Surface gravity

Astronaut John Young jumping on the Moon, illustrating that the gravitational pull of the Moon is approximately 1/6 of Earth's. The jumping height is limited by the EVA space suit's weight on the Moon of about 13.6 kg (30 lb) and by the suit's pressurization resisting the bending of the suit, as needed for jumping.

The surface gravity, g, of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. Surface gravity may be understood as the acceleration due to gravity experienced by a hypothetical test particle located very close to the object's surface, which has negligible mass so as not to disturb the system. For objects where the surface lies deep within an atmosphere and the radius is not well defined, the surface gravity is given at the 1-bar pressure level in the atmosphere.

Surface gravity is measured in units of acceleration, which, in the SI system, are meters per second squared. It may also be expressed as a multiple of the Earth's standard surface gravity, which is equal to

g = 9.80665 m/s2

In astrophysics, the surface gravity may be expressed as , which is obtained by first expressing the gravity in cgs units, where the unit of acceleration and surface gravity is centimeters per second squared (cm/s2), and then taking the base-10 logarithm of the cgs value of the surface gravity. Therefore, the surface gravity of Earth could be expressed in cgs units as 980.665 cm/s2, and then taking the base-10 logarithm ("log g") of 980.665, giving 2.992 as "log g".

The surface gravity of a white dwarf is very high, and that of a neutron star is even higher. A white dwarf's surface gravity is around 100,000 g (106 m/s2), while the compactness of a neutron star gives it a surface gravity of up to 7×1012 m/s2, with typical values on the order of 1012 m/s2. This is more than 1011 times that of Earth. One consequence of such immense gravity is that neutron stars have an escape velocity of around 100,000 km/s, about one-third of the speed of light. Since black holes do not have a surface, their surface gravity is not defined.

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