Comet Halley    
Previous appearances of Halley's Comet have, at times, terrified the public. Its appearance in 1909 and 1910 was among them, after it was announced that Earth would pass through the comet's tail. To make matters worse, scientists announced that cyanogen had been found in the comet's tail. Although astronomers tried to convince the public that the poisonous cyanogen would not penetrate Earth atmosphere it didn't stop a run on gas masks and anti-cyanogen pills, or survivalists from preparing for the end of the world.