  The stability of a light sail riding on a laser beam is analyzed both
analytically and numerically. Conical sails on Gaussian beams, which have been
studied in the past, are shown to be unstable without active control or
additional mechanical modifications. A new architecture for a passively stable
sail-and-beam configuration is proposed. The novel spherical shell design for
the sail is capable of "beam riding" without the need for active feedback
control. Full three-dimensional ray-tracing simulations are performed to verify
our analytical results.
