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Comment by 24gttghh | Hacker News Reader

>Of course, it's important to bear in mind that "just about to explode" means something different in cosmic terms versus human terms. It could happen at any time in the... next few hundred thousand years.

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I don't know what sort of luminosity a GRB would have at 8k LY. But if it's strong enough to be an ELE, that's still a pretty short timeframe, in case the expected GRB jets will be pointed at us. We'd need to not only get off the planet, but to another star system entirely.

I doubt another solar system would be far enough away

Ironically, in the specific case of a imminent gamma ray burst, we might actually have better luck staying on the planet, with eight thouand miles of rock and metal to act as ablative shielding. I'm not sure how far underground we'd need to be to survive the backscatter from eg the moon, though, and I doubt the planet would be habitable afterward anyway.

It's not the gamma rays that kill you. In particular even with a supernova strength source, it's unlikely that the gamma rays would even have enough energy to get through the atmosphere to the point of meaningfully heating the ground. However, they would butcher and greatly change the atmosphere which, in turn, is what kills you - probably through a mechanism of massive amounts of UV and other high energy radiation now happily making its way down to us. As an example, a gamma ray burst is one possible hypothesis for the Ordovician extinction.

Best case for survival would be to become multiplanetary. In particular the conditions that would be created would be survivable by the exact same technology that will be the baseline for human colonization on Mars.

> Best case for survival would be to become multiplanetary.

How far away do you have to be to avoid getting gamma'd?

Depends primarily on how far away it is. Scientists are pretty sure they're only 2-3 degrees wide at the source, so the closer you are, the less distance you would need to travel. Of course, the farther you are, the less energetic it is.

So, if GRB are 2 degrees wide at the source, and it occurs 8000LY away, and is pointed directly at us, we'd need to travel 136LY away.

The solar system is moving at 1/1300 the speed of light. I'm not sure how Apep is moving relative to us, but assuming it's stationary, we'd need ~175k years to move out of the crosshairs by sitting here on the planet.

I have to admit, I was not expecting to have been underestimating the intensity of a gamma ray burst.

> even with a supernova strength source

In my defence, my reference power source was "supermassive black hole falling into another supermassive black hole" (probably due to having heard either GRBs in general or one specific one attributed to the collapse of a quasar).

Out of curiosity, just what would happen to a person standing on Earth, staring up at the night sky right at the moment that a GRB jet hit the planet?

Also, how wide, in terms of arcseconds, is the jet? Is it a narrow laser beam that has no chance of actually hitting a planet, or is it wide enough that anything within 30 degrees of north is going to be sterilized?

Most likely (as in, very most likely) not bad [1], though it's theorized that the second largest mass extinction event on Earth may have been due to a GRB [2].

[1] https://en.wikipedia.org/wiki/Gamma-ray_burst#Rate_of_occurr... [2] https://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_ex...

There is a documentary about a comparable situation called The Incredible Hulk

Think about the odds of the beam being perfectly aligned and aimed to hit the planet with dead-on accuracy though. Even if the spread of the beam is as wide as Pluto's orbit, we'd have to be pretty unlucky to get hit.

If it's as narrow as the arc of a season, one quarter of the Earth's orbit, it could hit us during Autumn or miss us during Winter, or not at all if it's not pointed into the path of our orbit.

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