What Is A Solar Sail?
by Fraser Cain on January 20, 2014
Here’s how they work: Light is made up of photons. Even though they have no mass at rest, they have momentum when they’re moving, well, light speed. When they reflect off a surface, like a mirror or a shiny piece of metal, they impart some of this momentum to that surface. This effect is negligible here on Earth, but out in space, with forces perfectly in balance, that additional momentum can really add up.
A spacecraft flying to Mars gets pushed off course by several thousand kilometers because of light pressure from the Sun.If mission planners didn’t compensate for this drift, their spacecraft would miss the planet, or even worse, crash into it. Even though the total amount of pressure per square meter on a solar sail is minuscule, it’s constantly streaming from the Sun, and it’s totally free….
And propulsion that you don’t have to carry with you is the best kind there is.
IKAROS – solar sail from Japan. Image: JAXA
This is more than just an idea. Solar sails have already been launched and deployed in space. The Japanese Ikaros satellite unfurled a 14-meter solar sail back in 2010. NASA launched its own Nanosail-D spacecraft in 2011. An even bigger solar sail, the Sunjammer, is planned for launch in 2014. The Planetary Society is working on a solar sail project as well.
http://www.universetoday.com/108310/what-is-a-solar-sail/
The Kepler space telescope has been kept working to so extent using solar pressure. Its not working the way it was intended but they are able to use it by orienting it with solar pressure.