While no astronaut has been unwise enough to unclasp and remove their helmet in the vacuum of space (which is very bad for longevity), astronauts have reported a smell upon returning from space. Specifically, many astronauts report different smells in the airlock after participating in spacewalks.
"The best description I can come up with is metallic; a rather pleasant sweet metallic sensation," wrote astronaut Don Pettit, according to Space.com. "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." Pettit participated in several EVAs (extravehicular activities or spacewalks) during his NASA career, accumulating repeated experience with the smell.
Other astronauts have described it in similar yet varying ways: "burning metal," "a distinct odor of ozone, an acrid smell," "walnuts and brake pads," "gunpowder" and even "burnt almond cookie." Much like all wine connoisseurs smell something a bit different in the bottle, astronaut reports differ slightly in their "smelling notes" but have one thing in common: a burnt smell.
What might explain why space smells burnt? There are two possible explanations.