# Two-body problem

A problem dealing with the motion of two material points $ P _ {1} $
and $ P _ {2} $
with masses $ m _ {1} $
and $ m _ {2} $,
respectively, moving in three-dimensional Euclidean space $ E ^ {3} $
when acted upon by the mutual Newton attracting forces. The problem is a special case of the $ n $-body problem, which may be described by a system of ordinary differential equations of order $ 6n $,
and has 10 independent integrals: 6 of motion of the centre of inertia, 3 of law of areas (equivalently, conservation of angular momentum) and 1 of energy conservation [1]. The two-body problem also has three Laplace integrals (one of which is independent of the preceding ones) and is completely integrable [2].

The integration of the two-body problem is more conveniently effected in a special system of coordinates, in which these integrals are employed. If the origin of the Cartesian coordinates $ x , y , z $ is placed at the centre of mass $ ( m _ {1} \vec{r} {} _ {1} + m _ {2} \vec{r} {} _ {2} ) / ( m _ {1} + m _ {2} ) $ and the axis $ z $ is directed along the relative angular momentum vector, then the motion of the relative position vector $ \vec{r} {} _ {1} - \vec{r} {} _ {2} = ( x, y, z) $ takes place in the plane $ z = 0 $ and satisfies the system

$$ \tag{1 } \mu \ddot{x} = - f x r ^ {- 3} ,\ \mu \ddot{y} = - f y r ^ {- 3} , $$

where $ r = \sqrt {x ^ {2} + y ^ {2} } $, $ \mu = m _ {1} m _ {2} /( m _ {1} + m _ {2} ) $ is the reduced mass and $ f $ is the gravitational constant. The system (1) has four integrals:

$$ x \dot{y} - y \dot{x} = c \ \textrm{ (law of areas), } $$

$$ \frac{1}{2} \mu ( {\dot{x} } {} ^ {2} + {\dot{y} } {} ^ {2} ) - f r ^ {- 1} = h \textrm{ (energy) } , $$

$$ \mu ^ {2} c \dot{y} - \mu f x r ^ {- 1} = \lambda _ {1} \ \textrm{ and } \ \ \mu ^ {2} c \dot{x} + \mu f y r ^ {- 1} = - \lambda _ {2} \ \textrm{ (Laplace) }, $$

which are interconnected by the relation

$$ \lambda _ {1} ^ {2} + \lambda _ {2} ^ {2} = \ 2 \mu ^ {3} h c ^ {2} + \mu ^ {2} f ^ { 2 } . $$

Here

$$ \tag{2 } c ^ {2} = \lambda _ {1} x + \lambda _ {2} y + \mu r , $$

i.e. the orbits of the relative position vector are conical sections with parameter $ p = c ^ {2} / \mu $, major semi-axis $ a = - \mu / ( 2h) $, eccentricity $ e = \mu ^ {- 1} \sqrt {1 + 2hc ^ {2} } $, longitude of pericentre $ \omega $ ( $ \lambda _ {1} = \mu e \cos \omega $, $ \lambda _ {2} = \mu e \sin \omega $), and with focus at the coordinate origin. The location of the relative positive vector on the orbit is determined by the true anomaly $ v $, counted from the direction towards the pericentre; (2) then implies that $ r = p / ( 1 + e \cos v ) $. If $ c \neq 0 $, three types of orbits are possible:

I) If $ h < 0 $, they are ellipses.

II) If $ h > 0 $, they are hyperbolas.

III) If $ h = 0 $, they are parabolas.

If $ c = 0 $, the motion is rectilinear. The two-body problem describes an unperturbed Kepler motion of a planet with respect to the Sun or of a satellite with respect to a planet, etc.

#### References

[1] | C.L. Siegel, "Vorlesungen über Himmelmechanik" , Springer (1956) |

[2] | V.K. Abalakin, et al., "Handbook of celestial mechanics and astrodynamics" , Moscow (1971) (In Russian) |

[a1] | H. Poincaré, "Les méthodes nouvelles de la mécanique céleste" , 1–3 , Gauthier-Villars (1899) |

[a2] | C.L. Siegel, J. Moser, "Lectures on celestial mechanics" , Springer (1971) |

[a3] | V.I. Arnol'd, "Mathematical methods of classical mechanics" , Springer (1978) (Translated from Russian) |

[a4] | R. Abraham, J.E. Marsden, "Foundations of mechanics" , Benjamin (1978) |

**How to Cite This Entry:**

Two-body problem.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Two-body_problem&oldid=54207