The vastness of space presents an unexpected sensory experience, one that astronauts consistently describe with contrasting olfactory notes. Far from being a scentless void, the cosmos leaves a distinct aroma on objects exposed to it, a phenomenon repeatedly reported by those returning from spacewalks. This peculiar scent is often characterized as metallic, sweet, or burnt. Deep space observations also suggest a surprisingly fruity essence akin to raspberries and rum, an occurrence tied directly to the complex chemistry of stars and the physics of a vacuum.
The Paradox of Smell in Space
The notion of smelling space is inherently contradictory because olfaction relies on a medium that space fundamentally lacks. To perceive an odor, molecules must travel through the air and dissolve in the moist lining of the nasal cavity. Outer space, however, is a near-perfect vacuum with virtually no atmospheric pressure or air molecules to transport scent compounds.
If an astronaut removed their helmet outside the spacecraft, the lack of air would prevent any molecules from reaching the nose. Therefore, the reported “smell of space” is not detected directly in the void. It is a secondary effect, experienced only after cosmic materials are brought back into the pressurized, breathable environment of a module or airlock.
The Chemical Culprit: Esters and Aromatic Compounds
The diverse descriptions of space’s aroma point to two main classes of organic molecules found throughout the universe. The metallic and burnt notes are primarily attributed to Polycyclic Aromatic Hydrocarbons (PAHs). These sturdy, carbon-based molecules are produced by the incomplete combustion that occurs as stars die and release heavier elements.
PAHs are found on Earth in soot, charred food, and car exhaust, and are believed to be the source of the smoky, gunpowder-like smell astronauts report. Astronomers have also detected surprisingly pleasant molecules in dense interstellar clouds, such as Sagittarius B2 near the center of the Milky Way. This region contains ethyl formate, a chemical compound classified as an ester.
Ethyl formate is responsible for giving raspberries their characteristic flavor and is also a component of the scent of rum. The presence of this and other complex organic molecules demonstrates that the building blocks for sweet and fruity odors exist throughout the galaxy. While the metallic scent is tied to our immediate solar system environment, the raspberry-like aroma is a chemical signature of deep space.
How the Odor Reaches Astronauts
The unique smell is perceived by astronauts only after they have completed an extravehicular activity and repressurized the airlock. As the astronaut enters, the exterior of their spacesuit, tools, and the airlock walls are coated in microscopic residue collected from the space environment, consisting of PAHs and other particles.
When the airlock is filled with oxygen and pressurized, the volatile compounds collected on the surfaces are released into the cabin air. The resulting aroma is often described by astronauts as “hot metal,” “seared steak,” “welding fumes,” or “spent gunpowder.” Astronaut Don Pettit specifically noted a “pleasant sweet metallic sensation” that reminded him of welding fumes.
The reaction of highly reactive atomic oxygen, which is prevalent in low Earth orbit, is another likely contributor to the metallic and acrid scent. These single oxygen atoms adhere to the suit fabric and, upon re-entry, combine with the air to form ozone, a molecule known for its sharp, metallic smell. This process of off-gassing and chemical reaction allows the human nose to detect the lingering scent of the vacuum’s surface contamination.