Design and optimisation of orbits in proximity of asteroids using solar sails

Close-up observations of asteroids (particularly near-Earth asteroids, or NEAs) are important both for scientific reasons (many characteristics are largely unknown today, and asteroids are very different one another), for planetary protection and for future exploitation of their resources (metals, water). A possible way to visit multiple asteroids is with a solar sail spacecraft. A solar sail is a large, reflective and lightweight membrane that is deployed from a spacecraft and provides a thrust by reflecting the photons from the sun. This is appealing because it generates a small, but continuous, acceleration over time without propellant expenditure, enabling high-v missions.

Research in this group has shown the potential of solar sails to enable missions that can visit up to five near-Earth asteroids within ten years. In addition, trajectories were found for a variety of combinations of NEAs, as well as launch dates, demonstrating the flexibility offered by this type of propulsion. While the interplanetary journey is possible, a further research challenge is how to orbit around (or in proximity of) the asteroids: this PhD will investigate the dynamics and trajectories in proximity of the asteroids, together with the transfer to/from the interplanetary legs.

Asteroid orbits are challenging for several reasons: asteroids are highly irregular bodies, and their shape and density are not known in advance; in addition, most are tumbling, generating an irregular and time-varying gravity field. An additional challenge comes from the use of the solar sail, whose acceleration can only be controlled, through attitude manoeuvres, within certain limits and constraints.

Multi- and irregular body dynamics will be used. It is likely that an initial approach will be based on the energy levels and zero-velocity curves, to ensure bounded trajectories near the asteroid. Further research will involve numerical optimisation to target specific trajectories that maximise scientific return, reliability, and/or other merit figures. The ultimate goal is to design and optimise complete trajectories that inject into asteroid orbit starting from the interplanetary phase, orbit around the asteroid for a desired amount of time, and eventually depart into the next interplanetary transfer.