Star Wars tragics will of course be aware that solar sails are deployed on several spacecraft in George Lucas’s fictional universe.
They appear, for instance, on Ginivex-class starfighters and Jemlaat-class sail yachts; and who could forget Count Dooku’s pimped-up Punworcca 116-class interstellar sloop in which he escaped from the First Battle of Geonosis?
Okay, most people don’t delve into sci-fi movies quite that deeply, but Lucas’s love of solar sails as a method to propel spacecraft is no mere figment of the imagination.
“Solar sails use the Sun for propulsion,” explains David Siegal Bernstein in his 2017 book Blockbuster Science: The Real Science in Science Fiction (Prometheus Books). “Photons shooting from the Sun carry energy and momentum. A solar sail captures the momentum and reflects it off, resulting in continuous acceleration.”
It was pioneering astronomer Johannes Kepler who suggested, the best part of four centuries ago, that comet tails were created by breezes, and that therefore “ships and sails proper for heavenly air should be fashioned”. James Clerk Maxwell in 1873 demonstrated that such sails would be propelled not by wind but by the pressure of photons bouncing off their surface.
NASA proved the principle – out of necessity – in 1974, when its Mariner 10 spacecraft ran out of fuel. Mission Control angled the craft so that sunlight hit its flat-panel arrays, giving it the photon boost it needed to keep moving.
Since then several organisations have experimented with solar sail designs. One of the challenges in doing so is that the technology, although very simple in one respect, is extremely difficult to test on Earth. Solar sails only work beyond our planet’s atmosphere, where the particle-filled winds created by the Sun blow unfettered. Even when all the technical problems are ironed out, therefore, you will never see craft powered by solar sails taking off from Cape Canaveral. They must rely on some form of rocket propellant to get them out of the atmosphere before the sails can be unfurled, or dock in near-Earth orbit and have passengers ferried to them on smaller, feeder craft.
Work continues apace, however, because solar sails once fully developed will offer a low-cost, renewable, fuel-free way to travel through space.