Solar Sails

A staple of science fiction,solar sails take the trade-off between thrust and efficiency to an extreme. They are pushed along by the gentle pressure of sunlight—feeble but free.To deliver 25 tons from Earth to Mars within a year,a sail would have to be at least 4 square kilometers in size. Its material must be no denser than about 1 gram per square meter.Carbon fibers are now nearly that wispy.The next challenge will be deploying such a large but fragile structure. In 1993 the Russian Space Regatta Consortium unfurled the 300-square-meter Znamya space mirror,but in a second test last year it got tangled.NASA recently funded an analogous idea for a magnetic "sail"to catch the solar wind (charged particles streaming from the sun) rather than sunlight.

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