Most toilets in a space station

The most toilets in a space station is three, a tally reached by the International Space Station with the arrival of the Russian Nauka module on 29 July 2021.

During the early stages of ISS construction, the station had just one toilet, located in the Zvezda module that also houses the station's life support systems. Another toilet (called the "Waste and Hygiene Compartment") was added in 2001 with the berthing of the US-made Destiny module (the toilet in the US section was later moved to Tranquility). The last of the station's on-board facilities was added with the arrival of the Russian Nauka module on 29 July 2021, which contains another toilet.

The toilets in the Zvezda and Nauka modules are both ASU-8A Waste Management Systems, designed by Russian aerospace contractor NPP Zvezda. This model was originally developed for the Soviet Buran orbiter, and was used throughout the lifespan of the Mir space station. In the absence of gravity, it uses fan-assisted suction to separate people from their waste.

The toilet in the US section was originally also an ASU-8A – purchased by NASA at a cost of $19 million – but it has since been replaced by a toilet called the Universal Waste Management System (UWMS), which was delivered to the station in 2020. The UWMS was designed to be quieter, more hygienic, and – most importantly – easier for female astronauts and cosmonauts to use. (Using the toilet in zero gravity is difficult for everyone, but the design of systems like the ASU-8A make it particularly awkward for women.)

The UWMS system cost $23 million to develop, but astonishingly that's not quite enough to make it the most expensive toilet of all time. That record belongs to the upgraded version of the Space Shuttle's Waste Collector Subsystem, first flown in 1993, which cost $23.4 million.

In addition to the toilets that are a permanent fixture of the station, there are also typically additional toilets located in docked spacecraft, such as the Space Shuttle (formerly), Crew Dragon, Starliner or Soyuz. Usual procedures require for there to be two of these crew transport ships docked at all times so the station can be evacuated in an emergency. The toilets on these ships are only used in emergencies, such as in 2008, when the toilet on Zvezda module stopped working.