The sensory experience of space has long captured the human imagination. Since opening a helmet in the vacuum is fatal, direct tasting is impossible, but astronauts consistently report a distinct smell associated with objects returned from spacewalks. This unique phenomenon is not the smell of the vacuum itself, but the result of volatile compounds carried back into the pressurized spacecraft environment. These consistent reports offer indirect insight into the chemical nature of the near-Earth environment.
The Reported Sensory Experience of Space
The descriptions of the space scent are remarkably consistent across missions, frequently blending the familiar with the metallic. Only astronauts who have completed an Extravehicular Activity (EVA) and returned to the airlock encounter this scent. The odor is not noticeable during the spacewalk itself, but only once the airlock has been repressurized and equipment is removed.
Former NASA astronaut Don Pettit described the sensation as a “pleasant sweet metallic” smell, comparing it to the fumes from arc welding equipment. Other crew members have likened the scent to spent gunpowder, burnt almond cookies, seared steak, or the sharp, acrid odor of ozone. Another common comparison is the smell of hot metal.
The collective reports indicate a strong, lingering scent that clings to the fabric of spacesuits and tools. This consistency suggests the odor is a tangible chemical signature of the space environment, not an individual perception. NASA commissioned a chemist to recreate this scent for astronaut training.
The Science Behind the Cosmic Odor
The unique scent is a byproduct of chemical reactions occurring when spacecraft materials interact with the harsh conditions of Low Earth Orbit (LEO). One primary culprit is Atomic Oxygen (AO), which is abundant in the upper reaches of Earth’s atmosphere. AO consists of single, highly reactive oxygen atoms that bombard and erode spacecraft surfaces at orbital velocities.
When equipment is exposed during a spacewalk, these single oxygen atoms adhere to the surfaces. Upon re-entry, repressurization introduces molecular oxygen into the airlock. This molecular oxygen combines with the reactive AO atoms, quickly forming ozone. Ozone produces the sharp, electric, metallic smell often associated with lightning strikes or electrical arcing, accounting for many astronaut reports.
Another proposed source for the “burnt” smell is Polycyclic Aromatic Hydrocarbons (PAHs), common organic molecules found throughout the universe. PAHs are formed by stellar processes, such as the cooling of dying stars, and are abundant in interstellar dust and nebulae. When PAHs adhere to the spacesuit and are exposed to oxygen and radiation, they break down into volatile fragments that off-gas into the cabin air. These PAH fragments are chemically similar to compounds found in charred foods or spent gunpowder, providing a scientific basis for the smoky descriptions.
Why We Can Only Smell Space, Not Taste It
The question of what space tastes like is a matter of physics and human physiology. Taste, or gustation, requires chemical compounds to dissolve in saliva before they can interact with the taste receptors on the tongue. The near-perfect vacuum of space makes this process impossible.
In a vacuum, the lack of atmospheric pressure would cause any liquid exposed to space, including saliva, to instantly sublime, turning directly from a liquid to a gas. Therefore, tasting molecules cannot be dissolved or transported to the tongue’s receptors. Even within the pressurized cabin, the sense of flavor is significantly dulled.
Microgravity causes bodily fluids to shift upward, leading to facial swelling and nasal congestion, similar to a severe head cold. Since the perception of flavor is an integrated experience where the sense of smell (olfaction) accounts for up to 80% of what is perceived as taste, this congestion effectively blocks the primary pathway for flavor detection. The “smell of space” is purely an olfactory event involving volatile compounds that have off-gassed into the air.