The idea occurred to German astronomer Johannes Kepler in the 17th century when he detected a comet flying across the night sky trailing what looked like a plume of fire. If space had so much wind, why not build ships to sail the heavens?

Indeed. Tomorrow, barring delay or mishap, a U.S. filmmaker, an international association of space buffs and Russian aerospace organizations will use a leftover Soviet ballistic missile to put the first "solar sail" into orbit.

This unusual device, which looks like a 6,500-square-foot flower with eight triangular, mirrorlike petals, does not use wind, as Kepler predicted. Instead, it hopes to show that sunlight's gentle push might one day enable a spacecraft to reach speeds far greater than anything achieved by a mere rocket. Deployed, the petals are about 11/2 times the size of a basketball court.

"The design life for this mission is only a month," said Louis Friedman, project director for the venture known as Cosmos 1. "It could go longer, but not much. What we want to do is prove that it works -- that we can increase orbit energy and make it fly higher."

Solar sails work on a relatively simple principle -- that beams of light bouncing off a reflective surface will transmit a push to the surface, driving it forward. Although the force is tiny, it is also constant and cumulative. And in the vacuum of space, there is no atmospheric friction to slow it down.

Speed rises, and eventually will build well beyond the 25,000 mph needed to free an object from Earth's gravity. If they handle it properly, ground-based engineers can steer a solar sail back and forth in space, Friedman said, "tacking it, like a sailboat -- although the physics are different."

"The basic trick is to get a large enough sail surface and a spacecraft that's light enough so you can move," said NASA In-Space Propulsion Technology Manager Les Johnson, who is overseeing two NASA solar-sail projects expected to be ready sometime after 2010. "In the 1970s, we didn't have either the materials or the structures, and we've only gotten them in the last five or 10 years."

Space visionaries talk about using sails to travel between the stars, but beyond Jupiter, sunshine is not strong enough to provide the impulse, making it necessary to develop some kind of Earth- or space-based laser or microwave "ray gun" to provide light as the sails move into deep space. This technology is decades away.

In the relatively near term, however, solar sails might offer tremendous advantages over conventional propulsion for satellites orbiting the sun closer in than Earth. Instead of using up precious fuel in station-keeping "burns," solar-sail spacecraft will be able to change position at any time using the force imparted by the sun's rays.

Friedman said he first began thinking about solar sails in the 1970s as a NASA mission planner at the Jet Propulsion Laboratory (JPL). He was evaluating a proposal to use a solar sail to rendezvous with Halley's Comet, an idea that was "way too audacious at the time," Friedman said, but fascinating enough for him to write a book about it.

In 1980, Friedman joined planetary scientist Bruce Murray and astronomer and author Carl Sagan in founding the Pasadena, Calif.-based Planetary Society, an international advocacy organization dedicated to space exploration. Sagan, who died in 1996, was the Society's first president; Friedman is its executive director.

The Society is leading the solar-sail project, bankrolled by Ithaca, N.Y.-based Cosmos Studios, a scientific documentary film and entertainment company headed by Sagan's widow, Ann Druyan. "We allotted $4 million for the project," she said in a telephone interview. "We are still under budget."

The remaining partners are NPO Lavochkin, the quasi-independent Russian agency that designed the spacecraft, and the Russian Academy of Science's Space Research Institute. In addition, the Russian government will provide the submarine that will launch Cosmos 1 as the nose-cone payload aboard a modified Soviet-era Volna ballistic missile.

Undertaking the project "was an easy decision," Druyan said. "We were using a real weapon of mass destruction as a means to ride the light to the stars. We could make history for the price of a New York apartment."

The submerged submarine will launch Cosmos 1 from the Barents Sea and needs to put it into orbit 500 miles above Earth to get it above the atmosphere's residual drag. "Anything lower, and it won't move," said former JPL engineer Harris M. "Bud" Schurmeier, the project's leading systems engineer.

The Cosmos 1 spacecraft weighs 231 pounds, and contains the electronics that enable the sail to send and receive signals from the ground. The solar-sail assembly, which weighs an additional 88 pounds, is composed of eight sail segments made of thin Mylar-like material. Once unfurled, the array looks a bit like the vanes of a windmill. The segments are approximately 50 feet long on each side.

NASA is working on two designs to put a 531-pound payload into space with a 287-pound solar-sail system. Johnson said NASA is looking at four-segment sail configurations about 120 yards long and 53 yards wide in all -- the size of a football field.

Friedman said the Cosmos 1 ground team will wait several days before trying to deploy the sail segments. "When you pack them, there is always a little air left," he said. "If you just opened the compartment, there would be an explosive decompression. You want to gradually let the air leak out."

Each sail segment is held rigid by inflatable struts, which unroll "like one of those New Year's Eve noisemakers you blow into" when they are pumped full of nitrogen, Schurmeier said. A successful deployment is the mission's first goal.

The second is to move the sail. Once deployed, the spacecraft's orbit should grow larger as the sail adds speed. Friedman said the Cosmos 1 team will know whether this happens from micro-accelerator readings inside the spacecraft. People everywhere on Earth might eventually be able to see the sail with the naked eye, but chances will be better the further south one is.

Even if successful, however, Cosmos 1 will not last long. The beating from ultraviolet radiation will degrade the sail material, and within a few weeks, the nitrogen will start leaking from the struts, making it difficult and ultimately impossible to trim the sail segments so they continue to catch the sunlight.

But a few weeks will be enough. "This is proof of concept," Schurmeier said. "We want to make a contribution and move this method of propulsion forward."