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What are Solar Sails?
A solar sail is a large, lightweight mirror used to propel a spacecraft using the pressure exerted by sunlight. In essence, it works like a sailboat. Whereas a sailboat is propelled by deflecting the wind, a solar sail is propelled by deflecting sunlight. Solar pressure is very weak - about 9 millionths of a Newton (micro-Newtons) or 2 millionths of a pound (micro-pounds) of force on a square meter at Earth's distance from the sun. This is far too little pressure to have any effect on Earth, because other forces are much larger, like air drag and gravity driving us into the ground. In space there is no air drag and objects fall freely under the influence of gravity without the ground to constrain them. Sunlight can have a significant effect on objects, depending on how lightweight they are. Large and lightweight objects are affected more. Dust given off by comet is pushed into brilliant tails millions of kilometers long. Spacecraft accumulate small errors due to sunlight in their orbits and their spin over long time periods. Even asteroids gradually change their spin over millions of years. A solar sails can accelerate quickly enough to travel between planets or change the behavior of its orbit around a planet or the sun - without consuming any propellant
Sailing away from or towards the sun
Solar force (blue arrow) can spiral sail towards or away from the sunSpacecraft normally coast in orbits around the sun or a planet. Rockets are used to apply a quick force to change the orbit to transfer to another area. Once reaching the new location, a second rocket burn places the spacecraft into the destination orbit. Solar sails, using the tangential force, can also change their orbits like a rocket. The difference is that the solar sail generates very low thrust that can run over the entire orbit transfer, rather than just a short powerful burst at either end. Instead of coasting to the destination, the sail gradually alters the orbit over time. Despite having a force component pushing away from the sun, the tangential force allows a sail to both travel towards and away from the sun. Like a rocket, if the tangential force is in the same direction as the spacecraft's orbit, the orbit will expand and the sail will travel out from the sun. If the sail is tilted the other direction so that the tangential force goes against the orbit, the orbit will contract and the sail will spiral in closer to the sun. This is able to overcome any complications caused by the portion of the force pushing out.
