SR-71 Blackbird over South African Sky

In August 1977, a US Lockheed SR-71 Blackbird flew a high-speed reconnaissance mission over South Africa to investigate a suspected secret nuclear test site at Vastrap in the Kalahari Desert. The mission followed an urgent Soviet intelligence warning to Washington that South Africa could be preparing for an underground nuclear test, bringing Cold War rivals together over a common concern.

In the mid-afternoon on Saturday, Aug. 6, the acting chief of the Soviet embassy, Vladilen M. Vasev, called at the White House with an urgent personal message from Leonid I. Brezhnev to Jimmy Carter. South Africa, according to Soviet intelligence, was secretly preparing to detonate an atomic explosion in its Kalahari Desert. Brezhnev asked for Carter's help to stop it.

U.S. reconnaissance satellite with high-resolution cameras was also urgently programmed for low-orbit passes over the area in question. US intelligence confirmed the existence of the nuclear test site with an overflight of a Lockheed SR-71 spy plane.

The Flight operated from Diego Garcia Air Base and is about 5000km from South Africa; supported by an air-to-air refueling tanker, this flight would have been done with no difficulty.

About Vastrap Testing / Bombing Range

Vastrap on Google Maps 27°50'05.0"S 21°37'50.0"E ( https://goo.gl/maps/WhW5xQioCP7vtLBo8 ) 

Vastrap (Stand firm) is a small military airfield situated in the Kalahari Desert northeast of Upington inside a 700 square kilometre weapons test range of the same name[1] belonging to the South African National Defence Force. It was constructed to allow the SAAF to practice tactical bombing operations, and for aircraft to service the SADF's defunct underground nuclear weapons test site

Atomic testing

The area was selected for nuclear weapons testing due to its remoteness, low population density, stable geological formations and lack of underground rivers.

Two underground shafts 385 metres and 216 metres in depth and 1 metre in diameter were drilled from 1975-1977. Neither was ever used to perform a detonation, although instrumented tests were performed. The shafts were sealed with sand and concrete under the supervision of the International Atomic Energy Agency in July 1993.

Detection

The site was first detected by the Soviet spy satellite, Cosmos 922, when it photographed the area from 21–25 July 1977, and reported to the Americans on 6 August, who in turn confirmed its existence with an overflight of the Lockheed SR-71 spy plane. The US then applied pressure on the South Africans for it to be closed; France also insisted on closure, threatening cancellation of the Koeberg nuclear power station contract.

David Albright reported that South African officials believed that an attempt to re-use the site in the late 1980s was detected by Western or Soviet intelligence agencies, and that this discovery influenced the Tripartite Accord. In an effort to mask activities, a shed was built over one of the shafts, and the water that was pumped out in preparation for a test was hauled away.

About the SR-71 Blackbird

Few aircraft have captured the imagination quite like the Lockheed SR-71 Blackbird. Built for speed, altitude, and intelligence gathering, the Blackbird could cruise above 80,000 feet at more than Mach 3, allowing it to cover enormous distances at speeds few aircraft could match. During the Cold War, its unique combination of speed, altitude and advanced reconnaissance systems made the SR-71 one of the most remarkable aircraft ever to take to the skies.

About Diego Garcia Air base 

Diego Garcia is a remote, heavily militarised atoll in the central Indian Ocean and part of the British Indian Ocean Territory. After the local Chagossian population was forcibly removed between 1968 and 1973, the United States and United Kingdom established a major military base on the island.

The base features a 3,659-metre (12,000-foot) runway, one of the longest military runways in the region, capable of supporting large long-range aircraft. Its strategic location and substantial runway made Diego Garcia an important base for US military and reconnaissance operations during the Cold War, including missions involving the SR-71 Blackbird.

Sources:

https://en.wikipedia.org/wiki/Vastrap

https://www.washingtonpost.com/archive/politics/1977/08/28/how-west-soviets-acted-to-defuse-s-african-a-test/eada5bcc-7727-4c32-b8e2-f21d55a9d4c3/

https://en.wikipedia.org/wiki/South_Africa_and_weapons_of_mass_destruction

https://en.wikipedia.org/wiki/Diego_Garcia