1962: From Johnston Island, a Thor IRBM carried a megaton-plus hydrogen bomb above 250 miles in altitude. The detonation marked the highest thermonuclear blast by a US bomb, and perhaps, the highest of any nation. It also provided the U.S. public with the first display of the major EMP effects of a high-alt nuclear burst.
Starfish Prime was a high-altitude nuclear test conducted by the United States, a joint effort of the Atomic Energy Commission (AEC) and the Defense Atomic Support Agency.