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Planetary Society Set To Launch Solar Sail Experiment

The Planetary Society is preparing to launch a tiny satellite into orbit later this month as the first phase in testing a solar sail as a means of spacecraft propulsion — an idea that has been kicking around in the science (and science-fiction) literature for at least a century.

The satellite, LightSail, no larger than a loaf of bread, is scheduled to lift off aboard an Atlas V rocket on May 20.

The concept states that:

"Solar sails use the sun's energy as a method of propulsion — flight by light. Light is made of packets of energy called photons. While photons have no mass, a photon traveling as a packet of light has energy and momentum.

"Solar sail spacecraft capture light momentum with large, lightweight mirrored surfaces — sails. As light reflects off a sail, most of its momentum is transferred, pushing on the sail. The resulting acceleration is small, but continuous."

The first LightSail won't reach a high enough orbit to try out the sail in the solar wind, but it should be able to test the mechanism for deploying the 345-square foot tissue-thin Mylar sail. A mission set for next year should put a second LightSail in a high enough orbit to fully test the concept.

A decade ago, the Planetary Society, the non-profit founded by the late Carl Sagan and now headed by Bill Nye ("The Science Guy"), made its first attempt to launch a solar sail, but the satellite was lost when the Russian launch vehicle it was on failed to reach orbit.

LightSail won't be the first spacecraft to test the solar sail idea. Both NASA and the Japanese space agency have experimented. Japan's Ikaros, launched in 2010, was the first to demonstrate the concept. "While each photon is small and only generates a small push, over time the accumulated energy from each one of the strikes pushes the solar sail (and anything attached to it) forward," according to

ExtremeTech adds: "Solar sails fall into the same category as ion engine technology in that they are very low thrust, but highly efficient. In the case of solar sails, you don't need to bring any fuel at all, so it's really infinitely efficient if you don't mind the long acceleration times. You get just a few newtons of force from even a large solar sail, which is why some of the proposed designs for interplanetary sails have surface areas many times bigger — in the hundreds or thousands of square meters. Scientists are still working on how you'd deploy something that size."

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Corrected: May 11, 2015 at 9:00 PM PDT
A previous version of this post incorrectly said that the satellite LightSail is contained within the somewhat larger Prox-1 satellite developed by the Georgia Institute of Technology. In fact, it is the second LightSail satellite that will be deployed via the Prox-1 spacecraft. It also stated that the LightSail is pushed along by the solar wind, but is in fact is powered by photons.
Scott Neuman is a reporter and editor, working mainly on breaking news for NPR's digital and radio platforms.