I understand that it’s impossible to bring a photon to a state of rest, therefore impossible to collect them into a cup to weigh them and calculate their mass. A guy in a pub explained to me, quite smugly, that photons are just expressions of energy and that’s that. From my understanding solar sails would be just large surface areas being hit by photons, pushing the spacecraft in a desired direction, just like normal ship sails are being pushed by wind. But air particles do have mass. How could photons push the spacecraft if they don’t weigh anything?
In: Physics
Solar sails are certainly not science fiction. Japan launched IKAROS in 2010 with a 14m square sail that carried it to Venus in just 7 months. NASA’s ACS3 with an 80m square sail was completely unfurled just a few days ago and is currently in orbit. It may even be visible from your backyard (go to [NASA.gov](http://NASA.gov) for details).
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