Solar Radiation Shielding for Space Flight

Interplanetary adventurers must contend with deadly solar radiation – but the moon’s magnetic memories may hold the key to safe space flight

BORED on their six-month journey to Mars? Not a bit of it. Whenever the astronauts look out of the window, they find themselves mesmerised by the glowing, shimmering sphere of plasma that surrounds their spacecraft. Hard to believe that the modest electromagnet at the heart of their ship can produce something so beautiful.

Not that the magnet’s raison d’être is aesthetic, of course. Its main function is to keep the astronauts from a slow, horrible death by radiation sickness.

NASA is nervous about sending astronauts to Mars – and understandably so. Six months’ exposure to the wind of high-energy particles streaming from the sun could indeed prove deadly. But a team of researchers at the Rutherford Appleton Laboratory (RAL) near Oxford, UK, has hit upon a phenomenon that might just solve the problem. They have shown that a magnet no wider than your thumb can deflect a stream of charged particles like those in the solar wind. It gives new life to an old idea about shielding spacecraft, and might just usher in a new era of space travel. “Space radiation has been called the only showstopper for the crewed exploration of space,” says Ruth Bamford of RAL. “Our experiment demonstrates there may be a way the show can go on.”

The inspiration behind the idea is as old as the Earth. Life thrives on our planet because its core is a churning cauldron of molten iron. The result is our magnetosphere, the magnetic field that wraps itself around the Earth and deflects the solar wind. Without this shield some of the particles spat out by the sun would charge through our bodies, shattering the machinery of our cells. In the absence of our protective magnetic field, complex life on Earth would probably be unsustainable.

Without Earth’s magnetic shield, particles from the sun would shatter the machinery of our cells

Beyond the magnetosphere – on a mission to Mars, for example – we leave that protection behind. The solar wind can give rise to blasts of radiation 1000 times as powerful as that released by the atomic bombs at Hiroshima and Nagasaki. That is not something a space agency wants to inflict on its astronauts, or the taxpayers funding the trip. “Imagine a ship flying to Mars with a cargo of dead astronauts and the whole world watching in horror,” Bamford says.

Hence the decades-long search for a suitable shield. Wernher von Braun, the rocket pioneer who created the Apollo programme, first thought about a magnetic shield for spacecraft in the 1960s. He eventually dismissed the idea because he thought it would require an impractically large magnet. He was wrong. “The physics is more subtle than those simple ‘back of the envelope’ calculations,” Bamford says.

That subtlety has come to light through a series of discoveries made by roaming spacecraft. We initially thought that the only magnetospheres in the solar system belonged to bodies large enough to keep an iron core molten and churning. But it turns out that our solar system is littered with small but surprisingly powerful magnetic shields.

It seems our solar system is littered with small but surprisingly powerful magnetic shields

There seem to be several on the moon, for a start. The particles of the solar wind have gradually darkened most of the moon’s surface, but lighter-coloured swirls are also visible at various points. In 1998, NASA’s Lunar Prospector flew over one. The probe was skimming only 18 kilometres above the surface when its sensitive instruments indicated it had crossed through a region of bunched-up magnetic field lines and moved into a cavity where there was a sharp drop in the density of charged particles. It had entered a mini-magnetosphere that the solar wind’s particles could not penetrate.

This field most probably arose when the heat from an asteroid impact melted the lunar surface. This would have created a plasma – a cloud of hot, ionised gas. Plasmas carry a magnetic field, and when the lunar surface resolidified, the rock would have preserved an imprint of the plasma’s magnetism.

The field the Lunar Prospector found is a few hundred kilometres across and extends tens of kilometres out into space (Science, vol 281, p 1480). Most interesting of all is the protection this not particularly strong field seems to offer from the ravages of solar radiation, judging by the colour of the soil beneath the bubble (Planetary and Space Science, vol 56, p 941). “It’s as if, for billions of years, the rock has been partially shielded from the chemical etching of the solar wind,” says Bamford.

The moon is not the only place where this happens. Mars, for instance, has localised pockets of magnetic field left over from when it was hot enough to generate its own magnetosphere. Some regions still carry a field imprinted in the rocks, forming protective shields that rise hundreds of kilometres above the surface.

Even more exciting is a discovery that NASA’s Galileo spacecraft made in the early 1990s. En route to Jupiter, it flew by the asteroids Ida and Gaspra, tiny rocks about 30 and 20 kilometres across respectively. Contrary to all expectations, both bodies were found to sport weak magnetic fields. It seems that they were once part of a larger body big enough to have a molten interior and a magnetic field – perhaps even a whole planet or moon that got smashed apart in a cataclysmic collision (Advances in Space Research, vol 16, p 59).

Around both asteroids Galileo also spotted buffer zones free of charged particles, just like our magnetosphere. But these buffer zones were much further out from the asteroids’ surface than would be expected, given the weakness of their magnetic fields. What could be going on?

The answer lies in the details of what happens when a charged particle hits a magnetic field. First, it experiences a force perpendicular to the field lines. The momentum of the particle keeps it travelling forwards, but not straight along the field lines; instead, it spirals around them with a characteristic radius called the “Larmor radius”.

The Larmor radius depends on the field strength, the mass and charge of the particle, and the speed at which it is moving. Out where the solar wind meets the Earth’s magnetic field, a typical solar wind particle – a proton – has a Larmor radius of between 20 and several hundred kilometres. In theory at least, that means a magnetosphere that is 20 to several hundred kilometres in extent is necessary to stop that particle hitting Earth’s surface.

On the face of it, there is no way that bodies as puny as Ida or Gaspra can produce a magnetosphere that size. The reason that they can and do lies in the solar wind itself.

Because the solar wind is a plasma made up of charged particles, it too carries a magnetic field. When the solar wind’s field meets the rocks’ mini-magnetosphere, the two fields clash, exerting a force on each other. Something has to give. Because the solar wind’s field is created by free-moving particles, it is the one that yields, altering its orientation to minimise conflict with the mini-magnetosphere’s field.

Some parts of the solar wind shift more easily than others. The positively charged protons have nearly 2000 times the mass of the negatively charged electrons, so the latter are much more easily deflected. The electrons stay at the surface of the magnetic bubble, while the positive charges penetrate further in.

This separation of positive and negative charges generates intense electric fields up to a million times stronger than the magnetic fields that created them. Subsequent solar wind particles hit these electric fields and are strongly deflected. The result is a shielding effect far more powerful than the magnetic field alone might be expected to provide.

This is the effect that Bamford and her team have now observed in the lab. They suspended a magnet 2.5 centimetres in diameter in a long cylindrical vacuum chamber at RAL, then blew a plasma towards the magnet at supersonic speeds. The Larmor radius of the particles in that weedy magnetic field is about 120 millimetres: they should have smashed right into the magnet. But they didn’t. Despite the pathetic field strength, none of the particles reached the magnet (Plasma Physics and Controlled Fusion, vol 50, p 124025).

Probes inserted into the plasma wind tunnel showed the magnetic field lines to be bunched up around the magnet, creating a mini-magnetosphere. Further in there was a sharp drop in the density of charged particles: they were held at arm’s length by a glowing protective bubble of plasma that stretched 25 millimetres out from the magnet. “The electrically charged plasma not only went around the magnet, there was a clear cavity you could see with your eyes,” she says. This is precisely what computer models prepared by the team had predicted.

It is a breakthrough that could change the outlook for human space flight. NASA is already looking at a couple of ways of dealing with the space radiation that is holding up further human exploration of the moon, Mars and beyond. One is to find a drug that will reduce astronauts’ susceptibility to cancer or even repair damaged DNA. Another is to carry heavy shielding made of metal or tanks of water to mimic the effect of Earth’s atmosphere, which absorbs most of the high energy or uncharged particles that make it past our magnetosphere.

Now NASA could have a third option: to build spacecraft that carry their own protective magnetosphere, generated by an on-board electromagnet.

It’s a neat idea, according to Andrew Coates of the Mullard Space Science Laboratory in Holmbury St Mary, UK. “I think that scaling a magnetosphere to a spacecraft could work,” he says. He has spotted a few problems, though. Energy is always a scarce resource on spacecraft and, he adds, it’s still not clear that the mini-magnetosphere would be able to deflect higher energy particles. “I think the energetic particles would plough straight through,” he says.

Tito Mendonça of the Instituto Superior Técnico in Lisbon, Portugal, is similarly intrigued – and similarly cautious. “It is well known that our natural magnetosphere protects us very efficiently from the solar wind,” he says. “But how far this protection goes for mini-magnetospheres is still quite early to say.” It’s not just high-energy charged particles that might get through, he points out: a magnetosphere does nothing to deflect neutral particles such as high-energy photons.

Bamford is well aware of these problems, but points out that a mini-magnetosphere stretching a few hundred metres beyond the craft could be used in conjunction with the heavy shields that would stop neutral and high-energy radiation from frying the astronauts. “If you go out in the rain, you can wear a coat – but you can also carry an umbrella,” she says. “That’s what a mini-magnetosphere is – a plasma umbrella held up by magnetic fields. Even if it screened only 50 per cent of the solar particles, it could still help protect a big-mass shield, enabling it to be lighter,” she says. That would allow the craft to carry less fuel.

Bamford is in talks with the European Space Agency and NASA about the possibilities her team’s experiment raises, though she can’t give many details at this stage. “There are confidentiality and patent issues,” she says. What she will say is that NASA agrees that the old assumptions about the limits of magnetic shielding need to be revisited. “They want to work with us on this – a solution to their biggest problem with crewed exploration of space.”

Not that it will be straightforward. It’s still not clear whether the effect will scale up: there’s a big difference between protecting a thumb-sized magnet and a spaceship. Nor do we know whether it would cope with the violence of a solar storm, which would throw off particles with energies of millions, maybe even billions of electronvolts. The atom bombs at Hiroshima and Nagasaki produced gamma rays with energies up to 10 million electronvolts. Nobody knows the precise effect of full-body exposure to particles with energies 10 times as big, but it can’t be good news. “It would be like putting your head in a particle accelerator,” Bamford says.

Bamford and her team’s lab experiments and computer modelling show the electric field contribution can deflect more energetic ions (Plasma Physics and Controlled Fusion, vol 50, p 074017), but the key question is how much more energetic. “We cannot put numbers on any of this yet,” Bamford says. “This is precisely what we are seeking to do now.”

As they crank out those numbers using computer simulations and further lab experiments, we might watch with cautious optimism. The magnetic shield lives again, and maybe, just maybe, that trip to the Red Planet will finally get the green light.

[Via:newscientist]

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