By Paul Rincon Science reporter, BBC News, Preston

The plasma-filled shield would offer protection from harmful particles

They argue that magnetic shields could be deployed around spacecraft and on the surfaces of planets to deflect harmful energetic particles.

Several countries' space agencies have announced their intentions to resume human exploration of the Solar System.

Scientists hope to mimic the magnetic field which protects the Earth.

Details have been presented at the Royal Astronomical Society's National Astronomy Meeting in Preston, UK.

There are a variety of risks facing future space explorers, not least of which is the cancer-causing radiation from cosmic rays and solar flares that astronauts will encounter when they venture beyond the Earth's protective magnetic envelope, or magnetosphere.

The nice thing is that magnet technology is really quite evolved here on Earth. The question is can you take it into space? Mike Hapgood, Rutherford-Appleton Laboratory

The Earth's magnetosphere deflects many of the energetic particles from space; others are largely absorbed by the atmosphere.

Between 1968 and 1973, the Apollo astronauts were only in space for about 10 days at a time.

They were simply lucky not to have been in space during a major eruption on the Sun that would have flooded their spacecraft with deadly radiation.

Crew members on the International Space Station can retreat to a thick-walled room during times of increased solar radiation.

Stable field

But these protective shelters would not be practical on long-duration space journeys, since the "drip-drip" of energised particles is thought to be as harmful to the health of astronauts as large solar storms.

Potentially damaging solar activity is hard to predict

The harmful particles come from the Sun, in the form of the solar wind, and from sources outside our Solar System.

To create the deflector shield around a spacecraft or on the surface of a planet or moon, scientists need to generate a magnetic field and then fill it with ionised gas called plasma.

The plasma would be held in place by a stable magnetic field (without the magnetic field, the plasma would simply drift away). This shield could be deployed around a spacecraft or around astronauts on the surface of a planetary body such as the Moon.

As energetic particles interact with the plasma, energy is sapped away from them and they slow down.

"You don't need much of a magnetic field to hold off the solar wind. You could produce the shield 20-30 kilometres away from the spacecraft," explained Dr Ruth Bamford, from the Rutherford-Appleton Laboratory in Didcot, UK, one of the scientists on the team.

Dr Mike Hapgood, from the Didcot-based research centre, told BBC News: "The nice thing is that magnet technology is really quite evolved here on Earth. The question is can you take it into space?'"

The team from Rutherford-Appleton plans to build an artificial magnetosphere in the laboratory. They would eventually like to fly a test satellite which would test the technology in space.

'Shields on'

The idea has been likened to the deflector shields which protect the USS Enterprise and other spacecraft in Star Trek. Like their fictional counterparts, these shields could also be switched on and off.

The planned moon base will be exposed to solar radiation

An artificial magnetosphere could come in handy anywhere in the Solar System where humans would need to be for long durations.

A permanent Moon base, of the type Nasa plans to build, could be buried under lunar soil to protect the occupants and equipment from space radiation. But inhabitants will still be vulnerable when venturing outside in their spacesuits.

"Our warning systems aren't very good [for solar flares]. You might be able to say: 'this is a dangerous period in terms of solar activity', but you might be on red alert for weeks," said Dr Hapgood.

"If you've got a problem, you might not want to wait a week to fix it. You might want a device to deploy on the surface as a shield that would blunt the effect of a flare at ten minutes' notice, it adds an extra level of safety."

The idea for the shields draws on technology pioneered in experimental nuclear fusion reactors. Nuclear fusion is not yet a mature technology.

It works on the principle that energy can be released by forcing together atomic nuclei rather than by splitting them, as in the case of the fission reactions that drive existing nuclear power stations.

At the Jet experimental fusion facility at Culham in the UK, magnetic fields were used to keep plasma away from the interior wall of the reactor.

This represents a reversal of that technology: "We want to use the same technique to keep an object in the middle away from plasma that's on the outside," said Dr Bamford.