 UltraSail is a proposed type of robotic spacecraft that uses radiation pressure exerted by sunlight for propulsion.It builds upon the "heliogyro" concept by Richard H. MacNeal, published in 1971, and consists of multiple rotating blades attached to a central hub.The Heliogyro spacecraft's attitude (orientation), and therefore thrust direction, would be controlled by changing the cyclic and collective blade pitch similar to a helicopter.