This paper presents the design of solar-sail transfer trajectories to solar-sail displaced libration point orbits in the Earth- Moon system. The existence of families of solar-sail displaced libration point orbits in the Earth-Moon system has recently been demonstrated. These families originate from complementing the dynamics of the classical Earth-Moon circular restricted three-body problem with a solar-sail induced acceleration. Previous work has furthermore demonstrated the applicability of these orbits for high-latitude observation of the Earth and Moon. To not only demonstrate the existence and applicability of these orbits, but also their accessibility, this paper investigates the design of solar-sail transfers from Earth-bound parking orbits to a subset of these orbits. Initial guesses for the transfers are generated using reverse time propagation of the dynamics, where the control is provided by a locally optimal steering law. These initial guesses are subsequently used to initialize a 12th-order Gauss-Lobatto collocation method to satisfy a large number of constraints: departure from specific high Earth orbits, a minimum altitude with respect to the Earth and the Moon, and a maximum rotation rate of the solar sail. As an application of the developed methodology, this paper shows results for transferring two spacecraft to a constellation of displaced vertical Lyapunov orbits at the Earth-Moon L2 point. This constellation has been shown to provide continuous coverage of the lunar Aitken Basin and the lunar South Pole while maintaining a continuous line of sight with Earth. Sets of feasible trajectories for both spacecraft with identical launch conditions are produced in order for the constellation to be initiated using a single Soyuz launch. Such a Soyuz launch can deliver two 1160-kg spacecraft into the found transfer trajectories. One of the spacecraft subsequently requires a transfer time of 53.06 days to enter its constellation orbit, while the transfer of the other spacecraft takes 67.89 days. These results prove the accessibility of solar-sail displaced libration point orbits in the Earth-Moon system, thereby reaffirming the potential of solar-sail technology to enable novel scientific missions in the Earth-Moon system.

A pole-sitter is a satellite that is stationed along the polar axis of the Earth, or any other planet, to generate a continuous, hemispherical view of the planet’s polar regions. In order to maintain such a vantage point, a low-thrust propulsion system is required to counterbalance the gravitational attraction of the planet and the Sun. Previous work has considered the use of solar electric propulsion (SEP) or a hybrid configuration of an SEP thruster and a solar sail to produce the required acceleration. By subsequently optimising the propellant consumption by the thruster, estimates of the mission performance in terms of the payload capacity and mission lifetime have been obtained. This paper builds on these results and aims at lifting the pole-sitter concept to the next level by extending the work both from a technical and conceptual perspective: from a technical perspective, this paper will further improve the mission performance by optimising the pole-sitter orbits for the payload capacity or mission lifetime instead of for the propellant consumption. The results show that, at Earth, this allows improvements in the order of 5-10 percent in terms of payload capacity and mission lifetime. Furthermore, on a conceptual level, this paper will, for the first time, investigate the possibility of so-called quasi-pole-sitter orbits. For quasi-pole-sitter orbits the requirement to be exactly on the polar axis is relaxed to allow some movement around the polar axis as long as continuous observation of the entire polar region at a desired minimum elevation angle is achieved. This ultimately enables solar sail-only pole-sitter orbits that are no longer limited in performance by the SEP propellant consumption. Finally, this paper extends all analyses to other inner Solar System planets, showing that Mars provides excellent conditions for a pole-sitter platform with its low mass and relatively far distance from the Sun. With this extension of the pole-sitter concept to other planets as well as considering, for the first time, the option of quasi-pole-sitter orbits, the concept is lifted to the next level, strengthening the feasibility and utility of these orbits for continuous planetary polar observation.