In 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.

In the laboratory, cosmic rays first presented themselves as a minor annoyance. They were discovered when physicists noticed that electrically charged bodies do not stay that way; their charge slowly leaks away through the air. Something had to be ionizing the air, allowing it to conduct electricity. Many researchers blamed the ambient radioactivity of the soil and rocks underfoot. Austrian physicist Victor Hess settled the issue in 1912, when he went aloft in a balloon and showed that the higher he rose, the faster the charge leaked off his electroscope. So the cause of the ionized air was something mysterious coming in from space--thus the name "cosmic rays."