Reconfiguration of Sun-Earth Libration Point Formations Using Solar Radiation Pressure

Abstract

The use of solar radiation pressure for reconfiguration of spacecraft formations near one of the collinear sun-Earth libration points is examined. Specifically, it is considered whether spacecraft that are equipped with sun-facing thermal shields can exploit the solar radiation pressure force that acts on them to perform gradual relative maneuvering. Small shield off-pointing angles give rise to differential solar radiation pressure forces that are essentially perpendicular to the sun line: The effect of these small differential forces on the relative motion of the spacecraft in the formation is, therefore, quite different from that of the total (down-sun) solar force on the absolute orbital motion. First, the linearized equations of relative motion for libration point formations under differential solar radiation forcing are set up, with the sun shield off-pointing angle taken as the control input variable. The range of validity of these equations is examined. The effect of solar radiation pressure on the absolute motion is also analyzed, and the station-keeping issue is avoided by shifting the original libration point to a new equilibrium point. These equations are then used to study the existence and properties of solution trajectories that take the spacecraft from a given initial formation geometry to a desired final one. When the vector control produced by solar radiation pressure is reduced to a scalar one, the existence and properties of time-optimal trajectories based on bang-bang control theory are analyzed. Comparisons made between these two types of transfer trajectories show that the former possess simpler steering of sun shields, whereas the latter have a shorter transfer time and more reachable points in the target orbit.