The rays of light emanating from the Sun provide tremendous momentum that could push the space sail about 240,000 kilometres an hour - four to six times faster than the propulsion systems used on today's spacecraft. Sunlight is made of photons - a part of visible light that doesn't have mass or an electric charge but possesses energy and momentum. Photons would propel the spacecraft by pressing against the sail in the same way that wind pushes a sailboat. Astronomer Terrence Dickinson says the space sail is a very simple principle. But, it only works in space.
Sunlight has a very slight pressure when it lands on something.
In particular, the movement of the air in the atmosphere overwhelms any pressure sunlight would have on Earth. In space, when you're away from the atmosphere and that kind of interference is not there, then this very feeble pressure of sunlight starts to add up. In order to capture as much sunlight as possible, the space sail would have to be huge - about 400 metres across. On Earth, if the wind propelling a sailboat stops, so does the boat. Any momentum it gained from the wind would eventually be lost because of the friction created by the water.