The mushroom cloud reached 17 km in 90 seconds.© U.S. DoE.

"It's a totally different scheme and it will change the course of history," mathematician Stanislaw Ulam told his wife. Fifty years ago today his scheme - the hydrogen, or thermonuclear bomb - was tested. Ulam was right on both counts.

Now, however, the experiment is seen as having been a step too far. The power of a runaway thermonuclear reaction shocked those who witnessed it.

The secret US test - codenamed Mike, for megaton - took place on remote Elugelab Island in the Pacific on 1 November 1952. In an instant, the experiment yielded more explosive power than all the bombs dropped by Allied forces during the Second World War - the equivalent of 10.4 million tons of TNT.

Mike produced a fireball 5 kilometres wide. A mushroom cloud sprouted 17 kilometres high within 90 seconds. After five minutes, the cloud had reached the top of the stratosphere (an altitude of 41 km) and had a stem 13 kilometres wide.

"It would have eliminated a metropolis," says physicist Phil Morrison, who worked on the wartime development of the first nuclear bomb in Los Alamos, New Mexico. Elugelab Island was vaporized; only an underwater crater remains.

Of the 11,650 personnel - 2,300 of them civilians - involved in project Mike, only 408 were later said to have received no radiation following the explosion.

Fusion of ideas

The H- bomb was a natural extension of the 15- and 20-kiloton atom bombs dropped on the Japanese cities of Hiroshima and Nagasaki in 1945. An A-bomb's power comes from heavy atoms of uranium or plutonium splitting into lighter ones, releasing vast amounts of energy in the process.

In a thermonuclear bomb, light hydrogen nuclei fuse to form helium. Because the fuel is much lighter, and the nuclear reactions more efficient, hydrogen bombs are vastly more powerful than atom bombs.

<mediar rid='m2'/>Morrison recalls a meeting at Los Alamos in 1946 to discuss whether to build Mike. When it became clear that development was to proceed, he and many others left the project. "One nuclear war was enough for me," he says.

The Russians, British, Chinese and French all followed with H-bomb tests, but the nuclear arms race soon turned to smaller, more sophisticated atomic bombs. Unlike Mike, which was 22 feet long, these could sit atop a rocket.

Morrison now advocates nuclear non-proliferation. Mike, and similar demonstrations of nuclear might, were for some, but not all leaders, the beginning of the realization "that unlimited power will not preserve you for ever", he says.