The Manhattan Project and the Trinity Test

At 5:29:45 in the morning of July 16, 1945, in a remote stretch of the New Mexico desert known as the Jornada del Muerto, a steel tower one hundred feet tall held the first atomic weapon ever detonated by humans. The plutonium device, code named Gadget, was the culmination of three years of work by more than one hundred thousand people at a cost of nearly two billion dollars, the largest scientific project ever attempted up to that point. When the device detonated, it produced an explosion equivalent to approximately twenty one thousand tons of TNT, vaporising the steel tower and turning the asphalt and sand around it into a green glassy mineral that would later be called trinitite. The heat from the explosion was felt by observers more than ten thousand yards away. The light from the explosion was visible for hundreds of miles. The sound of the explosion was heard across much of southern New Mexico. J. Robert Oppenheimer, the scientific director of the project, watched the flash from a concrete bunker ten thousand yards away and remembered lines from the Bhagavad Gita, the Hindu scripture he had learned from his graduate school readings. Now I am become Death, the destroyer of worlds. The Trinity test was the moment when the United States demonstrated that it could build atomic weapons. Three weeks later, the United States dropped atomic weapons on Hiroshima and Nagasaki, and the Second World War came to an end.

This is a long essay about the Manhattan Project, the programme that produced the first atomic weapons, and about the Trinity test, the moment when those weapons were first demonstrated. The Manhattan Project was the largest scientific project of its time and one of the largest scientific projects ever attempted. It involved more than one hundred thousand people at dozens of sites across the United States, Canada, and the United Kingdom. It cost nearly two billion dollars in 1945 money, more than twenty billion dollars in present value. It produced three different types of atomic weapons, two of them based on uranium 235 and one on plutonium 239, in less than four years. And it did so under conditions of wartime secrecy that have no parallel in the history of science.

What follows is structured in five parts. The opening section traces the origins of the Manhattan Project, from the discovery of nuclear fission in 1938 through the Einstein Szilard letter of 1939 to the establishment of the Manhattan Engineer District in 1942. The second section examines the wartime organisation of the project, the partnership between the military director Leslie Groves and the scientific director J. Robert Oppenheimer, and the network of laboratories and production facilities that the project established. The third section follows the science of the project, from the first sustained nuclear chain reaction in Chicago Pile 1 in December 1942 to the production of fissionable material at Oak Ridge and Hanford in 1944 and 1945. The fourth section examines the Trinity test itself, the preparation of the Gadget, the explosion, and the scientific observations that were made. The closing section considers the legacy of the project, the bombs dropped on Hiroshima and Nagasaki, the establishment of the postwar nuclear weapons establishment, and the long consequences of the project for science, for international relations, and for the world.

From Fission to the Einstein Szilard Letter

The Manhattan Project began with a discovery in Berlin. In December 1938, the chemists Otto Hahn and Fritz Strassmann, working at the Kaiser Wilhelm Institute for Chemistry in Berlin, demonstrated that uranium, when bombarded with neutrons, split into two roughly equal fragments and released a large amount of energy. The discovery was communicated to Lise Meitner, a physicist who had recently fled Nazi Germany and who was working in Sweden. Meitner, together with her nephew Otto Frisch, calculated that the energy released per fission was enormous, on the order of two hundred million electron volts, and that the process might be self sustaining if each fission released more than one neutron that could go on to cause another fission. The discovery of nuclear fission, the splitting of the heavy atomic nucleus into two roughly equal fragments, was announced in January 1939 and was quickly confirmed by laboratories in several countries.

The possibility that fission could be made into a self sustaining chain reaction was recognised almost immediately by physicists on both sides of the Atlantic. In the United States, the Hungarian émigré physicists Leo Szilard and Enrico Fermi, both of whom had fled fascist Europe, began to design experiments to test the possibility. Szilard and Fermi worked at Columbia University in New York, where they applied for the first patent on a nuclear reactor in 1940 and where they conducted the first controlled nuclear chain reaction experiments in 1941. The British physicist James Chadwick, who had discovered the neutron in 1932 and who was now working on the British nuclear programme, communicated to American physicists the British view that a nuclear bomb might be feasible and might be powerful enough to win a war.

The prospect that Nazi Germany might be working on an atomic bomb alarmed a group of physicists who had fled Europe and who were now working in the United States. The most prominent of these physicists was Albert Einstein, who had himself fled Germany in 1933 and who was now living in Princeton. In August 1939, Leo Szilard drafted a letter to President Franklin Roosevelt, signed by Einstein, that warned of the possibility that Germany might develop atomic weapons and that urged the United States government to begin its own atomic weapons programme. The letter, the famous Einstein Szilard letter, was delivered to Roosevelt on October 11, 1939, more than two years before the United States would enter the Second World War.

Roosevelt responded to the Einstein Szilard letter by establishing the Advisory Committee on Uranium, which was placed under the National Bureau of Standards and which was initially given a budget of only six thousand dollars. The committee coordinated a small research programme at Columbia University and at several other universities, but the programme was underfunded and made slow progress. In June 1940, after the fall of France, the committee was reorganised under the Office of Scientific Research and Development, headed by Vannevar Bush, an engineer and inventor who had been a leading figure in American science policy for two decades. Bush and his colleagues accelerated the uranium research programme and began to consider the possibility of separating uranium 235 from natural uranium, a difficult technical problem that would require enormous industrial facilities.

The decisive moment came in December 1941, with the attack on Pearl Harbor and the entry of the United States into the Second World War. The uranium research programme was rapidly expanded, and Bush and his colleagues began to consider the possibility of an industrial scale effort to produce atomic weapons. In June 1942, Bush recommended to Roosevelt that the uranium programme be placed under the direct control of the Army Corps of Engineers, the military organisation that was responsible for the largest construction projects in the United States. Roosevelt approved the recommendation, and on August 13, 1942, the Manhattan Engineer District was established under the Corps of Engineers. The district was given the code name Manhattan because much of the early work had been done at Columbia University in New York. The name was misleading. The Manhattan Project was based in dozens of sites across the United States, not in Manhattan.

Groves, Oppenheimer, and the Sites

The military director of the Manhattan Engineer District was Colonel Leslie Groves, a career officer in the Army Corps of Engineers. Groves had previously overseen the construction of the Pentagon, which had been completed in 1943, and he had a reputation for being able to drive large construction projects through to completion on impossible schedules. Groves was assigned to the Manhattan Project on September 17, 1942, and within weeks he had taken control of the programme and begun to expand it on the scale that the science seemed to require.

The scientific director of the project was J. Robert Oppenheimer, a theoretical physicist who had been a professor at the University of California at Berkeley and who had worked on problems in nuclear physics for more than a decade. Oppenheimer had been considered for the role of scientific director earlier in the year, but his appointment had been blocked by security concerns related to his past associations with left wing political organisations. Groves interviewed Oppenheimer in October 1942 and decided to override the security concerns, on the grounds that Oppenheimer was the only scientist who had the combination of scientific understanding, organisational ability, and personal dynamism that the project required. The appointment of Oppenheimer was the most consequential personnel decision of the war.

Oppenheimer chose a site for the principal weapons laboratory on a mesa in northern New Mexico, near the town of Los Alamos. The site was on a former boys school that had been taken over by the Army, and it was sufficiently remote to provide security for the work. The laboratory was established in early 1943 and was designated as Site Y of the Manhattan Project. Los Alamos grew rapidly over the next two years, with several thousand people working on the design of the atomic weapons by the end of the war. The scientific staff included some of the most distinguished physicists of the twentieth century, including Hans Bethe, Edward Teller, Enrico Fermi, Richard Feynman, and many others.

The Manhattan Project established several other major sites for the production of fissionable material. The first was at Oak Ridge, Tennessee, where the project built enormous facilities for the separation of uranium 235 from natural uranium. The principal technique used at Oak Ridge was gaseous diffusion, in which uranium hexafluoride gas was forced through porous membranes that preferentially passed the lighter uranium 235. The technique was developed at the laboratory level by Harold Urey and his colleagues at Columbia University and was scaled up at Oak Ridge to industrial proportions. By 1945, the Oak Ridge facility was the largest gaseous diffusion plant in the world.

The second major production site was at Hanford, Washington, on the Columbia River in the eastern part of the state. Hanford was the site of the production reactors that converted uranium 238 into plutonium 239 by neutron capture. The plutonium was then separated from the unconverted uranium by chemical processing. The Hanford site was established in 1943 and was brought into operation in 1944. The site was located in a sparsely populated part of Washington state, and the Columbia River provided the cooling water that the reactors required. The Hanford site was, by any measure, one of the largest industrial facilities ever built up to that point.

The third major site was at the University of Chicago, where the first sustained nuclear chain reaction had been achieved on December 2, 1942, in an experiment known as Chicago Pile 1. Chicago Pile 1 was a pile of uranium metal and uranium oxide, stacked in a squash court under the stands of the Stagg Field stadium at the University of Chicago, with cadmium control rods to absorb excess neutrons and with graphite as a moderator. The pile was designed and built by Enrico Fermi and his colleagues, and the experiment that brought it to criticality was the first demonstration that a self sustaining nuclear chain reaction was possible. The Chicago site continued to be important for the Manhattan Project as a centre for the development of reactor physics and for the design of the production reactors that were later built at Hanford.

The Science of the Bomb

The science of the atomic bomb was, in its broad outlines, straightforward, even if the engineering was extraordinarily difficult. The principle of the bomb was that fission of uranium 235 or plutonium 239 would release energy and neutrons, and that the neutrons would cause further fissions in a self sustaining chain reaction. The challenge was to assemble enough fissionable material in one place to achieve a self sustaining chain reaction, and to do so quickly enough that the chain reaction would release its energy before the assembly flew apart.

The critical mass of uranium 235, the minimum amount needed to sustain a chain reaction, was estimated in 1940 to be in the range of tens of kilograms. The critical mass of plutonium 239 was estimated to be similar, although the actual figure depended on the geometry of the assembly and on the presence of neutron reflecting materials around the assembly. The principal difficulty was to assemble the critical mass quickly. In a reactor, the critical mass is achieved gradually and the chain reaction is controlled by neutron absorbing control rods. In a bomb, the chain reaction must be allowed to proceed without control, and the assembly must be brought together quickly enough that the chain reaction produces its energy before the assembly flies apart.

Two principal bomb designs emerged from the work at Los Alamos. The first, known as the gun assembly, used a cannon to fire one sub critical mass of uranium 235 into another sub critical mass to create a super critical assembly. The gun assembly was mechanically simple and was the basis of the bomb dropped on Hiroshima, which used uranium 235 produced at Oak Ridge. The second design, known as the implosion design, used shaped charges of conventional explosive to compress a sub critical mass of plutonium 239 into a super critical density. The implosion design was mechanically more complex and required much more development, but it was the only practical way to use plutonium 239 in a bomb because plutonium 239 would predetonate if assembled slowly.

The development of the implosion design was the principal scientific challenge of the Manhattan Project. The challenge was to design shaped explosive charges that would compress a sphere of plutonium 239 symmetrically and rapidly enough to achieve a super critical density. The problem was solved by a combination of mathematical analysis, explosive testing, and experiment. The mathematical analysis was led by John von Neumann, the Hungarian American mathematician who had previously worked on the theory of detonation waves. The explosive testing was done at a remote site in New Mexico, where the implosion of small scale models could be observed. The experimental work was led by Seth Neddermeyer, who first proposed the implosion design, and by George Kistiakowsky, a Harvard chemist who led the development of the explosive lenses.

The work at Los Alamos was complicated by the recognition, in mid 1944, that the plutonium produced at Hanford would not be usable in a gun assembly bomb. The reason was that plutonium 239 produced in a reactor is contaminated with plutonium 240, an isotope that fissions spontaneously and that would cause the gun assembly to predetonate. The contamination made the implosion design the only practical option for the plutonium bomb, and the work at Los Alamos focused increasingly on the implosion design as the project progressed. The gun assembly design continued in parallel because the uranium 235 produced at Oak Ridge was usable in the gun design and because the project wanted to have a bomb ready as soon as possible.

The development of the implosion design was also complicated by a 1944 accident that killed one of the scientists working on the project. The accident occurred during an experiment with high explosives at Los Alamos, when a chemical explosion in a laboratory injured several scientists. One of them, the physicist Harry Daghlian, died of radiation exposure nine days later after a separate accident with the same materials. A second physicist, Louis Slotin, was killed in a similar accident in 1946, after the war had ended. These accidents were an early indication of the dangers of working with the materials involved in nuclear weapons, and they led to the establishment of the safety procedures that have governed the handling of fissionable materials ever since.

The production of uranium 235 at Oak Ridge was itself an enormous technical achievement. The separation of uranium 235 from natural uranium, which is more than 99 percent uranium 238, required enormous industrial facilities. The gaseous diffusion process used at Oak Ridge required thousands of porous membranes, each one a specially prepared metal film with pores of controlled size, through which uranium hexafluoride gas was forced under high pressure. The membranes were the limiting factor in the process, and the production of the membranes was one of the most difficult technical problems of the war. The electromagnetic separation process, developed by Ernest Lawrence at Berkeley and also deployed at Oak Ridge, was used as a backup to the gaseous diffusion process and produced smaller quantities of highly enriched uranium.

The Trinity Test

By the summer of 1945, the Manhattan Project was ready to test the implosion design. The plutonium produced at Hanford had reached the quantities needed for a bomb, and the implosion design had been refined to the point that the project leadership was willing to risk a test. The test site was chosen on the Alamogordo Bombing and Gunnery Range, southeast of Socorro, New Mexico, in a remote stretch of the Jornada del Muerto desert. The site was code named Trinity by Oppenheimer, in reference to a line from John Donne’s Holy Sonnet Batter my heart, three personed God.

The Trinity test device, code named Gadget, was a sphere of plutonium 239 surrounded by shaped explosive lenses, designed to compress the plutonium to super critical density. The Gadget was assembled at the Trinity site by a team led by the physicist Norris Bradbury, who would later succeed Oppenheimer as director of Los Alamos. The Gadget was hoisted to the top of a one hundred foot steel tower, designated as Zero, on July 14, 1945. The tower was painted white to reflect the heat of the New Mexico sun and to prevent the device from being heated unevenly before the test.

The test was scheduled for 4:00 in the morning on July 16, 1945, to take advantage of the cool early morning air and to allow the test to be completed before the desert became too hot. The test was delayed by weather, and by uncertainty about whether the assembled device would work. At 4:51 in the morning, two and a half hours behind schedule, the countdown reached zero. The explosive lenses fired, the plutonium was compressed, the chain reaction began, and the device detonated at 5:29:45 a.m. Mountain War Time.

The explosion produced a flash of light that was visible for hundreds of miles, a fireball that rose rapidly into the sky, and a shock wave that was felt by observers more than ten thousand yards away. The explosion was observed by a small number of scientists and military officers at the base camp, by an additional group of observers at a site ten thousand yards from the tower, and by General Groves and other senior officers at the control bunker at Compania Hill, twelve miles from the tower. The observers at the ten thousand yard site included Oppenheimer, who watched the explosion from a concrete bunker with Edward Teller and Ernest Lawrence. The observers at the base camp included Enrico Fermi, who performed an experiment to estimate the yield of the bomb by dropping small pieces of paper and observing how far they were blown by the shock wave.

The yield of the Trinity test was estimated, in the days after the test, to be approximately twenty one thousand tons of TNT equivalent. The actual figure, refined in later years, was approximately twenty one kilotons. The yield was at the upper end of the estimates that had been made before the test, and it confirmed that the implosion design worked as expected. The Trinity test also confirmed that the bomb was capable of producing an explosion on a scale that no weapon previously imagined had been capable of. The observers at the base camp were showered with a fine dust, the trinitite that had formed from the vaporised sand and steel.

The scientific instruments deployed at the Trinity test gave the observers a great deal of data about the explosion. The instruments included high speed cameras that recorded the growth of the fireball, pressure gauges that recorded the strength of the shock wave, radiation detectors that recorded the intensity and spectrum of the radiation released by the explosion, and seismographs that recorded the ground motion produced by the blast. The data from the instruments were used to refine the estimates of the yield and to characterise the behaviour of the device. The data also confirmed that the implosion design had compressed the plutonium to a density close to the design value, and that the chain reaction had proceeded as predicted by the theoretical models.

The Trinity test also produced lasting effects on the landscape. The explosion created a crater approximately ten feet deep and several hundred yards wide in the desert sand. The sand around the crater was fused into the green glassy mineral that would be called trinitite. The radioactive contamination around the test site persisted for decades, and the area was closed to the public for many years after the war. The crater has since been filled in and the area has been partly cleaned up, but the test site, now part of the White Sands Missile Range, is still accessible to visitors on a limited basis and the trinitite is still found scattered across the area. The Trinity site is now a national historic landmark, and it is visited every year by a small number of tourists who come to see the place where the atomic age began.

The Aftermath

The Trinity test was the proof of concept for the atomic bomb. Three weeks later, on August 6, 1945, the United States dropped an atomic bomb on Hiroshima. The bomb, code named Little Boy, was a gun assembly device using uranium 235 produced at Oak Ridge, and it was dropped from the B 29 bomber Enola Gay. The bomb detonated at an altitude of approximately two thousand feet over the centre of the city, and it produced an explosion of approximately fifteen kilotons. The immediate death toll at Hiroshima was estimated at approximately seventy thousand people, with a comparable number dying later from injuries and radiation.

Three days later, on August 9, 1945, the United States dropped a second atomic bomb on Nagasaki. The bomb, code named Fat Man, was an implosion device using plutonium 239 produced at Hanford, and it was dropped from the B 29 bomber Bockscar. The bomb detonated over the industrial area of the city and produced an explosion of approximately twenty one kilotons. The immediate death toll at Nagasaki was estimated at approximately forty thousand people, with a comparable number dying later from injuries and radiation.

The bombings of Hiroshima and Nagasaki brought the Second World War to an end. Japan announced its surrender on August 15, 1945, and the formal surrender ceremony took place on September 2, 1945 aboard the USS Missouri in Tokyo Bay. The use of atomic weapons against Japan has been debated ever since. Some historians have argued that the bombings were necessary to bring the war to a quick end and to save the lives that would have been lost in a planned invasion of the Japanese home islands. Other historians have argued that the bombings were unnecessary and that Japan would have surrendered even without them. The debate is unlikely to be resolved, but the bombings remain the only use of nuclear weapons in warfare in the more than eighty years since.

The Manhattan Project also established the institutions of postwar American science. The wartime laboratories at Los Alamos, Oak Ridge, and Hanford were converted into the permanent national laboratories of the postwar nuclear weapons establishment. Los Alamos became Los Alamos National Laboratory, which continues to be one of the principal nuclear weapons laboratories in the United States. Oak Ridge became Oak Ridge National Laboratory, which has been a leading centre for nuclear energy research. The wartime network of academic scientists was maintained and expanded in the postwar period, and the National Science Foundation, established in 1950, was in large part a product of the wartime experience.

The Manhattan Project also initiated the nuclear arms race with the Soviet Union. The Soviet Union had its own nuclear weapons programme during the war, directed by the physicist Igor Kurchatov, and the Soviet Union tested its first atomic bomb in August 1949, less than four years after the Trinity test. The result was the beginning of the nuclear arms race that would dominate the international politics of the next half century and that would eventually produce more than sixty thousand nuclear weapons worldwide. The Manhattan Project had created the technology, and the technology was impossible to keep secret for long. The question of how to control the technology has remained one of the central questions of international politics since.

The legacy of the Manhattan Project is mixed. On one hand, the project demonstrated what science could achieve when given adequate resources and clear goals. The project produced three different types of atomic weapons in less than four years, and it did so with techniques and institutions that became the model for the large scale science of the postwar period. On the other hand, the project produced weapons of unprecedented destructiveness, and the use of those weapons against Hiroshima and Nagasaki has been a moral burden on the United States and on the world ever since. The scientists who worked on the project, including Oppenheimer, were divided in their views about what they had done. Oppenheimer himself was investigated by the Atomic Energy Commission in 1954 on charges of disloyalty, in a hearing that has become one of the most famous episodes in the history of American science and that turned on Oppenheimer’s earlier opposition to the hydrogen bomb. The hearing ended with Oppenheimer’s security clearance being revoked, and he never returned to government service.

For the world, the Manhattan Project is a reminder of the ambivalent relationship between science and society. The same science that produced the atomic bomb has also produced the peaceful uses of nuclear energy, the medical uses of radioisotopes, and the basic understanding of the universe that has come from particle physics and nuclear physics. The Manhattan Project cannot be reduced to a single lesson, but the lesson that is most often drawn from it is that scientists and engineers have a responsibility to consider the consequences of the work they do. That lesson has been drawn repeatedly in the years since, in debates about nuclear weapons, about genetic engineering, about artificial intelligence, and about other technologies that have the potential to do great harm. The Manhattan Project is, in this sense, the prototype for the ethical dilemmas of the technological age.

The most striking thing about the Manhattan Project, looked at from the perspective of more than eighty years later, is how small the project was in human terms relative to what it accomplished. The total scientific staff at Los Alamos at the height of the project numbered about six thousand people. The total staff at Oak Ridge at the height of the project numbered about fifty thousand. The total staff at Hanford at the height of the project numbered about fifty thousand. The combined scientific and engineering staff of all the Manhattan Project sites numbered perhaps one hundred thousand. By the standards of a modern industrial project, that is a small number. The Manhattan Project achieved what it did because the people who worked on it were extraordinarily talented, because the project had clear goals and adequate resources, and because the wartime environment allowed the project to bypass the normal constraints of bureaucratic and regulatory oversight.

The Manhattan Project also marked a transition in the relationship between science and the state. Before the war, most scientific research in the United States was conducted at universities and was funded by private foundations. The war showed that the federal government could be the principal funder of scientific research, and that large scale, mission oriented science could be carried out in dedicated laboratories outside the universities. The postwar science establishment, with its national laboratories, its federal grant making agencies, and its network of academic scientists funded by the Department of Defense, the Department of Energy, and the National Science Foundation, is in many ways a continuation of the wartime pattern established by the Manhattan Project. The relationship between science and the state that we take for granted today is, in a real sense, the legacy of the wartime project.

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