Abstract
This paper discusses a new family of non-Keplerian orbits for solar sail spacecraft displaced above or below the Earth's equatorial plane. The work aims to prove the assertion in the literature that displaced geostationary orbits exist, possibly to increase the number of available slots for geostationary communications satellites. The existence of displaced non-Keplerian periodic orbits is first shown analytically by linearization of the solar sail dynamics around a geostationary point. The full displaced periodic solution of the nonlinear equations of motion is then obtained using a Hermite-Simpson collocation method with inequality path constraints. The initial guess to the collocation method is given by the linearized solution, and the inequality path constraints are enforced as a box around the linearized solution. The linear and nonlinear displaced periodic orbits are also obtained for the worst-case sun-sail orientation at the solstices. Near-term and high-performance sails can be displaced between 10 and 25 km above the Earth's equatorial plane during the summer solstice, and a perforated sail can be displaced above the usual station-keeping box (75 x 75 km) of nominal geostationary satellites. Light-levitated orbit applications to space solar power are also considered.