Grilled steak, gunpowder, hot metal: what does space really smell like

The smell of space: what astronauts actually breathe when they return from a spacewalk

It's a childhood question, one you rarely dare ask once adult, and yet it has a real answer, documented by NASA and told by those who have gone up there. What does space smell like? What do astronauts actually breathe when they come back from a walk in the void? The answer is surprising, a little unsettling, and deeply fascinating for anyone, like me, interested in everything the nose can perceive. An investigation into the perfume of the cosmos.

You cannot smell the vacuum

First fact to clear up. Space, the interstellar vacuum, cannot be smelled directly. And for a simple reason: there are almost no molecules to breathe, and above all, no astronaut will remove their helmet in the vacuum to sniff. Smelling requires air, molecules that reach the nose's receptors. In the vacuum, none of that exists.

And yet, astronauts are categorical, and they have said so for decades. Space has a smell. The moment they perceive it is always the same: upon returning from an extravehicular activity, once back in the airlock, when the pressure is restored and they take off their helmet. Then, something strikes them. A persistent odor, clinging to the suit, the gloves, the tools, everything that spent time outside. It is not the vacuum they smell, but what the vacuum deposited on their equipment.

Grilled steak, gunpowder and hot metal

The most striking thing is the consistency of the descriptions. American, European and Russian astronauts, who did not coordinate their accounts, have used remarkably similar vocabulary for fifty years. The smell of space, they say, evokes hot metal, ozone, gunpowder, and especially, repeatedly, grilled steak or welding fumes.

The most famous description is that of NASA astronaut Don Pettit. He describes it as a metallic sensation, sweet and rather pleasant, which reminded him of summers as a student spent handling a blowtorch to repair equipment, an odor of sweet welding fumes. European André Kuipers spoke of a distinct seared-steak smell, American Thomas Jones of an acrid ozone. Others have mentioned toasted almonds, and some, more surprisingly, a fruity note. Far from being repulsive, this odor is often described as curiously pleasant by those who have breathed it.

Where does this smell come from?

Science has several leads, and they are fascinating. The first involves atomic oxygen. In low orbit, the Sun's ultraviolet radiation breaks oxygen molecules in the upper atmosphere into isolated, highly reactive atoms that stick to the astronaut's suit during a spacewalk. When the airlock is repressurized and air returns, these atoms recombine, notably forming ozone, that molecule with a metallic and electrical smell that one sometimes senses after a thunderstorm or near an electrical device. That explains the metallic facet.

The second, more flavorful lead concerns polycyclic aromatic hydrocarbons, which chemists call PAHs. These are carbon molecules extremely widespread throughout the universe, produced by stars, present in comets and cosmic dust. These same carbon structures also form when we grill a food, during the famous Maillard reaction, the process that gives taste to seared meat or browned bread. Our brain, recognizing these molecules, spontaneously interprets the signal as a grilled smell. Space would smell like steak, in short, because it shares chemistry with our cooking. Analyses performed on suits after a spacewalk have in fact identified more than eighty volatile compounds, present in tiny amounts, on the order of a few parts per billion.