Although photons don't have mass, they can still impart momentum. Solar sails rely on this fact to accelerate continuously on their journeys through space, reflecting photons off of their paper-thin surfaces to push themselves onward. Though they start slow, after a few years, the sails can travel at astonishing speeds. What's more, they don't need to carry any fuel with them (though some may come equipped with lasers to provide photon-powered momentum when the sail travels too far from the sun).
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Key Facts In This Video
Solar sails rely on photons to give them momentum. 01:00
A solar sail is designed to catch billions of photon collisions per second, and will continuously accelerate until it can no longer catch photons. 01:56
Solar sails could potentially steer asteroids away from Earth, or propel spacecrafts on interstellar missions. 03:26