First detection of cosmic gamma ray bursts

The Vela satellites were developed by the USA to monitor Earth for nuclear tests that contravened the 1963 Partial Test Ban Treaty. On 2 July 1967, Velas 3 and 4 detected a flash of gamma rays that did not match the signature of any known nuclear test. Although the Velas were not designed to be astronomical instruments, they had accidentally discovered gamma ray bursts, which we now know are associated with supernovae in distant galaxies. This discovery remained classified until 1973.