In 1963, to lessen the rapid proliferation of nuclear weapons, the United States and the Soviet Union signed a treaty prohibiting testing such weapons in the atmosphere and in space. To assure that there were no violations of this treaty, in the late 1960s the United States deployed a series of spacecraft, the Vela satellites, as monitors. Several spacecraft were positioned so that all of near- Earth space was always viewed by at least one set of detectors.
A nuclear explosion in space produces an intense prompt burst of X-rays, neutrons and γ rays. This signal is bright enough, and with a distinctive enough time signature, that there should be no confusion with natural events. Also, as in a supernova explosion, debris is ejected in all directions at high velocity. The primary detectors on the Vela spacecraft were designed to detect and recognise the prompt signals. Still, a clandestine test might be hidden from the promptburst detectors by detonating the device behind the Moon. The debris, however, which contains highly radioactive, rapidly decaying fission fragments, would be thrown from the vicinity of the explosion and free of the Moon's shadow. Gamma-ray detectors were therefore included which were capable of detecting radiation from nuclear debris.
In 1972, after 3 years of operation, the Los Alamos group responsible for the various detectors realised that the system was detecting bursts of γ rays that were real events, not some strange combination of background noise.