Minimizing Mass of Diffractive Space Sunshades in Non-Keplerian Orbits at Artificial Equilibrium Points Using Electric Propulsion

Sammanfattning

Mitigating global warming is one of the most pressing challenges of our time. One temporary solution is the implementation of space sunshades. An early proposal of a sunshade to limit solar radiation was given by James T. Early in 1989. The implementation methods are numerous, but one specific solution that could be possible to implement is a diffractive sunshade considered by Bourge and Hein [1]. These sunshades have considerably lower mass than alternatives like solar sails. Moving the sunshade closer to Earth reduces its required area, and therefore its structural mass. The aim of this study is to further reduce the mass by moving the sunshades from their natural equilibrium points, the so-called sub-Lagrangian L1′, to orbits around artificial equilibrium points. This is done by adding constant thrust using electric propulsion, so while the dry mass may decrease, the propellant mass increases. Numerical costs for station-keeping are added to analytical calculations for displacement acceleration to find locations where the overall mass is reduced. For a near-term practical diffractive sunshade (rho = 0.18 g/m², R = 6·10⁻⁴), it is found that displacing the sub-Lagrangian could become feasible depending on the available electric propulsion systems. Xenon-based systems (Isp = 3200) do not yield an advantage in mass over a non-displaced orbit. In contrast, argon-based systems such as VASIMR (Isp = 5000) could reduce dry mass by 60%, by displacing L1′ = 1.596 × 10⁶ km to X0 = 1.0 × 10⁶ km. One year’s worth of argon propellant could sustain this displacement for 14 years. For an ideal theoretical sunshade (rho = 0.0273 g/m², R = 6·10⁻⁴), the gains are even larger. A reduction in dry mass of close to 50% for 70 years could be possible with the same cost, displacing it from L1′ = 2.36 × 10⁶ km to X0 = 1.7 × 10⁶ km. The results should be interpreted with caution, as they are generated under key simplifying assumptions. They do, however, indicate that under certain conditions a reduction in mass may be achievable by displacing the sunshade equilibrium point. Future work should include the effects of time-varying thrust and realistic structural systems for more realistic values.