A mysterious event detected by U.S. satellites in 1979 was the detonation of an atomic bomb. The incident correlates with an equally mysterious rise in radioactivity in sheep in nearby Australia, confirming that a nuclear test took place. Experts believe that Israel and South Africa conducted a secret joint test of a tactical nuclear weapon in the Indian Ocean, hoping it would escape detection.

On September 22, 1979, a U.S. Vela satellite, part of the globe-scanning U.S. Nuclear Detonation Detection System (NDS), detected the characteristic “double flash” of a nuclear weapon. Nuclear bombs produce two distinct flashes: the superheating of air caused by the detonation and the shockwave itself, which emits light. The second flash is barely noticeable to human observers, but optical sensors known as bhangmeters can differentiate between single flash events, such as conventional explosions, and the double flash of a nuclear explosion.

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The Vela satellite passed the data to NDS ground stations manned by the U.S. Air Force and then quickly passed up the chain of command to then-President Jimmy Carter. The explosion was in a curious spot, between Antarctica and the southern tip of South America, ruling it out as a hostile act. It was also located far from where any of the declared nuclear powers tested their weapons. No one claimed responsibility for the event.

There were many theories, from a malfunctioning satellite to a meteorite landing in the Indian Ocean. Some, including President Carter himself, believed the explosion was the result of a joint Israeli-South African nuclear test, meant to be concealed from Vela by cloud cover that parted just moments before the bomb went off.

Now, data from an unlikely source corroborates the nuclear theory: Australian sheep. Nuclear explosions release radioactive iodine into the atmosphere, which are absorbed by the thyroid gland. According to the Bulletin of Atomic Scientists:

“Using data first gathered by Distinguished Professor Lester Van Middlesworth of the University of Tennessee on radioactivity found in the thyroids of sheep in Australia within the time period following the “flash,” plus meteorological data from the time and some radionuclide and hydro-acoustic data released by the US government, Wright and De Geer have produced an analysis of the Vela event that removes virtually all doubt that the “flash” was a nuclear explosion."

The explosion was reportedly a 2-3 kiloton nuclear bomb, compared to the 15-16 kilotons of the Hiroshima bomb. It was likely an artillery shell designed for battlefield use.

Whose bomb was it? None of the known nuclear powers fessed up to the event, nor would they have any reason to test a bomb in such a remote area. While the confirmation that a nuclear explosion did take place is fairly certain, the Israeli-South African theory is more an exercise in deductive reasoning. The proximity to South Africa, and Pretoria’s military ties to Israel made a South African-Israeli test a leading theory.

Israeli Army 175-millimeter howitzer firing in the Golan Heights, 1973. Getty Images

Why the test? In 1973, just six years earlier, Israel had barely stopped a surprise attack by Arab air and ground forces that threatened to overrun the country. Israel’s nuclear arsenal, which it denies having to this day, was aircraft delivered, and the Israeli Air Force lost more than one hundred planes during the war to enemy fire. An aircraft-delivered nuke might never reach its destination, and when you need a nuke, well, you really, really need a nuke.

An artillery shell on the other hand can be fired from behind friendly lines and can’t be stopped. Like Israel, South Africa was also ringed by hostile countries and its policy of racial segregation made defense cooperation with other countries unlikely.

Whoever set off an atomic bomb in the middle of nowhere in September 1979 went to great lengths to conceal it—only to be found out by flocks of sheep 40 years later.

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