How the Manhattan Project Built the First Atomic Bomb

The Letter That Started Everything

On August 2, 1939, Albert Einstein signed a letter to President Franklin D. Roosevelt warning that Nazi Germany might develop an atomic bomb. The letter, drafted largely by physicist Leo Szilard, described the potential for a nuclear chain reaction in uranium to produce "extremely powerful bombs." Roosevelt responded by forming the Advisory Committee on Uranium, which eventually grew into the largest secret scientific project in history: the Manhattan Engineer District, code-named the Manhattan Project. Between 1942 and 1945, the project employed 125,000 people, operated across 30 sites in the United States, Canada, and the United Kingdom, and cost approximately $2 billion—about $28 billion adjusted for inflation.

The Science of Fission

Nuclear fission—the splitting of a heavy atomic nucleus into lighter nuclei, releasing enormous energy—was discovered in December 1938 by Otto Hahn and Fritz Strassmann in Berlin, with theoretical explanation provided by Lise Meitner and Otto Frisch. The discovery electrified the physics community because fission of uranium-235 released not only energy but also additional neutrons, which could split other uranium nuclei in a self-sustaining chain reaction.

• Uranium-235 constitutes only 0.7% of natural uranium (the rest is non-fissile U-238)
• Enriching uranium to weapons-grade (>80% U-235) requires separating two isotopes with nearly identical chemical properties
• Plutonium-239, produced in nuclear reactors from U-238, offered an alternative fissile material
• The critical mass of U-235 is approximately 52 kg (a sphere roughly the size of a grapefruit)
• The critical mass of Pu-239 is approximately 10 kg—significantly less, making it more efficient per unit mass

Both paths to the bomb—uranium enrichment and plutonium production—were pursued simultaneously because no one knew which would succeed first.

Three Secret Cities

The Manhattan Project built entire cities from scratch, hidden in remote locations and erased from maps.

Oak Ridge consumed roughly 1/7 of all electricity produced in the United States during peak operations. Hanford's B Reactor became the world's first full-scale plutonium production reactor, going critical on September 26, 1944. Workers at both sites often had no idea what they were building. Compartmentalization was absolute—each worker knew only their specific task.

Oppenheimer and the Scientists

J. Robert Oppenheimer, a theoretical physicist from the University of California, Berkeley, was appointed scientific director of Los Alamos in 1943. He was 38 years old. His task was to recruit the finest scientific minds in the Western world and transform abstract physics into a deliverable weapon under wartime pressure.

• Enrico Fermi achieved the first controlled nuclear chain reaction at the University of Chicago on December 2, 1942
• Hans Bethe led the theoretical division at Los Alamos
• Richard Feynman, age 24, worked on the implosion calculations
• Edward Teller pushed for a hydrogen "super" bomb during the project (ultimately built in 1952)
• Niels Bohr, who had escaped Nazi-occupied Denmark, contributed to the theoretical framework

Oppenheimer managed a community of Nobel laureates and future Nobel laureates, balancing scientific disagreements with military deadlines. General Leslie Groves, the Army engineer overseeing the project, handled security, logistics, and the billions in funding.

Two Bomb Designs

The project produced two fundamentally different weapon designs, each using a different fissile material.

The gun-type design was simpler but required scarce enriched uranium. The implosion design was far more complex—requiring precisely shaped explosive lenses detonated within microseconds of each other—but used less fissile material. The implosion design's complexity is why it required a test.

Trinity: July 16, 1945

At 5:29 a.m. on July 16, 1945, in the Jornada del Muerto desert of New Mexico, the world's first nuclear device—code-named "Gadget"—detonated atop a 100-foot steel tower. The explosion produced a yield of approximately 21 kilotons of TNT equivalent, vaporized the tower, created a crater 5 feet deep and 30 feet wide in the desert floor, and turned the surrounding sand into a glassy substance later named trinitite.

The flash was visible from 200 miles away. The mushroom cloud rose to 40,000 feet. Oppenheimer later said the moment reminded him of a line from the Bhagavad Gita: "Now I am become Death, the destroyer of worlds." Test director Kenneth Bainbridge offered a blunter assessment: "Now we are all sons of bitches."

The Ethical Aftermath

The bombings of Hiroshima and Nagasaki killed an estimated 110,000 to 210,000 people by the end of 1945, with tens of thousands more dying in subsequent years from radiation-related illness. The decision to use the bombs remains one of the most debated moral questions of the twentieth century.

• Proponents argued the bombs ended the war and prevented a ground invasion of Japan that could have cost millions of lives on both sides
• Critics argued Japan was already near surrender, the Soviets' entry into the Pacific war was the decisive factor, or the bombings constituted an atrocity against civilians
• The Franck Report, signed by project scientists before Trinity, urged a demonstration blast on an uninhabited area rather than use against a city
• Many Manhattan Project scientists later became vocal advocates for nuclear arms control

Szilard, who had helped initiate the project, spent the rest of his life campaigning against nuclear weapons. Oppenheimer's security clearance was revoked in 1954 after he opposed the hydrogen bomb—a political humiliation that symbolized the fraught relationship between scientists and the military state they had empowered. The Manhattan Project built the bomb in 27 months. Humanity has spent the eight decades since trying to decide what to do with the knowledge it produced.