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It might seem impossible for the vacuum of space to have a scent, but astronauts consistently report a distinct aroma upon returning from spacewalks. This isn't the smell of space itself, as a vacuum has no odor. Instead, the intriguing scents cling to their spacesuits, tools, and the airlock walls, becoming noticeable as the cabin repressurizes. Descriptions vary, but common notes include a metallic sensation, like welding fumes or burning metal, alongside hints of seared steak, burnt charcoal, or even spent gunpowder.
Scientists propose a couple of theories for these peculiar smells. One leading explanation involves atomic oxygen, a highly reactive form of oxygen present in low Earth orbit. Ultraviolet radiation from the sun splits ordinary oxygen molecules (O2) into single, aggressive oxygen atoms. These atomic oxygen particles adhere to the surfaces of spacesuits and equipment. When astronauts re-enter the spacecraft and the airlock is refilled with air, these atomic oxygen particles react with the cabin's molecular oxygen (O2) to form ozone (O3), which has a sharp, metallic odor. This process is akin to oxidation or a combustion-like reaction without flame.
Another theory points to polycyclic aromatic hydrocarbons (PAHs). These carbon-based molecules are byproducts of combustion, found on Earth in things like coal, oil, and even grilled foods. They are also prevalent in space, formed when stars burn and die, drifting throughout the galaxy. Astronauts essentially bring these traces of cosmic chemistry back inside with them, where they interact with the air to produce the "burnt" or "charred" smells. Beyond our solar system, regions like the Sagittarius B2 molecular cloud, near the center of the Milky Way, are known to contain vast amounts of ethyl formate, a chemical compound responsible for the flavors of raspberries and the smell of rum, suggesting even more diverse cosmic aromas.