Starfish Prime was a high-altitude nuclear test conducted by the US on 9 July 1962, at Johnston Atoll in the Pacific, about 1400 km (870 mi) southwest of the Hawaiian islands. Starfish Prime was conducted as a part of Operation Dominic. The Johnston Atoll was chosen as a test site because of its relative isolation.
Operation Dominic was the largest series of nuclear tests ever conducted by the United States, with a total of 105 bombs exploded at various altitudes. At the time, Cold War paranoia was at its highest, and the United States was quickly rushing to develop missiles capable of striking into the heart of Russia from Allied launch pads in the UK. Starfish Prime was just one of these tests.
Launched on board a Thor rocket (an Intermediate Range Ballistic Missile) was a W49 thermonuclear warhead with a design yield of 1.44 megatons. The warhead was exploded 400 km (250 miles) above the island, well above the international definition for outer space, which is at only 100 km above the surface. The electromagnetic pulse generated knocked out numerous electronics in nearby Hawaii, setting off burglar alarms, breaking three hundred street lights, wreaking havoc with radios and TV sets, and fusing power lines. The explosion created an artificial aurora that persisted for seven minutes and could be seen from as far away as Samoa, 3200 km (2000 mi) distant.
Because of the lack of air at the altitude where Starfish Prime was staged, there was no resulting fireball, though numerous other malevolent effects occurred, some planned, some not. Three weeks prior, at another test associated with Operation Dominic, Bluegill Prime, the Thor booster failed and the test had to be aborted, raining radioactive metal down on Johnston Atoll.
During this test, Starfish Prime was successful, but it released so much energy into the Earth's radiation belts that the delicate electronics on seven satellites was ruined, a total of 1/3 of all satellites in low earth orbit at the time. Lacking any way of measuring the radiation in the Earth's inner radiation belts, this led to the launch of more delicate measurement satellites. It is said that high-energy electrons from Starfish Prime persisted in the atmosphere for as long as five years.