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Astronaut Scott Kelly smells space aboard the International Space Station. PBS

Space may be a giant airless vacuum, but astronauts swear that it has an odor.

Those who have sniffed the aroma liken it to burning metal, steak, and welding, among other peculiar olfactory memories.

"Space has its own unique smell," NASA astronaut Scott Kelly said in PBS' Year in Space documentary. "Whenever a vehicle docks, the smell of space when you open up the hatch is very distinct."

Astronauts obviously can't smell space when they're in it (they'd suffocate). But astronaut Don Pettit described what fellow crew members smelled like when they returned from space walks in a NASA blog post:

It was more pronounced on fabrics than on metal or plastic surfaces. It is hard to describe this smell; it is definitely not the olfactory equivalent to describing the palette sensations of some new food as "tastes like chicken." The best description I can come up with is metallic; a rather pleasant sweet metallic sensation. It reminded me of my college summers where I labored for many hours with an arc welding torch repairing heavy equipment for a small logging outfit. It reminded me of pleasant sweet smelling welding fumes. That is the smell of space.

Mmm, space. Shutterstock

Louis Allamandola, director of the Astrophysics and Astrochemistry Lab at NASA's Ames Research Center, told Popular Science that this smell is due to the polycyclic aromatic hydrocarbons floating around in space.

These hydrocarbons can be found in "tobacco smoke, car exhaust, and sometimes in heavily browned foods," here on Earth, according to chemist Kevin Boudreaux from Angelo State University.

NASA Science News has another explanation: Materials once exposed to the vacuum of space can react strongly with oxygen pumped back into a spacecraft. When you burn materials in air, the same process occurs — it's called oxidation — just at a much faster rate. So this might help explain the "burnt" character of the smell.

You can see crew on the International Space Station experience the smell when the Japanese cargo vehicle arrives in PBS' Year in Space documentary. It starts at 36:54:

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DNews also has a great compilation of what astronauts say space smelled like to them in this YouTube video:

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