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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.

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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.

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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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