Are Gamma Ray Bursts a Threat to Earth?
It's a subject no one likes to consider: swift death, rushing upon us from space and leaving humanity nowhere to hide. We know about asteroids, but there may be an even greater threat looming halfway across the galaxy. Such is a gamma ray burst; a massive explosion of energy that, if fired off close enough, could cook the very ground we stand on and decimate the atmosphere. Earth would have no warning before the blast hit, just a sudden flash of light as gamma and ultraviolet radiation pounded the planet for several seconds. The results would be devastating, but what are the odds of a gamma ray burst striking the Earth? To understand this, we must first understand the very nature of these ferocious energy beams and the stars that produce them.
What is a Gamma Ray Burst?
Currently, several hundred gamma ray bursts are recorded every year, all of them coming from outside the Milky Way Galaxy. They are the most intensely energetic processes known to exist, making them highly noticeable even in galaxies at the very edges of the visible universe.
It is believed that a GRB is the product of a star falling into itself and forming a black hole. The massive amounts of energy produced by this collapse throw a beam of radiation from the poles of the dying star, blasting everything in its path. This onslaught can take as long as a fraction of a second or stretch out to a full minute. Waves of ultraviolet radiation tend to follow the initial burst in a weaker but more sustained output.
The good news is that the beam is, relatively speaking, quite narrow. Imagine pointing an extremely long-range gun into the sky at random and consider the odds of hitting any particular object in the galaxy. Not good, right? But, it certainly isn't impossible, forcing us to ask ourselves: what if?
Possible Effects on Earth
Let's say that one day, out of the blue, Earth draws the short straw and finds itself the sudden victim of a gamma ray burst from within our own galaxy. What would happen?
A lot depends on proximity. If a star collapsed close enough, the entire planet could be reduced to a cinder. There wouldn't be even a chance of survival. Luckily for us, the odds of that are basically nil, and scientists have yet to find a risky star in our general area. So, what happens if we get hit from across the galaxy?
The first thing we'd notice is a flood of light as the radiation strikes the atmosphere. The dying star would appear as bright as our own sun. The first blast would not do much damage to the surface of the Earth. Instead, it would deplete a large part of the ozone layer. Where humans have contributed to a loss of about 5% of our ozone layer, a gamma ray burst from even a long distance could strip our atmosphere of up to 30% of the ozone. The destroyed molecules would recombine into smog, causing a massive global cooling. Even worse, the lack of an ozone layer would leave Earth wide open to UV radiation from the dying star and our own sun. The entire planet would be doused in radiation, along with anyone and anything on its surface.
Faced with a new ice age and genetic mutations from the UV rays, citizens of Earth would be in a bad position, but recovery should be possible. Those on the other side of the planet (not facing the burst,) will have time to take protective measures and find shelter. The marvelous thing about the atmosphere is its tendency to heal itself, given a few decades. Earth could right herself after a gamma ray burst, but it would be up to humans to survive until then.
So Should You be Building a Bunker?
Gamma ray bursts are a frightening prospect, but the odds of one hitting the Earth in any one lifetime are almost zero. We are blessed to live in a metal-rich galaxy, full of heavy elements that tend to prevent the volatile conditions needed for these catastrophic explosions. This is true for most hypothetical life forms in the universe as well. The large, element-dense galaxies prime for supporting life are far less likely to host gamma ray bursts. While there is still a chance of the Earth being struck, it is happily very small. There are plenty of other threats more likely than gamma ray bursts. If you must lose sleep- worry about them!