An Etymological Dictionary of Astronomy and Astrophysics

The region of space around one of the bodies in a system of two celestial bodies where a third body of much smaller mass is influenced by the gravitational field of that body. The sphere of influence of a planet with respect to the Sun has a radius given by: R = R_P(M_P/M_S)^2/3, where R_P is the radius of the planet’s orbit around the Sun, M_P is the mass of the planet, and M_S is the solar mass. The sphere of influence of the Earth has a radius of about 927,000 km or slightly under 150 Earth radii. Beyond this limit, a space probe will come under the influence of the Sun.

See also: → sphere; → influence.