STAN

MOSCOW – It was just past midnight and Stanislav Petrov had been called in on his day off to cover for one of his colleagues who was sick. He settled into the commander's chair inside the secret bunker at Serpukhov-15, the installation where the Soviet Union monitored its early-warning satellites over the United States. 

Then the alarms went off. On the panel in front him was a red pulsating button. One word flashed: "Start." 

It was Sept. 26, 1983, and Petrov was playing a principal role in one of the most harrowing incidents of the nuclear age, a false alarm signalling a U.S. missile attack. 

Although virtually unknown to the West at the time, the false alarm at the closed military facility south of Moscow came during one of the most tense periods of the Cold War. And the episode resonates today because Russia's early-warning system has fewer than half the satellites it did back then, raising the spectre of more such dangerous incidents. 

As Petrov described it in an interview, one of the Soviet satellites sent a signal to the bunker that a nuclear missile attack was underway. The warning system's computer, weighing the signal against static, concluded that a missile had been launched from a base in the United States. 

The responsibility fell to Petrov, then a 44-year-old lieutenant colonel, to make a decision: Was it for real?