mass-extinction

Sixty-six million years ago, a rock roughly 12 kilometers wide hit the Yucatán Peninsula at 20 kilometers per second, releasing energy equivalent to roughly one billion Hiroshima bombs. Within hours, wildfires swept continents. Within months, the sky went dark. Within a few thousand years, three-quarters of all species on Earth were gone — including every non-avian dinosaur.

A gamma-ray burst aimed at Earth from within a few thousand light-years would deplete the ozone layer in seconds, expose the surface to lethal UV, and possibly trigger a mass extinction. The good news: no such cosmic gun is currently aimed at us.