Solar sails will use the sun's energy to propel spacecraft. 

Photo courtesy The Planetary Society

In the 1970s, NASA scientists proposed sending a probe to Halley's comet that would be propelled by the pressure of sunlight against a giant solar sail. Although the proposal was rejected as being too risky and unproven, the idea of solar-sail-propelled spacecraft has endured. Numerous developments and tests of solar-sail materials have been conducted over the years, but no one had designed, successfully launched and sailed such a spacecraft.

In June 2005, The Planetary Society, in collaboration with several Russian space organizations, will launch the Cosmos-1 spacecraft into Earth orbit.

What is a solar sail? How can you use sunlight to move a spacecraft in outer space? In this article, HowStuffWorks will show you how solar sail technology works, take an in-depth look at the Cosmos-1 mission and find out what solar-sails mean for future space travel.