On July 9, 1962, a Thor rocket carrying a 1.4-megaton nuclear warhead lifted off from Johnston Atoll in the central Pacific Ocean. The missile climbed 248 miles into space before detonating in a brilliant flash visible across the entire Hawaiian Islands. The explosion created an artificial aurora that painted the night sky green and red, a ghostly light show born from thermonuclear fire. Scientists watching from the ground witnessed what they had theorized but never seen: a nuclear weapon detonating in the vacuum of space, vaporizing instruments and releasing a cascade of X-rays and gamma radiation across the upper atmosphere.

Starfish Prime was the largest of five high-altitude nuclear tests the United States conducted between 1958 and 1962, a program designed to understand how nuclear weapons would function beyond Earth's protective atmosphere. The Cold War context was stark: the Soviet Union had launched Sputnik four years earlier, and both superpowers were racing to master space. Military planners wanted to know whether nuclear weapons could reliably detonate in the cosmic environment, whether radiation effects would propagate differently than on Earth, and what the electromagnetic pulse might do to electronics and communications. The test was officially a joint venture of the Atomic Energy Commission and the Defense Atomic Support Agency, combining civilian and military expertise.

The consequences rippled far beyond the test site. The explosion created a belt of trapped radiation in Earth's magnetosphere that persisted for months, actually damaging satellites and disrupting radio communications across the Pacific. The artificial radiation belt, sometimes called the Starfish belt, remained measurable for years. The test shocked many scientists and ordinary citizens alike: the idea that humans could create artificial radiation belts orbiting Earth crystallized the awesome and terrifying power of nuclear technology. Within weeks, international pressure mounted, and the Soviet Union called for a nuclear test ban. By October 1963, the Partial Nuclear Test Ban Treaty was signed, banning atmospheric and space tests. Starfish Prime would be the last American nuclear detonation in space, a spectacular and sobering finale that helped push the nuclear powers toward restraint.