In 1962, the US detonated a 1.4 megaton nuclear bomb 250 miles above the Pacific in a test called Starfish Prime, and the electromagnetic pulse knocked out streetlights, burglar alarms, and a telephone company microwave link in Honolulu nearly 900 miles away, on an island most engineers had assumed was safely out of range (opens in new tab)

On July 9, 1962, a Thor rocket carrying a W49 thermonuclear warhead detonated roughly 250 miles above Johnston Atoll in the central Pacific. The yield was 1.4 megatons. The test was called Starfish Prime, and within seconds its electromagnetic pulse reached Hawaii, blowing fuses in streetlights, setting off burglar alarms, and damaging a telephone company […]