In an online question and answer session last summer, astronaut Mike Hopkins, who spent six months on the International Space Station, said that “Space has a smell … Once the pressure is equalized and the hatch is opened, you have this metallic ionization-type smell. It’s quite unique and distinct.” This one quote has found its way into many popular articles about Hopkins’ space trip, centered on answering readers’ unasked question: how can space, an airless vacuum, have a smell?
But first, a little background on olfaction, or the human sense of smell. Human olfaction is highly sensitive; in general, humans can detect most smells quite easily at 1 part per million — for example, humans can smell ethanol if there is one ethanol molecule in every million molecules of air (near their nose). Hydrocarbons are most often responsible for producing burning or metallic types of smells, and for these the threshold is even lower: polycyclic organic matter can be detected at 5-500 parts per billion, or one molecule of hydrocarbon in anywhere from 2 million to 2 billion atoms of air.
When the outer door of an airlock is open, the airlock is a vacuum, just like empty space. The astronaut drifts into the airlock, along with any gaseous or solid matter drifting in space nearby (in this case, hydrocarbon particles). When the outer airlock door closes, the airlock is filled with air, which mixes with the hydrocarbons on the astronaut’s suit. Because these particles can easily be smelled, we know that, as explained above, polycyclic hydrocarbons are present at a minimum concentration of one molecule for every two billion molecules of air. This means, in a volume of one liter of space, there may be as many as several million of these molecules. This may sound like a large number, but air on earth normally contains around a sextillion (1021) molecules of gas in one liter (about a quadrillion times more), so by comparison there are very few of these molecules in space.
Billionaire space tourist Anousheh Ansari experienced a similar smell to Mike Hopkins during her space trip in 2006. The astronauts onboard allowed her to smell the air in the main airlock, which she described as smelling like “burnt almond cookies.” However, she claims that the smell was due to cyanide fumes from the fuel and that she does not believe that space has a smell.
Popular Science writer Lizzie Schiffman wrote an article based on Hopkins’ experience of the smell, offering a different, intriguing theory. She quotes NASA Astrophysicist Louis Allamandola, who claims that the smell is due to combustion in stars. Normally, combustion produces carbon dioxide and water, if the fuel burns perfectly (i.e., if there is a perfect ratio of carbon, oxygen, and hydrogen present.) However, space contains much more carbon than oxygen, which means stars — by far the largest sources of combustion in the known galaxy — combust imperfectly, producing complex organic molecules, some of which are polycyclic aromatic hydrocarbons (PAHs).
The difficulty of quantifying smell or identifying dilute molecules in air makes this question difficult to resolve. It may seem a small problem, in comparison to the discoveries that the ISS and space exploration give us on a daily basis. However, the presence of PAH molecules has far reaching consequences. Common molecules, like cholesterol, have similar structures to the PAHs.
In fact, some theorize that PAHs are the precursors of many biological molecules. The presence of PAHs in space has even led some to infer the presence of extraterrestrial life. Scientist David McKay, in particular, has used the presence of PAHs on two recently analyzed asteroids to support his theory that the asteroids contain remnants of life. Though McKay has been heavily criticized, PAHs tell us that space contains a lot of complex chemistry that imitates or maybe even precedes the chemistry of life.
The problem is of somewhat more grounded, practical importance to NASA as well: Omega Ingredients, a flavoring company, was hired to chemically recreate the smell of space. Chemist Steve Pearce reported that the smell they synthesized was based on descriptions of “seared steak … hot metal … [and] arc welding on their motorbike”: other examples of incomplete combustion. The perfume has been used to prepare astronauts more realistically for life in space.
The presence of PAHs in space has produced a variety of reactions on Earth, from speculation about aliens to space perfumes. Such popular fascination with space should remind us that beyond all the practical and scientific advancements that can come from new discoveries, the next few decades will be a very exciting time for anyone with a deep curiosity about the human experience in space.