The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects.
Two events in later decades increased the alarm: the increasing acceptance of the Alvarez hypothesis that an impact event resulted in the Cretaceous-Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter.