factors in earth orbit which must be considered: heat transfer, vacuum, particulate and ultraviolet radiation and tenuous plasma interactions. Heat transfer is essentially limited to thermal radiation, since conduction and convection effects are generally non-existent. In normal operation, the MPTS is most affected by the thermal environment due to the waste heat generated and due to the daily cycle of orientation relative to the incident solar flux. Since the waste heat must ultimately be radiated away to space, the characteristic operating temperature levels of a tapered microwave emission profile can range from 550°K at the center to ~300°K at the edge. The centerline temperature imposes a limit to the local power emission and, therefore, the extent of emission tapering. Temperature levels can limit systems designs, material selections and lifetime characteristics. An important aspect to structural design is the distribution of temperatures and the time variations brought about by changes in orientation relative to the sun or shadowing effects (local or system-wide via occultation). Structural temperature levels in space are greatly affected by surface properties, overall geometric configurations and orientation relative to the sun. Temperature differences can give rise to significant local structural distortions, degraded structural performance and overall configuration distortion. These effects are influenced by structural material, structural design and overall configuration. They can be particularly significant to the flatness of the MPTS transmission surface and the dynamic behavior of the entire system. To illustrate the magnitude of thermal environments, temperature differences across simple structural members can be on the order of 50°K (sunside to space-side), temperature differences between structural elements can easily be greater than 100°K (due to orientation relative to the sun), and the temperature changes due to an occultation are nominally 200°K and can be 400°K. To accommodate this thermal environment, the structural material must be insensitive to temperature gradients and transients (low coefficient of thermal expansion, ~2 x 1O'^/°K), or the structure must be active, or the structural design and configuration must be insensitive to thermal effects (environment
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