Sphere of influence (astrodynamics)

Short description: Region of space gravitationally dominated by a given body

A sphere of influence (SOI) in astrodynamics and astronomy is the oblate-spheroid-shaped region around a celestial body where the primary gravitational influence on an orbiting object is that body. This is usually used to describe the areas in the Solar System where planets dominate the orbits of surrounding objects such as moons, despite the presence of the much more massive but distant Sun. In the patched conic approximation, used in estimating the trajectories of bodies moving between the neighbourhoods of different masses using a two body approximation, ellipses and hyperbolae, the SOI is taken as the boundary where the trajectory switches which mass field it is influenced by. It is not to be confused with the sphere of activity which extends well beyond the sphere of influence.[1]

The most common base models to calculate the sphere of influence is the Hill sphere and the Laplace sphere, but updated and particularly more dynamic ones have been described.[2][3] The general equation describing the radius of the sphere rSOI of a planet:[4] rSOI≈a(mM)2/5 where

* a is the semimajor axis of the smaller object's (usually a planet's) orbit around the larger body (usually the Sun).
* m and M are the masses of the smaller and the larger object (usually a planet and the 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). Because 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 and requires the mass of the primary body to be much greater than the mass of the secondary body. This changes the three-body problem into a restricted two-body problem.

## Contents

* 1 Table of selected SOI radii
* 2 Increased accuracy on the SOI
* 3 Derivation
* 4 See also
* 5 References
* 6 General references
* 7 External links

## Table of selected SOI radii

Dependence of Sphere of influence rSOI/a on the ratio m/M

The table shows the values of the sphere of gravity of the bodies of the solar system in relation to the Sun (with the exception of the Moon which is reported relative to Earth):[4][5][6][7][8][9][10]

Body  | SOI  | Body Diameter  | Body Mass (1024 kg)  | Distance from Sun

|
|
|
|

(106 km) | (mi) | (radii)  | (km) | (mi)  | (AU) | (106 mi) | (106 km)
Mercury | 0.117 | 72,700 | 46 | 4,878 | 3,031 | 0.33 | 0.39 | 36 | 57.9
Venus | 0.616 | 382,765 | 102 | 12,104 | 7,521 | 4.867 | 0.723 | 67.2 | 108.2
Earth \+ Moon | 0.929 | 577,254 | 145 | 12,742 (Earth) | 7,918 (Earth) | 5.972
(Earth) | 1 | 93 | 149.6
Moon | 0.0643 | 39,993 | 37 | 3,476 | 2,160 | 0.07346 | See Earth + Moon
Mars | 0.578 | 359,153 | 170 | 6,780 | 4,212 | 0.65 | 1.524 | 141.6 | 227.9
Jupiter | 48.2 | 29,950,092 | 687 | 139,822 | 86,881 | 1900 | 5.203 | 483.6 | 778.3
Saturn | 54.5 | 38,864,730 | 1025 | 116,464 | 72,367 | 570 | 9.539 | 886.7 | 1,427.0
Uranus | 51.9 | 32,249,165 | 2040 | 50,724 | 31,518 | 87 | 19.18 | 1,784.0 | 2,871.0
Neptune | 86.2 | 53,562,197 | 3525 | 49,248 | 30,601 | 100 | 30.06 | 2,794.4 | 4,497.1

An important understanding to be drawn from the above table is that "Sphere of Influence" here is "Primary". For example, though Jupiter is much larger in mass than say, Neptune, its Primary SOI is much smaller due to Jupiter's much closer proximity to the Sun.

## Increased accuracy on the SOI

The Sphere of influence is, in fact, not quite a sphere. The distance to the SOI depends on the angular distance θ from the massive body. A more accurate formula is given by[4] rSOI(θ)≈a(mM)2/511+3cos2(θ)10

Averaging over all possible directions we get: rSOI‾=0.9431a(mM)2/5

## Derivation

Consider two point masses A and B at locations rA and rB, with mass mA and mB respectively. The distance R=|rB−rA| separates the two objects. Given a massless third point C at location rC, one can ask whether to use a frame centered on A or on B to analyse the dynamics of C.

Geometry and dynamics to derive the sphere of influence

Consider a frame centered on A. The gravity of B is denoted as gB and will be treated as a perturbation to the dynamics of C due to the gravity gA of body A. Due to their gravitational interactions, point A is attracted to point B with acceleration aA=GmBR3(rB−rA), this frame is therefore non-inertial. To quantify the effects of the perturbations in this frame, one should consider the ratio of the perturbations to the main body gravity i.e. χA=|gB−aA||gA|. The perturbation gB−aA is also known as the tidal forces due to body B. It is possible to construct the perturbation ratio χB for the frame centered on B by interchanging A↔B.

| Frame A | Frame B

|
|

Main acceleration | gA | gB
Frame acceleration | aA | aB
Secondary acceleration | gB | gA
Perturbation, tidal forces | gB−aA | gA−aB
Perturbation ratio χ | χA=|gB−aA||gA| | χB=|gA−aB||gB|

As C gets close to A, χA->0 and χB->∞, and vice versa. The frame to choose is the one that has the smallest perturbation ratio. The surface for which χA=χB separates the two regions of influence. In general this region is rather complicated but in the case that one mass dominates the other, say mA≪mB, it is possible to approximate the separating surface. In such a case this surface must be close to the mass A, denote r as the distance from A to the separating surface.

| Frame A | Frame B

|
|

Main acceleration | gA=GmAr2 | gB≈GmBR2+GmBR3r≈GmBR2
Frame acceleration | aA=GmBR2 | aB=GmAR2≈0
Secondary acceleration | gB≈GmBR2+GmBR3r | gA=GmAr2
Perturbation, tidal forces | gB−aA≈GmBR3r | gA−aB≈GmAr2
Perturbation ratio χ | χA≈mBmAr3R3 | χB≈mAmBR2r2
Hill sphere and Sphere Of Influence for Solar System bodies

The distance to the sphere of influence must thus satisfy mBmAr3R3=mAmBR2r2 and so r=R(mAmB)2/5 is the radius of the sphere of influence of body A

## See also

* Hill sphere
* Sphere of influence (black hole)
* Clearing the neighbourhood

## References

1. ↑ Souami, D.; Cresson, J.; Biernacki, C.; Pierret, F. (2020). "On the local and global properties of gravitational spheres of influence". Monthly Notices of the Royal Astronomical Society 496 (4): 4287–4297. doi:10.1093/mnras/staa1520.
2. ↑ Cavallari, Irene; Grassi, Clara; Gronchi, Giovanni F.; Baù, Giulio; Valsecchi, Giovanni B. (2023). "A dynamical definition of the sphere of influence of the Earth". Communications in Nonlinear Science and Numerical Simulation (Elsevier BV) 119: 107091. doi:10.1016/j.cnsns.2023.107091. ISSN 1007-5704. Bibcode: 2023CNSNS.11907091C.
3. ↑ Araujo, R. A. N.; Winter, O. C.; Prado, A. F. B. A.; Vieira Martins, R. (2008-12-01). "Sphere of influence and gravitational capture radius: a dynamical approach". Monthly Notices of the Royal Astronomical Society (Oxford University Press (OUP)) 391 (2): 675–684. doi:10.1111/j.1365-2966.2008.13833.x. ISSN 0035-8711. Bibcode: 2008MNRAS.391..675A.
4. ↑ 4.0 4.1 4.2 Seefelder, Wolfgang (2002). Lunar Transfer Orbits Utilizing Solar Perturbations and Ballistic Capture. Munich: Herbert Utz Verlag. p. 76. ISBN 3-8316-0155-0. https://books.google.com/books?id=NVg_vYHePt0C&pg=PA76. Retrieved July 3, 2018.
5. ↑ Understanding Space: Artemis I Flight Day Eight: Orion Exits The Lunar Sphere Of Influence., 23 November 2022, https://blogs.nasa.gov/artemis/2022/11/23/artemis-i-flight-day-eight-orion-exits-the-lunar-sphere-of-influence
6. ↑ The Size of Planets, 23 May 2013, https://planetfacts.org/size-of-planets-in-order
7. ↑ How Big Is the Moon?, 4 June 2012, https://planetfacts.org/how-big-is-the-moon
8. ↑ The Mass of Planets, 9 May 2012, https://www.outerspaceuniverse.org/the-mass-of-planets-how-much-do-the-planets-in-our-solar-system-weigh.html
9. ↑ Moon Fact Sheet, https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html
10. ↑ Planet Distance to Sun, How Far Are The Planets From The Sun?, 5 March 2021, https://www.cleverlysmart.com/planet-distance-to-sun-how-far-are-the-planets-from-sun/

## General references

* Bate, Roger R.; Donald D. Mueller; Jerry E. White (1971). Fundamentals of Astrodynamics. New York: Dover Publications. pp. 333–334. ISBN 0-486-60061-0. https://archive.org/details/fundamentalsofas00bate/page/333.
* Sellers, Jerry J.; Astore, William J.; Giffen, Robert B.; Larson, Wiley J. (2004). Kirkpatrick, Douglas H.. ed. Understanding Space: An Introduction to Astronautics (2nd ed.). McGraw Hill. pp. 228, 738. ISBN 0-07-294364-5. https://archive.org/details/understandingspa00sell.
* Danby, J. M. A. (2003). Fundamentals of celestial mechanics (2. ed., rev. and enlarged, 5. print. ed.). Richmond, Va., U.S.A.: Willmann-Bell. pp. 352–353. ISBN 0-943396-20-4.

## External links

* Project Pluto

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