“To function properly, we need gravity. Without it, the environment is less demanding on the human body in several ways, and this shows upon the return to Earth. Remember the sight of weakened astronauts emerging after the Apollo missions? That is as nothing compared with what would happen to astronauts returning from Mars.”
…To the extent humans can agree on what being stuck on earth means, ie needing gravity, whatever species that leaves Earth one day might not actually be human, or not one that a modern human would recognize. We have been built and designed to survive on this planet.