atomic bomb

A test explosion of an atomic bomb in the 1950s. The characteristic mushroom cloud could rise to 10,700 m/35,000 feet if the bomb was detonated at ground level. To maintain secrecy, tests were carried out in remote areas; safety precautions were often remarkably lax, in the light of what is now known about the dangers of radiation.

The Atomic Bomb Dome, in Hiroshima. This building is all that remains of central Hiroshima after the atomic bomb that was dropped on it on 6 August 1945. It survived the blast because it was almost directly beneath the hypocentre of the explosion. It has been intentionally preserved as a reminder of the effects of nuclear warfare, and is a designated UNESCO World Heritage Site.

Bomb deriving its explosive force from nuclear fission (see nuclear energy) as a result of a neutron chain reaction, developed in the 1940s in the USA into a usable weapon.

Research began in the UK in 1940 and was transferred to the USA after its entry into World War II the following year. Known as the Manhattan Project, the work was carried out under the direction of the US physicist J Robert Oppenheimer at Los Alamos, New Mexico.

After one test explosion, two atomic bombs were dropped on the Japanese cities of Hiroshima (6 August 1945) and Nagasaki (9 August 1945); the bomb dropped on Hiroshima was as powerful as 12,700 tonnes of TNT, that on Nagaskai was equivalent to 22,000 tonnes of TNT. Following the strikes on Hiroshima and Nagasaki and the magnitude of the destruction there, the threat of nuclear war became a public concern. Bringing about what was called the nuclear, or atomic, age, the psychological impact of the development and use of the atomic bomb was felt around the world, particularly during the 1950s at the height of the Cold War and during the Cuban missile crisis of 1962.

The USSR first detonated an atomic bomb in 1949 and the UK in 1952.