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

NASA's NanoSail-D is expected to test a type of propulsion that taps the momentum of photons in sunlight. Advocates say solar sails provide the best way toward interstellar travel.

A representation of what the NanoSail-D will look like when deployed.

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.

Ironically, although NanoSail-D's systems are identical to those required for solar propulsion, the craft will be demonstrating something different over the next 70 to 120 days: the use of such sails for braking.

NASA's first attempt to loft NanoSail-D came in 2008 aboard Falcon 1, the first in a growing stable of rockets and capsules built by Spacex, one of a new generation of rocket-makers. The company currently has a contract with NASA to resupply the International Space Station once evaluation flights end for its larger Falcon 9, whose first two launches were successful.

Unfortunately, the Falcon 1 carrying the first NanoSail-D failed. The NanoSail-D currently in orbit is a back-up unit that engineers have continued to modify over the past two years.

The craft was one of six payloads lofted by Orbital Science Corporation's Minotaur IV rocket on Nov. 20. The six payloads rode into space on a common "bus." NanoSail-D was to have ejected from the bus Dec. 6.

"The door opened, but nothing came out," says Dean Alhorn, an engineer at the Marshall Space Flight Center in Huntsville, Ala., and the project's lead investigator.

For more than a month, his team was in limbo, trying to figure out what might have caused the apparent failure.

Then, to everyone's surprise, the craft phoned home Jan. 19, indicating that it somehow, finally, worked free of the bus. With the help of amateur-radio operators in the US, including the Marshall Space flight Center, and in Germany, who had equipment capable of receiving NanoSail-D's encoded communications, the team gathered up the data and judged NanoSail-D to be in good shape, if somewhat tardy.

"I'm pleased with how everything has worked out," Mr. Alhorn says.

The craft is orbiting some 350 nautical miles above Earth. There, drag from Earth's extended atmosphere exerts more influence on a spacecraft's speed than do photons from the sun. So the goal is to see how well a sail can guide a craft to a controlled reentry into Earth's atmosphere, where it would incinerate.

By international agreement, satellite operators must design their craft to carry enough fuel to either boost themselves into an higher orbit reserved for dead spacecraft or to slow the craft for reentry. The goal is to reduce the likelihood that derelict spacecraft in low-Earth orbit will collide, adding to an already worrisome collection of spent boosters and dead satellites orbiting Earth. Collisions between these objects generate a tenuous but troubling cloud of debris that has threatened active spacecraft, including the International Space Station and the space shuttle.

Solar sails are far lighter and cost far less than the motors and fuel craft currently must carry for deorbiting, Alhorn says.

Even as NanoSail-D prepares to spread its wings, the Planetary Society has embarked on a three-step program of solar-sail development. It comes on the heels of a 2005 attempt to launch the organization's Cosmos 1 solar-sail demonstration craft. The Russian rocket lofting the craft failed before the craft could reach orbit.

LightSail 1, which the Planetary Society says it hopes to launch during the first half of this year, would head directly for an orbit roughly 440 nautical miles above Earth. There, the influence of sunlight on the craft would exceed that of Earth's atmosphere, allowing for the controlled solar-sail flight the group hopes to achieve. LightSail 2 would be larger, last longer, and carry scientific payloads for earth observation. If all goes well, LightSail 3 would be designed to travel farm from Earth to provide early warning of solar storms that erupt from the sun.

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