You can't smell your sniff

In a July 2008 interview produced by NASA, Canadian astronaut Julie Payette was asked to discuss the most surprising thing about her first spaceflight.

She said, “On my first flight we had a space walk, and...when they came back in, I was the person responsible to repressurize the airlock and then open the hatch. And when I opened the hatch, I thought wow, I'm smelling this kind of cold, aseptic smell, and I thought wow, this is the smell of space. Because the airlock had been exposed to the vacuum of space for several hours, and when we opened the hatch from inside, well, this cold smell, this smell of nothing really, because there's nothing left in there, was the smell of space....”

“Not so fast, Julie,” says Dr. Puff Pants, senior research fellow at the Man-Bunny Center for Astronomy and Cosmology. “I'm sure you smell something, but it ain't space.”

And apparently, among NASA flight crews this perception is fairly widespread. This year, in a March 29 interview with SPACE.com, astronaut Thomas Jones said, “When you repressurize the airlock and get out of your suit, there is this distinct odor of ozone, a faint acrid smell...I like to think of it as getting a whiff of a vacuum!”

“Impossible,” says Dr. Pants. “Vacuums cannot be whiffed.”

So what's going on?

“Well, whatever they’re smelling is the result of materials being exposed to the environment of space. Substances that astronauts are bringing into space are reacting in that environment; producing some substance or set of substances that have a particular odor.”

And according to Dr. Poofy Too, associate professor of otolaryngology at the Man-Bunny Medical Center, “In order for the brain to generate the perception of smell, there has to be present what are called ‘odorant molecules’, which bind to receptors on the nasal epithelium lining the inside of your nose. This sets off an electrochemical cascade, which the brain ultimately interprets as the subjective experience of odor.”

And according to Dr. Pants, it would be impossible for odorant molecules to exist in a vacuum, because there are no molecules in a vacuum.

“Otherwise,” Dr. Pants says, “it wouldn't be a vacuum.”

But in his interview, Dr. Jones offered part of an explanation that the rabbits we spoke to said could be the first step in solving this mystery. According to SPACE.com, Dr. Jones “thinks the odor could stem from atomic oxygen that clings to spacesuit fabric.”

“That's quite likely to be part of the story,” Dr. Pants says.

Inside the shuttle, Dr. Pants explains, atomic oxygen is nearly non-existent, and astronauts are breathing air rich in molecular oxygen, O2, the same substance that keeps both Man and Bunny alive here on Earth. This gas would certainly permeate, and potentially remain trapped in, tiny microscopic spaces inside the fabric of a space suit. By contrast, atomic oxygen, O, is extremely unstable, and so is very rare under atmospheric conditions–those here on Earth, and within the carefully designed artificial atmosphere of the Space Shuttle.

“But,” says Dr. Pants, “once an astronaut steps outside the spacecraft, he or she is exposed to radiation, really tremendous solar radiation.” Much of this is high-energy radiation, what scientists term “ionizing radiation”, and it will split the molecular oxygen, generating large quantities of atomic oxygen.

“So at this point,” Dr. Pants explains, “once the space-walker re-enters the airlock, and the airlock is pressurized with air from inside the shuttle, you have a whole herd of lonely oxygen atoms, and they quickly combine with normal molecular oxygen.” This process is highly favored, Dr. Pants says, and generally produces high concentrations of “tri-atomic oxygen.”

And this would explain why astronauts consistently describe this “smell of space” as similar to the smell of ozone, because tri-atomic oxygen, O3, is precisely that: ozone.

“And yes,” Dr. Too says. “Ozone has a very distinctive odor.”

There still exists a likelihood that there are substances inherent to materials used in space–those generated by solar radiation reacting with compounds other than oxygen, or even from chemicals used in pre-flight cleaning–being forced by the vacuum of space from their microscopic hiding places, and stirred into this molecular cocktail inhaled by our astronauts. But one thing is certain: reports of bizarre odors in space are the result of the astronaut's equipment and activities in space, and are not at all the “smell of space.”

“The astronauts smelt it,” Dr. Pants says, “and they most definitely dealt it.”