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In a NASA first, NanoSail-D spacecraft to set sail on the sunlight

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Solar sails operate on the same general principles as conventional sails operate. But where a sailboat gets its push from wind, a solar sail gets its push from sunlight – a possibility first envisioned after physicists figured out that while particles of light, known as photons, have no mass, they do carry momentum. When they strike an object, such as a reflective sail in space, they can transfer momentum to the sail and thus to the object hoisting it.

A craft propelled exclusively via solar sails travels at a snail's pace when it starts. But with no air resistance in space, momentum would rapidly build. By some estimates, a mission to Pluto, currently a 10-year trip, could reach the dwarf planet in five years.

A solar-sail craft also could devote more of its payload to scientific experiments rather than mass-costly motors and fuel, which today's craft carry for course corrections on a long voyage or altitude changes to maintain orbit around a planet or moon.

So far, Japan has lofted the most sophisticated solar-sail craft to date. The craft, IKAROS, launched in May 2009 with the country's Venus climate orbiter, Akatuski. A month later, IKAROS deployed a square solar sail roughly 19 feet long on each side, which has propelled the craft on a trajectory that will put it in orbit around the sun.

Thin-film solar cells on the sail provide electricity for the craft. But one of its most ingenious features, Mr. Nye says, involves steering. Instead of moving the sail's angle relative to the incoming sunlight, the craft uses strategically placed arrays of liquid crystals – much like those in a digital watch – to alter the ability of a given section of the sail to reflect light.

The approach allows the craft to alter course, but slowly. The system takes roughly 24 hours to achieve a one-degree change in course.

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