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.
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.
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.