Why does astronauts wear diapers?

A spacesuit is a sealed pressure vessel. Once the helmet is locked and the suit is at working pressure, the person inside is committed for as long as the task takes: strapped in for a launch that may sit on the pad through delays, or working outside the station on a spacewalk that runs most of a working day. Depressurising to visit a lavatory is not an option, so the absorbent garment goes on beforehand, under the water-cooled undersuit.

The lesson was learned awkwardly. Alan Shepard's Freedom 7 flight in May 1961 was planned to last about fifteen minutes and carried no provision at all; delays on the pad left him with no dignified choice, and he relieved himself inside the suit. Apollo crews used a collection device with a condom-like cuff and a bag, a design that excluded half the human race and leaked with some regularity. The Shuttle era replaced it with a high-capacity absorbent garment, and today's maximum absorbency garment uses essentially the superabsorbent polymer found in babies' nappies, worn by everyone regardless of sex.

Nobody wears one all day. The station has a proper lavatory that works by suction rather than gravity: a fan draws air, and with it waste, into a sealed container, with a separate funnel and hose for urine. That urine is not thrown away. It goes into the station's water recovery system and comes back as drinking water, a loop the crews describe with the line that yesterday's coffee becomes tomorrow's coffee.