Why do astronauts go to space?

The first answer is research that only works in continuous free fall. Remove convection and sedimentation and flames burn as spheres, alloys mix without separating, protein crystals grow with fewer defects and fluids behave in ways no drop tower can sustain for more than seconds. The station also studies its own occupants: bone loss, fluid shifts towards the head, vision changes and immune response are measured on the only subjects who spend months without weight. Crews have kept that programme running without a break since November 2000, which is what makes the data comparable across decades.

Robots are cheaper and safer, but they improvise badly. Astronauts assembled a station from modules built on three continents, and they repaired and upgraded the Hubble Space Telescope in orbit, replacing instruments and rescuing a mission that would otherwise have been written off. When a coolant pump fails or a bolt seizes, a crew can diagnose the problem, fabricate a workaround and try again the same day. Presence also buys judgement: someone on the spot notices what no camera happened to be pointed at.

Beyond the science there is preparation. Every rotation in orbit is a dataset on how bodies, equipment and crews hold up far from resupply, which is the central unknown for missions to the Moon and Mars. Agencies also send people for reasons that are not technical: national capability, international partnership, industrial skills that atrophy if nobody flies, and the plain human interest in seeing a place first-hand. Those motives paid for the hardware long before any experiment did, and the two strands have travelled together ever since.