text: In celestial mechanics, the standard gravitational parameter μ of a celestial body is the product of the gravitational constant G and the total mass M of the bodies. For two bodies, the parameter may be expressed as , or as  when one body is much larger than the other:
  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 . However, the unit  is frequently used in the scientific literature and in spacecraft navigation.
  Definition
  Small body orbiting a central body
  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 . 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: