Solar sail $𝑯$-reversal trajectory: A review of its advances and applications

The set of the orbital angular-momentum reversal, or $H$-reversal, sailcraft trajectory was born as a type of unconventional precursor interstellar mission trajectory by using high-performance solar sails. Starting from an outline of the $H$-reversal sail trajectory, this paper mainly focuses on the 2D reversal-mode solution to the general solar-photon sail motion equations. The feasible region for $H$-reversal trajectories in fixed sail attitude angles is illustrated. Some interesting applications of the $H$-reversal trajectory are presented in detail to show its advantages. As a special case, a precursor interstellar probe can be delivered with a constant sail orientation in the $H$-reversal trajectory to be compared with the direct-motion sail flyby of the Sun. Of importance are the heliocentric periodic orbits in double $H$-reversal modes, obtained via both fixed and time-varying sail attitude angles. Two more applications involving $H$-reversal trajectories are discussed in terms of asteroid deflection and transfer trajectory to rectilinear orbits. Finally, some items of the mathematics behind the 3D motion-reversal trajectories are summarized.