Control of libration point orbits using lunar gravity-assisted transfer

The Interplanetary Physics Laboratory, WIND, will be placed in a small-amplitude halo orbit in late 1995. A lunar swingby is used to achieve the halo orbit. Using the lunar swingby reduces the fuel required to achieve the desired orbit. The spacecraft's position and velocity with respect to the Moon near the time of swingby are shown to determine the characteristics of the halo orbit. The shape of the halo orbit, its x-, y-, and z-amplitudes, must be designed to meet mission constraints. A convenient set of parameters for displaying the dependence of the halo orbit's shape upon the lunar swingby is formulated. The use of the lunar swingby adds additional constraints to the trajectory in terms of attainable swingby parameters. Strategies for obtaining the desired swingby parameters in view of these mission constraints are discussed. The limits on attainable halo orbit shapes using the lunar swingby technique are discussed in terms of minimum and maximum x-, y-, and z-amplitudes. The relevance of previous work on this topic is discussed.