Solar Propulsion
Interplanetary Solar Sails
A solar sail is a spacecraft that has an immense, lightweight mirror attached to it. It derives its propulsion by being pushed by light reflecting off of the mirror, instead of traditional rocket engines. The light used could be supplied by sunlight or lasers placed in orbit. 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 kilometre on a side would only feel about 9 Newton 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 manoeuvre, and there is no noticeable friction to interfere with your 9 Newton of thrust.Some rockets can push millions of times harder, but the sail keeps pulling as long as light shines on it.
Months or years after the rocket runs out of fuel, the sail will still operate.
Sails designed for robotic exploration missions would have dimensions on the order of several 10s of meters to a few kilometres. Sails designed to transport cargo in support of piloted missions would have dimensions of several kilometres because the cargo payloads are heavier than robotic spacecraft. For comparison, the engineering test prototype sail developed by the World Space Foundation was a square sail 30 meters on a side.
Links:
Solar Sailing School
Solar Spacecraft �kit�
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