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.
Solar Sailing Powered by Reflection
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.
Sailing on a Beam
Other rocket scientists, meanwhile, are investigating a form of space sailing that wouldn't rely on energy from the sun. Robert Winglee of the University of Washington in Seattle says his concept could get people to Mars and back in an astounding 90 days (current technology would require about two-and-a-half years of travel to make the trip).
The new concept would deploy an intermediate space station that would beam a stream of plasma, or magnetized particles. The space station would use solar energy to generate the beam of magnetized particles from a nozzle about 100 feet wide. By capturing these particles in its sail, the spacecraft would be propelled as the particles bounce from its surface. Winglee estimates the system could propel a craft to spectacular speeds of about seven miles per second.
Winglee and his students are refining models for the idea with funding from NASA's Institute for Advanced Concepts.
"Standard chemical propulsion systems would require long missions to Mars," said Winglee. "What we're saying is you can put the power on a space station, lighten up the spacecraft and increase speed. This will give you major savings in cost."
Even though it might be the fastest idea out there, it's unlikely to play an immediate role in President Bush's mandate, announced last January, to launch manned missions to the moon and on to Mars. Despite the advantage in speed, the science, says Friedman, is still far out.
One of the main challenges now, is making sure the sails unfurl without a hitch in space and can then be adequately manipulated by remote control. In a test three years ago, Friedman's group launched a suborbital version of Cosmos 1 that never managed to open its two-bladed sail. This time around, Friedman says he's more hopeful the test will work. But, he adds, there's still a lot of work ahead.
"It will take time," he said. "But in the long run I think sailing of some form will be what space exploration will look like. This is just the beginning."