"Every square centimeter on the top of the Earth's atmosphere is hit by several cosmic rays per second," Fields says. "This is forever going on."

At present, the average human receives the equivalent of about 10 chest X-rays per year from cosmic rays. We shouldn't be alarmed by this, since it is just part of the natural background radiation under which humans and our ancestors have been exposed to for eons. Indeed, cosmic-ray-induced mutations may sometimes be beneficial.

"It is clear that in some way cosmic rays shaped evolution of organisms on Earth," says Franco Ferrari from the University of Szczecin in Poland.

Although 30 light-years is small on a galactic scale, Fields thinks it likely that Earth has been caught in a supernova "kill radius" as many as a dozen times over our 4.5-billion-year history.

However, a nearby supernova is not the only way to increase the cosmic ray intensity. As our Sun orbits around the galactic center, it regularly passes through one of th