Brian Robertson, William Herschel Society, on 4 October 2002.

The use of a sail was one means of propulsion for a space ship proposed by Robert L. Forward, a writer of science fiction. Recently, it has been the subject of a paper to the American Association for the Advancement of Science at the conference in Boston this year by Geoff Landis from Jet Propulsion Laboratory.

A sail would be a good means for visits to the planets of stars; it would also be cost effective for sending unmanned supply ships to Mars. There are 12 stars within 10 light-years of Earth that could have planets of interest to us; the nearest is 4 light-years away.

The average speed of a space ship for such a visit is likely to be about 10% of the speed of light, so that it could take up to 100 years to reach a suitable planet — three generations of the crew.

A visit to Mars in 2018, when the alignment is favourable, could provide our first experience of extended space travel, although only for a light-hour or so, not even one light-year. People have lived on Mir for 12 months and suffered bone loss and other health problems so a very long journey could be hazardous.

There are a few basic design criteria to be established:

How many crew should there be? It is considered that 80 - 180 females may be needed to provide for healthy genetic diversity of the population on the new planet.

What ratio of men to women? Women have been found to be more adaptable to living in space; perhaps the crew should be all women, with frozen sperm used with artificial insemination to produce the next generations of the crew. This would also reduce launch weight.

How big is the space ship for a crew of this size who have to maintain themselves for 100 years? One suggestion is a cylindrical vessel where the crew live on the inside of the outer casing under artificial gravity caused by rotation of the cylinder. Food would be grown in the next layer inwards and recycling equipment would fill the centre space. A ship weighing about one million tonnes is likely to result.

How will the ship be driven and decelerated as it approaches the chosen planet?

It is proposed that the sails will consist of sheets of diamond crystals a few atoms thick, made on board the space ship. Many sails each perhaps a kilometre square would be the target of a number of very high-powered lasers located on or near Earth, perhaps on the Moon. The pressure of the laser light on the sails would move the space ship and accelerate it. To decelerate it, a mirror might be launched from the space ship and the sails parted so that the laser beam struck the mirror and was reflected on to the sails, providing the braking force. But there are doubts whether this method would be effective; would the light have lost energy to the mirror?

There are problems enough about the propulsion system, such as how you finance and run many Giga-watt lasers for decades. It is estimated that this cost is $1.5 trillion - but that is what is spent on armaments in only one year now.

How do the crew visit the chosen planet, exist on it, and return?

A small space ship `lifeboat' is suggested, perhaps with an ion engine to decelerate it. The main space ship is left to continue empty into space so there is no return until the new colony have built a suitable vehicle. Mars visits will have taught us how to land on, exist on and return from another planet.

One of the major problems is communications: if a message takes up to 10 years to reach the space ship as it travels to the destination, there will be no point in communicating with it, except by automatic telemetry from the ship to Earth; it will have to be a self-contained world. Will the crew or the world care to communicate after decades of silence.

It was suggested that it was inevitable, if a suitable planet was discovered, for a visit to it to be undertaken -`just because it's there'. The conditions were likened to emigrating to Australia from England in the 18th century. There is likely to be the added incentive of conditions on Earth deteriorating. Would the crew be multi-national? How would the project be financed?

During the discussion it was suggested that if men were going to be eliminated except as a source of sperm, applying this procedure to Earth would remove the need to find another planet for humans to live on.

The effect of the `solar wind' from the star being visited was also questioned.

Donald Lovell