Colonel Petrov's good judgment
by Geoffrey Forden, Pavel Podvig and Theodore A. PostolThis paper was published in IEEE Spectrum, March 2000, V37, Number 3.
Lieutenant Colonel Stanislav Petrov was in command of an early-warning bunker south of Moscow shortly after midnight on 26 September 1983. This was a period of high political tensions between the Soviet Union and the West. The United States was planning openly to deploy in Europe long-range Pershing II ballistic missiles and ground-launched cruise missiles in response to deployments of Soviet intermediate-range SS-20 ballistic missiles.
A particular concern of the moment was that the United States and NATO were organizing a military exercise for later that fall--code-named Able Archer 83--that centered on using tactical nuclear weapons in Europe. Soviet leaders worried that Able Archer was a cover for an actual invasion.
The Soviet Union had constructed a series of ground-based radars ringing the country to detect incoming warheads as they rose above the limb of the earth. The radars' warning might give Soviet leaders as much as 15 minutes to decide on their course of action. In an effort to extend this period of warning to perhaps 30 minutes, the Soviets had just that year incorporated a new space-based early-warning system into their strategic command and control.
Petrov's center was charged with validating any warnings of a surprise nuclear attack that those satellites might generate. Once validated, those warnings would be forwarded to the main Soviet early-warning command center.
Nine Oko satellites had been placed in highly elliptical orbits so they could take turns scanning the skies above U.S. missile fields. On that night in September, Cosmos 1382, whose turn it was to watch the United States, was just then reaching the highest point of its orbit, almost 32 000 km above the earth's surface. Directly below Cosmos 1382, northern Europe was engulfed in night. To a casual observer looking down from space, the snow and ice fields of the Arctic reached out toward the United States, which would have been unrecognizable, since the curve of the earth compressed it into a thin line. The line separating night and day stretched across the North Pole.
If Colonel Petrov had drawn a line from Cosmos 1382 to Malmstrom Air Force Base in Montana--the main U.S. intercontinental ballistic missile (ICBM) field--and continued it out into space, he would have intersected the sun right at the moment the klaxon went off in his control room indicating the beginning of "World War III."
What was really going on? The guess now is that there were scattered high-altitude clouds above Malmstrom on that day. Such clouds could easily have reflected sunlight into the infrared sensors aboard Cosmos 1382, imitating the bright light given off by the hot gases in a missile's plume. If there were indeed high clouds, the absorbing effects of the atmosphere would have been greatly reduced. Usually infrared light from the sun is reflected off clouds diffusely, which spreads the intensity in every direction.
Near the autumnal equinox, however, the sun could line up with the U.S. missile fields and the satellite to give specular reflections. The clouds would then act as mirrors, reflecting many times more light than they would if the reflection were diffuse. The Soviets had wisely chosen a grazing viewing angle for Cosmos 1382 to increase the effect of atmospheric absorption and remove the unwanted naturally occurring sun background. But instead, this unexpected effect led to a vast increase in the sun-background signal, triggering the alarm in Petrov's center.
Consistent with these speculations based on our analysis, the next year the Soviet Union apparently started dedicating one early-warning satellite in geostationary orbit (positioned over the eastern Atlantic Ocean) to act as a backup to these satellites. This new effort guaranteed that U.S. ICBM fields could always be seen from two very different viewing angles, at least one of which would certainly be free of reflections at any given instant.
Other modifications have since been made to the hardware and software of the early-warning system to filter out these rare events. And, in the process, the Russians have gained more than 15 years' experience and an enviable database of natural phenomena of use in designing future early-warning networks.
When Colonel Petrov had to make his decision, though, none of these improvements had been made. In his control room, he began to receive warnings that U.S. ICBMs were being launched. First one launch, then two, and then others, as different clouds started to reflect light. Eventually five launches were reported. It is possible that these warnings were automatically sent on to the Soviet General Staff, since Petrov's later account suggests that they were calling him, asking for further information.
It is unclear what launch authority arrangements were in place at that time, but it appears that Petrov was under pressure to take some form of action in response to the alert. His understanding was that the United States would only start a war with a massive nuclear attack. Colonel Petrov decided that nobody starts a war with just five missiles.
And so, despite the political tensions at the time, and what appeared to be a limited U.S. nuclear launch, Petrov took no action. He was later investigated for his conduct during the incident. It is Petrov's belief that the investigators tried to make him a scapegoat for the false alarm. Rather than admit that the hardware had been rushed into service and had flaws, the investigators tried to blame it on human error.
This unexpected and all but disastrous incident should add yet another note of caution about the enormously complex and unpredictable warning systems that continue to be operated by both the United States and Russia. Petrov nowadays lives outside Moscow on a small military pension. The Cold War had many unsung heroes on both sides. Surely Colonel Petrov is one of them.
See the previous page: False alarm, nuclear danger
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