Standard gravitational parameter

Body	μ (km3s−2)
Sun	132,712,440,018(8)
Mercury	22,032
Venus	324,859
Earth	398,600.4418(9)
Moon	4,902.7779
Mars	42,828
Ceres	63.1(3)
Jupiter	126,686,534
Saturn	37,931,187
Uranus	5,793,939(13)
Neptune	6,836,529
Pluto	871(5)
Eris	1,108(13)

In astrodynamics, the standard gravitational parameter μ of a celestial body is the product of the gravitational constant G and the mass M of the body.

$\mu=GM \$

The SI units of the standard gravitational parameter are m³s⁻².

Small body orbiting a central body

The relation between properties of mass and their associated physical constants. Every massive object is believed to exhibit all five properties. However, due to extremely large or extremely small constants, it is generally impossible to verify more than two or three properties for any object.

• The Schwarzschild radius (r_s) represents the ability of mass to cause curvature in space and time.
• The standard gravitational parameter (μ) represents the ability of a massive body to exert Newtonian gravitational forces on other bodies.
• Inertial mass (m) represents the Newtonian response of mass to forces.
• Rest energy (E₀) represents the ability of mass to be converted into other forms of energy.
• The Compton wavelength (λ) represents the quantum response of mass to local geometry.

Under standard assumptions in astrodynamics we have:

$m \ll M \$

where m is the mass of the orbiting body, M is the mass of the central body, and G is the standard gravitational parameter of the larger body.

For all circular orbits around a given central body:

$\mu = rv^2 = r^3\omega^2 = 4\pi^2r^3/T^2 \$

where r is the orbit radius, v is the orbital speed, ω is the angular speed, and T is the orbital period.

The last equality has a very simple generalization to elliptic orbits:

$\mu=4\pi^2a^3/T^2 \$

where a is the semi-major axis. See Kepler's third law.

For all parabolic trajectories rv² is constant and equal to 2μ. For elliptic and hyperbolic orbits μ = 2a|ε|, where ε is the specific orbital energy.

Two bodies orbiting each other

In the more general case where the bodies need not be a large one and a small one (the two-body problem), we define:

• the vector r is the position of one body relative to the other
• r, v, and in the case of an elliptic orbit, the semi-major axis a, are defined accordingly (hence r is the distance)
• μ = Gm₁ + Gm₂ = μ₁ + μ₂, where m₁ and m₂ are the masses of the two bodies.

Then:

• for circular orbits, rv² = r³ω² = 4π²r³/T² = μ
• for elliptic orbits, 4π²a³/T² = μ (with a expressed in AU and T in years, and with M the total mass relative to that of the Sun, we get a³/T² = M)
• for parabolic trajectories, rv² is constant and equal to 2μ
• for elliptic and hyperbolic orbits, μ is twice the semi-major axis times the absolute value of the specific orbital energy, where the latter is defined as the total energy of the system divided by the reduced mass.

Terminology and accuracy

Note that the reduced mass is also denoted by $\mu.\!\,$.

The value for the Earth is called the geocentric gravitational constant and equals 398,600.4418±0.0008 km³s⁻². Thus the uncertainty is 1 to 500,000,000, much smaller than the uncertainties in G and M separately (1 to 7,000 each).

The value for the Sun is called the heliocentric gravitational constant and equals 1.32712440018×10²⁰ m³s⁻².

See also

• Astronomical system of units
• Planetary mass