The concept of a pole-sitter has been under investigation for many years, showing the capability of a low-thrust propulsion system to maintain a spacecraft at a static position along a planet's polar axis. From such a position, the spacecraft has a view of the planet's polar regions equivalent to that of the low- and mid-latitudes from geostationary orbit. Previous work has hinted at the existence of pole-sitters that would only require a solar sail to provide the necessary propulsive thrust if a slight deviation from a position exactly along the polar axis is allowed, without compromising on the continuous view of the planet's polar region (a so-called quasi-pole-sitter). This paper conducts a further in-depth analysis of these high-potential solar-sail-only quasi-pole-sitters and presents a full end-to-end trajectory design: from launch and transfer to orbit design and orbit control. The results are the next steppingstone towards strengthening the feasibility and utility of these orbits for continuous planetary polar observation.

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.

The oscillating sail is a novel solar sail configuration where a triangular sail is released at a deflected angle with respect to the Sun-direction. As a result, the sail will conduct an undamped oscillating motion around the Sun-line due to the offset between the centre-of-pressure and centre-of-mass. In this paper, the resulting oscillatory motion of the acceleration vector is exploited to design new families of periodic orbits in the Earth-Moon circular restricted three-body system. In particular, the effect of adding an oscillating sail to the family of Lyapunov orbits at the L1- and L2-points as well as the family of distant retrograde orbits (DROs) is investigated. Because the solar sail Earth-Moon system is non-autonomous (due to the apparent orbital motion of the Sun), the sail’s oscillating period, the orbital period and the period of the Sun around the Earth-Moon system all need to be commensurable in order for the orbits to be repeatable over time. Using a differential correction technique, orbits that satisfy these constraints can be obtained and the results comprise new families of periodic orbits that are parameterised by the required sail performance. In addition to exploiting the oscillating sail for generating new orbit families, this paper also investigates its potential for orbital transfers. By combining a systematic search method with a local optimiser, oscillating sail parameters and orbital parameters can be obtained that enable transfers between classical Lyapunov orbits at the L1-point, connections between classical Lyapunov orbits at different Lagrange points as well as transfers between orbits within the family of classical DROs.