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Illustration Credit: courtesy NASA 
Explanation: Featured in this artist’s illustration, NASA’s NanoSail-D  finally unfurled  a very thin, 10 square meter reflective sail on January 20th, becoming the first solar sail spacecraft in low Earth orbit. Often considered the stuff of science fiction , sailing through space was suggested  400 years ago by astronomer Johannes Kepler who observed comet tails blown by the solar wind. Modern solar sail  spacecraft designs, like NanoSail-D or the Japanese interplanetary spacecraft IKAROS , rely on the small but continuous pressure from sunlight  itself for thrust. Glinting in the sunlight as it circles planet Earth, the NanoSail-D solar sail will periodically be bright and easily visible to the eye. In fact, skygazers are urged to participate in an ongoing contest to capture images of NanoSail-D . The images will help NASA monitor the satellite before it reenters the atmosphere in April or May.