IF ASTRONAUTS ever make it to Mars, they’ll need a roof over their heads. Bricks are too heavy to lug all the way from Earth, but with just a few bucket-loads of polyethylene powder, they could rustle them up when they get there. “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.”