Laser sail propulsion is an idea that won't go away. By aiming powerful Earth-based lasers at tiny spacecraft with light sails, tiny spacecraft can be accelerated to near-relativistic speeds without carrying fuel or an energy source, and without carrying any kind of propulsion system at all. There are clear advantages to this idea, if it can be implemented. Two spacecraft have partly implemented the idea. JAXA's IKAROS spacecraft and the Planetary Society's Lightsail-2 used sails, but they were powered by photons from the Sun, not from a laser. They showed that the concept can work, at least partly. The Breakthrough Starshot program, now defunct, aimed to build a fleet of 1,000 miniature spacecraft with photon sails and send them on a 20-year journey to Alpha Centauri, the star system closest to us.…