Courtesy National Nuclear Security Administration / Nevada Site Office

Operation Argus was a series of nuclear weapons tests and missile tests secretly conducted during August and September of 1958 over the South Atlantic Ocean by the United States's Defense Nuclear Agency, in conjunction with the Explorer 4 space mission. Operation Argus was conducted between the nuclear test series Operation Hardtack I and Operation Hardtack II. Contractors from Lockheed Aircraft Corporation as well as a few personnel and contractors from the U.S. Atomic Energy Commission were on hand as well. The time frame for Argus was substantially expedited due to the instability of the political environment [bans on atmospheric and exoatmospheric testing were forthcoming]. Consequently, the tests were conducted within a mere half year of conception (whereas "normal" testing took one to two years).

http://en.wikipedia.org/wiki/Operation_Argus