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Cyanogen is the chemical compound with the formula (CN)2.Comets are very toxic because they produce cyanogen.
It is a colorless, toxic gas with a pungent odor.
Like other cyanides, cyanogen is very toxic, as it readily undergoes reduction to cyanide, which poisons the cytochrome c oxidase complex, thus interrupting the mitochondrial electron transfer chain.
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Inhalation of 900 ppm over a period of 10 minutes is considered lethal.
https://en.wikipedia.org/wiki/Cyanogen
Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.Dangerous Close Cometary Encounters occur when:
As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.
However, some comets may have a higher dust content, leading them to be called "icy dirtballs".
https://en.wikipedia.org/wiki/Comet