When asked to think up a bad smell, your mind will likely drift toward the putrid stench of rotten eggs. That tell-tail smell is caused by hydrogen sulfide and is produced by bacteria feasting on rotting organic matter.
Meet Rosetta's Beautiful Lumpy Comet: Photos
It also turns out that, to the surprise of scientists studying data from the European Rosetta mission, Comet 67P/Churyumov-Gerasimenko is venting hydrogen sulfide. But that's not all. Churyumov-Gerasimenko has gone all-out on the sniff-o-meter - it's also venting ammonia, methane, hydrogen cyanide and formaldehyde, chemicals that have smells associated with urine, alcohol, bitter almonds and vinegar.
In short, if we were to return a sample of this nasty cometary cocktail to Earth, it would not only bring a tear to the eye if you were unfortunate enough to sniff it, you'd also likely gag to death.
This delightful array of chemicals in the comet's coma (the comet's ‘atmosphere' surrounding the nucleus) were detected by Rosetta's mass spectrometer, the Rosina-DFMS instrument.