Reuters reports that a private U.S. group is preparing to launch into space what would be the first spacecraft using solar sails.
Basically, the sails reflect light and the action of photons hitting it causes the vehicle to accelerate. It sounds very sci-fi, but there is no reason in principle that it shouldn’t work. On the other hand, it probably will never be a viable method of space travel.
The problem with solar sails is that they just don’t scale very well. If you want to move even modest payloads at significant speeds the sails end up having to be enormous or you have to use an auxiliary source of light. A lot of futuristic solar sail plans, for example, speculate on building laser arrays on the moon or an asteroid — the laser would be aimed at the solar sails and give it much more acceleration than sunlight.
Unfortunately by the time you’re done working out the kinks from that sort of system — not to mention finding a way to pay for such an elaborate system — it’s a lot cheaper and more efficient to go with other equally exotic but definitely workable propulsion systems such as fusion reactors or matter/anti-matter engines.
For the near term, nuclear power plants will be the main source of long range, fast spacecraft although viable fusion reactors are a lot closer than most people think.
To make space exploration and colonization viable we will have to develop craft capable of speeds approaching 5 to 10 percent of the speed of light which is certainly doable but not with solar sails.