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S. Lubow (STScI)
NGST will likely be placed in an elliptical orbit around the external collinear Lagrange point L2. However, the orbit is dynamically unstable and on-board thrusters must be used to make corrections to the orbit over time. The limited fuel available for such corrections may well limit the mission lifetime. Due to its large (200 square meters) heat shield, NGST will be subject to significant perturbations by solar radiation pressure. We present an analysis for controlling the orbit against various perturbations, including solar radiation pressure. Ignoring effects of nonlinearity and noncoplanarity, we obtain an analytic description of the orbital evolution subject to arbitrary time-dependent perturbations. Orbital instability can be controlled by minimizing the effects of secular growing terms. Several control strategies are described. These strategies include the optimal use of thrusters and the selection of an appropriate orbit which compensates for perturbations.