Sunlight pushes, and a sail has flown on it
Light carries momentum. A sail can accelerate a spacecraft with no propellant. Japan’s IKAROS demonstrated it in interplanetary space in 2010.
Electromagnetic waves carry momentum equal to their energy divided by the speed of light. Absorb a watt and you feel a force of a few nanonewtons. Reflect it and you feel about twice that. A thin mirror of hundreds of square metres, facing the Sun near Earth, gathers enough light for a force around a millinewton — the weight of a mosquito, applied without stopping and without fuel. IKAROS, a square sail about 14 metres on a side, unfurled on the way to Venus and measured the acceleration. The spiral of its course changed in the way a sail predicts and a pure ballistic arc does not.
Later sails, including LightSail 2, have shown controlled flight in Earth orbit, fighting the fact that half of every orbit is in shadow and that atmospheric drag is a rival force down low. The useful place for a sail is further out, or inward, where the light is steady and the months are available. It will not lift a cargo this decade. It has already done the thing that sounded like a metaphor.
Caveats
- The force is real and tiny. A sail is not a feeling you would notice if you held the sheet, and it does not work as a boat on a lake. Air does not let the light win.
- IKAROS also carried a small thruster for other jobs. The published result that matters here is the measured solar acceleration of the sail itself.
Sources
- 1Tsuda Y. et al.. Achievement of IKAROS — Japanese deep space solar sail demonstration mission. Acta Astronautica 82, 2013.