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Imagine an extremely thin sheet of reflective material as vast as a football field, tensioned by fine wires and struts and deployed in Earth orbit. This is a solar sail. Driven only by the pressure of sunlight, a solar sail could carry a scientific payload as heavy as an eight-wheel truck on a tour of the asteroids or transport supplies for a manned expedition to Mars. Before that, though, this unique means of spacecraft propulsion could make its debut in unmanned races to the Moon and Mars in 1992.

The wind in the rigging of a solar sail is light emanating from the Sun. The light from the Sun is reflected off the solar sail, transferring some of its momentum to the sail. That momentum can either slow it down or speed it up, depending on the sail’s orientation to the Sun. By changing the angle of the sail, it is possible to steer a spacecraft.

By the time the light from the Sun reaches the Earth, the pressure it exerts has dropped to about a thousandth that of a feather resting on a hand. So, any solar sail in Earth orbit is bound to get off to a slow start. The Planetary Society in the US says that a solar sail one-third of a mile square would accelerate to less than 10 miles an hour after one hour. But, in the vacuum of space, nothing would oppose the sail, and the tiny pressure is enough to initiate a steady acceleration. After 18 days, says the Planetary Society, the sail would be travelling at one mile per second.

In his short story, The Wind from the Sun, Arthur C Clarke’s hero, John Merton, sends a solar sail, ‘glittering splendidly in the sunlight that would be hers for centuries to come’, out of the Solar System. From a slow start in Earth orbit, the sail flashes past the Moon, gaining two thousand miles per hour.

At the beginning of the 17th century Johannes Kepler, the German astronomer, surmised that the pressure of sunlight pushed comet tails away from the Sun. Yet it was not until the beginning of this century that Peter Lebedew, a Russian physicist, succeeded in measuring the effect. He shone an intense beam of light onto a metal disc mounted at the end of a fine balance, and measured the disc’s displacement when it was in the light. He placed the equipment in a vacuum chamber so that air currents could not interfere with his readings.

The idea of putting the radiation pressure of sunlight to work to drive a spacecraft dates back to the 1920s and Konstantin Tsiolkovsky, the father of astronautics. Tsiolkovsky and hus fellow scientists, Fridrich Tsander, wrote in their notes of using ‘tremendous mirrors of very thin sheets’ and of ‘using the pressure of sunlight to attain cosmic velocities’. But with the exception of a few science fiction wriers, their enthusiasm was ignored. Then in 1958, Richard Garwin, a consultant to the US Department of Defense, published a paper on the subject. Garwin recognised the unique elegance of solar sailing.

Solar sails carry no propellant, which makes them particularly useful for long missions with large payloads. By the mid-1960s, space scientists were exhibiting a growing interest in solar sails. Initial technical studies confirmed the feasibility of effectiveness of the concept, but budget cuts in the 1970s, following the end of the Apollo programme, brought the work to a standstill.

Then, a few years later, the prospect of a mission to Halley’s Comet, which passed round the Sun in the spring of 1986, revived the falling fortunes of solar sails. To make the mission exciting to the public as well as scientifically valuable, NASA decided to plan a spacecraft that could match speeds with the comet and rendezvous, rather than flying past. Such a mission, said the agency, required the development of a new and advanced propulsion system.

NASA’s first thought was that ion propulsion might be the answer. Spacecraft with ion drives reach high velocities because an electric field accelerates charged particles to form a high-speed jet that provides steady acceleration. Unfortunately, calculations showed that if an ion-driven spacecraft was to attain the velocities needed for a rendezvous, it would have to be launched 10 years before Halley’s Comet was due to arrive in the inner solar System. That was far too long; the rendezvous appeared doomed.

Not so, said Jerome Wright, from the Battelle Institute in Ohio. He discovered a trajectory for a solar sail that would meet Halley’s Comet in only four years. ¿ìè¶ÌÊÓƵs at NASA’s Jet Propulsion Laboratory in Pasadena were amazed by the short flight time. Wright succeeded in capturing Bruce Murray’s attention. Murray, who was then director of JPL, adopted solar sailing as one of his ‘purple parrot projects’ or bright new birds of the future. Also, given the importance of attracting public interest, the solar sail would be visible from Earth for months after its launch.

In 1977, scientists at JPL began a $4 million project (about 2 million Pounds) to design a sail with the launch date set for late 1981. They also continued to develop the ion-electric propulsion, so the two systems were in direct competition.

The initial sail design measured 800 square metres. Small vanes at the edge of the sail would allow NASA’s engineers to steer the craft in space. NASA planned to make its solar sail out of Kapton, a lightweight plastic, coated with reflective aluminium. Du Pont makes Kapton as an insulator for semiconductor devices.

When NASA examined the square-sail design in more detail, problems emerged. How was the agency to pack 640,000 square metres of delicate solar sail into a shuttle’s payload bay? And, once packed, how was NASA to get the sail out without causing damage, and deploy it in space?

The answer was to divide the sail into 12 sheets spread over blades. The resulting design, dubbed a heliogyro, looked like a helicopter. Each blade was 10 metres wide, and 7 kilometres long. The blade design turned one large, possibly insoluble problem of packaging and deployment into 12 more tractable problems. Richard MacNeal, of the MacNeal-Schwendler Corporation, and John Hedgepath, of Astro Engineering in California, first invented this design of solar sail in the mid-1960s.

controlling a heliogyro, though, cannot be achieved with small vanes. For this craft, the theory of helicopter rotors was more suitable. By pitching the blades, engineers could control the direction of the spacecraft.

Even though solar sails offered advantages for a rendezvous with Halley’s Comet, NASA abandoned them in September 1977 to concentrate on the ion-electric propulsion system, which it saw as the ‘interplanetary automated shuttle for use within the Solar System in the 1980s and beyond’.

Shortly afterwards, the ion-drive project was also cancelled. However, the work for the mission to Halley’s Comet showed solar sailing to be a viable method of propulsion. Wright proposed solar sails as an ‘interplanetary shuttle’.

The aim was to find ways to ferry scientific instruments around the Solar System. In theory, the technology developed for the Halley’s Comet rendezvous could transport 42 tonnes to Mercury in less than four years. Unlike conventional spacecraft which have to carry large amounts of propellant, a solar sail could bring its cargo of scientific instruments back home. After each mission, the solar sail could return to Earth to be refurbished in Earth orbit.

Solar sails can also achieve exotic orbits with far less fuss than conventional spacecraft. For example, a solar sail could carry scientific instruments out of the plane of the ecliptic (the plane in which the planets orbit the Sun). No rocket or chemical propellant yet devised can provide sufficient power to send a spacecraft directly out of the ecliptic.

The designers would need to balance the sail’s size and mass and to arrange its orientation so that the radiation pressure would lift the spacecraft out of the ecliptic. Ulysses, the satellite that the European Space Agency and NASA are sending to observe the solar poles, exemplifies the difficulties a conventional spacecraft must overcome to escape the ecliptic. Ulysses is travelling to Jupiter, where the planet’s massive gravitational field will sling the spacecraft into an orbit around the Sun and out of the plane of the ecliptic.

Many of the exotic applications require high-performance sails constructed from very lightweight materials. The sail material for the Halley’s Comet rendezvous was a relatively thick plastic with a reflective aluminium coating. If, though, the solar sail could be constructed in the weightless conditions of space, say at space station Freedom when it is eventually launched, virtually no plastic would be needed to support the aluminium. An almost pure film of the metal could be deployed as a sail.

The ultimate sail would be aluminium film perforated with tiny holes. Providing that these holes are smaller than the wavelength of light, solar radiation would still reflect from the surface and supply the momentum to accelerate the sail.

Going out on a wing

Despite the scuttling of the Halley’s Comet project in 1977, a great enthusiasm for solar sailing remains. In 1979, several of the engineers at JPL who had been working on the project formed the World Space Foundation to develop a sail privately. In their spare time, they have designed a small 30-square-metre sail. They have now constructed a one-third scale model of their spacecraft to test the unfurling mechanism.

In 1981, a group of French scientists from the French national space agency formed a club to promote solar sails and to promote a race of solar sail spacecraft from Earth to the Moon, to mark the International Space Year in 1992. The International Astronautical Federation has agreed to set rules and guidelines for the race.

The race has attracted Japanese scientists, who formed the Solar Sail Union of Japan in 1985. This group has designed a 70-square-metre sail, and won public support by showing a film about a race to the Moon with manned spacecraft in the next century. Both the French and Japanese groups have created their designs with industrial partners. The designers from each of the three groups hope to have spacecraft ready for a simultaneous launch on an Ariane 4 rocket in 1992.

During 1992 we may also see a race to Mars to commemorate the quincentenary of Columbus’s voyage to the New World. This idea is the brainchild of members of the US’s Quincentenary Jubilee Committee. Each entrant must carry a plaque weighing 1 kilogram. The winner will be the first of the entrants to pass within 10,000 kilometres of Mars.

Klaus Heiss, the race organiser, hopes to have one entrant from Europe, from where Columbus set sail; one from America, where he ended up; and one from Asia, to where he thought he was going. Funding and construction must be in collaboration with industry. Heiss says, ‘one entry in the Americas Cup can cost $15 million’, which is less than the amount some of the entrants will need to raise.

John Hopkins University in Baltimore has designed a sail with 480 individual elements, shaped like a sunflower. Some of the elements will move to allow the craft to be steered. The craft will study the asteroids, and the sail will reflect sunlight onto the dark areas of asteroids. The university wants amateur astronomers to photograph the sail against the stars.

The Massachusetts Institute of Technology’s entrant is a heliogyro with a 170-metre diameter, eight blades and a mass of only 20 kilograms. Heiss has christened the craft the ‘gadfly of the fleet’. By having such a low mass, MIT hopes that it can be launched on a Pegasus rocket. Pegasus is launched from beneath the wing of a B52 bomber, and is designed to put small satellites in space. A launch on Pegasus might be cheaper than hitching a lift on a larger rocket.

In Britain, Cambridge Consultants, a small high-technology consultancy, has patented a design that it hopes to enter into the race. The race organisers have identified the Cambridge design as the most technically advanced. It is a circular sail of 276 metres diameter, which packs like a concertina into a 4-cubic-metre cylinder for the launch. Once in orbit, the flat disc can be modified by flexing plastic ribbing reinforced by carbon fibre that runs through the sail. The designers, Gordon Oswald and Steve Temple, calculate a flight time to Mars of less than a year, making their sail one of the leading contenders.

The future, then, appears bright for solar sails. The 1990s could be the decade when the visions of the 1920s come to fruition.

Colin McInnes is a research student in the department of physics and astronomy at the University of Glasgow. He is studying potential missions and trajectories for solar sails.