Space is full of radiation. It's impossible to escape. Imagine standing in the middle of a dust storm, with bits of gravel constantly swirling around you, whizzing by, pinging against your skin. That's what radiation is like in space. The problem is that, unlike a pebble or a speck of dirt, ionizing radiation doesn't bounce off human flesh. It goes right through, like a cannonball through the side of the building, leaving damage behind.