Search query: Solar sail spacecraft projects
Video title:  momentum without any Mass instead a photon's momentum is proportional to its frequency how quickly it oscillates back and forth as a wave instead of to its mass that means that different colors of light have different amounts of momentum blue light has a higher frequency than red light so photons of blue light have more momentum than photons of red light and would push a solar sail more when light pushes it's called radiation pressure and it's something that space scientists have been using and accounting for since the early 1960s like when they were designing the Mariner 4 Mars probe for example which used radiation pressure to stabilize itself and make sure it stayed on course because they compensated for the push from sunlight Mariner 4 became the first probe to ever return pictures of another planet from deep space when it sent back images of Mars in 1967 radi pressure is also being used to keep the Kepler space telescope stable we've talked about Kepler before its main mission was to find planets around other stars by staring at them for years at a time to do this while orbiting our sun Kepler needed to be able to rotate slightly every once in a while so that it could keep an eye on the same patch of Sky while it moved but in the vacuum of space friction won't stop you from rotating once you've started so Kepler had what are called reaction Wheels which would basically spin in the opposite direction Toop sto the telescope from rotating out of control in 2013 when two of these reaction Wheels stopped working though it looked like Kepler's days were numbered but in 2014 the team working on the project was able to use the sun's radiation pressure to change where the telescope was facing and it's still working more than a year after the fix all thanks to light's ability to push it's one thing to keep a ship stable but for radiation pressure to be a practical way of propelling a spacecraft solar sails have to be huge like 10 or hundreds of square meters because there needs to be room for a lot of photons to hit it to give you an idea of how small a photon's momentum really is it takes about 2 million billion