David Lowry asks (Letters, 11 August) whether Israel has carried out a nuclear weapon test. Although there is no conclusive answer, he is not correct to say that there is no public information. Since 2004, the CIA report on the double flash detected by a US Vela satellite on 22 September 1979, originating in the south Atlantic, has been declassified, albeit heavily redacted. The purpose of the Vela satellites was to detect atmospheric nuclear tests, and the double flash is characteristic of nuclear explosions.

According to the report: "In September 1979 some special security measures were put into effect which indicate that certain elements of the South African navy were exercising or on alert. The harbour and naval base at Simonstown were declared on 23 August to be off limits for the period 17-23 September … Also, the Saldanha naval facility was suddenly placed on alert for the period 21-23 September."

A clandestine nuclear test by Israel would have been useful. According to the report: "The Israelis might have conceivably foreseen needs for more advanced weapons, such as low-yield nuclear weapons that could be used on the battlefield. Or they might have considered desirable a small tactical nuclear warhead for Israel's short-range Lance surface-to-surface missiles. Israeli strategists might even have been interested in developing the fission trigger for a thermonuclear weapon. If they were to have developed reliable nuclear devices for any of these weapons without access to tested designs, moreover, Israeli nuclear weapons designers would probably have wanted to test prototypes."

Taken with your coverage of Israeli-South African military collaboration during the 1970s (24 May), the evidence for an Israeli test is strong, if not conclusive.

Emeritus professor Norman Dombey

University of Sussex