Solar Sails Get Their Speed From Light, Not Fuel
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. (1:00)
A solar sail is designed to catch billions of photon collisions per second, and will continuously accelerate until it can no longer catch photons. (1:56)
Solar sails could potentially steer asteroids away from Earth, or propel spacecrafts on interstellar missions. (3:26)