One curious feature of the surface of the Moon is the existence of “lunar swirls”, wisp-like regions of the surface that are whiter than surrounding areas. Until recently, astronomers had puzzled over how such regions could form.

But a couple of years ago, a team of physicists suggested that mini-magnetic fields on the Moon could deflect high energy particles and thereby prevent the darkening of the surface they cause. The team went on to test the idea in the solar wind tunnel using a magnetic field to deflect high-energy particles in just the way that seems to happen on the Moon.

What was unexpected about this work was that it showed how relatively weak magnetic fields of just a few hundred nanoTesla have created radiation shelters over relatively large areas of the Moon’s surface. (By comparison, the Earth’s magnetic field has an intensity of about 40,000 nanoTesla.)

This has given the same team of physicists the idea that this trick could protect astronauts on long duration missions in space.

Until now, numerous studies of magnetic deflector shields have suggested that they would have to be hugely powerful to do this job. But if weak fields can protect parts of the moon, they ought to be able to do the same for astronauts. The question is how.

Today we get an answer thanks to the work of Ruth Bamford at the Rutherford Appleton Laboratory in the UK and a few pals. These guys have studied how an artificial mini-magnetosphere could act as a radiation shelter for astronauts in space and have even built a demonstrator to show how it would work.

The first stage in their work is to understand how a weak magnetic field can provide the necessary protection. Bamford and co say that previous studies of magnetic shields have neglected a crucial ingredient— the natural, low-density plasma that already exists in space.

This plasma is so weak that it consists of just a handful of positive and negative ions in each cubic centimetre of space. But a magnetic field moving through space would sweep these ions ahead of it, causing them to bunch up into a denser region of plasma in front of the spacecraft.

Because of the separation of charge within this plasma, it generates its own electric field. And this turns out to be crucial when it comes to deflecting high-energy particles from the Sun and beyond.

Bamford and co point out that it is not necessary for a magnetic shield to stop high-energy particles but merely deflect them. “Much like defending against a charging rugby-footballer, rather than stand in his way to protect the goal line, a better policy is to deﬂect the player sideways using a small amount of force so he is pushed into touch and out of the ﬁeld of play,” they say.

The electric field provides exactly this force, deflecting high energy particles and creating a region within the magnetic field that is shielded from them. Because previous studies have never considered this process, they have vastly overestimated the strength of the magnetic field required to provide protection.

Now Bamford and co argue that a much weaker, and therefore more easily achievable, field could do the job. And they have worked out how such a device might work in practice.

An important point is that the full protection is not necessary all of the time. The Sun produces a steady stream of energetic particles that are easily shielded using the body of the spacecraft itself. The problem arises during the occasional storm on the Sun that accelerates particles to energies that are many orders of magnitude higher than the background level.

During the late 60s and early 70s, the Apollo missions avoided these storms by pure luck. But had one hit, the results could have been fatal.

Today, NASA and other space agencies have the ability to spot these storms as they occur on the Sun and so forecast when the high-energy particles are likely to arrive. And this gives provides a warning that would allow the spacecraft to switch on a shield only when it is needed.

But that immediately introduces a problem. How does the magnetic field gather enough plasma to create a shield in the few minutes warning that the crew might have of an impending storm?

The answer, say Bamford and co, is to release a gas of say, xenon, into the space around the craft and allow the Sun’s ultraviolet light to ionise it to form a plasma that then becomes trapped in the magnetic field. They estimate that 0.5 kilograms of xenon would be enough to protect against the two or three storms likely to occur during a six-month trip to Mars.

They go on to show that such a system would provide more than adequate protection for astronauts. Their simulations show that the system would prevent all of the background radiation from entering the radiation shelter inside the spacecraft and that 95 per cent of the high-energy particles (with energies 1 million times higher than the background levels) would be excluded as well.

Just how practical would such a system be? Bamford and co give a back-of-an-envelope calculation suggesting that a superconducting coil capable of generating the required field would require about 16 kilowatts of power alone and the cooling and control systems would require another 5 kilowatts. The total mass of this deflector shield system would be about 1.5 tonnes.

That’s probably beyond what most mission planners would consider practical. But it is at least within an order of magnitude of what might one day be launched, unlike previous studies that have all required science fiction-type power sources.

Of course, Bamford and co are quick to point out that there are numerous complexities that need to be studied in much more detail before their deflector shield could be built. “Much has yet to be determined quantiﬁably before the full engineering standard of precision is available,” they say.

But their key point is that previous studies have dramatically overestimated the power requirements for a deflector shield. And their new approach radically reduces these power requirements.

This work is important because the world’s space agencies have begun to draw up plans for humans to journey beyond low Earth orbit to visit nearby asteroids, to revisit the Moon and even to go to Mars and beyond.

If these missions are to be successful, the problem of radiation shielding will need to be solved. This looks to be an important step in that direction.

And who knows what other tricks of plasma engineering could make these shields even more effective and more efficient in future.

Ref: arxiv.org/abs/1406.1159 : An Exploration Of The Eﬀectiveness Of Artiﬁcial Mini-Magnetospheres As A Potential Solar Storm Shelter For Long Term Human Space Missions