| Making the sheet reflective like a mirror is important. Sending most of the light back (nearly) doubles the pressure when the sail squarely faces the Sun. But more important, the sail can be tilted at an angle to the Sun's rays and reflect them in some chosen direction,. e. g. in the direction opposite from the one in which the spacecraft is moving, which gradually increases the velocity of the spacecraft. When the sail faces the Sun squarely, the pressure of sunlight counteracts the Sun's pull and only enables the satellite to circle the Sun in a slightly larger orbit. On the other hand, placing it at an angle can put the spacecraft on an outward or inward spiral, trajectories useful for interplanetary missions.
A solar sail can also be useful for a monitoring spacecraft, placed to warn Earth of approaching "storms" from the Sun at a greater distance from Earth than the Lagrangian L1 point where such spacecraft are usually placed (details here).
In general, solar sails are only expected to be useful where sunlight is sufficiently intense, e. g. inside the orbit of Mars. However, Robert Forward, science fiction author and physicist, has proposed the use of a solar sail in trips to the distant stars.
In his "Flight of the Dragonfly" a team of brave astronauts sets off in a starship to the water-covered half of Roche, a strange double planet inhabited by intelligent whale-like creatures. The starship is propelled by a huge solar sail, pushed by tight beams of light from lasers which are powered by sunlight and orbit the Sun. As the craft approaches its target, part of the sail is cut free. The laser beams then push it ahead of the starship, and as those beams are reflected back, they hit the remaining solar sail from the opposite direction, slowing down the ship and allowing its passengers to land. Let it never be said that any idea is too far-out for scientists to consider! In a technical appendix to the book, Forward presents the technical details of his fantasy.
The solar sail home page
Picture of a solar sail, with links.
Web page on the current state (2004) of solar sail work.
More about Robert Forward's laser-driven sail in "The Starflight Handbook" (subtitle: "A Pioneer's Guide to Interstellar Travel"), by Eugene F. Mallove and Gregory L. Matloff, John Wiley & Sons, 1989.
Arthur Clarke wrote in 1963 a short science fiction story "The Wind from the Sun" about an international race in the far future, between spaceships propelled by solar sails. It can be found in a collection of Clarke's stories, also titled "The Wind from the Sun," published in 1972 by Harcourt, Brace and Jovanovich, inc. Olivier Boisard has produced a charming picture-book version of the story on the world-wide web, at http://www.ec-lille.fr/~u3p/bd/bda.html.
Questions from Users:
Resistance to solar sails by interstellar gas
Holes in a Solar Sail