Solar sails (also called light sails or photon sails) are a
form of spacecraft propulsion using the radiation pressure (also called solar
pressure) from stars to push large ultra-thin mirrors to high speeds. Light
sails could also be driven by energy beams to extend their range of operations,
which is strictly beam sailing rather than solar sailing. Solar sail craft
offer the possibility of low-cost operations combined with long operating
lifetimes. Since they have few moving parts and use no propellant, they can
potentially be used numerous times for delivery of payloads. Solar sails use a
phenomenon that has a proven, measured effect on spacecraft. Solar pressure
affects all spacecraft, whether in interplanetary space or in orbit around a
planet or small body. A typical spacecraft going to Mars, for example, will be
displaced by thousands of kilometres by solar pressure, so the effects must be
accounted for in trajectory planning, which has been done since the time of the
earliest interplanetary spacecraft of the 1960s. Solar pressure also affects
the attitude of a craft, a factor that must be included in spacecraft design. The
total force exerted on an 800 by 800 meter solar sail, for example, is about 5
newtons at Earth's distance from Sol, making it a low-thrust
propulsion system, similar to spacecraft propelled by electric engines.
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