The United States conducted the Starfish Prime high-altitude nuclear test, the largest nuclear explosion in outer space.

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.

High-altitude nuclear explosions are the result of nuclear weapons testing within the upper layers of the Earth's atmosphere and in outer space. Several such tests were performed at high altitudes by the United States and the Soviet Union between 1958 and 1962.

A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, though to date all fusion-based weapons have used a fission device to initiate fusion, and a pure fusion weapon remains a hypothetical device. Nuclear explosions are used in nuclear weapons and nuclear testing.