Space may be a vacuum, but at least aboard the International Space Station, smells still have plenty of room to waft. And considering the ISS has 6 living, breathing, excreting human beings living in such close proximity, some of those smells could get to be a major problem. Fortunately, NASA's accounted for that.
NASA engineer Robert Frost and former ISS astronaut Clayton Anderson explain how NASA takes care of astronauts' more malodorous byproducts.
Odors can result from equipment off-gassing, crew metabolic processes, food, experiments, and returning EVA crew members. Two-hundred and sixteen such contaminants have been identified and designed for.
In the SM (Service Module), the Micropurification Unit (БМП) provides a regenerable means to remove both low and high molecular weight contaminants.
In the Lab, the Trace Contaminant Control Subassembly (TCCS) performs a similar function.
Both of these units are nominally operating. Either one is capable of providing the trace contaminant removal for the entire ISS. The БМП vents contaminants overboard while the TCCS traps them in replaceable beds.
Major components of the TCCS include an activated charcoal bed, a catalytic oxidizer assembly, a lithium hydroxide sorbent bed, a fan, and a flow meter. Although the TCCS removes most atmospheric contaminants with the charcoal bed, the high temperature catalytic oxidizer is required for removal of lower molecular weight compounds, such as methane.
TCCS inlet air is drawn directly from the open cabin atmosphere into the activated charcoal bed by the TCCS fan, which is downstream of the charcoal bed. The charcoal bed is impregnated with phosphoric acid, which enables it to absorb ammonia. Downstream of the fan, the process air is split into two flow streams; one going to the Catalytic Oxidizer Assembly, and one to a bypass line. The flow rate to the Catalytic Oxidizer Assembly, measured by a flow meter, is used to control the speed of the fan to provide a specified rate of flow to the oxidizer. The remaining flow is sent through the bypass.
A regenerative heat exchanger and a resistance heater are used to heat the air entering the oxidizer to approximately 400° C. The catalyst oxidizes the organic compounds to CO2 and water, and converts the inorganic compounds to acidic gases such as hydrogen chloride, hydrogen fluoride, and sulfur dioxide. Air leaving the catalyst bed is cooled in the regenerative heat exchanger and is circulated through the LiOH sorbent bed. This bed removes any acid by-products produced during the oxidation process. The air exiting the sorbent bed combines with the unprocessed bypass air and returns directly to the ducts.
From Clayton C. Anderson, Retired ISS Astronaut for NASA:
As a two-time space flier with over 167 days in space (all but 8 on the International Space Station (ISS)), I have definite personal knowledge of the odors existing inside her anodized aluminum hull.
The systems mentioned by Robert typically perform flawlessly, with the crew performing routine maintenance on those systems per the schedule provided by the ground. This, in and of itself, gives us a significantly "odor-free" environment on the ISS. That's not to say we are without smells however. Our standard atmosphere, composed of the very same percentages of constituent gases that we have here on earth, is regulated to 14.7 psi and gives us a shirt-sleeved environment maintaining about 72 degrees F, 55% humidity, with a slight breeze out of the south!
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. 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.
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!
Food odors were also present, but they didn't seem unusually overpowering to me.
Eating a fish dish often produced the most pungent odor, especially the US version of seafood gumbo. It might take a couple of hours to "purge" that smell from the airflow of the ISS. On shuttle missions, many commanders outlawed the eating of seafood gumbo due to its distinctive—and disliked—smell.
And don't worry too much about the stink from the toilets. The airflow systems there (especially the US segment... not so much for the Russian side) were very effective the shuttles had the absolute best system for containing poop odors), pulling the stench quickly and completely into the bowels (from one set to another) of the ISS where they were absorbed efficiently by filters.
Finally, there is the "smell of space." Oft mentioned by astronauts, the smell of space is somewhat hard to describe. Ever distinct—I would know it instantly if I smelled it—it has been likened to smells associated with welding or burning of ozone (now who the heck really knows what that smells like?!). Most noticeable following a spacewalk, when crews and their equipment returned to the inside of the ISS, I remember being able to smell traces of this unique scent for several days following an excursion into the unforgiving vacuum of space.
Top image via NASA
This answer has been lightly edited for grammar and clarity.