Background

Operation Starfish prime was enabled because the U.S. wanted to see what would happen if a nuclear bomb exploded in space. During the Cold War, the U.S. was rivals with the Soviets and tested bigger and stronger Bombs. The bomb was to be launched off of Johnston atoll neighbor to Bikini atoll where the U.S. tested other nuclear bombs. People in the Pacific held Bomb watching parties ready to see a Nuclear Bomb explode. The bomb was a 1.4-megaton Nuclear bomb. 500 times more powerful than the one that fell on Hiroshima.

Explosion and Aftermath

The test was there to help the U.S. understand how to detect nuclear detonations in space and build a system, later called Vela Hotel, to monitor tests by other countries. Other potent sources of radiation in outer space. There is a very small chance,  a solar flare at just the right moment could hit the planet with a similar amount of radiation.