and/or distortion). The latter is difficult to achieve without compromising other structural requirements. The significance of the other environmental effects, vacuum, particulate and ultraviolet radiation and plasma interaction, is difficult to assess due to our limited experience with exposure to this environment. The vacuum environment mainly affects the loss of volatile ingredients and surface deposition of effluents. Prime concern with particulate and ultraviolet radiation and plasma interaction is the stability of surface properties such as solar absorptance and infrared emissivity. It is possible that structural properties may be affected particularly for minimum gauge materials. There are spacecraft which have maintained operational performance in this environment for a number of years and there are spacecraft which have suffered degradations of performance which can be correlated to these environmental effects. System Dynamics - The low structural mass fraction of SPS concepts to date, the stringent pointing and flatness requirements of the MPTS, cyclic disturbances (such as gravity gradient and the configuration kinematics) and the seasonal occultations dictate consideration of the SPS system dynamics and the associated configuration requirements such as structural stiffness. For the array, the most straightforward approach to achieving dynamic stability with a passive structure is to have the natural frequency of the solar blankets be greater than that of the overall array which must in turn be greater than the control frequency which is in turn greater than the disturbance frequency (gravity gra- dient~2 x 10 HZ). The array natural frequency for a given size array and a given structural material is established by the geometry of the structure (e.g., depth), while the solar blanket natural frequency is controlled by the tension level ( ~4 N/m). This requirement of passive dynamic stability is the source of design structural compression loads (nominally ~10 N). If the array structure material has a high coefficient of thermal expansion (such as aluminum), the dynamic response of the SPS system to occultation would set the stiffness requirements and many of the design loads.
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