Shielding space travelers.

M A R C H 2 0 0 6 C R E D IT Shielding Space Travelers I n science fiction, the worst threats to space travelers are large ones: careening asteroids, ravenous creatures, imperial battle cruisers. In reality, though, the scariest menaces for humans in space are the tiniest: fast-moving elementary particles known as cosmic rays. On a long journey, they would give astronauts a dose of radiation serious enough to cause cancer. Unlike most of the other challenges of venturing into deep space, which engineers should be able to solve given enough time and money, cosmic rays pose irreducible risks, and dealing with them involves fundamental trade-offs. They could be the show-stopper for visiting Mars.