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Astronauts say that they have smelled unusual scents in space. However, that's not surprising due to the difference between the chemistry of Earth and space. 

Technically, one cannot smell anything in space as it is an airless vacuum. However, it's not entirely a vacuum, as it has all sorts of molecules, some of which have strong odours, according to Space.com.

Gunpowder, Ozone, Burnt Steak

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Some of the most prominent odours that astronauts have reported smelling in space are gunpowder, ozone and burnt steak. According to scientists, there can be two reasons behind it. First, while an astronaut is on a spacewalk, single oxygen atoms can cling to their spacesuit and enter the airlock. It repressurises molecular oxygen, which floods into the airlock and reacts with the single atoms of oxygen to form ozone, explaining the sour, metallic smell.  

Secondly, most interstellar carbon has PAHs (Polycyclic aromatic hydrocarbons) found in charred food like burned toast or barbecued meat. Astronauts could've picked these easily, as there are plenty in the solar system, and brought them inside the space station or the capsule. 

Rotten eggs and urine

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However, these are not the only strange odours reported in space. Europen Space Agency's Rosetta crossed comet 67P/Churyumov-Gerasimenko in 2014, detecting many different molecules in the comet's gaseous halo surrounding its nucleus. These molecules include hydrogen sulphide, which gives rotten eggs their unpleasant stink, and ammonia, whose smell reminds one of urine.

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Hydrogen cyanide, an almond-like smelling toxic molecule, sweet-smelling carbon disulphide and pickled aroma of formaldehyde were also present in the halo. Although one would shut their nostrils at this combination of smells, the odour of this comet was weak, as most of the comet's coma was water vapour and carbon dioxide. 

Gasoline and booze

An astronaut can't smell Saturn's moon Titan, as it has no oxygen and a minus 176.6 degrees Celsius temperature. However, scientists have found that Titan reeks of gasoline. 

NASA discovered that Titan's hazy atmosphere contains a chemical, identified by Cassini spacecraft, consisting of nitrogen, methane and benzene, hailing from a family of molecules called polycyclic aromatic nitrogen heterocycles (PANHs). Thus, the presence of benzene gives Titan its petroleum stench. 

Furthermore, Sagittarius B2, a massive interstellar molecular cloud of star-forming gas and dust, located less than 400 light-years from the centre of the Milky Way, contains large amounts of alcohol, including vinyl alcohol, methanol, and ethanol, which is used to make beer.

Astronomers also discovered ethyl formate, a sweet-smelling chemical found in raspberries, in Sagittarius B2 in 2009. 

(With inputs from agencies)