Space itself does not have a smell, as it is essentially a vacuum containing extremely low densities of particles. However, astronauts returning from spacewalks and other extravehicular activities have reported smelling some unusual odors upon re-entering their spacecraft and removing their helmets.
The most commonly described space smell is likened to seared steak, hot metal, gunpowder, and welding fumes. The smells have also been compared to more unusual odors like burnt almond cookies and walnuts. While each astronaut seems to smell something slightly different, there is consensus that space has a distinct scent.
The causes of these space smells are not yet fully understood, but a few explanations have been proposed. One is that atomic oxygen particles become attached to an astronaut’s spacesuit fabrics in the vacuum of space. When the astronaut returns to the oxygenated interior of the spacecraft, these oxygen atoms rapidly react and oxidize with other particles, producing ozone and other substances that give off strange smells.
Materials off-gassing in the vacuum environment may also play a role. The lack of air pressure causes substances to evaporate from the spacecraft’s and spacesuits’ interiors, which the astronauts then smell when they return inside. Changes in air pressure and circulation when opening hatches and airlocks could concentrate and intensify these off-gassed particles to become more noticeable.
While space itself lacks smell, the unusual scents astronauts encounter serve as olfactory reminders of their extra-vehicular activities beyond the confines of their spacecraft.
