Next challenge in space: hitching a ride on a sunbeam

A spacecraft powered by a sail the size of a playground but far thinner than a human hair could be riding on a sunbeam by 2004.

A private venture, the space yacht will carry drawings, messages, photographs and DNA signatures from millions of earthlings. But the experiment also has backing from US climate scientists, who believe that sail-powered satellites can visit parts of space no rocket-driven craft can reach.

"You get the most bizarre orbits out of these things. You can have a Saturn orbiter that was always above the rings. You could always be looking down at the rings without ever crossing the ring plane," says Carl Murray, an astronomer at Queen Mary College, London. "I think maybe the time has come for these things."

Details of the sail - 70 metres by 70 metres in area but only one seventysixth of a human hair in thickness - will be outlined in London this week at an international conference led by Professor Murray and Colin McInnes, an engineer at the University of Glasgow, on the next great adventure in space: the space clipper.

Most of the cost of space is in getting there: in accelerating an object to the 5 miles a second needed to escape from the well of gravity. But once in orbit, spacecraft need fuel to manoeuvre, stay in position or begin the voyage to distant planets. So part of the payload must be more rocket fuel. There is another catch. Still prisoners of planetary or solar gravity, conventional orbiters are condemned to stay in the plane of the solar system.

But light itself is a force. Sunbeams exert pressure. For decades theorists have dreamed of small spacecraft with huge sails, slowly gathering speed in free fall, driven by the rays of the sun. The Planetary Society, founded by the late Carl Sagan, hopes to launch the tiny experimental sailor Cosmos-1 in orbit later this year.

Team Encounter of Houston, Texas, a commercial venture, hopes to launch a packet of human memorabilia aboard a spacecraft weighing only 18kg, but with a sail area of 1.2 acres, in perhaps two years. It could gradually get up to 67,000 miles per hour - four times the speed of the space shuttle.

The European and German space agencies have plans for a solar sail measuring 20 metres by 20 metres. It would weigh 35kg, and be launched folded up into a parcel measuring 60cms by 60cms by 40cms. Once in space, it would be unfurled like a parachute, and gently begin to drag its payload at increasing speed under the continuous impact of tiny packets of light from the sun.

Professor McInnes in Glasgow has been working with Nasa, the European space agency and Lockheed Martin and others on a sail-powered satellite that would always stay within the Earth's magnetic field. "It is quite hard to do experiments in the Earth's magnetic field because it always points away from the sun," he says.

Because the Earth goes round the sun, and because the magnetic field shifts with it each day, the elliptical orbit of conventional spacecraft would only line up with the field once a year. But a spacecraft with a sail 40 metres across could use the sun's "wind" to keep itself on station all the time.

"It is a very simple mission to do, but the science benefit is tremendous. You get data all year round." He and colleagues are working with Nasa on plans for a sail-powered mission to the planet Mercury. But Nasa has more far-reaching ambitions. By 2010, it hopes to launch a sail-powered craft with a "canvas" more than 400 metres across.

Each square metre would weigh about a tenth of an ounce - the mass of a raisin. Under the pressure of the sun's rays, the spacecraft would begin accelerating at only millimetres per second, every second. But in just a day, an acceleration of 1mm per second would build up to 100 metres per second.