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

A new NASA craft is due to set sail, literally, Thursday night.

What's more, it hopes to be unfurling its sail in outer space.

What, you may ask, is NASA doing with a sail-powered vehicle?

The answer is that the bread-loaf-size satellite, built on a shoe-string budget, is designed to test a space propulsion technology that until the past few years has dwelt in the realm of science fiction.

The satellite, NanoSail-D, is expected to open its thin, square, reflective sail at 10 p.m. Eastern Standard Time, representing what would be the first successful on-orbit deployment of a solar sail in the history of the US space program.

IN PICTURES: Harnessing the sun's energy

It make lack the pizzazz of warp drive, the fictional propulsion system known to Star Trek fans. But many of its advocates argue that solar sails represent the best path to eventual interstellar travel. More immediately, the technology also holds the promise of reducing the amount of space junk orbiting Earth, boosters say.

The rooting section for Wednesday night's sail-deployment attempt may be small, but it's enthusiastic.

"The solar-sailing world is such a small world that we're all rooting for each other," says Bill Nye, executive director of the Planetary Society, a space-exploration advocacy group in Pasadena, Calif. The society has its own solar-sail demonstration program underway.

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.

That works well for deep-space travel. But for orbital work, a craft would have to be more agile, requiring a mechanical means of trimming the sail.

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