When Halley’s Comet visited in 1910, it was not well received. That’s because, with the help of the spectroscope, chemists and physicists had recently discovered cyanogen in comet tails. There was now a chemical explanation for their green color, and it seemed deeply ominous: Cyanide (one of the components of cyanogen) had recently surpassed arsenic in reputation as the deadliest of all poisons. The news that the Earth would brush through the comet’s outer tail on the night of May 18, 1910, was enough to rouse concern that the atmosphere would be poisoned.
Astronomers tried to reassure the public that a comet tail was so skimpy and rarefied, no one would even notice when it met the Earth’s far thicker atmosphere. Tabloids nonetheless proclaimed a coming cataclysm: It would be the death of all earthly life. The public’s reaction was an uncanny mixture of panic and celebration as the date approached. Tonics and “comet pills” that would supposedly protect against the cyanide were peddled widely. Many people took to storm cellars or sealed off their homes, shutting the flues in chimneys, stuffing cloth into door cracks, and trying not to breathe too deeply. Others went out to pubs and dances and partied the night away, having decided to spend their final hours in the merry company of friends and strangers. The apocalypse, of course, didn’t happen—although some believed that the comet was related to the outbreak of the Great War four years later.
— Adapted from Earth-Shattering: Violent Supernovas, Galactic Explosions, Biological Mayhem, Nuclear Meltdowns, and Other Hazards to Life in Our Universe, by Bob Berman, published by Little, Brown