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The Chain Reaction Idea
Leo Szilard conceives the idea of using a chain reaction of neutron collisions with atomic nuclei to release energy. He also considers the possibility of using this to make bombs. This predates the discovery of fission by more than six years. -
The Einstein Letter
Pres. Roosevelt receives "The Einstein Letter" warning about the prospect of an atomic bomb. Written by Szilard in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner, the letter warned that Germany might develop atomic bombs and suggested that the United States should start its own nuclear program. -
S-1 Executive Committee
The first meeting of the Advisory Committee on Uranium in Washington, DC, which was created at President Roosevelt's order. The physicists argue for urgent government attention, but Adamson is hostile. Teller requests $6,000 for research on preliminary uranium-graphite slow neutron experiments, which is grudgingly approved. -
Neutron Experimentation
After much prodding by Leo Szilard, Lyman Briggs, head of the Uranium Committee, finally releases a promised $6,000 budgeted for conducting neutron experiments with Enrico Fermi at Columbia University. -
Britain's Awakening
First meeting of the British committee (later code-named the MAUD Committee) organized by Henry Tizard to consider Britain's actions regarding the "uranium problem". Research into isotope separation and fast fission is agreed upon. -
A Careful Process
Second meeting of Lyman Briggs' The Uranium Committee. Briggs' decision is that neither research on fast fission, nor work on building a critical uranium-graphite assembly, should begin until the small scale lab experiments, just getting underway, are finished. -
Expansion and Approval
The newly founded The National Defense Research Committee, headed by Vannevar Bush, takes over responsibility for uranium research. In his final report, Lyman Briggs requests $140,000 for further work: $40,000 for lab measurements, and $100,000 for large scale uranium-graphite studies. Bush approves only $40,000. -
Work Begins in Manhattan
The $40,000 contract from the NDRC finally comes through. Work begins at Manhattan, NY to assemble a large subcritical pile made of graphite and uranium oxide. -
Vannevar's Report
After months of growing pressure from scientists in Britain and the U.S. (particularly University of California at Berkeley's Ernest O. Lawrence), Vannevar Bush at The National Defense Research Committee decides to review the prospects of nuclear energy further and engages Arthur H. Compton and the National Academy of Sciences for the task. The report is issued May 17 and treats military prospects favorably for power production. -
The MAUD Report
The MAUD Committee approves its final report and disbands. It describes atomic bombs in some technical detail, provides specific proposals for developing them, and includes cost estimates. Although the contents of The MAUD Report reach Bush at The Office of Scientific Research and Development immediately, he decides to wait for the report to be transmitted officially before taking any further action. In short, the report concludes that an atomic bomb is indeed feasible. -
Churchill's Endorsement
With PM Winston Churchill's endorsement, the British Chiefs of Staff agree to begin development of an atomic bomb. -
The Prelude to Cold War
Peter L. Kapitza, one of the leaders of Soviet physics, remarked at an international "anti-Fascist" meeting of scientists in Moscow that the recent discovery of nuclear energy could be useful in the war against Germany and that the theoretical prospects of a uranium bomb seemed promising. The Soviet Union would later launch its own atomic program. -
It Can Be Made
Arthur H. Compton holds a meeting in Schenectady, NY with Ernest O. Lawrence, J. Robert Oppenheimer, George Kistiakowsky, and James B. Conant reviewing The MAUD Report and the most recent US work. The meeting ends by concluding that an atomic bomb could be made. -
Approval and Relocation
Pres. Roosevelt approves the production of an atomic bomb. Arthur H. Compton creates the Metallurgical Laboratory at the University of Chicago to act as a consolidated research center. He transfers work on "uranium burners" (reactors) to it. J. Robert Oppenheimer organizes a program on fast neutron theoretical physics at the University of California at Berkeley. -
Enter Oppenheimer
J. Robert Oppenheimer joins the Chicago Met Lab to lead an effort on fast neutron physics, and prepares an outline for the entire neutron physics program. Production of plutonium through marathon irradiation by cyclotron begins. Chicago Met Lab engineering council begins developing plans for large scale plutonium production reactors. President Roosevelt approves a plan for spending $85 million for a weapon development program. -
Design
Oppenheimer assembles theoretical study group in Berkeley to examine the principles of bomb design. Included are J. Robert Oppenheimer, Hans Bethe, Edward Teller, John Van Vleck, Felix Bloch, Robert Serber, and Emil Konopinski. During the summer the group develops the principles of atomic bomb design, and examines the feasibility of fusion bombs. Oppenheimer emerges as a natural leader. -
Irradiated Uranium Arrives
First shipment of irradiated uranium arrives at the Chicago Met Lab (300 lb.). Enrico Fermi's group demonstrates an experimental pile with a projected k value of close to 1.04. Achieving a chain reaction is now certain. -
Manhattan Leadership
Col. Leslie Groves is notified at 10:30 a.m. by Gen. Brehon Somervell that his assignment overseas has been cancelled and that he will take another assignment - command of the Manhattan Engineer District. Groves' previous assignment had required overseeing ten billion dollars’ worth of construction projects, including the construction of the Pentagon. -
Groves' Touch
Col. Leslie Groves buys 1250 tons of high quality Belgian Congo uranium ore stored on Staten Island. At his insistence the Manhattan Project is granted approval by the War Production Board to use the highest emergency procurement priority in existence (AAA) when needed. He selects Oak Ridge, TN as the site for the pilot plant. He buys Site X, 52,000 acres of land on the Clinch River. Preliminary construction work begins soon after. -
Chicago Pile-1
After 17 days of work, Enrico Fermi's group completes Chicago Pile-1. It contains 36.6 metric tons of uranium oxide, 5.6 metric tons of uranium metal, and 350 metric tons of graphite. Construction is halted sooner than planned when Fermi projects that a critical configuration has been reached. -
The Price Tag
Vannevar Bush provides Roosevelt with an estimate placing the total cost for the Manhattan Project at $400 million (almost 5 times the previous estimate). Roosevelt approves the expenditure. Plans and contracts are made for the construction of an experimental reactor, plutonium separation plant, and Electromagnetic separation facility at Oak Ridge, TN. -
Construction Begins
Construction begins at Oak Ridge, TN on buildings for the Y-12 Plant -- the electromagnetic U-235 separation plant. -
Roosevelt and Churchill Agree
President Franklin Roosevelt and British Prime Minister Winston Churchill sign the Hyde Park aide-memoire, pledging to continue researching atomic technology. -
A Test is Approved
J. Robert Oppenheimer approves plans for a bomb test in the Jornada del Muerto valley at the Alamagordo Bombing Range. General Leslie Groves approves 5 days later, provided that the test be conducted in Jumbo. -
Fat Man
First Fat Man bomb assembly is completed as production gets underway. Explosive lenses and nuclear material are not yet available, the bomb assemblies are used for airdrop and ground handling practice. -
Notification
Admiral Nimitz, Commander in Chief, Pacific Ocean Areas, is notified of the nature of the atomic bomb project. -
Possible Targets
The first meeting of the Target Committee to select targets for atomic bombing. Seventeen targets are selected for study: Tokyo Bay (for a non-lethal demonstration), Yokohama, Nagoya, Osaka, Kobe, Hiroshima, Kokura, Fukuoka, Nagasaki, and Sasebo (some of these are soon dropped because they had already been burned down). -
Little Boy
Little Boy is ready for combat use, except for the U-235 core. It is estimated sufficient material will be available by 1 August. -
The Three
Target Committee meets with Lt. Col. Paul Tibbets in attendance. The meeting reviews preparation for delivering atomic bombs, and status of conventional bombing of Japan. Tibbets estimates that by Jan. 1, 1946 all major cities of Japan will have been destroyed by fire bombing. The target list is now Kyoto, Hiroshima, and Niigata. -
The Trinity Test
At 5:29:45 a.m., as part of The Trinity Test, Gadget is detonated in Alamogordo, NM in the first atomic explosion in history. The explosive yield is 20-22 Kt (initially estimated at 18.9 Kt), vaporizing the steel tower. -
Final Decisions
Secretary of War Henry Stimson, in Potsdam for meeting between President Truman and Soviet Premier Stalin, receives current target list. In order of choice it is: Hiroshima, Kokura, and Niigata. He also receives an estimate of atomic bomb availability: Little Boy should be ready for use on Aug. 6, second Fat Man-type by Aug. 24, 3 should be available in September, and more each month - reaching 7 or more in December. -
A Warning
President Truman issues the Potsdam Declaration, which warns Japan of "prompt and utter destruction" and requires unconditional surrender of the Japanese armed forces. USS Indianapolis delivers Little Boy bomb units, and the U-235 projectile to Tinian Island. Five C-54 transport planes leave Kirtland Air Force Base, Albuquerque with: the Little Boy U-235 target (its final component); the Fat Man plutonium core, and its initiator. -
Hiroshima
Final briefing, the target of choice is Hiroshima. Lt. Col. Paul Tibbets is pilot, Robert Lewis is co-pilot. 0245: Enola Gay begins takeoff roll. 0730: The bomb is armed. 0850: Flying at 31,000 ft Enola Gay crosses Shikoku due east of Hiroshima. Bombing conditions are good, the aim point is easily visible, and no opposition is encountered. Little Boy explodes at an altitude of 1850 feet, 550 feet from the aim point, the Aioi Bridge, with a yield of 12.5-18 Kt (best estimate is 15 Kt). -
Nagasaki
Nagasaki is covered with clouds, but one gap allows a drop several miles from the intended aim point. 1102 (Nagasaki time): Fat Man explodes at 1950 feet near the perimeter of the city, scoring a direct hit on the Mitsubishi Steel and Arms Works. Yield is 19-23 Kt (best estimate is 21 Kt).
