The unfurling of a Japanese solar sail, the first demonstration of a new space propulsion technology, went exactly according to plan.
Ã¢â‚¬Å“This is the first sail ever deployed in space, and if they succeed in using it for solar-sail flight Ã¢â‚¬â€ itÃ¢â‚¬â„¢ll still be a few weeks before we know that Ã¢â‚¬â€ itÃ¢â‚¬â„¢ll be a milestone,Ã¢â‚¬Â said Louis Friedman, outgoing 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 thatÃ¢â‚¬â„¢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.
Ã¢â‚¬Å“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,Ã¢â‚¬Â Friedman said. Ã¢â‚¬Å“ThatÃ¢â‚¬â„¢s where a sail really does its work is long missions.Ã¢â‚¬Â
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 itÃ¢â‚¬â„¢s hard to deploy. Ã¢â‚¬Å“The things weÃ¢â‚¬â„¢re watching for are all their dynamical behaviors that you ultimately canÃ¢â‚¬â„¢t model and that might cause undue stress on the material,Ã¢â‚¬Â 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 doesnÃ¢â‚¬â„¢t require fuel, which makes it the leading (and basically) only candidate for very long-distance travel.
Ã¢â‚¬Å“ItÃ¢â‚¬â„¢s the only way we know Ã¢â‚¬â€ that anybody knows Ã¢â‚¬â€ to ultimately do practical interstellar flight because you donÃ¢â‚¬â„¢t have to carry your propellant along with you,Ã¢â‚¬Â Friedman said. Ã¢â‚¬Å“Anything else you do, whether itÃ¢â‚¬â„¢s nuclear or advanced engines, youÃ¢â‚¬â„¢re always carrying propellant and the mass becomes too great.Ã¢â‚¬Â