On September 22, 1979, an American Vela satellite detected a mysterious double flash over the remote waters of the southern Atlantic and Indian Ocean region. The pattern resembled the distinctive optical signature the satellites had been designed to detect, an atmospheric nuclear explosion.

This episode examines why the detection immediately alarmed American intelligence, why a White House scientific panel questioned the nuclear explanation, and why later analysis and declassified records kept the possibility of a secret weapons test alive. More than four decades later, researchers still disagree about exactly what produced the signal.

Was the Vela Incident an unusual sensor event, or did a nation secretly detonate a nuclear weapon while the world was watching from orbit?