Starfish Prime was a high-altitude nuclear test conducted by the United States on July 9, 1962, a joint effort of the Atomic Energy Commission (AEC) and the Defense Atomic Support Agency (which became the Defense Nuclear Agency in 1971).
Launched via a Thor rocket and carrying a W49 thermonuclear warhead (manufactured by Los Alamos Scientific Laboratory) and a Mk. 2 reentry vehicle, the explosion took place 250 miles (400 km) above a point 19 miles (31 km) southwest of Johnston Island in the Pacific Ocean. It was one of five tests conducted by the USA in outer space as defined by the Fédération Aéronautique Internationale (FAI). It produced a yield equivalent to 1.4 megatonnes of TNT.
It's a bird! It's a plane! It's... something far more frightening. 'It' was code-named 'Starfish Prime,' a test conducted in the summer of 1962, when a 1.45-megaton nuclear weapon was launched 250 miles into space above the Pacific Ocean and then detonated.
The resulting nuclear explosion in space, reports Discover Magazine, was a pulse of energy so strong it affected electrical circuits, power lines, and streetlights in Hawaii, nearly 600 miles away.
The purpose of this test was, basically, just to see what would happen, notes HowStuffWorks. Specifically, researchers wanted examine how the explosion woul daffect the Van Allen Radiation Belts -- bands of high-energy protons and electrons that follow the Earth's natural magnetic field -- to see if they could be manipulated for national defense purposes.
Scientists learned plenty from the experiment, and bystanders from Hawaii to New Zealand were treated to a view of "rainbow skies," but according to NPR, this "greatest man-made light show" actually resulted from radioactive particles coming into contact with oxygen and nitrogen in Earth's atmosphere. And the particles took years to eventually return to normal levels.
Now, 50 years after Starfish Prime, a handful of nuclear non-proliferation treaties are in place to prevent similar experiments from ever happening again.