“So jet flight at 39,000 ft should be out of the question, eh?”
It proves that to do so you have to be surrounded with technology, supported by a massive earth-based infrastructure, thousands of unseen workers making it possible, and even then you are limited to just several hours.
Try occupying the sea floor near Antarctica to get an idea how much technology you need and how you would not live without all the people and supplies required daily to keep you alive there. That’s peanuts compared to what’s needed for people to go to dead cold planets.