frac\; \backslash right)^2$$and alternately,$$v\_p^2\; =\; GM\; \backslash left(\backslash frac\; -\; \backslash frac\; \backslash right)\; =\; \backslash frac\; \backslash left(\backslash frac\; \backslash right)\; =\; \backslash frac\; \backslash left(\backslash frac\; \backslash right)\; =\; \backslash frac\; \backslash left(\backslash frac\; \backslash right)^2$$

Therefore, specific angular momentum

h = r _{p} v _{p} = r _{a} v _{a} = b \sqrt{ (GM)/(a) }, and

Total angular momentum

L = mh = mb \sqrt{ (GM)/(a) }

Practical applications

Given the total mass and the scalars and at a single point of the orbit, one can compute:

• and at any other point in the orbit;^[4] and

\varepsilon , allowing an object orbiting a larger object to be classified as having not enough energy to remain in orbit, hence being "suborbital" (a ballistic missile, for example), having enough energy to be "orbital", but without the possibility to complete a full orbit anyway because it eventually collides with the other body, or having enough energy to come from and/or go to infinity (as a meteor, for example).

The formula for escape velocity can be obtained from the Vis-viva equation by taking the limit as

a approaches inf ty :$$v\_e^2\; =\; GM\; \backslash left(\backslash frac-0\; \backslash right)\; \backslash rightarrow\; v\_e\; =\; \backslash sqrt$$

Notes and References

1. Ivanov, Stefan: XXV Национална олимпиада по астрономия, Бургас, 06-08.05.2022, Полезни формули и справочни данни (Useful formulas and reference data)
2. Book: Tom Logsdon. Orbital Mechanics: Theory and Applications. 1998. John Wiley & Sons. 978-0-471-14636-0.
3. Book: Fundamental Planetary Sciences : physics, chemistry, and habitability . Lissauer. Jack J. . de Pater. Imke . 2019 . Cambridge University Press . 9781108411981 . New York, NY, USA . 29–31 .
4. For the three-body problem there is hardly a comparable vis-viva equation: conservation of energy reduces the larger number of degrees of freedom by only one.