How Do Solar Sails Work?
Solar sails would, theoretically, ride the radiation pressure of light that are being emitted by the Sun, the sails being large, ultra-thin mirrors that will efficiently reflect the photons to provide the push. Note that this is different than solar wind, which provides a slightly smaller force. While this exerts relatively little force upon the sails, over time it could accelerate the craft to huge speeds, estimated to above 200,000 kilometers per hour within three years! This set up is analogous to Earthly sailboats that rode the winds back in the Age of Discovery, this endless source of propulsion will enable easy exploration amongst our planets. Even old school navigation techniques are applicable, such as tacking into the wind. On top of that, it has even been suggested that the surface area of the sails could double as a large antenna, optimum for deep space travel. Pretty cool, right?
Sort of. While seemingly easy enough of a concept, it is difficult to execute, and not necessarily as useful as it might seem. While solar sails could potentially reach great speeds, it takes considerable time for them to do so, which is impractical for human travel and any unmanned satellites that require fast deployment. Also, for this to be at all effective, even at a small scale, the sails have to be quite large, which has some serious logistical deployment problems.
The issue of speed may be alleviated by proposed hybrid models, some of which involve both a chemically powered rocket to give the satellite an initial boost, and then the solar sails kicking in to provide even more speed in the long run. Another model yet, developed by the Japanese, involves both a solar-powered ion drive and solar sails.
The deployment issue is still one that engineers are wrestling with. There are many competing designs for solar sails, from square sails to heliogyros to circular disks to circular rings, and which one works best is still up in the air. Engineering solar sails has been largely theoretical up to this point, spotted with a few unhelpful failures.