Phil PlaitNovember 16, 2015 A new study says that violent space weather that could cost $2 trillion in damage is more common than previously thought In the years 774 and 993, the Earth was attacked from space. Not by aliens, but by a natural event—and it was very, very powerful. Whatever it was, it subtly altered the chemistry of our planet’s atmosphere, creating trace amounts of radioactive elements like chlorine-36, beryllium-10, and carbon-14. And those provide the clue to what the event was: Those isotopes are created when high-energy protons slam into our air. That means the source must have...