Solar Sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push spacecraft.  Solar Sailing is an inexpensive method of propelling spacecraft because no fuel or engines would be required. Solar Cells are also called light sails or photon sails.

How do they work?

Solar Sails propel a spacecraft by utilizing the pressure created by the stream of photons (tiny units of light energy) from the sun. Once a spacecraft is in orbit, a lightweight sail would unfurl. Changing the position of the sail would increase or decrease speed. The thrust created by the photon stream is very low and interplanetary journeys would take years. For long missions, an on-board laser or microwave transmitter would be fitted to provide power when the Sun is distant.

History

On February 4, 1993, Znamya 2, a 20-meter wide aluminized-mylar reflector, was successfully tested from the Russian Mir space station. Although the deployment test was successful, the experiment only demonstrated the deployment, not propulsion. A second test, Znamaya 2.5, failed to deploy properly.

On August 9, 2004, the Japanese ISAS successfully deployed two prototype solar sails from a sounding rocket. A clover type sail was deployed at 122 km altitude and a fan type sail was deployed at 169 km altitude. Both sails used 7.5 micrometer thick film. The experiment was purely a test of the deployment mechanisms, not of propulsion.

In June 21, 2005 a Volna rocket launched from a Russian submarine in the Barents Sea launched the privately built Cosmos-1 spacecraft. However, a rocket failure prevented it from reaching its intended orbit. The Cosmos-1 spacecraft was designed to use solar sails to move through space. Had the mission been successful, it would have been the first ever orbital use of a solar sail to speed up a spacecraft, as well as the first space mission by a space advocacy group (Planetary Society).

The IKAROS probe is the world's first spacecraft to use solar sailing as the main propulsion.

LightSail-1, a second orbital spacecraft by the Planetary Socity is under construction and is expected to be ready by the end of 2010.

Reference:

Picture from http://src.space.ru/page_30e.htm (Space Regatta Consortium)

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