Few things in science fiction look as straight up fantasy as solar sails. The solar sail in fiction essentially mimics the surfboard-plus-sail combo used in windsurfing here on Earth. Except where windsurfers catch gusts of air to carve their way through waves, solar sails would harness radiation pressure--the light and gases emitted by a star--to surf through the blackness of space. Well, turns out that solar sails aren't that fantastical--in fact, NASA plans to launch one as early as 2014.
NASA's solar sail project couldn't have a more perfect name. It's called Sunjammer, after a 1963 Arthur C. Clarke which coined the term solar sail. According to NASA, the in-development solar sail will measure approximately 124 feet to a side for a total surface area of about 13,000 square feet. But "when collapsed, it's the size of a dishwasher and weighs just 70 pounds. Attached to a 175-pound disposable support module, the Sunjammer is easily packed into a secondary payload on a rocket bound for low-Earth orbit."
Solar sails could offer an alternative to heavy and costly rocket fuels, but they don't exactly offer the same degree of thrust. The maximum thrust would amount to less than a Newton of force, but because the surface area of NASA's sail is so large, it will still be able to move. A solar sail wiki writes:
"Solar pressure is very weak - about 9 millionths of a Newton (micro-Newtons) or 2 millionths of a pound (micro-pounds) of force on a square meter at Earth's distance from the sun. This is far too little pressure to have any effect on Earth, because other forces are much larger, like air drag and gravity driving us into the ground. In space there is no air and objects fall freely under the influence of gravity without the ground to constrain them. Sunlight can have a significant effect on objects, depending on how lightweight they are. Large and lightweight objects are affected more. Dust given off by comets is pushed into brilliant tails millions of kilometers long. Sunlight causes small errors to accumulate over time in spacecraft orbits and spin. Even asteroids gradually change their spin over millions of years. This gentle force is enough for a solar sail that is sufficiently large and light weight to travel between planets or change the behavior of its orbit around a planet or the sun - without consuming any propellant."
Monolith Magazine writes that "When held in orbit, Sunjammer will act as a type of forward observatory for both NASA and the UK Space Agency, with British scientists developing two instruments on board to study solar wind." Sails could eventually be used to clear debris from Earth's orbit or even deflect dangerous asteroids, though it's hard to predict what kind of thrust would be required for that.
Clarke's vision of ships racing under the power of the sun won't come true anytime soon, but NASA launching a real solar sail? That's a good first step.