- Sphere of influence (astrodynamics)
A sphere of influence (SOI) in
astrodynamicsand astronomyis the spherical region around a celestial bodywhere the primary gravitational influence on an orbiting object is that body. This is usually used to describe the areas in our solar systemwhere planetsdominate the orbits of surrounding objects (such as moons), despite the presence of the much more massive (but distant) Sun. In a more general sense, the patched conic approximation is only valid within the SOI.
The general equation describing the
radiusof the sphere of a planet:: where: is the semimajor axisof the planet's orbit relative to the largest body in the system, usually the Sun: and are the masses of the planet and Sun, respectively.
In the patched conic approximation, once an object leaves the planet's SOI, the primary/only gravitational influence is the Sun (until the object enters another body's SOI). Since the definition of rSOI relies on the presence of the Sun and a planet, the term is only applicable in a three body or greater system.
Table of planetary SOI radii
coauthors= Donald D. Mueller, Jerry E. White
title= Fundamentals of Astrodynamics
coauthors= Astore, William J., Giffen, Robert B., Larson, Wiley J.
editor=Kirkpatrick, Douglas H.
title= Understanding Space: An Introduction to Astronautics
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