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. It was launched from Johnston Atoll on July 9, 1962, and was the largest nuclear test conducted in outer space, and one of five conducted by the US in space.

A Thor rocket carrying a W49 thermonuclear warhead (designed at Los Alamos Scientific Laboratory) and a Mk. 2 reentry vehicle was launched from Johnston Atoll in the Pacific Ocean, about 900 miles (1,450 km) west-southwest of Hawaii. The explosion took place at an altitude of 250 miles (400 km), above a point 19 miles (31 km) southwest of Johnston Atoll. It had a yield of 1.4 Mt (5.9 PJ). The explosion was about 10° above the horizon as seen from Hawaii, at 11 pm Hawaii time.: 3