Outer space is usually thought of as an empty void, with the occasional planet or star mixed in. In reality even what appears to be "empty space" still has some particles floating around in it.
When an astronaut enters an airlock from outer space, the chamber is initially empty... or it appears empty. In reality it contains those same particles (space dust) that I mentioned. So when the hatch is closed, those particles are STILL inside the hatch, even as it is flooded with air.
The interesting thing is that space dust does have a smell.
Anousheh Ansari, a space tourist:
Quote: The time went by really slowly, but finally the moment arrived and they were ready to open the hatch. Mike and Misha called me closer and told me to take a good whiff because this would be the first time I would smell “SPACE.”
They said it is a very unique smell. As they pulled the hatch open on the Soyuz side, I smelled “SPACE.” It was strange… kind of like burned almond cookie. I said to them, “It smells like cooking” and they both looked at me like I was crazy and exclaimed:”Cooking!”
I said, “Yes… sort of like something is burning… I don’t know it is hard to explain…”
Jerry Linenger, astronaut
Quote: Flying into Mir, it smells sort of like dirty sweat socks in a guys’ locker room. Actual smell of space, though, that’s a very interesting question. When we would open a hatch, for example, that was exposed to the vacuum of space, uh, there’s always a double hatch, and so you open the one hatch, you now have the pure smell of space. And it’s a uh, tough — you know, any aroma is tough to describe, but it has a distinct smell, and it’s sort of a burned-out, uh, after-the-fire, the next-morning-in-your-fireplace sort of smell. And that’s the real smell of the vacuum of space.
http://jennifersaylor.wordpress.com/2006/09/30/what-does-outer-space-smell-like/
Don Pettit, ISS Science Officer
Quote: Few people have experienced traveling into space. Even fewer have experienced the smell of space. Now this sounds strange, that a vacuum could have a smell and that a human being could live to smell that smell. It seems about as improbable as listening to sounds in space, yet space has a definite smell. Being creatures of an atmosphere, we can only smell space indirectly. Sort of like the way a pit viper smells by waving its tongue in the air and thenpressing it to the roof of its mouth where sensors process the molecules that have been adsorbed onto the waggling appendage. I had the pleasure of operating the airlock for two of my crewmates while they went on several space walks. Each time, when I repressed the airlock, opened the hatch and welcomed two tired workers inside, a peculiar odor tickled my olfactory senses. At first I couldn't quite place it. It must have come from the air ducts that re-pressed the compartment. Then I noticed that this smell was on their suit, helmet, gloves, and tools. 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.
http://spaceflight.nasa.gov/station/crew/exp6/spacechronicles4.html
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