The smell of the Universe

Does it have a fragrance? Does it stink? The idea of smells in the cosmic vacuum might seem bizarre, but scientists and astronauts swear that space has a surprisingly varied “bouquet” of odours. This sensory investigation isn’t just a curiosity; analysing these cosmic aromas provides important details about the chemical composition of the Universe and offers clues as to where life beyond Earth might be found. One distinctive smell of space has been reported by many astronauts after a spacewalk. It’s a scent that, astronauts testify, clings to their suit, helmet, gloves and equipment. Some describe it as a metallic smell, reminiscent of welding in a mechanical workshop. Other astronauts have said the smell is similar to burnt meat, gunpowder or scorched electrical cables. Some have likened it to the aroma of nuts or the scent of motorcycle brake pads. The exact origin of this smell remains a mystery. One possible explanation is the oxidation of substances. The extremely thin atmosphere surrounding the space station, although almost a vacuum at that altitude, contains atomic oxygen. These single oxygen atoms can attach themselves to an astronaut’s spacesuit or tools. On returning inside the station, the single oxygen atoms combine with the O₂ present in the cabin, forming ozone (O₃), which produces an “ozone smell” as soon as it reacts. It’s the smell of electricity that one notices during a thunderstorm. Another possibility is that astronauts are detecting the scent of polycyclic aromatic hydrocarbon compounds. These molecules drift through the Universe as products of dying stars and contribute to the formation of new comets, planets and stars. On Earth, PAHs are present in fossil fuels (coal, crude oil, petrol) and often form during the incomplete combustion of organic materials, like a steak left too long on the barbecue. This explains the association with smells similar to solvents, mothballs, burning plastic or bitumen, as well as grilled meat.