The thought of dying isn't the inherently problematic part, but rather the extreme isolation. People who go to war are there for only some months and have plans for going home, back to friends and family. People who dive and explore the oceans are isolated for a few days at worst; perhaps months in a nuclear sub, but there you have already a pretty large crew around you. Compared to decades spent isolated with a handful of people, and no return ticket to the rest of humanity, those are nothing.
Huh?
"The first crewed Mars mission would be expected to have approximately 12 people, with the primary goal to "build out and troubleshoot the propellant plant and Mars Base Alpha power system" as well as a" rudimentary base." In the event of an emergency, the spaceship would be able to return to Earth without having to wait a full 26 months for the next synodic period."
https://en.wikipedia.org/wiki/Interplanetary_Transport_Syste...
NASA, I believe, is currently trying to solve the problem with more efficient engines for interplanetary travel. Ion drives seem to be the preferred method, but there is still some research going on in nuclear thermal rockets.
Who said they're never coming back to Earth?
>no perfectly rational, mentally healthy human would volunteer for such a mission
That's an easy problem to solve, as there are no "perfectly rational" and/or absolutely "mentally healthy humans", and have never been anyway.
And people can still be quite rational and mentally healthy and yet cherish the possibility to take such a mission, even if there's a possibility to die, etc.
In fact, it's needs no more troubled people than those that go to the army and are OK to be sent to war, or people that dive and explore the oceans, etc etc.