Station-keeping method for libration point trajectories

Three-dimensional orbits in the vicinity of the interior libration point (Li) of the Sun-Earth/Moon barycenter system are currently being considered for use with a number of missions planned for the 1990s. Since such libration point trajectories are, in general, unstable, spacecraft moving on these paths must use some form of trajectory control to remain close to their nominal orbit. The primary goal of this effort is the development of a Stationkeeping strategy applicable to such trajectories. A method is presented that uses maneuvers executed (impulsively) at discrete time intervals. The analysis includes some investigation of a number of the problem parameters that affect the overall maneuver costs. Simulations are designed to provide representative station- keeping costs for a spacecraft moving in a libration point trajectory, and preliminary results are summarized. RAJECTORY planning for a number of scientific mis- sions scheduled for launch in the 1990s includes the possi- ble use of three-dimensi onal halo or Lissajous orbits in the vicinity of the interior L! libration point of the Sun-Earth/ Moon barycenter system.1 This effort is directed toward the development of a Stationkeeping strategy that can be used to maintain spacecraft near such nominal libration point trajec- tories. A significant number of analyses have been completed that involve Stationkeeping methods for Earth orbiting satel- lites; maintaining a spacecraft on a libration point orbit, how- ever, has received limited attention. In the late 1960s, Far- quhar2 developed a number of possible Stationkeeping strat- egies for libration point orbits. Later, in 1974, a station- keeping method for spacecraft moving on halo orbits in the vicinity of the Earth-Moon translunar libration point L2 was published by Breakwell et al. 3 In contrast, specific mission requirements influenced the design of the Stationkeeping strat- egy for the first libration point mission. When the Interna- tional Sun-Earth Explorer-3 (ISEE-3) satellite was injected into a halo orbit associated with the interior L! libration point of the Sun-Earth system in 1978, a series of maneuvers exe- cuted at approximately three-month intervals was used for Stationkeeping.4 More recently, a series of papers have pre- sented results from studies that use Floquet and invariant manifold theories to develop a ''loose Stationkeeping strat- egy for halo-type orbits.58 Similar to ISEE-3, the method also uses discrete maneuvers, applied at varying time intervals, that control the trajectory near the nominal path. A significant goal of this study is the development of a potentially ''tight control strategy for Stationkeeping that can be applied to both halo and Lissajous trajectories (as well as other possible types of libration point paths). In this approach, the allowable deviation of the actual trajectory relative to the nominal path can be varied over a wide range depending on mission require- ments. Of course, low costs are desirable as well.