Louis Friedman imagines the day when people rocketed into weightlessness will unfurl lightweight, football field-sized sheets from their spacecraft and set sail for Mars.
"It's a realistic prospect," said Friedman, head of the Planetary Society, a Pasadena, Calif.-based organization that was co-founded in 1980 by space visionary Carl Sagan. "I can imagine it working well for cargoes on long trips."
Friedman's group plans to perform the first-ever orbital test of a solar sail this coming spring with the help of another Sagan-affiliated group, Cosmos Studios, based in Los Angeles, and Russia's Babakin Space Center in Moscow.
Cosmos 1, a 3-foot-wide spacecraft, will be launched by a converted Russian ballistic missile from a submarine in the Barents Sea. Once the craft is in Earth's orbit, inflatable tubes will force out a lightweight "sail" that will fan out into its orbital form. The eight blades of the craft's beach umbrella-like sail are programmed to lock into position, and can then be manipulated to allow light particles from the sun, or photons, to nudge the sails from any angle.
The concept of "sailing" using the sun's rays was first devised nearly 400 years ago by astronomer Johannes Kepler, who noticed that comets' tails are blown by an apparent solar wind. He figured the same force could be harnessed to propel space vessels.
In fact, the solar sail designed by Friedman's group isn't exactly blown by solar winds. Unlike a breeze, which pushes the canvas of a traditional sail, light particles generate their force by striking the mirror-like surface of the ultra-thin sail and reflecting from it. This reflection creates a very tiny force (about the force of a postage stamp resting in your palm), but it is constant and, in the vacuum of space, can accelerate over weeks and months to reach velocities faster than any chemically fueled spacecraft to date.
To maximize the amount of energy taken from reflecting light photons, each of the eight blades of Cosmos 1's sail will span 47 feet. The sail, itself, however will be fairly flimsy and made of a Mylar material that's not much stronger than typical plastic wrap. Friedman isn't sure how long the sail will persist in its orbit, but he says even if it lasts just a half-hour he will consider the mission a success. Since it will be deployed in the cluttered environs of Earth's orbit, Friedman anticipates some minor damage to the sail, although it will be reinforced with rip-stop features.
"Space debris will definitely put holes in it," he said. "But it will be going so fast, they should be clean holes, not big tears. That is a concern, but just for this test, it should work."
NASA researchers at the Jet Propulsion Laboratory in Pasadena have been working on their own space sail concept and had planned to launch a probe that would zip around the outer solar system using the force of solar particles.
JPL's craft would use a sail made of unusually thin, but strong, carbon fiber material. While about as thin as a piece of notebook paper, the carbon fiber could withstand intense heat from the sun and so could fly closer to the fireball for maximum propulsion. A flight test of the craft had been scheduled for 2007, but was indefinitely postponed due to a lack of funding, according to JPL's Sarah Gavit.