The best thing about solar sails is that they're effectively free spaceflight. Most spacecraft need a significant amount of space to be set aside for fuel. This means that solar sails may well be the most economical way to travel. After all, the sun already provides a huge amount of energy to everything within an 18 billion kilometer (11 billion mile) radius. Why not spacecraft too?
VIDEO: NASA Unfurls Solar Sail
Solar sails aren't a new concept. In fact, the idea is surprisingly old. The first to notice that comet tails always point away from the sun was Johannes Kepler. In a letter to Galileo in 1610, he noted: "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void." In the 17th century, of course, powering a ship with anything other than sails would likely have seemed ludicrous.
400 years after Kepler's original inspiration, JAXA launched IKAROS, the first true solar sail ever built; a few sails had been created previously, but none had successfully demonstrated propulsion. Deployed in June 2010, IKAROS had a sail 200 square meters in size, constructed from polyimide with LCD Panels built in to allow attitude control by adjusting the sail's reflectance. Reaching a velocity of around 100 meters per second (224 miles per hour), IKAROS reached its destination at Venus roughly 6 months after its sail deployed.