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Here Is How A Spacecraft Can Be Propelled With Nothing But Light

A single light particle (or photon) isn't precisely the miracle propellant we've been looking for. A single photon can only push an object with about 1 quintillionth of the force of a snowflake falling on your cheek. But once out in space, where countless photons are continuously being ejected from the Sun, and there's no friction to hold back, the mutual energy and momentum of all photons released by sun, might just be enough to push a complete spacecraft around.

That's the exact thinking behind the Planetary Society's Lightsail - a solar-powered spaceship armed with a big, 32-square-metre sail that quite accurately sails on sunlight same as a boat sails on the wind. Presented back in June, the spacecraft moves, when the photons reflect off the Lightsail, they pass a simple transferal of momentum, bouncing off the extremely reflective surface instead of being absorbed.

What’s better is that Japan's Ikaros lightsail essentially uses photons to maneuver it around by altering the reflectivity of certain parts on the sail. The actual way they do this is purely genius - liquid crystals, which are alike to the ones in your phone's LCD screen, can increase or decrease the reflectivity of a surface, and whichever side is 'shiniest' is the one that acquires more thrust.
You might be thinking that photons have no mass, so how can something with zero mass have any momentum at all?

Well, think back to high school physics, and you would recall the classic "momentum equals mass times velocity" equation i.e p = mv. Well if mass equals zero, momentum has to equal zero too, correct? Not precisely, and that’s because of because of Einstein's general theory of relativity. According to Einstein's general theory of relativity, something can have momentum even if it has no mass at all - it only has to have some amount of energy.

You can also watch the video below to get a good understanding of how this all works.

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