http://www.sciam.com/2001/0701issue/0701scicit2.html

Sailing on Sunlight
A LOW-COST MISSION TO LAUNCH THE FIRST SOLAR SAIL

Pssst. Want to launch a spacecraft on the cheap? All you need is a group of space enthusiasts, a few million dollars and a Russian ballistic missile. The Planetary Society, a Pasadena, Calif.–based nonprofit organization dedicated to space exploration, is taking this low-budget approach to conduct the first demonstration of solar sailing—using the pressure of sunlight to propel a spacecraft.

The Planetary Society, which has more than 100,000 dues-paying members, contracted the Babakin Space Center, located just outside Moscow, to construct and launch its spacecraft. Russian scientists had proposed the idea of boosting the craft into orbit using a submarine-launched ICBM called the Volna. Arms-control agreements require the Russians to either discard the rockets or convert them to other uses. "We are literally taking these missiles out of the battlefield," says Louis Friedman, executive director of the Planetary Society. "The plan was so practical and inexpensive that we were able to find private funding for the mission."

That funding came from Cosmos Studios, a media company founded by Ann Druyan, the widow of Carl Sagan. The company furnished $4 million to develop Cosmos 1, a 40- kilogram spacecraft containing eight triangular panels of ultrathin Mylar. Once the craft is in orbit, it will unfurl the Mylar panels to form a 30-meter-wide sail that will be turned toward the sun. If all goes well, persistent pressure from the sun's photons will slowly push Cosmos 1 to a higher orbit. The goal of the mission is to sail the craft for one week, but it could conceivably keep cruising on the sun's rays for months.

So far, however, the progress of Cosmos 1 has been anything but smooth. In April an electrical short occurred during ground testing, damaging some of the craft's components and cables. The accident prompted the rescheduling of a suborbital test flight as well as the orbital mission, which is now planned for late this year or early 2002. For both the test flight and the orbital mission, the submarine launches will take place in the Barents Sea, north of the Russian port of Murmansk. Cosmos Studios will use images from both flights to prepare a two-hour documentary on the mission, which will be broadcast by A&E Television Networks next year. "It's amazing that a small company like ours could finance this," Druyan says. "I feel like I'm outfitting the Wright brothers' bicycle shop."

The great advantage of solar sails over conventional propulsion systems is that they do not require huge loads of rocket fuel. In fact, NASA is studying the use of solar sails to propel the Interstellar Probe, a mission tentatively scheduled for launch in 2010. A giant solar sail could accelerate the probe to speeds as high as 90 kilometers per second, allowing it to travel far beyond the solar system in just a few years. Cosmos 1 could serve as a stepping-stone for such efforts. Says Friedman: "I hope this will be the beginning of something grand."

--BY MARK ALPERT

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BASICS OF
SOLAR SAILING

The bigger the sail, the stronger the push. At Earth's orbit, the maximum thrust from solar pressure is only about nine millionths of a newton per square meter—equivalent to about one thousandth the weight of a paper clip. That's why NASA is planning to build a 400-meter-wide sail for the Interstellar Probe, which will be capable of traveling five times faster than any rocket-powered craft. Navigators will be able to tack the probe by adjusting the angle of its sail as it orbits the sun. Tilting the sail about 35 degrees away from the sunlight will maximize the thrust in the direction of the probe's orbital motion, causing it to spiral outward to the farthest reaches of the solar system and beyond.