Weighing in at 100 kilos (220 pounds), Cosmos 1 is scheduled to lift off at 1946 GMT on its historic mission, the main project of the Planetary Society of Pasadena, California, founded in 1980 by legendary writer and astrophysicist Carl Sagan, and financed largely by Cosmos Studios, which Sagan's widow, Ann Druyan, created in his memory.

Cosmos 1 was conceived and built by NPO Lavochkin, a semi-independent Russian space agency. The Russian government provided the launch vehicle and platform.

The solar spaceship will be placed in orbit 900 kilometers (550 miles) above Earth later Tuesday and will remain there for one month.

It carries eight triangular sails made of very tough, highly reflective, ultra-thin Mylar, one-fifth the thickness of a plastic trash bag. Individually, the sails look like flower petals, but when they come together, they become an enormous mirror 30 meters (100 feet) in diameter.

Reflecting the powerful rays of the sun, Cosmos 1 will be visible from Earth with the naked eye as it completes one orbit every 100 minutes.

The solar arrays will open only after the satellite and its instruments become fully accustomed to the vacuum of space, which experts expect will happen late Saturday.

"Getting into orbit and opening the sail will be big milestones," said Planetary Society President and Cosmos 1 project director Louis Friedman.

"Beyond that, we'd be happy with any small change in acceleration we can measure" after turning the sails to face the sun, he said.

Friedman had already thought of a solar sail ship in the 1970s, when he worked for the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Pasadena. He envisioned a sail ship drifting toward Halley's comet, but the idea was too bold for the time, he said.

Thirty years later, technology has evolved and Friedman hopes Cosmos 1 will prove that a solar sail ship is no longer science fiction.

"It is very promising technology. ... All we are trying to do is to demonstrate that the technology can work," said Emily Lakdawalla, who is working on the project.

The principle behind it is relatively simple. A constant flow of photons, the elementary particles that make up light, bounce off the surface of the sails, pushing them forward, much like sails on a boat catch the wind to propel the vessel across the water.

Although the energy reaching the sails is very weak, it will be constant and will provide steady acceleration.

Scientists calculate that the solar sail ship can reach 10,000 kilometers (6,200 miles) per hour within 100 days and 36,000 kilometers (22,400 miles) per hour in one year.

After three years, the spaceship should travel at more than 100,000 kilometers (62,000 miles) per hour, allowing it to reach Pluto, in the farthest reaches of our solar system, in less than five years, the Planetary Society said in a statement.

A more traditional mission to Pluto, using chemically powered engines and relying on Jupiter's gravity to accelerate, would take nine years to reach its goal.

NASA is already busy designing a solar spaceship capable of carrying 240 kilograms (531 pounds), with a solar sail the size of a football field.

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