In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter or, if one object is much more massive than the other bodies in the system, its speed relative to the center of mass of the most massive body.
The term can be used to refer to either the mean orbital speed, i.e. the average speed over an entire orbit, or its instantaneous speed at a particular point in its orbit. Maximum (instantaneous) orbital speed occurs at periapsis (perigee, perihelion, etc.), while minimum speed for objects in closed orbits occurs at apoapsis (apogee, aphelion, etc.). In ideal two-body systems, objects in open orbits continue to slow down forever as their distance to the barycenter increases.
When a system approximates a two-body system, instantaneous orbital speed at a given point of the orbit can be computed from its distance to the central body and the object's specific orbital energy, sometimes called "total energy". Specific orbital energy is constant and independent of position.
In the following, it is assumed that the system is a two-body system and the orbiting object has a negligible mass compared to the larger (central) object. In real-world orbital mechanics, it is the system's barycenter, not the larger object, which is at the focus.
Specific orbital energy, or total energy, is equal to K.E. - P.E. (kinetic energy - potential energy). The sign of the result may be positive, zero, or negative and the sign tells us something about the type of orbit:
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.
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 in meters, T is the orbital period, and ?=GM is the standard gravitational parameter. This is an approximation that only holds true when the orbiting body is of considerably lesser mass than the central one, and eccentricity is close to zero.
When one of the bodies is not of considerably lesser mass see: Gravitational two-body problem
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.
For the instantaneous orbital speed of a body at any given point in its trajectory, both the mean distance and the instantaneous distance are taken into account:
where ? is the standard gravitational parameter of the orbited body, 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. This expression is called the vis-viva equation.
For the Earth at perihelion, the value is:
which is slightly faster than Earth's average orbital speed of 29,800 m/s, as expected from Kepler's 2nd Law.
the Earth's surface
|Speed||Orbital period||Specific orbital energy|
|Earth's own rotation at surface (for comparison-- not an orbit)||6,378km||0km||465.1m/s (1,674km/h or 1,040mph)||23h 56min||-62.6MJ/kg|
|Orbiting at Earth's surface (equator) theoretical||6,378km||0km||7.9km/s (28,440km/h or 17,672mph)||1h 24min 18sec||-31.2MJ/kg|
|Low Earth orbit||6,600-8,400km||200-2,000km||
||1h 29min - 2h 8min||-29.8MJ/kg|
|Molniya orbit||6,900-46,300km||500-39,900km||1.5-10.0km/s (5,400-36,000km/h or 3,335-22,370mph) respectively||11h 58min||-4.7MJ/kg|
|Geostationary||42,000km||35,786km||3.1km/s (11,600km/h or 6,935mph)||23h 56min||-4.6MJ/kg|
|Orbit of the Moon||363,000-406,000km||357,000-399,000km||0.97-1.08km/s (3,492-3,888km/h or 2,170-2,416mph) respectively||27.3days||-0.5MJ/kg|
...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.