What did Vela see?

On September 22, 1979 an aging satellite named Vela 6911 detected two very distinct flashes in the vicinity of the Indian or South Atlantic oceans that supposedly could be only one thing: a nuclear detonation. The Carter administration held an emergency meeting, other satellites were enlisted to see if they saw the detonation, which they did not, and utter pandemonium ensued for a short time as the US government scrambled to see who or what had set off a nuclear weapon that day. It was a small explosion, estimated at only three kilotons, and while the Soviets, Chinese, French and British are unlikely as the originators, the finger was tentatively pointed at Israel or South Africa for testing a weapon. Problem is, the whole thing made no sense.

The first problem was that no other satellites had detected the detonation, even though at least three were capable of it, if not more. It might have ended there, a very scary and potentially dangerous malfunction caused a false alarm. But it didn’t. Astronomers working at the Arecibo radio telescope in Puerto Rico detected an atmospheric shockwave that could have been linked to a nuclear explosion. The US government’s hydrophone network detected a very clear echo of a large explosion. But the one piece of foolproof evidence was never found: radiation. Dozens of flights were conducted to try to detect fallout, and none was ever found, though extremely low levels of a certain radioactive element might have been detected in Australia, some time later.

The blast remains a secret to this day, despite South Africa having given up all nuclear weaponry and testing decades ago. If they did it, they still aren’t saying, even though they have no motivation to keep quiet after all this time. Only if it involved Israel would it be worth keeping secret still.

It couldn’t have been a natural phenomena, meteorites do not make double flashes of light characteristic of a nuclear explosion. But given that all parties that may or may not have been involved state it wasn’t theirs, and the fact that radiation fallout was never confirmed, tends to point to something else.  ....

Its not likely we’ll ever know. The facts surrounding the event to this day do not add up, and any potential players in the nuclear arena remain tight-lipped. Most likely, Israel tested a nuclear weapon, but how it managed to not produce detected fallout is a mystery. Could it have been a UFO? Someone in government knows, but they aren’t going to say. - paranomala

Here is the wikipedia summary:

The flash was detected on 22 September 1979, at 00:53 GMT, by US Vela satellite 6911, which carried various sensors designed specifically to detect nuclear explosions. In addition to being able to detect gamma rays, x-rays and neutrons, the satellite also contained two bhangmeter sensors which were able to detect the dual light flashes associated with a nuclear explosion, specifically the initial brief, intense flash as well as the second longer flash that followed.^[2]

The satellite reported the characteristic double flash (a very fast and very bright flash, then a longer and less-bright one) of an atmospheric nuclear explosion of two to three kilotons, in the Indian Ocean between Bouvet Island (Norwegian dependency) and the Prince Edward Islands (South African dependencies) at 47°S 40°E / -47, 40Coordinates: 47°S 40°E / -47, 40. It should be noted that the explosion of some meteors as they are entering the atmosphere can produce energy measured from kilotons (Eastern Mediterranean Event) to megatons (Tunguska Event). However, the mechanism is different, and meteors do not produce the double flash characteristic of a nuclear detonation. - wiki

Wiki has a few other possibilities from US intelligence analysts:

... U.S. analysts also considered the possibility that it could have been a covert test by a known nuclear state. They concluded that there would be little motivation for the USSR or China in particular to test a nuclear weapon in such a way, unless they were attempting to make it look like South Africa or Israel were covertly testing weapons. As the flash could have occurred in the vicinity of the Kerguelen Islands, it is possible that France was testing a neutron bomb.^[10]