On 22 September 1979 at about 1 a.m. GMT, a US Vela satellite passing over the South Atlantic detected a double flash of light in the vicinity of Prince Edward Island. The satellite had been launched in 1969 in order to detect atmospheric nuclear tests. When a nuclear weapon explodes in the atmosphere, the heat of the fireball strips the electrons off the atoms and molecules of the surrounding air. For a fraction of a second the ionised air is opaque, until the blast blows it away. The resulting double flash is the signature of a nuclear explosion. At the time the Vela had successfully detected 41 such explosions. Guy Barasch of Los Alamos, the laboratory which ran the Vela programme, concluded that ‘naturally occurring signals would not be mistaken for that of a nuclear explosion’ and that
