(which is proportional to the distance from the center of the earth). The finite size of the SPS, therefore, gives rise to a distribution of body forces which, depending on the geometry and orientation, characteristically represents the largest operational applied forces and moments. For a rectangular, 5 GW ground output configuration in the worst orientation, control forces on the order of 300 N at the corners of the array would enable attitude control. The £ 9 solar radiation pressure of about 5 x 10 °N/m acts on illuminated surfaces as a function of the surface solar reflectance and orientation. The greatest influence of this force is a potential daily and six-month periodic perturbation of the orbit. There is also an antenna recoil from the microwave power transmission of about 25 N. Solar and lunar gravity and earth eccentricities give rise to potential orbit perturbation but insignificant structural loads. There is no atmospheric drag at geosynchronous orbit. However, in low earth orbit (~500 Km) this pressure (~10 N/m ) can give rise to a force which is significant to orbital decay. Operational system induced loads on the SPS structure must be considered, although they are quite dependent on configuration and system design. Current interaction forces are generally small, although their greatest influence is at the joint where the largest currents and least separation distance between conductors can occur. Depending on the configuration and system operation, centrifugal acceleration about the center of mass can also contribute to structural loading. The applied forces and moments for attitude control and MPTS pointing are significant inputs to the structural loading as discussed above. Structural loads associated with maintenance, construction, transportation, handling and all relative aspects of the SPS activities must be considered in the structural design. Any governing loads other than operational must, however, be weighed against the impact to the system and, ultimately, to the cost of delivering electricity. Environment - The normal environmental concerns of terrestrial structures (e.g., wind loading, oxidation and moisture effects, soil mechanics, etc.) are not encountered in earth orbit. However, there are environmental
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