How does an astronaut poop in space?

On Earth a toilet works because waste falls. In free fall nothing falls, so the plumbing has to create a directed flow of air. The seat opening on a spacecraft toilet is small — roughly the size of a dinner plate's centre — and beneath it a fan draws a continuous stream of air downwards, carrying solid waste into a bag lining a removable canister. The airflow is also what makes alignment critical; crews practise on a mock-up with a camera inside the bowl before flight, since there is no room for approximation.

Urine is handled separately, through a flexible hose ending in a funnel. Interchangeable funnels accommodate different anatomies, and the same suction principle applies. Foot restraints and thigh bars hold the user in place, because otherwise the act of sitting down would push them away.

The two streams diverge. Urine goes to the water recovery system, where a distillation assembly and filters turn it back into potable water; combined with condensate recovered from cabin humidity, the station recycles the large majority of the water it consumes, which is the only way to avoid launching every litre from the ground. The crews' standard line about drinking yesterday's coffee is accurate.

Solid waste is compacted, sealed and stored. It leaves the station inside a departing cargo vehicle that burns up on re-entry. Some samples, though, are kept and returned frozen for medical study of crew health and gut microbiology.

The Universal Waste Management System, installed on the ISS in 2020 and adopted for Orion, is smaller and lighter than its predecessor, with a titanium structure, a redesigned seat and funnel, and better odour control. Spacewalks are a different problem entirely: there the answer is an absorbent garment worn inside the suit.