How the Manhattan Project Worked

By: John Fuller

A photograph shows the first atomic bomb test on July 16, 1945, at 5:30am at the Trinity Site in New Mexico. See more nuclear bomb pictures.
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On Aug. 6, 1945, Paul Tibbets, pilot of the B-29 airplane named the Enola Gay, dropped an atomic bomb over the Japanese city of Hiroshima. Nicknamed "Little Boy," the bomb created an explosion equivalent to 15,000 tons of TNT, destroying nearly every building within a mile of ground zero and creating a massive firestorm that eventually engulfed the city. It's believed that 70,000 citizens died immediately after the blast, but the eventual death total may have reached as many as 100,000 by the end of the 1945 and 200,000 after 5 years due to the effects of radiation [source: U.S Department of Energy]. Three days later on Aug. 9 a second bomb was dropped on the industrial city of Nagasaki. Nicknamed "Fat Man," the second bomb killed about 40,000 people initially, and the death toll eventually reached 70,000 after the end of the year and 140,000 after 5 years [source: U.S. Department of Energy]. Japan surrendered to the Allied forces on Aug. 14, 1945, officially ending World War II.

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The development and use of the atomic bomb, the most powerful weapon created by the human race, is viewed as one of the most important and controversial events in the 20th century. Its terrifying ability to devastate an entire city and its symbol as a source of power sparked a tense nuclear arms race between the United States and the Soviet Union after the end of the war. Modern warfare had changed dramatically at the beginning of the century -- airplanes, machine guns and biological and chemical warfare were just a few of the technological advancements that caused widespread devastation and altered military tactics. But the atomic bomb was a different story. Some people thought its existence would put an end to all war, while others feared the potential annihilation of the human race.

The Manhattan Project, the code name for the United States' secret plan to develop atomic weapons for use in warfare, was a broad designation for the people, geographic locations and resources involved in atomic research during World War II. Many were, and still are, split on the decision to use the bomb in Japan, including the very people who helped build it. Some feel it saved lives and ended World War II, while others argue the Japanese would have surrendered anyway.

How did they do it? Who was involved? Why did they call it the Manhattan Project anyway? In this article we'll take a closer look at the Manhattan Project and how a large network of scientists and military personnel managed to create the most powerful display of energy the Earth has ever witnessed.


The Discovery of Nuclear Fission

To get to the Manhattan Project and the bombings of Hiroshima and Nagasaki, it helps to understand the advancements made in physics leading up to World War II. Between 1919 and the early 1930s, scientists were piecing together the important parts of the atom's structure. In 1919, at Manchester University in England, New Zealand physicist Ernest Rutherford discovered protons, positively charged particles located in the nucleus of the atom that, along with negatively charged electrons orbiting around the center, make up the atom.

There was one problem -- physicists couldn't explain why several elements weighed different amounts. This remained a mystery until 1932, when James Chadwick, one of Rutherford's colleagues, discovered the neutron, a third subatomic particle. With no charge, neutrons share space with protons in the atom's nucleus. While the number of protons and electrons is always the same for any given element -- carbon, for instance, always has 14 protons and 14 electrons -- there can be different numbers of neutrons. This explained why carbon could weigh different amounts, even though it was essentially the same element. These different weights of atoms are known as isotopes.


Around this time, scientists began using particle accelerators to bombard the nuclei of atoms in the hopes of splitting atoms and creating energy. Initially, they achieved very little success -- early particle accelerators shot out protons and alpha particles, both positively charged. Even at high speeds, these particles were easily repelled by the positively charged nuclei, and figures such as Rutherford, Albert Einstein and Niels Bohr felt that harnessing atomic power was close to impossible.

This changed when Italian physicist Enrico Fermi thought to use neutrons for bombardment in 1934. Since neutrons have no charge, they can hit an atom's nucleus without being repelled. He successfully bombarded several elements and created new, radioactive ones in the process. What Fermi had done, without recognizing it, was discover the process of nuclear fission. Two German scientists, Otto Hahn and Fritz Strassmann, were the first to officially acknowledge this process in 1938 when they successfully split uranium atoms into two or more parts.

Pellets of natural uranium oxide fuel used for nuclear power.
Pellets of natural uranium oxide fuel used for nuclear power.
Fritz Goro/Time Life Pictures/Getty Images

­Uranium, the heaviest natural element on Earth, was involved in many of these early processes and became a subject of great interest in physics for a few reasons. Uranium is the heaviest natural element with 92 protons. Hydrogen, in contrast, is extremely light and only has one proton. The interesting part about uranium, however, isn't so much the number of protons -- it's the unusually high number of neutrons in its isotopes. One isotope of uranium, uranium-235, has 143 neutrons and undergoes induced fission very easily.

When a uranium atom splits, it's essentially losing mass. According to Einstein's famous equation E = mc², where E is energy, m is mass and c is the speed of light, matter can be converted into energy. The more matter you have, the more energy you're able to create. Uranium is heavy since it has so many protons and neutrons, so when it's split into two or more parts it has more matter to lose. This loss of mass, as tiny as an atom may be, is equivalent to the creation of a great deal of energy.

On top of this, extra neutrons break off from the pieces of a split uranium atom. Since a pound of uranium contains trillions of atoms, the chances of a stray neutron hitting another atom of uranium are very high. This caught the attention of the physics world -- a controlled chain reaction could create safe nuclear power, while an uncontrolled reaction had the potential to devastate.

On the next page, we'll talk about the U.S. decision to build a nuclear bomb.

The German Threat

Albert Einstein and Leo Szilard in 1946, reenacting the signing of their letter to President Roosevelt warning him that Germany may be building an atomic bomb.
Albert Einstein and Leo Szilard in 1946, reenacting the signing of their letter to President Roosevelt warning him that Germany may be building an atomic bomb.
March Of Time/Time Life Pictures/Getty Images

News about nuclear fission traveled quickly from Europe to America, and by 1939, many leading physics labs in the United States, including Ernest Lawrence's at the Berkeley campus of the University of California, were testing the possibilities of generating power with uranium.

Although this was an exciting time for physics, it was also a tense and uncertain one. World War II was well under way by now, as Hitler had risen to power in Nazi Germany and invaded Poland on Sept. 1, 1939. Many feared the Germans were fast at work on a nuclear weapon, one they would undoubtedly use against their enemies during wartime. Prominent physicists such as Leo Szilard, Edward Teller and Eugene Wigner, all Europeans who fled to America to avoid the war, felt it necessary to warn the U.S. government about the dangers of Germany developing nuclear arms first.


Albert Einstein and Szilard were concerned enough to write a letter to U.S. President Franklin D. Roosevelt, describing the German threat and the possibility of constructing powerful weapons with uranium. After some discussion with economist Alexander Sachs, Roosevelt decided it was necessary to begin research on nuclear power, and he set up the Advisory Committee on Uranium, with Lyman J. Briggs as its head.

The next two years were fraught with uncertainty, because no one was sure how much uranium was needed, how much bomb construction would cost or how much time the U.S. had to complete a functional weapon. On top of this, research was still inconclusive regarding the extraction of uranium-235 from uranium.

Vannevar Bush on the cover of the April 3, 1944, issue of Time Magazine.
Vannevar Bush on the cover of the April 3, 1944, issue of Time Magazine.
Time Life Pictures/Getty Images

Things sped up with the help of Vannevar Bush, the president of the Carnegie Foundation, who was appointed chairman of the National Defense Research Committee by Roosevelt in the summer of 1940. Bush folded the Uranium Committee into this new government committee, giving scientists better funding and security. One more step on June 28, 1941, put Bush as director of the Office of Scientific Research and Development. The National Defense Research Committee became an advisory body to the Office of Scientific Research and Development, and the Uranium Committee was renamed the Office of Scientific Research and Development Section on Uranium, along with the codenam­e S-1. Find all the naming-switching confusing? So did anyone else trying to figure out what the White House was doing with the bomb program.

Later in July 1941, Bush received the boost he needed to get the project officially off of the ground. The MAUD Committee, the British's own version of a nuclear weapon plan, released the similarly titled MAUD Report. Although England's resources were stretched thin because of their involvement in World War II, their theoretical contributions to the bomb's design were invaluable, and the report ensured many who read it that a nuclear bomb and the enrichment of uranium-235 would definitely be possible. Bush set several research groups, mostly universities like Berkeley and Columbia, into motion with much more funding than before -- Lawrence alone received $400,000 for his work with electromagnetism. Secrecy was still the top priority despite the extra money, and scientists picked strange locations in order to conceal their efforts -- many people today are shocked to hear that physicists Enrico Fermi and Arthur Compton used space beneath the stands at Stagg Field, the racket courts at the University of Chicago, to conduct the first nuclear chain reaction in 1942.

Soon it was time for the Army to step in. To learn more about the organization of the Manhattan Project, read the next page.

Manhattan Project Organization

Nuclear physicist Robert Oppenheimer, left, with Major General Leslie Groves, by the remains of the tower from which an atom test bomb was ignited.
Nuclear physicist Robert Oppenheimer, left, with Major General Leslie Groves, by the remains of the tower from which an atom test bomb was ignited.
Keystone/Getty Images

By March 1942, the Army Corps of Engineers became directly involved with S-1 meetings, and on Sept. 18 Colonel Leslie R. Groves was appointed head of the project, now officially known as the Manhattan Project. With a strong background in engineering -- he oversaw with the construction of the Pentagon -- Groves turned out to be an incredibly skilled administrator and contributed greatly to the bomb's success within an impossibly short time span.

Over the next year Groves would select several sites across the United States that would aid in the bomb's completion, including Oak Ridge, Tenn. (Site X) and Hanford, Wash. (Site W). These locations were massive facilities meant for uranium and plutonium production. When Groves selected Robert Oppenheimer, professor of theoretical physics at Berkeley, to act as director of Project Y, the two chose Los Alamos, New Mexico, as the site that would be the central hub of the Manhattan Project.


Los Alamos, along with the sites in Tennessee and Washington State, were remote locations picked for maximum security, but you wouldn't know it if you saw pictures of them during peak production. The desolate New Mexican mesa in Los Alamos, for instance, was essentially turned into a small city, with laboratories, offices, dining halls and housing for everyone involved in the project. Oppenheimer worked hard on gathering the best scientific minds in the country, and for nearly three years between the fall of 1942 and the bombing of Hiroshima on Aug. 6, 1945, thousands of people worked through the challenges of constructing an atomic weapon.

Simple housing for the workers involved in the Manhattan Project at Los Alamos, N.M.
Simple housing for the workers involved in the Manhattan Project at Los Alamos, N.M.
Keystone/Getty Images

­Security at Los Alamos was extremely tight, as people were hardly allowed to contact family members and friends for their entire stay at Site Y. Guards were tough on clearance issues, and barbed wire surrounding the entire complex. The Manhattan Project was enveloped in so much secrecy, in fact, that some people didn't even know the nature of their work until they heard news of the bomb exploding over Hiroshima.

Two types of nuclear bombs were designed at Los Alamos -- an implosion bomb and a gun-triggered bomb. After major improvements were made on the implosion device, a site was finally chosen to test the first nuclear bomb. Alamogordo, a desert range about 210 miles south of Los Alamos, was nicknamed "Trinity" for the testing of a plutonium bomb design -- Oppenheimer allegedly recalled a John Donne poem that begins "Batter my heart three-person'd God" and felt the comparison fitting. At 5:30 a.m. on July 16, 1945, the bomb was detonated, creating a massive blast and temporarily blinding several of the observing scientists: the Atomic Age had begun.

Manhattan Project officials, including Dr. Robert J. Oppenheimer (white hat) and General Leslie Groves, inspect the detonation site of the Trinity atomic bomb test.
Manhattan Project officials, including Dr. Robert J. Oppenheimer (white hat) and General Leslie Groves, inspect the detonation site of the Trinity atomic bomb test.
Los Alamos National Laboratory/Time Life Pictures/Getty Images

Less than a month later, the United States used the implosion bomb and the untested gun-triggered bomb in order to coerce the Japanese into surrender. Although the bomb arguably ended the conflict overseas by ending ground combat in Japan, the bomb's existence ushered in a nuclear arms race that would dramatically alter the second half of the 20th century.

If you'd like to learn more about the Manhattan Project and nuclear weapons, follow the links on the next page.­ 

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More Great Links

  • Bird, Kai and Martin Sherwin. "American Prometheus." New York: Vintage Books, 2005.
  • Broad, William. "Why they called it the Manhattan Project." The New York Times. Oct. 30, 2007.
  • "The Manhattan Project: Making the Atomic Bomb."