  We consider the motion of a light sail that is accelerated by a powerful
laser beam. We derive the equations of motion for two proof-of-concept sail
designs with damped internal degrees of freedom. Using linear stability
analysis we show that perturbations of the sail movement in all lateral degrees
of freedom can be damped passively. This analysis also shows complicated
behaviour akin to that associated with exceptional points in PT-symmetric
systems in optics and quantum mechanics. The excess heat that is produced by
the damping mechanism is likely to be substantially smaller than the expected
heating due to the partial absorption of the incident laser beam by the sail.
