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==Introduction==

Hopefully by reading this document, you will be able to answer these questions when the topic of solar sails comes up:

* What is a solar sail?
* How does a solar sail work?
* Why does sunlight push a solar and not the solar wind?

==What is a Solar Sail?==

A solar sail is a large, lightweight mirror attached to a spacecraft that is pushed through space by the pressure of light reflecting off of it. Look here for [[Solar Sail Design]]. The light to push a sail can come from the sun or large lasers. Satellites in orbit around the Earth can survive for many years without any maintenance while using only a little bit of rocket propellant to hold their positions. Solar sails can be made to survive in space for many years as well. Solar sails use sunlight that never runs out like rocket propellant. During those years in space, a sail can move around as much as you want it to, such as from Earth to Mars and back, possibly several times if the sail remains in good condition. A similarly equipped rocket would either be ridiculously huge because it has to carry the fuel for all the trips, or would need to be refueled regularly.

==How Does Light Push a Solar Sail?==

===Electromagnetism===

Before anyone ever took a beam of light and measured how much it could push, there were predictions that light could exert a very gentle push on objects it hits. James Clerk Maxwell developed the laws describing electromagnetism and concluded that light is an electromagnetic wave. Maxwell predicted that when light hits an object and is absorbed or reflected, the light wave pushes on electric charges in the surface of the object, which in turn push on the rest of the object. If the light is reflected, the object gets pushed twice as hard, just as if you would be pushed twice as hard by a rubber ball hitting you as a ball of clay. In 1901-1903, the Americans Nichols and Hull and Russian Lebedev were able to measure light pressure as predicted by Maxwell. Find a physics text that discussed electromagnetism, like Physics, by Halliday, Resnick, and Krane, to see how the force is derived from Maxwell's equations.

===Einstein===

When Einstein developed his theories of relativity, and gave us the equation <tex>E = m c^2</tex>, it allowed us to calculate light pressure a lot easier. <tex>E = m c^2</tex> compares energy, which can be easily measured in light, to mass and movement, which can easily be used to find forces.

* E is an amount of energy
* m is an amount of mass
* c is the speed of light, about 300 million meters per second

If you fiddle around with <tex>E = m c^2</tex>, you find that the force light exerts is the power of the sunlight divided by the speed of light. Like I stated above, you get twice as much force from an object that reflects all the light as you do from an object that absorbs all the light. In order to get this simple formula, force equals power divided by speed of light, the steps taken by Maxwell and others had to be taken first.

==Very, Very, Gentle Force==

Sunlight exerts a very gentle force. The power of sunlight in space at Earth's distance from the sun is between 1.3-1.4 kilowatts per square meter. When you divide 1.4 kilowatts by the speed of light, about 300 million meters per second, the result is very small. A square mirror 1 kilometer on a side would only feel about 9 Newtons or 2 pounds of force. Fortunately, space is very empty and clean compared to Earth, so there is plenty of room for a 1 kilometer wide sail to maneuver, and there is no noticeable friction to interfere with your 9 Newtons of thrust. A sailboat on Earth wouldn't be going anywhere with that little force because of drag from the water and air. Some rockets can push millions of times harder, but the sail keeps pulling so long as light shines on it. Months or years after the rocket runs out of fuel, the sail is still pulling.

==Why Don't Solar Sails Use the Solar Wind?==

Many people assume that because here on Earth they feel wind but not sunlight, that solar sails must be pushed by the solar wind. However, there is a very big difference between space and Earth. Earth is wrapped in a thick layer of gas that is felt as wind whenever it moves. In space, there is no air to move around and cause strong winds like we feel on Earth. The solar wind is an extremely tenuous flow of particles ejected by the sun which exerts very little force on anything it hits. In fact, the solar wind can have 1000-10,000 times less force on a solar sail than sunlight. People worry about the solar wind for other reasons. Solar storms can hurt people and electronics in space. Increases in the solar wind can cause the upper atmosphere to swell and increase drag on low Earth orbit spacecraft. The solar wind can be used to push magnetic sail or plasma sail spacecraft.