- The Guardian,
- Thursday July 27 2000
A spacecraft could sail on a sunbeam to Mars and beyond. This is not the start of a fairy story but a theory confirmed by Nasa scientists, in two different experiments in two laboratories.
For more than two decades, theorists have tinkered with the mathematics. At the Jet Propulsion Laboratory in Pasadena, California, and at an air force base in Ohio, they demonstrated the reality. The JPL team fired 10 kw of microwave radiation at an ultra lightweight film of stiff carbon in a vacuum chamber. It took off and flew at least two feet. At the Wright Patterson air force base, they hung sails of the same stuff on a pendulum and pumped them with 7.9 to 13.9 kw lasers. The sails fluttered in the waves of light.
The biggest cost of space is the launch: in every second of liftoff, the space shuttle's solid rocket boosters burn enough energy to power 2 million family saloon cars. A shuttle has to carry its experiments as well as its human passengers, with their food, air and water for a fortnight. If it also had to carry fuel for a 100 million mile journey to Mars and back, the launch costs would rise even more extravagantly.
So the hunt is on for a less expensive, faster way of scudding across the cosmos. Instead of a push from exploding chemicals, a spacecraft could get a push from light itself. The earth is 93 million miles from the sun. At this distance, sunbeams exert a pressure of 9 newtons per square kilometre or 0.78 lb per square mile.
Down in gravity's bowl this force is undetectable. But, one expert calculated, that much sunlight reflected from a square mile of mirror would have enough force to support the weight of a cat. Out in the vacuum of free fall, it would start a sail moving, very slowly at first, but to within 1km a second in eight days.
The spacecraft would go on accelerating as long as there was light to push it, offering a low-fuel route not just to Mars, but out of the solar system altogether. The idea fell out of fashion as Nasa's troubles multiplied: there were huge problems, such as how to make a sail sufficiently large and light, then how to get it into space without damaging it, then how to save on journey time by cutting out the slow process of acceleration with some kind of catapult launch from high orbit.
But the wind has changed, and there are signs of activity in space harbour. By 2010 the JPL scientists could be unveiling a solar sail-driven called the Interstellar Probe. Its half-kilometre sail would be unfurled in space, and it would appear for a while to hang there, a kite drifting in infinity. But not for long. Pressure from the sun would accelerate it to speeds five times greater than possible with conventional rockets, and after a while it would be zooming towards the stars at 90km a second.
At such a speed, it could go from New York to Los Angeles in less than a minute. Such a sail-powered spaceprobe, launched in 2010, would overtake Voyager one in 2018. Voyager one, at that time would have been going for 41 years. The Interstellar P robe would have covered the same distance in just eight years.
There are ambitious plans to give such sailcraft a shove with microwaves or lasers. Robert Frisbee of JPL reckons that microwave transmitters could blast the sail, heat it to 2000 C, and get it going ultimately to one tenth of the speed of light. More than two decades ago a Nasa scientist worked out that a 20 ton solar sail could carry 180 tons of cargo to anywhere in the solar system. Like boats, solar sailing ships could "tack" back to earth using light pressure and solar gravitational tug just as real clipper ships tack by the pressure of wind and the resistance of water. With only six tons of payload, it could fly to Pluto, the furthest planet of the solar system, in one and a half years.
• Space sails, a science news exhibition, opens at the Wellcome Wing of the Science Museum, London, today.
