J. Allen Hynek got the call at 6:30 p.m., October 4, 1957.
The associate director of the Smithsonian Astrophysical Observatory, near Boston, hung up and told a colleague: "There's a Russian satellite up."
Sputnik's launch shocked the public: scientists were surprised only that the Russians did it first—earlier that year, researchers worldwide had agreed their countries would send up satellites to study the planet. In anticipation, observatory director Fred Whipple had summoned amateur astronomers—to be called Moonwatchers—to track any satellites. After Sputnik, 83 teams in 20 countries (above, in Pretoria, South Africa) rushed to their posts. By 1959, some 230 teams were tracking two dozen satellites; the teams' data led to an accurate measure of the Earth's size and shape.
Cameras replaced the Moonwatchers by 1975. Hynek, who died in 1986, went on to study UFOs. In 1972 he coined the phrase "close encounters of the third kind."
(Courtesy of the Smithsonian Institution Archives, image #96-960)