The question of what space smells like presents a fundamental paradox, as the vacuum of outer space is technically odorless. Smell relies on volatile molecules traveling through a medium like air to reach the olfactory receptors in the nose. However, astronauts consistently report a scent that is not experienced outside the spacecraft but only upon returning to the pressurized cabin environment. This aroma is a result of interstellar particles and chemical reactions that cling to spacesuits and equipment exposed to the near-perfect vacuum.
The Astronaut Experience
Astronauts who have completed spacewalks, or Extravehicular Activities (EVAs), share remarkably similar descriptions of the odor they encounter. The smell is often characterized as a harsh, metallic sensation, sometimes compared to the fumes from arc welding. Former astronaut Don Pettit described the scent as “a rather pleasant sweet metallic sensation,” reminiscent of welding fumes he experienced during his college summers.
Multiple veterans cite the smell as similar to spent gunpowder. Some Apollo-era astronauts noted the lunar dust they brought back had a strong, acrid odor, much like the residue after a firework has been set off. The smell has also been likened to seared steak, hot metal, or a blend of walnuts and brake pads. These consistent descriptions suggest a common chemical source.
The Mechanism of Odor Transfer
The smell does not exist in the vacuum itself but is generated the moment space-exposed materials re-enter the pressurized, oxygenated air of the cabin or airlock. During a spacewalk, volatile molecules and high-energy particles from the space environment adhere to the surfaces of the spacesuit fabric, tools, and the exterior of the spacecraft. These particles are essentially “baked” onto the surface by the harsh radiation and vacuum.
When the equipment is brought back into the airlock and the pressure is restored, the volatile compounds rapidly vaporize, or desorb, into the cabin air. This transition from a solid state directly into a gaseous state allows the molecules to be inhaled and smelled by the astronauts. The sudden release of these space-borne molecules creates the sharp odor that permeates the airlock.
The Specific Chemical Compounds
The metallic and burnt odor is primarily attributed to two chemical phenomena: Polycyclic Aromatic Hydrocarbons (PAHs) and the reaction of atomic oxygen. PAHs are complex organic molecules made up of fused benzene rings, and they are common in the universe. These molecules are formed when stars die and are found in interstellar gas and dust clouds.
When these PAHs encounter the oxygen in the spacecraft cabin, they undergo oxidation, which is chemically similar to combustion but without visible flame. This oxidation is thought to be the source of the “burnt” smell, such as spent gunpowder or seared meat.
The other primary contributor is the presence of atomic oxygen, the most abundant element in the low Earth orbit environment. Atomic oxygen is highly reactive, and single oxygen atoms can cling to the spacesuit material during an EVA. Upon re-pressurization, these single oxygen atoms immediately bond with the molecular oxygen (\(O_2\)) in the cabin air, forming ozone (\(O_3\)). Ozone is well-known for its sharp, metallic, and sometimes acrid scent.