SCIENCE / Sailing from Earth on a sunbeam: A Russian spacecraft driven by solar pressure is about to be tested, says Peter Bond

THREE months from now, a Russian Progress spacecraft will initiate a new chapter in the use of near-Earth space. Deployment of the world's first operational solar sail is scheduled for 8 February 1993. If all goes well, pollution-free space propulsion and giant mirrors orbiting our planet will be transferred from the pages of science fiction into reality.

The Znamia or 'banner' experiment will begin after Progress M-15 has completed its routine resupply mission to the Mir space station, operating in near-Earth space since February 1986. The aim is to unfurl a lightweight reflective sail from the nose drum of the robot spacecraft, allowing it to be steered like a yacht - but using light rather than wind. The structure's circular shape will be maintained by rotating it at low speed, without the need for heavy support frames or spokes.

Scientists have known for more than a century that photons or 'packets' of light exert a pressure on the surfaces they strike. Solar sails, which have been developed by several countries but have never made fully operational, rely entirely on light pressure to provide the energy which enables them to move through the vacuum of space.

The momentum generated when photons bounce off a reflecting surface pushes the mirror in the opposite direction. A larger reflecting surface should provide greater pressure, with a marked improvement in acceleration. By tilting the mirror, or sail, in different directions, the ship can be propelled along the required path.

Traditional glass or metal mirrors weigh far too much for this system to operate effectively. The most efficient type of solar sail must be smooth, highly reflective, thinner than household clingfilm and weigh only a few kilograms. The material used in the Russian project is only 5 microns (five one-thousandths of a millimetre) thick, and the eight-segment 20m diameter sail weighs just 4.2kg. On the other hand, solar sails must be durable enough to survive impacts from micro-meteorites and space debris, and to overcome the threat of tears caused by electrostatic discharges.

Critics argue that, even if these design problems are overcome, solar sailing craft will be unable to operate efficiently below an altitude of 700km, where they would suffer from the effects of atmospheric drag. In addition, the low thrust derived from light pressure limits the uses to which they can be put - during the February trial, only minor changes in the attitude of the heavy Progress craft will be generated by its small experimental sail.

Even fitted with a giant sail, a ship would be unlikely to accelerate faster than 1mm per second per second. At this rate, it would take about 100 days for the craft to reach a velocity of 8km per second. To break free of Earth's gravity, a solar-powered ship would have to travel in ever-increasing circles for months before it reached a high enough velocity to escape. A similar decrease in velocity by spiralling in towards Mars would increase travel time still further. On this basis, a craft weighing several tonnes would take up to 400 days to enter the planet's orbit.

Advocates argue that such long travel times are more than outweighed by the financial savings. The cost of developing solar sails is low compared with the vast sums spent on creating the traditional expendable rockets - the budget for the Znamia experiment is put at a modest dollars 60,000 (pounds 40,000). In the absence of power plants and on-board propellants, such craft offer a low-cost method of transport which could revolutionise space travel and open up interstellar travel.

In his book Space Sailing (Gordon and Breach pounds 13), the solar propulsion pioneer Jerome Wright explains: 'These sailing ships can carry cargo, robots and passengers . . . No other form of space transportation has the potential to operate between Earth and Mars at such low cost. These ships can open up Mars for settlers and commercial activities because of their low costs. Sailing ships can operate for decades, making numerous voyages in their lifetimes, all without the consumption of propellant.' For a manned mission to Mars, Jerome Wright says, the saving on propellant alone could come to dollars 10bn.

Despite this potential, solar sail technology has had a stuttering history. It was not until the 1920's that the Russians Konstantin Tsiolkovsky and Friedrich Tsander first proposed using large mirrors for spacecraft propulsion. Another 30 years passed before these ideas began to receive serious consideration.

Scientific research on solar propulsion in America culminated in a project to develop a solar-powered spacecraft which could be launched in 1981 and rendezvous with Halley's Comet. Unfortunately, a shortage of time and money led to its cancellation and the virtual abandonment of solar sail research outside the Soviet Union.

More recently, an international competition to design a solar sailing craft capable of flying to the Moon and Mars has been grounded through lack of finance. The International Space Regatta scheduled for this year has been put back until at least 1994. Groups from Japan, France and the US retain their dreams of sailing a sunbeam. But, according to Dr Gordon Oswald of Cambridge Consultants, a British design commended in 1990 as the most technically advanced and imaginative of all the proposals remains firmly on the drawing board.

While the Russians seem to have lost interest in the race, the Moscow-based Institute of Space Research has definite plans to use small solar sailing craft for scientific research within the next few years. They will be orientated and stabilised using a 5m diameter solar sail and small 'petals' - with one black and one shiny surface - which act as rudders.

Russian scientists intend to use the craft to study the Sun's activity - including solar flares and the charged particles ejected in the solar wind. There are proposals to market them in the West as a 'space patrol alert service', giving advance warning of major flares which could affect communications and power supplies on Earth. Later versions are expected to be adapted for astrometry - the accurate location of stars - and perhaps to rendezvous with the elusive small bodies of the solar system, the comets and asteroids.

But the Russians have other plans in mind, apart from the low-cost propulsion of research craft. Solar sails are ideal for use as mirrors which can redirect sunlight towards the Earth's surface. Although the Znamia mission is primarily an engineering test of the system, it will also entail an attempt to orient the sail to reflect sunlight towards Russian territory and other parts of the northern hemisphere. But the effect of this small prototype will hardly be noticeable on the ground, comparable to a bright star passing overhead.

This initial test, however, is only a small part of a larger project to deploy a network of 200m diameter reflectors in Earth orbit. Russian designers working on the Noviy Svet (New Light) technology development programme claim that four or five of these structures could outshine the full Moon.

The eventual aim of the Russian consortium is to install a ring of these huge orbital mirrors which will illuminate selected cities and areas on Earth from highly elliptical orbits.

Earlier this year the project director, Nikolai Sevestianov, commented: 'This could provide a sufficient level of light to allow work during night-time hours on oil or gas drilling rigs in northern Russia, or at the scene of major disasters.'

The Russians claim their system will provide major economic benefits by substantially reducing the cost of illuminating large areas using traditional electrical lighting. Promotional literature includes an estimated profitability within 18 months and a saving of 2m tons of oil over five years.

Aside from a novel means of brightening the dull, unhealthy lives of people burdened by lack of winter sunlight, solar reflectors have potential uses as large antennae for relaying telecommunications or for sweeping dangerous space debris away from satellites and manned craft. Even more significant, perhaps, is their potential as a possible replacement for chemically fuelled rocket engines in future generations of spacecraft.

As in all areas of Russian industrial and scientific enterprise these days, the scheme's proponents are keen to generate interest overseas and obtain hard currency. In an effort to increase awareness of their creation, the Energia Scientific and Industrial Enterprise and the 15-member Kosmitcheskaya Regatta Consortium will display their corporate logos on the reflector. The aim is to encourage foreign advertising on future Znamia sails.-

(Photographs omitted)