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The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body. It can be used to refer to either the mean orbital speed, i.e. the average speed as it completes an orbit, or the speed at a particular point in its orbit such as perihelia.
The orbital speed at any position in the orbit can be computed from the distance to the central body at that position, and the specific orbital energy, which is independent of position: the kinetic energy is the total energy minus the potential energy.
In the case of radial motion:
- If the specific orbital energy is positive, the body's kinetic energy is greater than its potential energy: The orbit is thus open, following a hyperbola with focus at the other body. See radial hyperbolic trajectory
- For the zero-energy case, the body's kinetic energy is exactly equal to its potential energy: the orbit is thus a parabola with focus at the other body. See radial parabolic trajectory.
- If the energy is negative, the body's potential energy is greater than its kinetic energy: The orbit is thus closed. The motion is on an ellipse with one focus at the other body. See radial elliptic trajectory, free-fall time.
Transverse orbital speed
The transverse orbital speed is inversely proportional to the distance to the central body because of the law of conservation of angular momentum, or equivalently, Kepler's second law. This states that as a body moves around its orbit during a fixed amount of time, the line from the barycenter to the body sweeps a constant area of the orbital plane, regardless of which part of its orbit the body traces during that period of time.
Mean orbital speed
For orbits with small eccentricity, the length of the orbit is close to that of a circular one, and the mean orbital speed can be approximated either from observations of the orbital period and the semimajor axis of its orbit, or from knowledge of the masses of the two bodies and the semimajor axis.
where v is the orbital velocity, a is the length of the semimajor axis, T is the orbital period, and μ=GM is the standard gravitational parameter. Note that this is only an approximation that holds true when the orbiting body is of considerably lesser mass than the central one, and eccentricity is close to zero.
Taking into account the mass of the orbiting body,
where m1 is the mass of the orbiting body, m2 is the mass of the body being orbited, r is specifically the distance between the two bodies (which is the sum of the distances from each to the center of mass), and G is the gravitational constant. This is still a simplified version; it doesn't allow for elliptical orbits, but it does at least allow for bodies of similar masses.
or assuming r equal to the body's radius
Where M is the (greater) mass around which this negligible mass or body is orbiting, and ve is the escape velocity.
For an object in an eccentric orbit orbiting a much larger body, the length of the orbit decreases with orbital eccentricity e, and is an ellipse. This can be used to obtain a more accurate estimate of the average orbital speed:
The mean orbital speed decreases with eccentricity.
Precise orbital speed
For the precise orbital speed of a body at any given point in its trajectory, both the mean distance and the precise distance are taken into account:
where μ is the standard gravitational parameter, r is the distance at which the speed is to be calculated, and a is the length of the semi-major axis of the elliptical orbit. For the Earth at perihelion,
which is slightly faster than Earth's average orbital speed of 29,800 m/s, as expected from Kepler's 2nd Law.
Tangential velocities at altitude
the Earth's surface
|Speed||Orbital period||Specific orbital energy|
|Standing on Earth's surface at the equator (for comparison – not an orbit)||6,378 km||0 km||465.1 m/s (1,040 mph)||1 day (24h)||−62.6 MJ/kg|
|Orbiting at Earth's surface (equator)||6,378 km||0 km||7.9 km/s (17,672 mph)||1 h 24 min 18 sec||−31.2 MJ/kg|
|Low Earth orbit||6,600–8,400 km||200–2,000 km||circular orbit: 7.8–6.9 km/s (17,450–14,430 mph) respectively
elliptic orbit: 6.5–8.2 km/s respectively
|1 h 29 min – 2 h 8 min||−29.8 MJ/kg|
|Molniya orbit||6,900–46,300 km||500–39,900 km||1.5–10.0 km/s (3,335–22,370 mph) respectively||11 h 58 min||−4.7 MJ/kg|
|Geostationary||42,000 km||35,786 km||3.1 km/s (6,935 mph)||23 h 56 min||−4.6 MJ/kg|
|Orbit of the Moon||363,000–406,000 km||357,000–399,000 km||0.97–1.08 km/s (2,170–2,416 mph) respectively||27.3 days||−0.5 MJ/kg|
- Gamow, George (1962). Gravity. New York: Anchor Books, Doubleday & Co. p. 66. ISBN 0-486-42563-0.
...the motion of planets along their elliptical orbits proceeds in such a way that an imaginary line connecting the Sun with the planet sweeps over equal areas of the planetary orbit in equal intervals of time.
- Wertz, edited by James R. Wertz; Larson, Wiley J. (2010). Space mission analysis and design (3rd ed.). Hawthorne, Calif.: Microcosm. p. 135. ISBN 978-1881883-10-4.
- Horst Stöcker; John W. Harris (1998). Handbook of Mathematics and Computational Science. Springer. p. 386. ISBN 0-387-94746-9.