A vacuum is a condition that is nearly devoid of molecules, and space is a molecular desert that makes the Empty Quarter of the Saudi Arabian peninsula seem like an oasis in comparison. But the space vacuum still has some molecules—residue from galactic processes, solar wind or atomic detritus spalled off from our atmosphere. And molecules, typically floating in the surrounding air, can be sensed via smell.

To talk about the smell of space makes no sense at all. Even if we had space-adapted noses, there is no air to transport the trace molecules. However, space does have a definite smell, and we can smell it in a roundabout way.

I have had the pleasure of operating our space station airlock for many crewmates while they went on spacewalks. Each time, when I repressurized the airlock, opened the hatch, and greeted my tired returning friends, a peculiar essence drifting about the newly repressurized chamber tickled my olfactory senses. I noticed that the smell was coming from the spacesuit fabric, the tools, and any other equipment that had been brought inside. It was more pronounced on fabrics than on metal or plastic surfaces. It most definitely did not come from the air lines that pressurized the chamber.

At first I couldn’t quite place the smell. The best description I can come up with is that it’s rather pleasantly metallic. It brought me back to my college summers, when I used an arc welding torch to repair heavy equipment for a small logging outfit. It reminded me of sweet-smelling welding fumes. To me this is the smell of space.

Reptiles have smell sensors located not within their nasal passage, but on the roof of their mouth. They smell by waving their moist tongue in the air, then pressing it against the roof of their mouth, thus indirectly transferring molecules from the air to the olfactory sensors. It occurred to me that I was smelling the essence of space through an indirect transfer, in a manner not unlike that of our lizard friends.