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Sails of the next century

Imagine solar sailing ships plying the trade routes between the planets, powered by light from the sun. David Whitehouse reports on a futuristic vision of space propulsion
Later this week, a group of hopeful people will gather for a conference at Nasa's Jet Propulsion Laboratory in Pasadena, California. The location is ironic; the method of space transport that they will be discussing doesn't involve jets at all. It uses the motive power of the sun's light to propel spacecraft of the future.

The conference title, "Solar Sails", revives a technology which has foundered in recent years. In the 1980s, George Bush asked a US event-planning committee to commemorate the 500th anniversary of Christopher Columbus's departure from Europe for the Americas. One bright idea was to launch a group of spacecraft using chemical rockets on Columbus Day, 1992, and let them race to Mars powered only by light pressure.

The idea never materialised. Yet the idea of solar sails is a robust one. Light applies a slight pressure on any illuminated object. In 1924, space pioneers Fridrikh Tsander and Konstantin Tsiolkovsky noted that in the vacuum of space, a large thin sheet of reflective material could work as a propulsion device requiring no propellant; sunlight would literally push it along.

For a highly reflective sail, the solar flux could produce a force of about nine Newtons for every square kilometre (one Newton is the force needed to give a one-kilogram mass an acceleration of one metre per second every second). That seems minimal - but it's actually quite reasonable, because it is continuous.

In 1973, Nasa sponsored a study for a solar-sailing probe to tow a platform of scientific instruments to intercept Halley's Comet. A team from JPL designed a number of solar sails. Their recommended sailcraft had a central mast and booms that would spread and support an aluminium-coated plastic sheet 850 square metres in size and two micrometres thick. Making one side of the sheet reflective and the other dark would set up a force imbalance that could be used for manoeuvring. It would weigh five tonnes and need little highly advanced technology. But it would have needed launching in 1985 to meet the comet.

Instead, others in Nasa decided that the Halley's Comet interceptor should be driven by a solar-powered electric propulsion system. Both studies were soon rendered academic when the project was cancelled by Congress.

In 1979, the World Space Foundation, a non-profit-making organisation of space enthusiasts (including many JPL scientists), took up the idea again. Two years later, the world's first solar sail, a half-scale prototype, was exhibited. By 1983, a full-size prototype was completed.

A spacecraft pulled by a solar sail - a large metal sheet, microns thick, that converts the momentum of sunlight into thrust - would be a very useful space vehicle. A sail of four square kilometres deployed from the Space Shuttle in Earth orbit could take a cargo-bay of payload to Mars before returning to Earth for more. An ordinary rocket motor would get the equipment there sooner, but would need up to three times as much fuel. It would also have to be lifted out of the Earth's "gravity well".

A "sunjammer" solar sail vehicle could carry any amount of cargo; the only difference would be to the journey time. Permanent bases on other planets such as Mars would be delighted to receive such provisions. Sunjammers could therefore form a bridgehead and lifeline for such colonists.

The Voyager 2 spaceprobe, now heading out of our solar system, will take some 80,000 years to travel the 4.3 light years (about 25 million million miles) to the nearest star - though it is not heading in the right direction. A conventional solar sail could perhaps reduce this to about 15,000 years because it would keep accelerating while the sun was behind it. In interstellar space, there would be nothing to slow it down. That time seems an eternity when compared with an individual's lifespan, but some scientists have suggested that we could speed sunjammers up by firing lasers or microwave beams at them.

Studies indicate that a solar sail 3.6 kilometres in diameter, trailing a one-tonne probe, could be powered by a 65 billion-watt laser fired from Earth orbit. Such energies are far higher than any yet obtained, but could conceivably be developed during the next century. Although the laser light would be a tight beam, it would tend to spread out over vast distances of space. For this reason, a large Fresnel lens 1,000 kilometres wide, located at a stationary point between Saturn and Uranus, would be used to focus the laser beam on the receding probe. Three years of such acceleration would give it a velocity 10 per cent that of light, meaning that it would reach the nearest star system - the Alpha Centauri group - in about 40 years.

Such ideas, although fascinating, are still deep in the realm of speculation. Other propulsion technologies may be better at propelling the first exploration trips to the stars, but it is not too fanciful to think of future space mariners plying the trade routes between the planets on sails of gossamer that ride the sunlight.

Surprisingly, perhaps, Tennyson saw it - or something like it - in the last century: For I dip't into the future/ far as human eye could see./ Saw the vision of the world,/ and all the wonder that would be,/ Saw the heavens fill with commerce,/ argosies of magic sails,/ Pilots of the purple twilight,/ dropping down with costly bales.

The writer is the BBC's science correspondent.