In July of 2012, the sun emitted a coronal mass ejection (CME), which is more or less an eruption of solar wind. The sun has large loops of magnetic fields that usually reach the surface and then go back into the interior. Sometimes these interact with each other in complicated ways, and the energy has to be released out into the solar system. That's usually what we call a solar flare. But if a larger amount of energy is ejected, we call it a CME.

These events happen all the time, but the one in 2012 was the largest one ever recorded. How large? Well, if we had been hit with it directly, we probably would have a blackout lasting several months at least. And by "we," I mean humanity. As in, a huge portion of us.

The last one of that magnitude to hit us was in 1859, called the Carrington Event (named after an amateur astronomer who noticed weird shit appearing on the sun's surface). When the earth was hit, a geomagnetic storm started all over the earth — aurorae could be seen all over the Earth, not just up north — but importantly for us, telegraph systems all over just failed.

Now, this is back in 1859 when pretty much the only electrical thing that mattered were telegraphs. Think about how that would affect us today. First off, most if not all satellites would just be fried, so no GPS, cell phones, whatever. Large transformers would be completely destroyed by the huge current of electricity flowing through them, and entire countries may be without power. Fixing all of this would take trillions of dollars.

It's just luck that this missed us, but it can happen again. We're not really ready for it; the best we can do is observe the sun and give fair warning to shut down the grid and move satellites, although I doubt anyone would listen if astronomers start making a fuss. (Certainly no one did anything in 2012.) It's just one of those things that keeps me up at night as I wonder what the hell we'll do if it happens.