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Solar Sail Trajectory Optimization for the Solar Polar Imager (SPI) Mission

The Solar Polar Imager (SPI) mission is one of several Sun-Earth Connection solar sail roadmap missions currently envisioned by NASA. A current SPI reference mission design is based on a 160 m x 160 m, 150 kg square solar sail assembly with a 250 kg spacecraft bus and a scientific payload of 50 kg (450 kg total mass), having a characteristic acceleration of 0.35 mm/s 2 . Using a conservative solar sail film temperature limit of 100°C to constrain the solar distance ("cold" mission scenario), our transfer trajectory to the SPI target orbit (circular orbit at 0.48 AU solar distance with 75 deg inclination) approaches the sun closer (to about 0.4 AU solar distance) than a current reference trajectory and therefore, exploiting the larger solar radiation pressure, takes - even with a lower hyperbolic excess energy for interplanetary insertion - only 6.4 instead of 6.7 years. For a higher sail temperature limit of 240°C ("hot" mission scenario), the optimal transfer trajectory approaches the sun much closer (to about 0.22 AU solar distance), resulting in an even shorter transfer duration of only 4.7 years. Based on this "hot" mission scenario, we perform several mission tradeoffs to gain a deeper insight into the trade space of the SPI mission: different sail temperature limits, different characteristic accelerations, different interplanetary insertion energies, and different sail degradation behaviors are investigated.