The unfurling of a Japanese solar sail, the first demonstration of a new space propulsion technology, went exactly according to plan.

According toJAXA's blog posts and photos from the event, the IKAROS spacecraft's sail appears to be in place. It's a big step in its attempt to travel driven only by sunlight.

"This is the first sail ever deployed in space, and if they succeed in using it for solar-sail flight – it'll still be a few weeks before we know that – it'll be a milestone," said Louis Friedman, executive director of the Planetary Society, an organization dedicated to promoting space exploration, which is readying its own solar-sailing mission.

A solar sail uses the pressure from photons striking its surface to push the spacecraft through space. Materially, the 650 square-foot sail is made of incredibly thin, aluminized plastic that's only 0.0003 inches thick, a little thicker than spider silk, or about the diameter of a red blood cell. When a photon strikes its surface, it bounces off, imparting its momentum to the sail. Each photon might not deliver much thrust, but over time, all that light adds up.

"The actual force might be just a few millionths of a g, but because it acts continuously, it allows you to build up large velocity changes over time," Friedman said. "That's where a sail really does its work is long missions."

The Japanese sail also has thin-film solar cells built into it. They could be used to generate electricity to drive an engine that would work alongside the sail.

The key difficulty with such a thin and large object is that it's hard to deploy. "The things we're watching for are all their dynamical behaviors that you ultimately can't model and that might cause undue stress on the material," Friedman said.

In the IKAROS design, the sail was unfurled by using centrifugal force generated by spinning the craft.

Space-travel proponents are particularly interested in the technology because it doesn't require fuel, which makes it the leading (and basically) only candidate for very long-distance travel.

"It's the only way we know – that anybody knows – to ultimately do practical interstellar flight because you don't have to carry your propellant along with you," Friedman said. "Anything else you do, whether it's nuclear or advanced engines, you're always carrying propellant and the mass becomes too great."

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