Hydrogen Bomb, also known as H-bomb or thermonuclear bomb, nuclear weapon in which a thermonuclear fusion reaction takes place among heavy isotopes of hydrogen (either deuterium or tritium) to produce an explosion. A hydrogen bomb produces an extremely large explosion, equivalent to millions of tons of TNT (see Trinitrotoluene).

In the fusion reaction in a hydrogen bomb, two atoms of deuterium or tritium collide to produce a helium atom and extra neutrons. The resulting energy is proportional to the difference in mass between the original atoms and the products of the collision. To ignite this fusion reaction, an environment of tremendous heat is needed, comparable in temperature to heat generated by the Sun. This condition is created by using a nuclear fission bomb as a trigger (see Atomic Bomb). The thermonuclear explosion resulting from the fusion creates great heat, enormous shock waves, high winds, and deadly radiation in the form of gamma rays and neutrons that destroys living matter and contaminates soil and water.

The hydrogen bomb came in part out of the creation of the atomic bomb. In 1939 physicists in the United States and in Europe realized that a powerful explosive weapon could be created through the splitting, or fission, of uranium atoms. In 1942 the U.S. government established the top secret Manhattan Project, which created the first atomic bomb. The first fission atomic bomb was exploded as a test in July 1945. On August 6, 1945, the United States dropped one fission bomb, called “Little Boy,” on the Japanese city of Hiroshima. Three days later it dropped a second, called “Fat Man,” on the city of Nagasaki. The bombs were extremely destructive, killing more than 100,000 people instantly. Radiation illness killed thousands more, and the eventual death toll from both explosions totaled about 340,000 people.

Several scientists who worked on the Manhattan Project, led by Hungarian-born American physicist Edward Teller, imagined an even more powerful weapon triggered by a fission reaction but fueled by the fusion of hydrogen. The United States detonated the first hydrogen bomb in a test on Enewetak Atoll, in a remote region of the Pacific Ocean, on November 1, 1952. Its explosive force was about 500 times greater than the Hiroshima or Nagasaki bombs. The Union of Soviet Socialist Republics (USSR) detonated its first hydrogen bomb on August 12, 1953. Today, most nuclear weapons are fusion thermonuclear devices.

The United States, Russia, the United Kingdom, France, China, India, and Pakistan are the only nations at present that admit to possessing nuclear weapons. South Africa admitted to having built and then dismantling a number of bombs. Several other nations, including Israel and North Korea, are thought to have them or to have the capability to assemble them quickly.

Military strategists classify nuclear weapons as either tactical or strategic weapons. Tactical nuclear weapons are intended for use in a limited area of conflict and are designed to halt an enemy advance with crippling force. Tactical bombs can be delivered from aircraft, submarines, or weapons on the ground. Strategic nuclear weapons, on the other hand, are designed for all-out battles of one nation against another across continents. They can be delivered to their targets by long-range bomber aircraft or in missiles fired from fixed or mobile launchers on land or from submarines. See also ICBM; SLBM.

The development of atomic and hydrogen bombs has had so great an impact on the world that historians draw a sharp distinction between the atomic age, or nuclear age, and all previous periods. The aftermath of Hiroshima and Nagasaki quickly made it apparent that humans had succeeded in harnessing enough energy from nature itself to destroy the planet and all its inhabitants. Beginning in the early 1960s, several nations negotiated limitations on testing, producing, distributing, and deploying nuclear weapons and fissile materials. In addition, over 180 nations have signed the Nuclear Nonproliferation Treaty of 1968, pledging not to acquire nuclear weapons or distribute nuclear weapons technology. See also Arms Control.