A person cannot smell in space because the human biological process of olfaction cannot function in a vacuum. However, the question is complex, as many astronauts who have spent time outside a spacecraft report a distinct odor. This apparent contradiction arises because the smell is perceived inside the pressurized habitat, not in the harsh, airless environment of outer space. The true scent comes from compounds that cling to objects returning from the void.
The Physical Barriers to Olfaction in Vacuum
The mechanism of human smell requires two fundamental components: air pressure and moist tissue. Olfaction begins when airborne odorant molecules are drawn into the nasal cavity during inhalation. These molecules must then dissolve in the mucus layer covering the olfactory epithelium for specialized receptors to detect them.
The near-perfect vacuum of space eliminates the possibility of this process occurring. Without an atmosphere, there is no air pressure to carry odorant molecules. Furthermore, any exposed moisture, such as the mucus in the nasal passages, would immediately vaporize due to the lack of external pressure, a phenomenon known as ebullism.
The physical conditions in space are instantly fatal to an unprotected human body, making biological sensation impossible. Trying to inhale in a vacuum would result in a rupture of the lungs, and the lack of pressure would cause bodily fluids to boil at body temperature. Therefore, the act of smelling, which relies on gas exchange and moist membranes, is biologically precluded in the vacuum environment outside a spacesuit.
Reports of Scented Particles
Despite the physical impossibility of smelling in a vacuum, a consistent observation has emerged from decades of human spaceflight. Astronauts frequently report a peculiar, strong odor, but only after completing an Extravehicular Activity (EVA) and returning to the airlock. The smell is detected when the pressure suit or tools are exposed to the oxygenated, pressurized atmosphere of the spacecraft cabin.
Astronaut Don Pettit described the scent as “metallic” and reminiscent of “sweet-smelling welding fumes.” Other common descriptions include “burnt steak,” “hot metal,” or the smell of spent gunpowder. One astronaut likened the aroma to a mixture of walnuts and motorbike brake pads.
These reports confirm that the smell is caused by particles adhering to the surfaces of the spacesuits and equipment, not a phantom sensation. When the airlock is repressurized, these space-exposed materials off-gas the accumulated compounds into the breathable air. The consistency of these descriptions indicates a common chemical source originating from the space environment.
The Chemical Sources of the Reported Smell
The distinctive scent results from chemical reactions involving materials bombarded by high-energy radiation and atomic particles in space. The International Space Station operates in low Earth orbit, encountering a faint atmosphere containing atomic oxygen. This highly reactive form of oxygen attaches to the outer surfaces of spacecraft and spacesuits.
When the suits and equipment are brought back into the airlock, the atomic oxygen and other residues react with the cabin’s atmosphere. This process is a form of oxidation, a chemical reaction similar to combustion, which releases volatile compounds. The reaction does not involve fire but produces similar byproducts to those created when materials are burned or heated.
The primary chemical contributors are likely Polycyclic Aromatic Hydrocarbons (PAHs), stable molecules composed of carbon and hydrogen. PAHs are ubiquitous throughout the universe, forming in the aftermath of stellar explosions and present in interstellar dust and nebulae. These compounds are also found on Earth in soot, exhaust fumes, and charred food, aligning with the “burnt” and “welding fumes” descriptions.
Our sense of smell is sensitive to small PAHs, meaning trace amounts brought into the airlock create a noticeable aroma. The PAHs, energized by solar wind and cosmic radiation, adhere to the suits’ fabric and metal. When they contact the oxygen-rich environment inside the spacecraft, the resulting chemical off-gassing produces the unique sweet and metallic aroma called the “smell of space.”