What does an astronaut do?

An astronaut's work in orbit looks more like that of a lab and maintenance technician than an explorer. The day is planned down to the minute from mission control, with task blocks assigned minute by minute. Part of it is devoted to science: setting up, running, and tearing down experiments designed by researchers on Earth, in fields ranging from cell biology to fluid physics or materials science. The astronaut is rarely the author of the experiment; they're the hands that carry it out in an environment impossible to reproduce on the ground.

Another large part, often larger than people imagine, is maintenance: filters, pumps, water recycling systems, panels, computers. On top of that come capturing and unloading cargo vehicles, using the robotic arm when docking isn't automatic, and spacewalks, which are infrequent and preceded by dozens of hours of preparation. Time is also set aside for outreach and contact with schools.

Without weight, bone and muscle atrophy, so the schedule sets aside roughly two hours a day for exercise on a treadmill with a harness, a bike, and a resistance device that simulates weights. That discipline is what allows a crew to return after six months and walk again within days, not months.

Most of a career takes place without ever leaving the planet. After selection comes two years of basic training: spacecraft and station systems, robotics, Russian, survival, flights in a training jet, and hundreds of hours in a pool to rehearse spacewalks. Then come support roles, including serving as the liaison with the crew in orbit, until assignment to a specific mission arrives, which usually means two more years of mission-specific training.