Solar sails are all about "propellantless" propulsion — using photons from the Sun (as opposed to an onboard fuel supply) to push a spacecraft through space. Building on the lessons learned from smaller solar sail missions, like NASA's NanoSail-D and JAXA's IKAROS, NASA is gearing up to to launch the biggest solar sail in history as early as next year. It's called Sunjammer, and at 13,000 square feet it is positively massive.
The Sunjammer mission—the name is borrowed from an Arthur C. Clarke short story about an interplanetary yacht race—will unfurl a solar sail that dwarfs those that have thus far been tested in space. Where NanoSail-D's diminutive sail measured just 100 square feet and Japan's IKAROS measures something like 2,000 square feet, Sunjammer's sail possesses a total surface area of nearly 13,000 square feet. Yet collapsed it weighs just 70 pounds and takes up about as much space as a dishwasher, making it easy to stow in the secondary payload bay of a rocket headed to low Earth orbit.
The sail will be made of Kapton, a super-thin film developed by DuPont that is used in all kinds of things, from space suits to flexible circuitry. The special layer of Kapton film developed by DuPont along with NASA is just 5 microns thick. Once unfurled, it is light enough that its own weight isn't a hindrance yet strong enough that it can tow a support module across space using pressure provided by the sun in the form of photons as propellant, much as a maritime sail uses the wind to pull a sailboat across a body of water.