Nearly 400 years ago astronome Johannes Kepler observedcomet tails blown by solarbreeze and suggested thatvessels might likewise navigate through space usingappropriatel fashioned sails.It is now widely recognized that sunlight does indeed produce a forcewhich moves comet tails and large, reflective sailcould be a practical means o propellinga spacecraft.In fact, the illustration above represents one concep explored by NASA centers to develop aninterstellar prob pushed alongby sunlight reflected from an ultrathin sail.Nearly half a kilometer wide, the delicat solarsail would be unfurled in space.Continuous pressure from sunlight would ultimately accelerate the craftto speeds about five times higher than possible with conventiona rockets -- without requiring any fuel!If launched in 2010 such a probe could overtak Voyager 1,the mos distant spacecraft bound for interstellar space,in 2018 going as far ineight years as the Voyager will have journeyed in 41 years.
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