# Orbital speed

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 the object is or relative to its center of mass.

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, = 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:

## 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.

This law implies that the body moves slower near its apoapsis than near its periapsis, because at the smaller distance along the arc it needs to move faster to cover the same area.

## 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.

$v\approx {2\pi a \over T}\approx {\sqrt {\mu \over a}}$ where v is the orbital velocity, a is the length of the semimajor axis in meters, T is the orbital period, and μ=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

So, when one of the masses is almost negligible compared to the other mass, as the case for Earth and Sun, one can approximate the orbit velocity $v_{o}$ as:

$v_{o}\approx {\sqrt {\frac {GM}{r}}}$ or assuming r equal to the body's radius

$v_{o}\approx {\frac {v_{e}}{\sqrt {2}}}$ 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:

$v_{o}={\frac {2\pi a}{T}}\left[1-{\frac {1}{4}}e^{2}-{\frac {3}{64}}e^{4}-{\frac {5}{256}}e^{6}-{\frac {175}{16384}}e^{8}-\dots \right]$ The mean orbital speed decreases with eccentricity.

## Instantaneous orbital speed

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:

$v={\sqrt {\mu \left({2 \over r}-{1 \over a}\right)}}$ 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. This expression is called the vis-viva equation. 

For the Earth at perihelion, the value is:

${\sqrt {1.327\times 10^{20}~{\text{m}}^{3}{\text{s}}^{-2}\cdot \left({2 \over 1.471\times 10^{11}~{\text{m}}}-{1 \over 1.496\times 10^{11}~{\text{m}}}\right)}}\approx 30,300~{\text{m}}/{\text{s}}$ 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

Orbit Center-to-center
distance
Altitude above
the Earth's surface
Speed Orbital period Specific orbital energy
Earth's own rotation at surface (for comparison— not an orbit) 6,378 km0 km 465.1 m/s (1,674 km/h or 1,040 mph) 23 h 56 min−62.6 MJ/kg
Orbiting at Earth's surface (equator) 6,378 km0 km7.9 km/s (28,440 km/h or 17,672 mph)1 h 24 min 18 sec−31.2 MJ/kg
Low Earth orbit 6,600–8,400 km200–2,000 km
• Circular orbit: 6.9–7.8 km/s (24,840–28,080 km/h or 14,430–17,450 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 km500–39,900 km1.5–10.0 km/s (5,400–36,000 km/h or 3,335–22,370 mph) respectively11 h 58 min−4.7 MJ/kg
Geostationary 42,000 km35,786 km3.1 km/s (11,600 km/h or 6,935 mph)23 h 56 min−4.6 MJ/kg
Orbit of the Moon 363,000–406,000 km357,000–399,000 km0.97–1.08 km/s (3,492–3,888 km/h or 2,170–2,416 mph) respectively27.3 days−0.5 MJ/kg