Outer space, often perceived as a silent and odorless void, holds a surprising secret: it has a distinct scent. While direct exposure to the vacuum of space prevents humans from perceiving this aroma, astronauts returning from spacewalks consistently report unique smells clinging to their suits and equipment. These insights offer a glimpse into the chemical composition of the cosmos, revealing space is far from a scentless environment. Exploring these cosmic odors helps us understand the complex chemistry occurring beyond Earth.
The Scent of the Cosmos
Astronauts who have spent time outside spacecraft describe a particular aroma upon re-entering their pressurized modules. This scent is not experienced directly in the vacuum, but rather as molecules that adhere to their spacesuits, tools, and the airlock off-gas once back inside. Descriptions of this “smell of space” vary, but often converge on similar notes.
Common comparisons include seared steak, hot metal, or welding fumes. Some astronauts have likened it to gunpowder or burnt cookies, while others noted a sweet, metallic sensation. Don Pettit, a NASA astronaut, described it as “pleasant sweet-smelling welding fumes,” reminiscent of working with an arc welding torch. These descriptions highlight a shared experience of a somewhat acrid, burnt, or metallic scent.
The Origin of Cosmic Odors
The unique smells detected by astronauts stem from specific chemical compounds and processes occurring in space. One primary contributor is polycyclic aromatic hydrocarbons (PAHs), complex molecules abundant throughout the universe. These PAHs are byproducts of the intense energy released by dying stars, forming as cosmic dust and debris are ejected. On Earth, PAHs are found in charred food, coal, and oil, explaining the burnt or smoky notes reported by astronauts.
Another proposed source for the metallic or acrid scent is atomic oxygen. In low Earth orbit, the faint atmosphere contains a significant amount of atomic oxygen. When astronauts conduct spacewalks, these single oxygen atoms cling to their suits and equipment. Upon re-pressurization inside the airlock, these atomic oxygen particles react with molecular oxygen (O2) in the cabin, forming ozone (O3), known for its sharp, metallic smell. Some comets also emit molecules like hydrogen sulfide (rotten eggs) and ammonia (urine), showcasing the diverse chemistry of celestial bodies.
Why We Can’t Smell Space Directly
Humans cannot directly smell space because olfaction requires the presence of air to transport odor molecules to our nasal receptors. Space is a near-perfect vacuum, meaning it contains very few molecules. This lack of air prevents the transmission of any scent.
The smells astronauts perceive are indirect, originating from residual molecules that attach to surfaces exposed to the space environment. These molecules remain inert in the vacuum but become detectable once brought back into a pressurized cabin where they off-gas into the breathable air. Chemical reactions, such as oxidation, that occur during re-pressurization also contribute to the release of these odors, making them noticeable only in an oxygen-rich atmosphere.