During the readout of Vela 6911, AFTEC personnel watched as a stylus drew a figure representing the variations in light intensity, as monitored by the two satellite bhangmeters. There was no data from a third optical sensor, whose mission was to provide the geographic origin of any noticeable flash of light, because it was out of commission. Nor would there be any reading from the satellite’s electromagnetic pulse sensors, which were no longer functioning. But what the technicians saw was sufficient cause for concern. The stylus drew a figure with a double hump, indicating a brief intense flash of light, a dramatic decline in intensity, and then a second, longer-lasting flash. Such double flashes had always been associated with nuclear detonations, where the fireball’s surface is rapidly overtaken by the expanding hydrodynamic shock wave, which acts as an optical shutter and hides the small but extremely hot and bright early fireball behind an opaque ionized shock front which is comparatively quite dim. The initial flash normally lasts only a millisecond and emits about only 1 percent of the total thermal energy, although it is the point of maximum intensity. It appeared that some nation or nations, in some part of the world covered by Vela 6911, had detonated a nuclear device in the atmosphere.
Richelson, Jeffrey T. Spying on the Bomb: American Nuclear Intelligence from Nazi Germany to Iran and North Korea. W.W. Norton and Company; New York. 2007. p. 285 (paperback)
Related:
The 1979 Vela Incident
What the U.S. Government Really Thought of Israel’s Apparent 1979 Nuclear Test – POLITICO Magazine
https://www.politico.com/magazine/story/2016/12/1979-vela-incident-nuclear-test-israel-south-africa-214507
There are indications (e.g. Vanunu’s testimony) that after the 1973 war Israel followed suit and significantly strengthened its nuclear commitment. It is believed that in the mid-late 1970s Israel took the path of its predecessors and built an arsenal that is qualitatively advanced and consists primarily of two-stage thermonuclear weapons. This may shed new light on one of the greatest mysteries in the history of proliferation: whether the “double flash” that was detected by an American Vela Hotel satellite at dawn September 22, 1979, over a remote swath of the Indian Ocean nearly halfway between South Africa and Antarctica, was produced by a low-yield nuclear test. Israel was the only country in 1979 that had a strong motivation to conduct such a test. If Israel was moving at that time toward an advanced two-stage arsenal, Israel had to test it somewhere. The technical complexity of two-stage weapons, in particular the working of the trigger (primary), requires a test. A nation cannot launch such an ambitious program without conducting a final test. If this story is true, then it was another indication of Israeli technological resolve.
Cohen, Avner. The Worst-Kept Secret: Israel’s Bargain with the Bomb. Columbia University Press, 2010. p. 82
Blast From the Past
Forty years ago, a U.S. satellite detected the telltale signs of a nuclear explosion. An analysis of the evidence today points to a clandestine nuclear test, a Carter administration cover-up, and only one country that was willing and able to carry it out: Israel.