Space Smells Like Seared Steak

While the vacuum of space itself means humans cannot directly perceive odors, astronauts consistently report a distinctive scent upon returning from spacewalks. This peculiar aroma clings to their helmets, suits, and equipment as they re-enter the pressurized environment of a spacecraft. Descriptions vary, often including notes of hot metal, seared steak, welding fumes, or even gunpowder. Some have also mentioned sweeter undertones, like raspberries and rum, or more unusual comparisons such as walnuts and brake pads.

Scientists believe the primary source of these "burnt" smells comes from polycyclic aromatic hydrocarbons (PAHs). These robust, complex molecules are incredibly common throughout the universe. On Earth, PAHs are familiar byproducts of combustion, found in things like soot, car exhaust, charred meat, and burnt toast. Their prevalence in space suggests that these microscopic particles, generated from the intense energy of dying stars, adhere to the astronauts' gear during extravehicular activity.

As stars reach the end of their lives, they expel heavy elements, including carbon, creating these aromatic compounds that drift through interstellar (Review) space, eventually contributing to comets, meteors, and cosmic dust. Another theory suggests the metallic or ozone-like scent might stem from single oxygen atoms in space reacting to form ozone (O3) when they combine with molecular oxygen during the repressurization of an airlock. NASA has even commissioned a chemist to recreate these unique smells for astronaut training, highlighting the importance of understanding the chemical environment of space.