About twelve hundred years ago, Earth may have experienced one of the rarest and most powerful cosmic events a planet can be exposed to: A Gamma Ray burst. We're not sure yet if it actually happened, but if it did...well, let's just say that we, as living things on Earth, are lucky it wasn't worse.

Gamma Rays are the most energetic form of light, and when explosions in space release them in sudden bursts, lasting anywhere from a few milliseconds to a minute or more, they release radiation that's a million trillion times as intense as the sun.  

Astronomers think these enormous blasts of energy come from a couple of different sources.  Some bursts last on average for about thirty seconds, and occur when massive stars die in a violent supernova, creating a black hole in the process.  But as gamma bursts go, thirty seconds can be a long time.  Short bursts, like the one astronomers think might have hit the Earth, last for just a second or two, and almost always occur when two neutron stars, or in some cases a neutron star and a black hole, collide.

Neutron stars are almost inconceivably dense.  Picture the entire mass of the sun crammed into just a couple of kilometers.  If a couple of these extremely dense stars collide, the result is so powerful that it releases some of its energy as gamma rays, which have the smallest wavelengths and the most energy of any wave in the electro-magnetic spectrum.  

These Gamma Ray Bursts, or GRBs, aren't exactly rare on a universe-wide scale.  Because they're so bright, they're easy to spot with telescopes, even at the outer reaches of the universe.  But within our galaxy, astronomers believe they only happen about once every million years.

 And GRBs only release a very narrow beam of powerful radiation, which means a planet has to be in just the right place at the right time to be struck by one. So what makes scientists think a GRB hit Earth?

The answer lies in ancient trees and ancient ice, where in 2012 scientists discovered unusually high levels of two particular isotopes: Carbon-14 and Beryllium-10. These isotopes form when radiation from space collides with nitrogen atoms in the atmosphere, causing them to decay. Because cosmic rays are constantly bombarding our atmosphere, we expect to find certain amounts of these isotopes lying around. But they turned up at levels ten times higher than normal in two distinct places. In the tree rings of old Japanese cedars, and in ancient layers of Antarctic ice. Both of these sources have been dated to about the same time: around the year 775.

Based on what they're seeing, scientists estimate that the energy that caused these anomalies was roughly equivalent to thirteen Hiroshima bombs spread out over half the planet's surface. Unlike the famous supernova of 1054, which was recorded by cultures all over the world, there doesn't appear to be any historical record of people noticing anything unusual in the sky around 775. But that doesn't actually tell us much, because the burst probably only lasted 2 seconds and may not have emitted any visible light.

But of course there are a couple other explanations. A historically large solar flare may have bombarded Earth with enough radiation to create the clues we're seeing. But it also would have cause a truly spectacularly large display of northern lights. Which again doesn't seem to have been noted by anyone. A nearby supernova is another possibility, but for such an event to send out comparable levels of radiation, the explosion would have been easily visible from Earth. And again there is no record of such a thing happening.

So we're left with the GRB scenario. Scientists believe the explosion that caused it must have occurred between three thousand and twelve thousand light-years from Earth. Because if it were any closer, the radiation would have caused the sudden extinction of at least some life.

Though highly unlikely, what would happen if a comparable GRB struck Earth today? Well it depends how far its source was from us. If it was more than a few thousand light-years away, Earth's atmosphere would have absorbed most of dangerous radiation, so humanity would be OK. But, our satellites and power grids would all be fried, likely causing some post-technological scenario worthy of a cable TV drama that you wouldn't be able to watch.

If the burst occurred closer than three thousand light-years away, the radiation could destroy a fair amount of our atmosphere. Particularly the ozone layer, which is made of triple oxygen molecules and protects our planet from the ultraviolet radiation from the sun. With our shields basically down, Earth's surface could be exposed to potentially lethal levels of radiation. Which, while not killing everything, would certainly add to the list of mass extinction events that our planet has endured so far.

So on the bright side, if a gamma ray burst did happen to graze Earth twelve hundred years ago, at least we don't have to worry about it for another nine hundred, ninety-nine thousand years or so.

Thanks for joining me for this episode of Sci-Show Space. If you'd like to learn how you can help us keep exploring the universe together, go to http://subbable.com/scishow. And don't forget to go to http://youtube.com/scishowspace and subscribe.