For several objects in the Solar System, the value of μ is known to greater accuracy than either G or M. The SI unit of the standard gravitational parameter is m3⋅s−2. However, the unit km3⋅s−2 is frequently used in the scientific literature and in spacecraft navigation.
Small body orbiting a central bodyedit
The central body in an orbital system can be defined as the one whose mass (M) is much larger than the mass of the orbiting body (m), or M ≫ m. This approximation is standard for planets orbiting the Sun or most moons and greatly simplifies equations. Under Newton's law of universal gravitation, if the distance between the bodies is r, the force exerted on the smaller body is:
Thus only the product of G and M is needed to predict the motion of the smaller body. Conversely, measurements of the smaller body's orbit only provide information on the product, μ, not G and M separately. The gravitational constant, G, is difficult to measure with high accuracy, while orbits, at least in the solar system, can be measured with great precision and used to determine μ with similar precision.
for elliptic and hyperbolic orbits, μ is twice the semi-major axis times the negative of the specific orbital energy, where the latter is defined as the total energy of the system divided by the reduced mass.
In a pendulumedit
The standard gravitational parameter can be determined using a pendulum oscillating above the surface of a body as:
where r is the radius of the gravitating body, L is the length of the pendulum, and T is the period of the pendulum (for the reason of the approximation see Pendulum in mechanics).
Geocentric gravitational constantedit
GMEarth, the gravitational parameter for the Earth as the central body, is called the geocentric gravitational constant. It equals (3.986004418±0.000000008)×1014 m3⋅s−2.
The value of this constant became important with the beginning of spaceflight in the 1950s, and great effort was expended to determine it as accurately as possible during the 1960s. Sagitov (1969) cites a range of values reported from 1960s high-precision measurements, with a relative uncertainty of the order of 10−6.
During the 1970s to 1980s, the increasing number of artificial satellites in Earth orbit further facilitated high-precision measurements,
and the relative uncertainty was decreased by another three orders of magnitude, to about 2×10−9 (1 in 500 million) as of 1992.
Measurement involves observations of the distances from the satellite to Earth stations at different times, which can be obtained to high accuracy using radar or laser ranging.
Heliocentric gravitational constantedit
GM☉, the gravitational parameter for the Sun as the central body,
is called the heliocentric gravitational constant or geopotential of the Sun and equals (1.32712440042±0.0000000001)×1020 m3⋅s−2.
The relative uncertainty in GM☉, cited at below 10−10 as of 2015, is smaller than the uncertainty in GMEarth
because GM☉ is derived from the ranging of interplanetary probes, and the absolute error of the distance measures to them is about the same as the earth satellite ranging measures, while the absolute distances involved are much bigger.
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