Avner Cohen and William Burr write: On the dawn of September 22, 1979, a U.S. Vela satellite used to detect nuclear explosions spotted a double flash somewhere in the South Atlantic. Normally characteristic of nuclear detonations, the double flash quickly set off a panic within the U.S. national security apparatus: Had a nation really detonated a nuclear weapon, possibly in violation of the Limited Test Ban Treaty? And if so, who had done it? Or was it simply a technical malfunction, or even a reflection of a natural cosmic phenomenon?

Over the months that followed, U.S. scientists and intelligence experts launched a series of investigations to determine what happened, but the results were never conclusive. While White House science advisers officially maintained that the double flash was a result of a technical malfunction, others in the government believed that it was a nuclear test, possibly by South Africa or more likely Israel. Today, U.S. government officials appear more interested in preserving secrecy about the incident than shedding light on what it might have known at the time.

What the Vela 6911 satellite actually detected on September 22 is one of the biggest unsolved mysteries of the nuclear age, and probably will remain so as long as significant intelligence reports on the Vela flash remain classified. But thanks to a new trove of declassified documents at the National Archives (from the files of Ambassador Gerard C. Smith, President Jimmy Carter’s special representative for non-proliferation matters) and a few items from the Carter Presidential Library — all published today for the first time by the National Security Archive — we are able to discover more about what really happened that morning, how the Carter administration reacted and why many in the intelligence community never accepted the official White House narrative. [Continue reading…]