Photogravimagnetic assists of light sails: a mixed blessing for Breakthrough Starshot?

• Published in: Monthly notices of the Royal Astronomical Society Vol. 474; no. 3; pp. 3212 - 3220
• Main Authors: Forgan, Duncan H, Heller, René, Hippke, Michael
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01.03.2018
• Subjects: Binary stars; Charging; Deceleration; Ejection; Lorentz force; Magnetic fields; Magnetism; Radiation pressure; Red dwarf stars; Sails; Stellar magnetic fields; Trajectories
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx2834

AbstractUpon entering a star system, light sails are subject to both gravitational forces and radiation pressure, and can use both in concert to modify their trajectory. Moreover, stars possess significant magnetic fields, and if the sail is in any way charged, it will feel the Lorentz force also. We investigate the dynamics of so-called 'photogravimagnetic assists' of sailcraft around α Centauri A, a potential first destination en route to Proxima Centauri (the goal of the Breakthrough Starshot programme). We find that a 10-m2 sail with a charge-to-mass ratio of around 10 μC g-1 or higher will need to take account of magnetic field effects during orbital manoeuvres. The magnetic field can provide an extra source of deceleration and deflection, and allow capture on to closer orbits around a target star. However, flipping the sign of the sailcraft's charge can radically change resulting trajectories, resulting in complex loop-de-loops around magnetic field lines and essentially random ejection from the star system. Even on well-behaved trajectories, the field can generate off-axis deflections at α Centauri that, while minor, can result in very poor targeting of the final destination (Proxima) post-assist. Fortunately for Breakthrough Starshot, nanosails are less prone to charging en route than their heavier counterparts, but can still accrue relatively high charge at both the origin and destination, when travelling at low speeds. Photogravimagnetic assists are highly non-trivial, and require careful course correction to mitigate against unwanted changes in trajectory.