Dr. Robert L. Forward has shown that much larger solar sails, on the order of a thousand square kilometers, might be used for effective interstellar travel. Away from a star, of course, there is no light to provide pressure against the solar sail. Engineers in the star system of origin could construct an array of lasers, a beam combiner and a giant Fresnel lens to project an incredibly powerful beam of collimated (laser) light. The light, when it hit the solar sail, would provide much more acceleration than normal sunlight at any distance. As the starship drew further away from the system of origin, the engineers back home would need to build more lasers and make the beam more powerful. Leaving your propulsion system at home like this is a really neat idea, since people on Earth have infinitely more resources than the people aboard the spacecraft and can tinker with the lasers, making use of technological improvements. Used in this way a solar sail might be better known as a lightsail, but either terminology is accepted.

Surprisingly enough, a solar sail used this way can also provide the means for deceleration. As the spacecraft approaches its destination, the habitation module located at the center of the solar sail detaches from the bulk of the lightsail and fractionally decelerates using rockets. The larger sail structure moves ahead of the habitation module and its smaller braking sail, and reflects laser light back onto the leading surface of the spacecraft's braking sail, slowing it down. Once the habitation module is in system, it uses the braking sail as a standard solar sail to navigate through the destination system.

                            main sail
             braking sail  /|
                        | / |
 huge beam of         Io|/       <--- Reflected light pushes back on
 laser light          Io|\            spacecraft to decelerate it
                        | \ |

In addition to being a consulting physicist, Dr. Forward is also a successful hard science-fiction writer. The bulk of his solar sail work is summarized in a lengthy but approachable appendix to his science-fiction novel Rocheworld, in which a solar sail starship crewed by colorful characters travels to the Barnard star system. The book is an excellent read, and is also a good illustration of the possibilities of solar sail propulsion.

For the more technically inclined, Forward has published an article on lightsail propulsion for interstellar probes in Jane's Spaceraft, the text of which is freely available at http://www.whidbey.com/forward/pdf/tp075.pdf.