Flight and spin of microwave-driven sails: first experiments

Microwave-driven acceleration by photon reflection has been suggested for propelling probes to very high speeds for science missions to the outer solar system and the nearby stars. Beam-driven probes have the advantage that energy is expended to accelerate only the sail and payload, not the propelling beam generator. We are using an intense microwave beam driving ultralight carbon sails to conduct initial experiments of key aspects of this concept: liftoff and flight against gravity, spinning of sails by coupling electromagnetic angular momentum from the beam and initial studies of beam/sail stability, known as ´beam-riding´. We send a 10 kW, 7.16 GHz beam into a microTorr vacuum chamber onto sails of mass density 5-10 g/m/sup 2/ at microwave power densities of /spl sim/kW/cm/sup 2/. We observe flight of ultralight sails of carbon-carbon microtruss material at several gees acceleration. Sails so accelerated reach >2000 K from microwave absorption. Photon reflection can account for 3 to 30% of the observed acceleration, so another cause must be present. The most plausible explanation is evaporation of absorbed molecules from the hot side of the sail. This effect must be understood and anticipated in future sail experiments and missions, including solar sails. To spin sails of aluminum and carbon, we converted the beam to circular polarization and tethered the sail with a carbon fiber above the circular horn. The sail angular speed increases with power and reverses with polarization reversal. We explored a variety of sail shapes, such as disk, pyramid, and cone. Remotely controlling a sail´s rotation with a beam increases its stability in flight. Combined, this work demonstrates the basics of microwave-beam-propulsion by showing beam driving of both the longitudinal and the azimuthal energies of the sail.