Vis-viva equation 1
Vis-viva equation
Astrodynamics
Orbital mechanics
• v
• t
• e [1]
In astrodynamics, the vis viva equation, also referred to as orbital energy conservation equation, is one of the
fundamental and useful 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.
Vis viva equation
For any Kepler orbit (elliptic, parabolic, hyperbolic or radial), the vis viva equation[2] is as follows:
where:
• is the relative speed of the two bodies
• is the distance between the two bodies
• is the semi-major axis (a>0 for ellipses, or =0 for parabolas, and a<0 for hyperbolas)
• is the gravitational constant
• is the mass of the central body
Derivation
In the vis-viva equation the mass m of the orbiting body (e.g., a spacecraft) is taken to be negligible in comparison to
the mass M of the central body (e.g., the Earth). 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 a and p to denote apoapsis (apogee)
and periapsis (perigee), respectively,
Rearranging,
Vis-viva equation 2
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 , thus
Isolating the kinetic energy at apoapsis and simplifying,
From the geometry of an ellipse, where a is the length of the semimajor axis. Thus,
Substituting this into our original expression for specific orbital energy,
Thus, and the vis-viva equation may be written
or
Practical applications
Given the total mass and the scalars r and v at a single point of the orbit, one can compute r and v at any other point
in the orbit.[3]
Given the total mass and the scalars r and v at a single point of the orbit, one can compute the specific orbital energy
, 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).
Vis-viva equation 3
References
[1] http:/ / en. wikipedia. org/ w/ index. php?title=Template:Astrodynamics& action=edit
[2] Tom Logsdon, Orbital Mechanics: theory and applications (http:/ / books. google. de/ books?id=C70gQI5ayEAC), John Wiley & Sons, 1998
[3] 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.
Article Sources and Contributors 4
Article Sources and Contributors
Vis-viva equation Source: http://en.wikipedia.org/w/index.php?oldid=561841711 Contributors: 0.39, Albmont, Brianhicks, CsDix, Cutler, Doradus, EDG, Fortdj33, Gaius Cornelius, Gjp23,
GregorB, JabberWok, Latifahphysics, Mihoshi, NOrbeck, Nehpyhxin, Nr4ps, Patrick, Plrk, Rich257, Salih, Stevan White, Steve Pucci, Stoph, Student7, Teapeat, The Thing That Should Not Be,
Wolfkeeper, Wwoods, 19 anonymous edits
Image Sources, Licenses and Contributors
File:Angular Parameters of Elliptical Orbit.png Source: http://en.wikipedia.org/w/index.php?title=File:Angular_Parameters_of_Elliptical_Orbit.png License: Creative Commons
Attribution-ShareAlike 3.0 Unported Contributors: Adam majewski, Aldaron, Docu, Peo, Snaily, W!B:, Ysangkok, 5 anonymous edits
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