Humans eat smelly things. Our bodies produce stinky outputs. And if we build up a sweat, our clothes start to smell too.
Theis a big, climate-controlled environment that houses six people at any given time. As you can probably imagine, these humans produce odors just like the rest of us. So how do NASA and other space agencies make the a decent-smelling place to live for for months at a time?
NASA engineer Robert Frost took to Quora to answer that very question, and his answer is filled with all sorts of engineering goodness. In the ISS' service module, for example, a micropurification unit removes both low- and high-molecular weight contaminants, and a "Trace Contaminant Control Subassembly "does this in the lab environment. Both are serviced regularly and help to keep the ISS smelling fresh.
"Either one is capable of providing the trace contaminant removal for the entire ISS," Frost said.
Astronaut Clayton C. Anderson, who's spent more than 167 days in space, including 159 aboard the ISS, said that though these systems work nearly flawlessly, that doesn't stop some smells from permeating the ISS. Anderson noted that if sweaty clothes aren't dried properly, the air smells a bit like a locker room.
"Oleg Kotov, my Expedition 15 Russian crewmate and our Soyuz commander, liked to stash his used workout clothes above the forward-facing FGB (Functional Cargo Block, Russian Module) hatch," Anderson said. "This was not my favorite choice for the stowage of sweaty workout gear as there was not a very good chance that they would dry out effectively."
Continued Anderson: "I chose to put my nasty shorts/socks/T-shirt onto a handrail in the US segment's Node 1 module. This handrail was near an A/C vent, meaning fresh, cold air would blow across my sweaty laundry for many hours until I donned them -- dry as a bone -- the next day. Decreasing their ability to generate any 'locker room' odors, that special placement also allowed for our environmental systems to easily soak up my sweat and turn it into drinking water for later!"
Some foods, like versions of seafood gumbo, were actually banned from several shuttle missions, and it could take the ISS' systems a few hours to clear that fishy smell out of the hull.
And, of course, the ISS distinctly smells like space, a scent that Anderson claims he could instantly identify, though struggles to describe. He notes that others have likened it to the smell of welding or the burning of ozone, neither smells most people are typically familiar with.
You can read Frost and Anderson's in-depth answers over at Quora.