Let's just be clear - a comet doesn't really 'smell' of anything. Were you to expose your nostrils to its odour in deep space, you would instantly freeze, boil, explode and very shortly after die.
But the chemical make-up of a comet has until now been relatively mysterious, and so discovering more precisely what they are made of can be revealing.
And yes, revealing about their smell.
So here we are - as the billion-euro Rosetta and Philae crafts currently in orbit around Comet 67P, make their final approach to land on its surface safely for the first time - this is what a comet... smells like: