A Photon-Propelled Spacecraft

Spacecraft have always been expensive to launch, largely because of the weight of the fuel they need once in orbit. NASA’s Advanced Composite Solar Sail experiment, launched in April, aims to fix that. Instead of using chemical engines to maneuver and accelerate, it uses a solar sail—a large metallic film that catches photons from the sun much the way a cloth sail catches wind. The spacecraft, about the size of a microwave oven, unfurled its sail—which billowed out to 860 square feet—in late August. Keats Wilkie, NASA’s principal investigator for the experiment, calls the mission an “opportunity to gain experience controlling and flying a working solar sail at a small scale.”