"In a couple of decades or so, humans may be going to Mars," says Richard Kiefer of the College of William and Mary in Williamsburg, Virginia. "But you're likely to be there for a long time, so you're going to need a habitat."

Because Mars has a very thin atmosphere and no magnetic field, astronauts would be exposed to cosmic radiation. "On Earth, the magnetic field and the atmosphere protect us on the surface from cosmic radiation," says Kiefer. Buildings on Mars would have to shield you from these. It's unlikely astronauts will have enough room on their spacecraft for the right materials to keep out high-energy particles. So Kiefer and some NASA researchers decided to try to take advantage of the local soils. "The idea is to see if we can use Martian topsoil mixed with polymer powder to make bricks," he says.

With no suitable equivalent to Martian topsoil, Kiefer tried the next best thing: some simulated lunar topsoil sold off-the-shelf by the University of Minnesota. "It has very similar characteristics," he says. He mixed it with polyethylene powder and heated the mixture to 110 �C for half an hour at a pressure of 100 pounds per square inch (690 kilopascals). The finished bricks were "good quality", as long as they weren't more than 93 per cent topsoil, says Kiefer. "Shielding against radiation is a tricky business because you can actually make matters worse by interposing materials," says Roger Emory of the Space Science and Technology Department at the Rutherford Appleton Laboratory near Oxford. That's because incoming radiation can break down the nuclei of the atoms in the shield, producing more harmful particles. But Kiefer says polyethylene bricks will minimise this risk. "Polyethylene has more hydrogen than any other polymer, and hydrogen is good because it can't break apart like this," he says.

Kiefer is now checking the shielding properties of his bricks by firing high energy particles at them at the Brookhaven National Laboratory in New York.

This article will appear inm the April 7, 2001 issue of New Scientist

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