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Long-duration crewed missions beyond low Earth orbit, such as a sustained human presence on the lunar surface or a future journey to Mars, present physiological challenges that go well beyond the technical difficulty of propulsion and navigation. Extended exposure to microgravity causes measurable bone density loss and muscle atrophy, as the skeletal and muscular systems no longer bear the loads they would under Earth's gravity, requiring astronauts to follow rigorous daily exercise regimens using specialized resistance equipment to slow this deterioration. Cosmic radiation poses an even more serious long-term threat, since beyond the protective magnetosphere that shields low Earth orbit, astronauts are exposed to high-energy particles linked to elevated cancer risk and potential cognitive effects, problems that current spacecraft shielding only partially mitigates. Psychological strain is a further concern, as prolonged isolation in a confined vehicle, combined with communication delays of many minutes on a Mars trajectory, limits real-time support from mission control and increases reliance on crew members' own conflict-resolution and problem-solving skills. Mission planners have responded to these constraints by developing autonomous medical diagnostic systems, closed-loop life support that recycles water and oxygen with minimal resupply, and crew selection processes that weigh psychological resilience alongside technical competence. Some proposals also call for artificial gravity, generated through rotating spacecraft sections, to reduce musculoskeletal decline during transit. While no single solution eliminates every risk, mission architects generally agree that combining engineering safeguards with careful crew preparation and selection offers the most realistic path toward sustaining human health on multi-year deep-space journeys.