Crewed spaceflight

A crewed spacecraft must keep people alive — pressure, oxygen, temperature, radiation shielding — and get them home if something goes wrong, which demands redundant systems and an abort path at every phase of flight. Those constraints add mass and cost on top of an already demanding launch problem. The hard part is not putting humans in orbit; it is doing so reliably enough that the risk per flight is acceptable. Beyond low Earth orbit, radiation exposure and mission duration compound the difficulty considerably.