I can’t imagine any other group who would be as calm as NASA astronauts. Maybe SEALs or other special forces.

It looks like there are a few astronauts that were SEALs, one returned December 9th from the ISS.

https://en.wikipedia.org/wiki/Jonny_Kim

Jonny Kim was indeed a SEAL, and a few more things as well, with a CV almost as impressive as Johnny Sins:

> American NASA astronaut, physician, U.S. Navy officer, dual designated naval aviator and flight surgeon, and former Navy SEAL.

Note that "physician" here means Harvard MD.

This guy is the Chuck Norris of NASA. His Wiki page is wild.

In the US, many astronauts start as Air Force pilots.

And for the preternaturally calm and confident who don't have the perfect eyesight required to enter the Air Force, many of them apparently serve instead on nuclear submarines...

The point of training someone to their breaking point is not to make them immune to breaking. It's to give them experience with a realistic battlefield situation and their own physiological responses during it so they stand a basic chance when it does occur.

Somewhat related to this topic: https://youtube.com/watch?v=Zo62S0ulqhA

Totally, they put a bunch of people in those giant spinning chair things and weed out the ones that puke or freak out. Those are the astronauts, they have the right stuff.