First paragraph of the article:
In the wake of the 1963 Partial Test Ban Treaty, the United States launched a series of satellites under the name Vela (after a constellation in the southern hemisphere sometimes called “the sails” because of its configuration). The Vela satellites were designed to monitor compliance with the treaty by detecting clandestine nuclear tests either in space or in the atmosphere. The first such satellite was launched in 1963, the last in 1969. They operated by measuring X-rays, neutrons and gamma rays, and, in the case of the more advanced units, emissions of light using two photodiode sensors called bhangmeters (derived from the Indian word for cannabis). These satellites had a nominal life of seven years, after which the burden of detection was to be shifted to a new series of satellites under the Defense Support Program (DSP), equipped with infra-red detectors designed to pick up missile launches as well as nuclear tests. The Vela satellites, however, kept operating long past the end of their nominal design life and one of them, designated Vela 6911, detected an event on September 22, 1979, that has become a subject of intense interest ever since.