Azero-velocity surface is a concept that relates to theN-body problem ofgravity. It represents a surface a body of given energy cannot cross, since it would have zero velocity on the surface. It was first introduced byGeorge William Hill.^[2] The zero-velocity surface is particularly significant when working with weak gravitational interactions amongorbiting bodies.

In the circular restrictedthree-body problem two heavy masses orbit each other at constant radial distance and angular velocity, and a particle of negligible mass is affected by their gravity. By shifting to arotating coordinate system where the masses are stationary a centrifugal force is introduced. Energy and momentum are not conserved separately in this coordinate system, but theJacobi integral remains constant:

${\displaystyle C={\omega }^2(x^2+y^2)+2\left(\frac{{\mu }_1}{r_1}+\frac{{\mu }_2}{r_2}\right)-\left({\dot{x}}^2+{\dot{y}}^2+{\dot{z}}^2\right)}$

where${\displaystyle \omega }$ is the rotation rate,${\displaystyle x,y}$ the particle's location in the rotating coordinate system,${\displaystyle r_1,r_2}$ the distances to the bodies, and${\displaystyle {\mu }_1,{\mu }_2}$ their masses times the gravitational constant.^[3]

For a given value of${\displaystyle C}$, points on the surface

${\displaystyle C={\omega }^2(x^2+y^2)+2\left(\frac{{\mu }_1}{r_1}+\frac{{\mu }_2}{r_2}\right)}$

require that${\displaystyle {\dot{x}}^2+{\dot{y}}^2+{\dot{z}}^2=0}$. That is, the particle will not be able to cross over this surface (since the squared velocity would have to become negative). This is the zero-velocity surface of the problem.^[4]

Note that this means zero velocity in the rotating frame: in a non-rotating frame the particle is seen as rotating with the other bodies. The surface also only predicts what regions cannot be entered, not the shape of the trajectory within the surface.^[3]

The concept can be generalized to more complex problems, for example with masses in elliptic orbits,^[5] the general planar three-body problem,^[6] the four-body problem with solar wind drag,^[7] or in rings.^[8]

The zero-velocity surface is also an important parameter in findingLagrange points. These points correspond to locations where the apparent potential in the rotating coordinate system is extremal. This corresponds to places where the zero-velocity surfaces pinch and develop holes as${\displaystyle C}$ is changed.^[9] Since trajectories are confined by the surfaces, a trajectory that seeks to escape (or enter) a region with minimal energy will typically pass close to the Lagrange point, which is used inlow-energy transfer trajectory planning.

Given a group ofgalaxies which are gravitationally interacting, the zero-velocity surface is used to determine which objects are gravitationally bound (i.e. not overcome by theHubble expansion) and thus part of agalaxy cluster, such as theLocal Group.^[10]

• Hill sphere
• Low-energy transfer
• Orbital mechanics

• Gravity

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