The Timeline for the Nuclear Research and Atomic Bombs Used in World War 2  
  Tuesday, December 29, 1931  
  Harold C. Urey announced the discovery of heavy water at the Christmas meeting of the American. Association for the Advancement of Science. The American physical chemist found that heavy water was highly enriched in the hydrogen isotope deuterium (2H), and was used in many applications such as nuclear magnetic resonance, neutron moderation in nuclear power plants, and organic chemistry as well as German research in creating an atomic weapon.  
   
  Saturday, December 10, 1938  
  Italian physicist Enrico Fermi received the Nobel Prize for discovering fission of uranium (actually then believed to be transuranic elements).  
   
  Saturday, December 17, 1938  
  On the evidence of a decisive experiment (the celebrated "radium-barium-mesothorium-fractionation",) German chemist Otto Hahn concluded that the uranium nucleus had "burst" into atomic nuclei of medium weight. This was the discovery of nuclear fission.  
   
  Thursday, December 22, 1938  
  German chemists Otto Hahn and Fritz Strassmann sent a manuscript to Naturwissenschaften reporting their radiochemical results, which were the irrefutable proof that the uranium had been split into fragments consisting of lighter elements. Simultaneously, they communicated these results to Austrian-born Jewish physicist Lise Meitner, who had escaped out of Germany earlier that year and was then in Sweden. Meitner, and her nephew, Austrian-British physicist Dr. Otto R. Frisch, correctly interpreted these results as being nuclear fission, a term coined by Frisch, which subsequently became internationally known.  
   
  Friday, January 6, 1939  
  The scientific journal Naturwissenschaften published the findings of Hahn and Strassmann, nuclear physicists at the Kaiser Wilhelm Gesellschaft (Kaiser Wilhelm Society) in Berlin. Their paper reported the proof of nuclear fission using uranium-235.  
   
  Friday, January 13, 1939  
  Austrian-British physicist Otto Robert Frisch experimentally confirmed the results Hahn and Strassmann experiments as being nuclear fission.  
   
  Sunday, January 22, 1939  
  The uranium atom was split for the first time using the cyclotron designed and built by John Dunning at Columbia University in New York City. Dunning, a Columbia professor and later Dean of the now School of Engineering and Applied Science, and two student assistants split the uranium atom in the basement of Pupin Hall. George Pegram, chair of the Physics Department, was also involved; he and Dunning collaborated with Columbia professor Enrico Fermi. The success of this experiment led to the Manhattan Project and eventually to the development of the atomic bomb.  
   
  Thursday, January 26, 1939  
  Danish physicist Niels Bohr publicly announced the discovery of fission at an annual theoretical physics conference at George Washington University in Washington, DC. This announcement was the principal revelation of nuclear fission in the United States.  
   
  February, 1939  
  Dr. J. Robert Oppenheimer, a physicist at the University of California, Berkeley, speculated in a letter that a chain reaction in a 10-centimeter cube of uranium deuteride "might very well blow itself to hell."  
   
  March 1939  
  Fermi described the recent fission experiments and their implication that uranium could be a potent energy source or explosive to a group of U.S. military officials in Washington including Rear Admiral Stanford C. Hooper, the Chief of Naval Operations. Fermi had trouble getting these officials to take him seriously. On one occasion, Fermi got no further than two young lieutenant commanders, who politely listened as he outlined his case in broken English, before kindly showing him to the door.  
   
  Monday, July 3, 1939  
  Hungarian physicist Leo Szilard composed a letter to U.S. President Franklin D. Roosevelt. The letter pointed out to Roosevelt the importance of research on chain reactions and the possibility that this type of research might lead to developing powerful bombs. The letter was signed by Albert Einstein because Szilard felt it would more likely be taken seriously if Einstein was associated with the letter. The letter would be delivered by Alexander Sachs, an economist and writer who had a friendly relationship with U.S. President Franklin D. Roosevelt, on October 11, 1939.  
   
  Wednesday, August 2, 1939  
  Hungarian émigré and physicist Leo Szilard wrote to Fermi describing the concept (uranium lattice in carbon) for creating a chain reaction.  
   
  Wednesday, October 11, 1939  
  Roosevelt had a meeting with Alexander Sachs, an economist and writer who had a friendly relationship with Roosevelt, where Sachs delivered and discussed a letter written by Hungarian physicist Leo Szilard and signed by physicist Albert Einstein on August 2, 1939 warning of the importance of research on chain reactions and the possibility that this type of research might lead to developing powerful bombs. Roosevelt would establish the Advisory Committee on Uranium the next day.  
   
  Thursday, October 12, 1939  
  Roosevelt established the Advisory Committee on Uranium with Lyman J. Briggs, director of the U.S. National Bureau of Standards, as the committee head. This action was in response to a letter Roosevelt received the previous day from physicist Albert Einstein warning of the importance of research on chain reactions and the possibility that this type of research might lead to developing powerful bombs.  
   
  Thursday, October 19, 1939  
  Roosevelt informed physicist Albert Einstein of the establishment of the Advisory Committee on Uranium with Lyman J. Briggs, director of the U.S. National Bureau of Standards, as the committee head. Roosevelt was responding to a letter he had received on October 11, 1939 from Einstein warning of the importance of research on chain reactions and the possibility that this type of research might lead to developing powerful bombs.  
   
  Saturday, October 21, 1939  
  The Advisory Committee on Uranium with Lyman J. Briggs, director of the U.S. National Bureau of Standards, as the committee head met for the first time. The committee had been established on October 12, 1939 after U.S. President Franklin D. Roosevelt received a letter from physicist Albert Einstein warning of the importance of research on chain reactions and the possibility that this type of research might lead to developing powerful bombs.  
   
  Wednesday, November 1, 1939  
  The Advisory Committee on Uranium recommended that the U.S. government purchase graphite and uranium oxide for fission research.  
   
  March 1940  
  American physicist John R. Dunning of Columbia University and his colleagues demonstrated that fission was more readily produced in the rare uranium-235 isotope, not the more plentiful uranium-238.  
   
  Otto Robert Frisch and Rudolf Ernst Peierls co-authored the Frisch–Peierls memorandum. This short paper was the first to set out how one could construct an atomic bomb from a small amount of fissionable uranium-235.  
   
  Wednesday, June 12, 1940  
  Dr. Vannevar Bush, president of the Carnegie Institution of Washington, met with Roosevelt and proposed the creation of the National Defense Research Committee. Roosevelt agreed with Bush’s proposal.  
   
  Saturday, June 15, 1940  
  Dr. Vannevar Bush was appointed the head of the National Defense Research Committee.  
   
  The University of California began building a giant cyclotron under the direction of American physicist and Nobel Laureate Dr. Ernest O. Lawrence. It was at this facility that American physicist Dr. Edwin McMillan would bombard uranium with neutrons and produce the first "transuranic" element, neptunium (Atomic Number 93). This led to the eventual development of plutonium (Atomic Number 94).  
   
  Thursday, June 27, 1940  
  The National Defense Research Committee was officially created as part of the Council of National Defense, which had been created during 1916 to coordinate industry and resources for national security purposes, by an order of Roosevelt. The Advisory Committee on Uranium would become a subordinate council to the NDRC.  
   
  Monday, February 24, 1941  
  University of California at Berkeley professor Dr. Glenn T. Seaborg's research group discovered plutonium.  
   
  Friday, March 28, 1941  
  Seaborg's research group demonstrated that plutonium was fissionable and could be used as a weapon.  
   
  Saturday, May 3, 1941  
  Seaborg's research group proved plutonium was more fissionable than uranium-235.  
   
  Saturday, June 28, 1941  
  Bush was named head of the Office of Scientific Research and Development. Dr. James B. Conant, President of Harvard University, replaced Bush at the National Defense Research Committee, which became an advisory body to the Office of Scientific Research and Development.  
   
  Wednesday, July 2, 1941  
  A British MAUD Committee report, “'Use of Uranium for a Bomb”, concluded that an atomic bomb was feasible. The report described the bomb in technical detail, providing specific proposals for developing a bomb and including cost estimates. The report concluded that building an atomic bomb would require a large skilled labor force that was also needed for other parts of the war effort. The report also suggested that the Germans could also be working on such a bomb, and so it recommended that the work should be continued with high priority in cooperation with the Americans.  
   
  Monday, July 14, 1941  
  Bush and Conant received the MAUD Committee report.  
   
  Thursday, October 9, 1941  
  Bush briefed Roosevelt and U.S. Vice President Henry A. Wallace on the state of atomic bomb research. Roosevelt instructed Bush to find out if a bomb could be built and at what cost. Bush received permission to explore construction needs with the Army.  
   
  Sunday, November 9, 1941  
  A third National Academy of Sciences report agreed with the British MAUD Committee report that an atomic bomb was feasible.  
   
  Thursday, November 27, 1941  
  Dr. Vannevar Bush forwarded the third National Academy of Sciences report that agreed with the British Committee MAUD report to President Roosevelt.  
   
  Tuesday, December 16, 1941  
  A meeting of the Top Policy Group, consisting of U.S. Vice President Henry A. Wallace, U.S. Secretary of War Henry L. Stimson, and Dr. Vannevar Bush was held. Mr. H. L. Smith of the Budget Bureau attended. Bush described the reorganization that was in progress and his plans were approved. In a memorandum to Dr. James B. Conant describing his meeting, Bush wrote, "It was definitely felt by the entire group that OSRD should press as fast as possible on the fundamental physics and on the engineering planning, and particularly on the construction of pilot plants," Bush estimated the cost of this aspect of the work would be four or five million dollars, and stated the Army should take over when full-scale construction was started, presumably when pilot plants were ready. He suggested the assignment of a technically trained Army officer to become familiar with the general nature of the uranium problem. It was made clear at this meeting that the international relations involved were in the hands of the President, with Bush responsible or liaison on technical matters only. Dr. James B. Conant and U.S. General of the Army General George C. Marshall who were members of this group were absent from the meeting.  
   
  Thursday, December 18, 1941  
  A meeting of the reorganized S-1 Executive Committee (which replaced the Uranium Committee in the Office of Scientific and Research Development) was held. Dr. James B. Conant was present and discussed new policy, which called for an all-out effort. He emphasized that such an effort was justified only by the military value of atomic bombs and that all attention must be concentrated in the direction of bomb development. The whole meeting was pervaded by an atmosphere of enthusiasm and urgency. Several methods of electromagnetic separation were proposed and discussed, and a number of new contracts were recommended including $400,000 to the work on electromagnetic isotope separation by American physicist Ernest Lawrence of the University of California, Berkeley.  
   
  June 1942  
  Production pile designs were developed at the Metallurgical Laboratory in Chicago.  
   
  Wednesday, June 17, 1942  
  Roosevelt approved the S-1 Executive Committee recommendation to proceed to the pilot plant stage and instructed that plant construction be the responsibility of the U.S. Army. The Office of Scientific Research and Development continued to direct nuclear research, while the U.S. Army delegated the task of plant construction to the Corps of Engineers.  
   
  July 1942  
  American biophysicist Dr. Kenneth Cole of Columbia University established the health division at the Metallurgical Laboratory.  
   
  Thursday, August 13, 1942  
  The Manhattan Engineer District was established in New York City under the command of Colonel James C. Marshall.  
   
  Tuesday, August 18, 1942  
  Dr. Burris B. Cunningham and L.B. Werner succeeded in isolating about a microgram of Pu 239 (plutonium) free of carrier material and all other foreign matter, in the Metallurgical Laboratory of the University of Chicago. This made plutonium the first synthetic element to be obtained in visible amounts.  
   
  Saturday, August 29, 1942  
  A status report by Conant is relayed to the Stimson by Bush indicating the very positive results of Oppenheimer's group. Bush added his concerns about the organization and leadership of the project, requesting new leadership be appointed.  
   
  Thursday, September 10, 1942  
  Cunningham and L.B. Werner isolated a sample of 2.77 micrograms in the Metallurgical Laboratory of the University of Chicago.  
  Sunday, September 13, 1942  
  The S-1 Executive Committee visited Lawrence's University of California, Berkeley laboratory and recommended building an electromagnetic pilot plant and a section of a full scale plant in Tennessee.  
   
  Tuesday, September 15, 1942  
  Starting on this date, and continuing until November 15, Fermi's group received shipments of uranium and graphite for the Chicago Pile 1 (CP-1) at the University of Chicago and prepared them for assembly.  
   
  Thursday, September 17, 1942  
  Colonel Leslie R. Groves was notified at 10:30 a.m. by General Brehon Somervell that his assignment overseas has been cancelled and that he would take another assignment. The new assignment was the 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.  
   
  Friday, September 18, 1942  
  Groves sent Colonel Kenneth D. Nichols to purchase the 1,200 tons of uranium ore owned by the Union Minière du Haut Katanga (UMHK), a Belgian mining company, and being stored at Staten Island. Nichols was met by Edgar Sengier, head of the UMHK, who reportedly said, “I have been waiting for your visit.” Nichols negotiated a contract to purchase all the uranium ore Sengier had brought to the United States as well as all he had remaining in the Belgian Congo.  
   
  Saturday, September 19, 1942  
  Groves bought Site X for a pilot uranium enriching plant. Site X was 52,000 acres of land on the Clinch River in Roane and Anderson counties, Tennessee and would become the future site of Oak Ridge. Preliminary construction work began soon after.  
   
  Groves requested that Donald Nelson, head of the War Production Board, sign a letter giving the Manhattan Project a AAA priority rating, the highest rating available. Nelson resisted Groves’ request. After Groves told Nelson that he would go to U.S. President Franklin D. Roosevelt if he did not sign it Nelson relented.  
   
  Wednesday, September 23, 1942  
  U.S. Secretary of War Henry L. Stimson created a Military Policy Committee to help make decisions for the Manhattan Project.  
   
  Colonel Leslie R. Groves was promoted to Brigadier General.  
   
  Saturday, September 26, 1942  
  At Groves' insistence the Manhattan Project was granted approval by the War Production Board to use the highest emergency procurement priority in existence (AAA) when needed.  
   
  Tuesday, September 29, 1942  
  Oppenheimer proposed that a "fast-neutron lab" to study fast neutron physics and develop designs for an atomic bomb be created. The idea at this point was for the lab to be a small research institution; it would not be involved in the engineering and production of nuclear weapons.  
   
  October 1942  
  Conant recommended to that information exchange with Britain, already largely one-way (United Kingdom to the United States), be sharply restricted. Bush would pass this recommendation to Roosevelt. As a result the U.S. lost access to British work in gaseous diffusion, which seriously delayed successful gaseous diffusion plant completion.  
   
  Saturday, October 3, 1942  
  E. I. du Pont de Nemours and Company agreed to build the chemical separation plant at Oak Ridge, Tennessee.  
   
  Monday, October 5, 1942  
  American physicist Arthur Compton, head of the S-1 Committee, recommended an intermediate pile at Argonne National Laboratory at the University of Chicago.  
   
  Groves visited the Met Lab and met the key scientists, including Oppenheimer. He ordered that key engineering decisions for plutonium production, under debate for months, be made in 5 days.  
   
  Thursday, October 15, 1942  
  Groves asked Oppenheimer to head Project Y, planned to be the new central laboratory for weapon physics research and design.  
   
  Monday, October 19, 1942  
  Bush approved Oppenheimer’s appointment in a meeting with Oppenheimer and Groves.  
   
  Groves decided to establish a separate scientific laboratory to design an atomic bomb.  
   
  Monday, October 26, 1942  
  Dr. James B. Conant, of the S-1 Executive Committee, recommended dropping the centrifuge method of producing uranium.  
   
  Tuesday, November 3, 1942  
  Dr. Glenn T. Seaborg reported that due to plutonium's high alpha activity, slight amounts of light element impurities can cause a serious problem with neutron emission from alpha to n reactions. This issue caused major concern with many project leaders, including Groves and Conant, chairman of the National Defense Research Committee, not only due to its own significance, but because it raised apprehension about the impact of other unexplored phenomena. (This issue later became moot due to the problems with Pu-240 contamination.) Later in the month the Lewis Committee was formed to review progress and make recommendations.  
   
  Thursday, November 12, 1942  
  On the recommendation of Groves and Conant, the Military Policy Committee decided to skip the pilot plant stage on the plutonium, electromagnetic, and gaseous diffusion projects and go directly from the research stage to industrial-scale production. The Committee also decided not to build a centrifuge plant.  
   
  Friday, November 13, 1942  
  Bush convinced Roosevelt to ratify the policy of restricted interchange of atomic research with Great Britain. “In accordance with your instruction,” Bush wrote, “our British relations have been discussed…and we have reached a tentative agreement…The basic principle observed was free interchange of all information provided the Nation receiving such interchange is capable of making use of it during the present war.”  
   
  Saturday, November 14, 1942  
  The S-1 Executive Committee endorsed the November 12 recommendations of the Military Policy committee.  
   
  Monday, November 16, 1942  
  Physicist Enrico Fermi's group began constructing the CP-1 at Staggs Field at the University of Chicago using round-the-clock shifts.  
   
  Brigadier General Leslie R. Groves and Dr. J. Robert Oppenheimer visit the Los Alamos mesa in New Mexico and selected it for "Site Y".  
   
  Wednesday, November 25, 1942  
  Groves selected Los Alamos, New Mexico as the laboratory for the atomic bomb (codenamed Project Y). Dr. J. Robert Oppenheimer of the University of California, Berkeley was chosen as laboratory director.  
   
  December 1942  
  Bush provided Roosevelt with an estimate placing the total cost for the Manhattan Project at $400 million (almost 5 times the previous estimate). Roosevelt would approve the expenditure..  
   
  Plans and contracts were made for the construction of an experimental reactor, plutonium separation plant, and electromagnetic separation facility at Oak Ridge.  
   
  Tuesday, December 1, 1942  
  After 17 days of work, Dr. Enrico Fermi's group completed CP-1 (the Chicago Pile). It contained 36.6 metric tons of uranium oxide, 5.6 metric tons of uranium metal, and 350 metric tons of graphite. Construction was halted sooner than planned when Fermi projected that a critical configuration has been reached.  
   
  Sunday, December 6, 1942  
  M. M. Sundt Company was appointed contractor to build the Los Alamos Laboratory in a handshake deal. The company began construction immediately, without plans or blueprints in order to finish as quickly as possible.  
   
  Wednesday, December 2, 1942  
  Below the bleachers of Stagg Field at the University of Chicago at 3:49 p.m., a team of scientists led by Italian physicist Enrico Fermi and American physicist Dr. Arthur Compton of Washington University of St. Louis successfully completed the first controlled nuclear reaction at Chicago Pile 1 (CP-1). A coded message, "The Italian navigator has landed in the new world" was sent to U.S. President Franklin D. Roosevelt. The pile demonstrated a k value of 1.0006, and was allowed to reach a thermal output of 0.5 watts. Ultimately it would operate at 200 watts maximum.  
   
  Thursday, December 10, 1942  
  A final review committee headed by Warren K. Lewis of the Massachusetts Institute of Technology compromised on the electromagnetic method. The Military policy committee decided to build the plutonium production facilities at a site other than Oak Ridge.  
   
  Monday, December 28, 1942  
  U.S. President Franklin D. Roosevelt approved detailed plans for building production facilities and producing atomic weapons.  
   
  Agreeing with his advisors, Roosevelt confirmed that information on research activities would not be provided to the British unless they were actively involved in the research. The policy was specifically targeted at the ultra-secret Manhattan Project, the development of the nuclear bomb. The British were upset by the decision.  
   
  January 13 - January 14, 1943  
  Plans for the Y-12 electromagnetic plant at Oak Ridge needed to separate uranium-235 were discussed. Groves insisted that Y-12's first racetrack be finished by July 1, 1943.  
   
  Saturday, January 16, 1943  
  Groves acquired the Hanford Engineer Works, 780 square miles of land on the Columbia River in Washington for plutonium production reactors and separation plants. Eventually three reactors, called B, D, and F, would be built at Hanford.  
   
  Thursday, February 18, 1943  
  Construction began at Oak Ridge on buildings for Y-12, the electromagnetic U-235 separation plant.  
   
  February 1943  
  Groundbreaking for the X-10 plutonium pilot plant took place at Oak Ridge.  
   
  March 1943  
  As the original construction program neared completion Oppenheimer began assembling his preliminary staff at Los Alamos. Work began with a series of lectures by the senior scientists who had already done work on the atomic weapon program. From this point on the site would grow non-stop through the end of the war.  
   
  Saturday, March 27, 1943  
  Dr. Richard C. Tolman wrote Dr. J. Robert Oppenheimer about using explosives to collapse a shell into a critical mass. This is the earliest surviving reference to the idea of implosion (although this term was not used).  
   
  April 1943  
  Work at Los Alamos began on the atomic bomb’s design.  
   
  Thursday, April 1, 1943  
  Fencing of the property at Oak Ridge was completed. Oak Ridge was now closed off to public access.  
   
  Construction began on the plant for manufacturing gaseous diffusion barriers in Decatur, Ill. although no barrier materials of usable quality had yet been produced.  
   
  A contract was concluded with the University of California to manage Los Alamos, acting as paymaster, accountant, and procurement agency. This contract (back dated to Jan. 1 for work already performed) is still in existence and serves as the basis for University of California management of both the Los Alamos and Lawrence Livermore laboratories.  
   
  Monday, May 10, 1943  
  The Los Alamos review committee approved the laboratory's research program.  
   
  June 1943  
  U.S. Navy Captain William Parsons arrived at Los Alamos as Ordnance Division leader to begin directing gun assembly research.  
   
  Tuesday, June 1, 1943  
  Site preparation for the K-25 gaseous diffusion plant commenced at Oak Ridge, Tennessee.  
   
  Thursday, June 24, 1943  
  Working with cyclotron produced plutonium, Dr. Emilio Segre determined that the spontaneous fission rate was 5 fissions/kg-sec. This was well within the assembly speed capability of a high speed gun.  
   
  Sunday, July 4, 1943  
  Dr. Seth Neddermeyer conducted the first explosion in the implosion research program at Los Alamos. At this time this program consisted of Neddermeyer and 3 informal assistants.  
   
  July 10 - July 15, 1943  
  The first nuclear physics experiment was conducted at Los Alamos (the measurement of Pu-239 fission neutron yield). This inaugurated Los Alamos as a functioning laboratory.  
   
  Tuesday, July 20, 1943  
  Groves personally waived the security requirements and issued Oppenheimer a clearance so that he could become head of the Manhattan Project.  
   
  August 1943  
  Despite the efforts of more than 1,000 researchers at Kellex and Columbia University, no suitable diffusion barrier material had yet been developed.  
   
  Due to lagging progress on gaseous diffusion, and continuing uncertainties about the required amount of U-235 for a bomb, Groves decided to double the size of the Y-12 electromagnetic plant at Oak Ridge.  
   
  The first Alpha electromagnetic uranium separation unit began operation. Construction staff at Oak Ridge now exceeded 20,000.  
   
  Construction began on the cooling systems for the production reactors at Hanford, Washington. Construction staff is about 5,000.  
   
  Friday, August 27, 1943  
  Groundbreaking for the water-cooling plant for the 100-B pile plutonium production pile at Hanford took place.  
   
  Thursday, September 9, 1943  
  Groves decided to double the size of Y-12 electromagnetic plant at Oak Ridge.  
   
  Friday, September 17, 1943  
  The first shot was fired in the gun assembly research program at Los Alamos. The focus at this point was on developing a high velocity gun for plutonium since a uranium gun would be much easier to make.  
   
  Monday, September 20, 1943  
  Hungarian-American mathematician Johann Von Neumann arrived on a visit to Los Alamos and pointed out the potential for high compression from implosion. This was a clear advantage for the technique which would make a bomb more efficient, and require a smaller critical mass. Hungarian-American theoretical physicist Dr. Edward Teller and German-American nuclear physicist Dr. Hans Bethe began investigating the subject theoretically. Oppenheimer and Groves became very interested in its potential, and efforts to accelerate the program began. Von Neumann agrees to work on the physics of implosion in his spare time.  
   
  Thursday, September 23, 1943  
  Oppenheimer suggested recruiting Ukrainian-American Harvard chemistry professor George Kistiakowsky, the leading explosives research director at Office of Scientific Research and Development, to aid an expanded implosion effort.  
   
  Monday, September 27, 1943  
  Construction began on the K-25 gaseous diffusion plant at Oak Ridge.  
   
  October 1943  
  The first Alpha racetrack (containing 96 units) was completed. A work force of 4,800 to run Y-12 had been assembled. The startup was unsuccessful due to unexplained shorts in the magnets.  
   
  Project Alberta, the full scale atomic bomb delivery program, began.  
   
  Harvard physics professor Dr. Norman F. Ramsey was appointed to select and modify aircraft for delivering atomic bombs.  
   
  Monday, October 4, 1943  
  Du Pont engineers released reactor design drawings for the first Hanford plutonium production pile, B-100, allowing construction to begin.  
   
  Sunday, October 10, 1943  
  Site preparation starts for the B-100 plutonium production reactor at Hanford.  
   
  Thursday, October 21, 1943  
  The first concrete was poured for the K-25 gaseous diffusion plant building at Oak Ridge.  
   
  November 1943  
  The top experts in England on fission weapons, many former members of the MAUD committee, departed England for the United States to assist the atomic bomb project. Included are Niels Bohr, Otto Frisch, Rudolf Peierls, James Chadwick, William Penney, George Placzek, P.B. Moon, James Tuck, Egon Bretscher, and Klaus Fuchs.  
   
  The U.S. Navy approved Philip Abelson's plan to build a liquid thermal diffusion pilot plant for enriching uranium.  
   
  The world's first sample of plutonium in metal form was produced by reducing PuF4 with Ba at the Met Lab.  
   
  Thursday, November 4, 1943  
  The X-10 pile went critical at Oak Ridge. This air-cooled experimental pile began producing the first substantial (gram) amounts of plutonium to assist research into its properties. The world supply of plutonium at this time is 2.5 mg, produced by cyclotrons. The plant would begin producing plutonium by the end of the month.  
   
  A Manhattan Project Governing Board meeting approved an ambitious implosion research program, intended to develop it to the point of usability in six months.  
   
  Monday, November 29, 1943  
  The first B-29 Superfortress modifications began at Wright Field, Ohio to adapt it for carrying atomic bombs.  
   
  December 1943  
  Dr. Emilio Segre measured the spontaneous fission rate of U-235 at Los Alamos, New Mexico and found it lower than expected. This allowed a substantial reduction in performance of the planned gun assembly method for uranium.  
   
  Chemical separation of reactor-produced plutonium begins, using fuel from the X-10 pile.  
   
  Wednesday, December 15, 1943  
  Groves arrived at Oak Ridge and shut down the first Alpha racetrack of the Y-12 electromagnetic plant. This was due to various failures including the vacuum tanks leaking and shimmying out of line due to magnetic pressure, failed welds, malfunctioning electrical circuits, and frequent operator mistakes. Most seriously, the magnet coils shorted out because of rust and sediment in the cooling oil.  
   
  January 1944  
  Kistiakowsky arrived at Los Alamos to assist Neddermeyer in implosion research. It became increasingly clear that Neddermeyer's academic research style was unsuited to directing a rapidly expanding research and engineering program.  
   
  The second Alpha racetrack of the Y-12 electromagnetic plant at Oak Ridge was started and demonstrated maintenance problems similar to those that disabled the first.  
   
  Problems with developing suitable diffusion barriers led Groves to switch planned production to a new type of barrier, creating months of delays in equipping the K-25 gaseous diffusion plant at Oak Ridge for operation.  
   
  Dr. Philip Abelson, at the Naval Research Laboratory at the Philadelphia Naval Yard, began constructing a thermal diffusion uranium enrichment plant. Upon learning about the problems with the Manhattan Project's gaseous diffusion plant, he leaked information about his technology to Oppenheimer at Los Alamos.  
   
  Groves and Oppenheimer decided to plan for a fission bomb test (none was envisioned before this). Groves stipulated that the active material must be recoverable if a fizzle occurred, so the construction of Jumbo, a 214 ton steel container (25 ft x 12 ft), was authorized.  
   
  Tuesday, January 11, 1944  
  An implosion theory group was set up with Dr. Edward Teller as head.  
   
  February 1944  
  The Alpha 2 racetrack at the Y-12 electromagnetic plant at Oak Ridge produced about 200 grams of twelve-percent uranium 235 by the end of February 1944. This was enough to send samples to Los Alamos as well as feed the new Beta unit but not enough to satisfy estimates of weapon requirements.  
   
  With the concrete building to house it complete, construction began on the first reactor at Hanford: the B pile.  
   
  The Los Alamos Governing Board reevaluated deuterium fusion research and determined that tritium would be necessary to make an explosive reaction. Priority of fusion bomb work was further downgraded.  
   
  Wednesday, February 16, 1944  
  Kistiakowsky became the full-time Los Alamos staff member, replacing Neddermeyer as leader of implosion research.  
   
  March 1944  
  The Beta building at the Y-12 electromagnetic plant at Oak Ridge was completed.  
   
  Dr. Emilio Segre had improved his spontaneous fission estimates in cyclotron plutonium (essentially pure Pu-239) to 11 fissions/kg-sec, this was still acceptable for gun assembly, but greatly narrowed the margin of security.  
   
  Models for the atomic bomb were tested at Los Alamos.  
   
  Friday, March 3, 1944  
  Drop tests with the specially modified B-29 Superfortresses and full-scale" dummy bombs began at Muroc, California.  
   
  April 1944  
  Oppenheimer informed Groves that Abelson’s experiments on thermal diffusion at the Philadelphia Naval Yard deserved a closer look. Abelson was building a plant to produce enriched uranium to be completed in early July. It might be possible, Oppenheimer thought, to help Abelson complete and expand his plant and use its slightly enriched product as feed for the Y-12 electromagnetic plant at Oak Ridge until problems with K-25 power plant could be resolved.  
   
  IBM calculating equipment arrived at Los Alamos, New Mexico and was put to work on implosion research.  
   
  British physicist James Tuck suggested the idea of using explosive lenses to create spherical converging implosion waves.  
   
  Wednesday, April 5, 1944  
  The first sample of reactor produced plutonium arrived from Oak Ridge, Tennessee. Dr. Emilio Segre immediately began monitoring its spontaneous fission rate. Monsanto would begin delivering polonium for initiator research. The rate is initially 2.5 curies/month.  
   
  Saturday, April 15, 1944  
  Segre made a preliminary estimate of a spontaneous fission rate of over 50 fissions/kg-sec (due to Pu-240 contamination), far too high for gun assembly. The report was kept quiet due to limited statistics, and observations continue.  
   
  May, 1944  
  The Los Alamos staff exceeded 1200 employees.  
   
  Six months after the start of accelerated implosion research, little progress towards successful implosion had been made. Inadequate diagnostic equipment prevented accurate measurement of the implosion process and no scheme to avoid asymmetry had yet shown promise. The current approach was to use many simultaneous detonation points over the surface of a sphere, and try different methods of inert spacers or gaps to suppress the shaped charge-like jets that form when detonation waves from adjacent initiation points merge. Spalling (the ejection of fragments) from the interior surface of the hollow core was a serious problem, as was simply getting precise simultaneous detonation.  
   
  Dr. Edward Teller was removed as head of the implosion theory group, and also from fission weapon research entirely, due to conflicts with Dr. Hans Bethe and his increasing obsession with the idea of the Super (hydrogen bomb).  
   
  Two British scientists joined Los Alamos who had important impacts on the implosion program. Geoffrey Taylor (who arrived May 24) pointed out implosion instability problems (especially the Rayleigh-Taylor instability), which ultimately led to a very conservative design to minimize possible instability. James Tuck brought the idea of explosives lenses for detonation wave shaping (two-D lenses for plane wave generation originally proposed by M. J. Poole in England, 1942), but suggested developing 3-D lenses to create a spherical implosion.  
   
  Tuesday, May 9, 1944  
  The 50 milliWatt Water Boiler reactor went critical at Los Alamos. Holding 565 g of U-235 (in the form of 14.7% enriched uranyl sulfate), dissolved in a 12" sphere of water, this was the world's first reactor to use enriched uranium, and the first critical assembly constructed at Los Alamos.  
   
  Sunday, May 28, 1944  
  The first test of the exploding wire detonator occurred, used to achieve precise, reliable simultaneous detonation for implosion.  
   
  June 1944  
  Oppenheimer replaces Neddermeyer with Kistiakowsky as director of implosion research.  
   
  Dr. Hans Bethe and Rudolf Peierls worked on developing explosive lens concept.  
   
  Johann Von Neumann provided a design breakthrough for the slow component for focusing.  
   
  Saturday, June 3, 1944  
  After visiting the uranium enrichment pilot plan at the Naval research Laboratory, a team of Manhattan Project experts recommended that a thermal diffusion plant be built to feed enriched material to the electromagnetic enrichment plant at Oak Ridge.  
   
  Sunday, June 18, 1944  
  Groves contracted to have S-50, a liquid thermal diffusion uranium enrichment plant, built at Oak Ridge in no more than three months.  
   
  July 1944  
  Experiments with explosive lens designs began by mid-month when 2-D models were fired. The design for the gun gadget neutron initiator was completed.  
   
  Tuesday, July 4, 1944  
  U.S. President Franklin D. Roosevelt granted the "The Manhattan Project" the highest project-wide procurement priority (AA-1).  
   
  Saturday, July 1, 1944  
  Oppenheimer revealed Segre's spontaneous fission measurements to the Los Alamos staff. The neutron emission for reactor-produced plutonium was too high for the gun assembly to work. The measured rate was 50 fissions/kg-sec, the fission rate in Hanford plutonium was expected to be over 100 times higher still. The planned plutonium gun had to be abandoned, and Oppenheimer was forced to make implosion research a top priority, using all available resources.  
   
  The decision was made to work on a calutron with a 30-beam source for use in Y-12 electromagnetic plant.  
   
  Monday, July 17, 1944  
  Work on the "Thin Man" (formally the Mark 2) nuclear bomb was abandoned. This proposed plutonium gun-type nuclear bomb was discontinued when it was discovered that the spontaneous fission rate of nuclear reactor-bred plutonium was too high for use in a gun-type design.  
   
  Thursday, July 20, 1944  
  The Los Alamos Administrative Board decided on a reorganization plan to direct the laboratory's full resources on implosion. Instead of being organized around scientific and engineering areas of expertise, all work was organized around whether it applied to implosion, or the uranium gun weapon, with the former receiving most of the resources. The reorganization was completed in less than two weeks.  
   
  August 1944  
  The U.S. Army Air Force began modifying 17 B-29 Superfortresses for combat delivery of atomic weapons at the Glenn L. Martin plant in Omaha, Nebraska.  
   
  U.S. Navy Capt. William Parsons assessed February 1945 as the earliest that an implosion lens system could be ready for full scale test "with extremely good breaks", and most likely late 1945.  
   
  Harvard geophysicist A. Francis Birch was placed in charge of designing and engineering the triggering mechanism for the Hiroshima atomic bomb code-named Little Boy. Birch was chosen because of his knowledge of metals and metallurgy.  
   
  Monday, August 7, 1944  
  Dr. Vannevar Bush, the director of the Office of Scientific Research and Development which oversaw the Manhattan Project, briefed U.S. General of the Army George C. Marshall, informing him that small implosion bombs might be ready by mid-1945 and that a uranium bomb would almost certainly be ready by August 1, 1945.  
  September 1944  
  Air Force Lt. Colonel Paul Tibbets began organizing the 509th Composite Group at Wendover Field, Utah. This unit would include the 393rd Bombardment Squadron which began test drops with dummy bombs called Pumpkins. The 509th Composite Group would deliver the atomic bombs on Japan in August of 1945.  
   
  The K-25 gaseous diffusion plant at Oak Ridge was half built at this point, but no usable diffusion barriers have been produced. The Y-12 electromagnetic plant was operating at only 0.05% efficiency. The total production of highly enriched uranium to date is a few grams.  
   
  Wednesday, September 13, 1944  
  The first uranium slug was placed in pile 100-B (a nuclear reactor) at Hanford, Washington by Italian physicist Enrico Fermi.  
   
  Saturday, September 16, 1944  
  The S-50 thermal diffusion plant at Oak Ridge began partial operation, but leaks prevented substantial output.  
   
  Tuesday, September 19, 1944  
  Roosevelt and British Prime Minister Winston Churchill met in Hyde Park, New York and signed an "aide memoire" concerning atomic policy. In the document it was agreed that the atomic bomb was remain an utmost secret, that the U.S. and Great Britain would have a postwar atomic monopoly, and that the enemy, most likely Japan, would be repeatedly warned that the use of the bomb would be repeated until surrender.  
   
  Friday, September 22, 1944  
  The first RaLa implosion test shot was made. This diagnostic technique used 100 curies of radiolanthanum produced by the X-Reactor at Oak Ridge to provide an intense gamma source for making observations of implosion (essentially an internal x-ray generator). This was the largest radioisotope source ever assembled in the world up to this time.  
   
  Tuesday, September 26, 1944  
  The first full scale plutonium reactor, the B pile, at Hanford was completed and loaded with uranium and the process for producing plutonium began. This reactor contained 200 tons of uranium metal, 1,200 tons of graphite, and was cooled by 5 m^3 of water/sec. It was designed to operate at 250 megawatts, producing some 6 kg of plutonium a month. Fermi supervised the reactor start-up.  
   
  Wednesday, September 27, 1944  
  The 100-B reactor at Hanford went critical for the first time and was operating at “a higher level of power than any previous chain reaction.” Despite this initial success, after less than a day’s operation, the reactor experienced a significant problem due to by-products of the fission process, which caused it to lose power and shut itself down. Within a few days this is determined to be due to poisoning by the highly efficient neutron absorber Xenon-135, a radioactive fission product. The reactor had to be modified to add extra reactivity to overcome this effect before production can begin.  
   
  Saturday, September 30, 1944  
  Bush and Conant advocated international agreements on atomic research to prevent an arms race.  
   
  Friday, October 27, 1944  
  Oppenheimer approved plans for a bomb test in the Jornada del Muerto valley at the Alamagordo Bombing Range.  
   
  November 1944  
  Output at the Y-12 electromagnetic plant reached 40 grams of highly enriched uranium a day.  
   
  Wednesday, November 1, 1944  
  Groves approved Oppenheimer’s plans for a bomb test in the Jornada del Muerto valley at the Alamagordo Bombing Range, provided that the test be conducted in Jumbo (a 214-ton steel container).  
   
  December 1944  
  Both 221T and 221U, the chemical separation buildings in the 200-West complex at Hanford, were finished by December 1944. Nicknamed Queen Marys by the workers who built them, the separation buildings were awesome canyon-like structures 800 feet long, 65 feet wide, and 80 feet high containing forty process pools.  
   
  Output at the Y-12 electromagnetic plant climbed to 90 grams of highly enriched uranium a day.  
   
  Work began on an implosion initiator for the solid core bomb although it was not clear at this point if one could be made.  
   
  Mid-December, 1944 - The first successful explosive lens tests were conducted at Los Alamos. These tests established the feasibility of making an implosion bomb.  
   
  Sunday, December 17, 1944  
  The D pile at Hanford went critical with sufficient reactivity to overcome fission product poisoning effects. Large scale plutonium production began.  
   
  Friday, December 22, 1944  
  The first Fat Man bomb assembly was completed as production got underway. Explosive lenses and nuclear material were not yet available. The bomb assemblies were used for airdrop and ground handling practice.  
   
  Tuesday, December 26, 1944  
  The processing of irradiated uranium slugs to separate plutonium began at Hanford.  
   
  Thursday, December 28, 1944  
  The modified B pile at Hanford was restarted.  
   
  January 1945  
  Output at the Y-12 electromagnetic plant at Oak Ridge reached an average of 204 grams of 80% U-235 a day. The projected production of sufficient material for a bomb (approximately 40 kilograms) was July 1.  
   
  Usable barrier tubes began arriving at the K-25 gaseous diffusion plant at Oak Ridge.  
   
  160 grams of plutonium from the X-Pile at Oak Ridge was on hand at Los Alamos. The first shipment from Hanford had not yet arrived.  
   
  Substantial production of approximately 0.85% enriched uranium began at S-50 thermal diffusion plant at Oak Ridge, with ten of 21 racks going in to operation.  
   
  Thursday, January 18, 1945  
  The Dragon experiment was conducted at Los Alamos in which a U-235 hydride slug was dropped through a ring of barely subcritical U-235 hydride and made it go critical for a fraction of a second. The object was to reach prompt criticality and thereby simulate bomb conditions.  
   
  Saturday, January 20, 1945  
  The first stage of the K-25 gaseous diffusion plant at Oak Ridge was charged with uranium hexafluoride and began operation.  
   
  Wednesday, January 31, 1945  
  American nuclear physicist Robert Bacher reported to Dr. J. Robert Oppenheimer that a Po-210/Be-9 implosion initiator was possible. The solid-core design of this type of trigger device was used for Gadget, the first nuclear explosive ever tested, and Fat Man, the atomic bomb dropped on Nagasaki.  
   
  February 1945  
  The F reactor went on-line at Hanford, raising theoretical production of capacity to 21 kg of plutonium a month.  
   
  Planning for an implosion bomb test began in earnest.  
   
  Initiator tests began. Demand for polonium rose to 100 curies/month. Initiators were the triggers that set off the atomic bombs and polonium the element that was used to set off the device.  
   
  Admiral Chester A. Nimitz, Commander in Chief, Pacific Ocean Areas, was notified of the nature of the atomic bomb project. “This sounds fine,” he told the courier, “but this is only February. Can't we get one sooner?”  
   
  Tinian Island in the Northern Marianas was selected as the base of operations for the atomic attacks on Japan.  
   
  Friday, February 2, 1945  
  After a month and a half of refining, the first batch of plutonium produced at Hanford was ready for transport to the Los Alamos testing site. It took three days to arrive in New Mexico where it was then tested by Oppenheimer and other physicists at the lab.  
   
  Friday, February 5, 1945  
  The first batch of plutonium produced at Hanford was arrived at the Los Alamos testing site testing by Oppenheimer and other physicists at the lab began tests with it.  
   
  Sunday, February 25, 1945  
  The first stage of the K-25 gaseous diffusion plant at Oak Ridge began operating.  
   
  Wednesday, February 28, 1945  
  A meeting between Dr. J. Robert Oppenheimer, Brigadier General Leslie R. Groves, Dr. George Kistiakowsky, Dr. James B. Conant, Dr. Richard C. Tolman, Dr. Hans Bethe, and Dr. Charles Lauritsen was held to fix the design approach for the plutonium bomb. It was agreed that work would focus on the solid core Christy gadget, use explosive lenses, use a modulated initiator, and electric detonators. The use of Composition B and Baratol for the lenses was also decided, as was the multiple lens configuration and detonator arrangement. However none of these approaches or components had been proved yet. The solid core compression had not been demonstrated at this time. A schedule for completing research, development, engineering, and testing was also established.  
   
  March 1945  
  The S-50 thermal diffusion plant finally began enriching uranium in quantity at Oak Ridge.  
   
  The first evidence of solid compression from implosion was observed (5%).  
   
  Thursday, March 1, 1945  
  The powerful Cowpuncher Committee was organized by Dr. J. Robert Oppenheimer to "ride herd" on implosion bomb development. The Cowpuncher Committee consisted of Dr. Hans Bethe, Dr. George Kistiakowsky, Rear Admiral William Sterling "Deak" Parsons, Dr. Robert Bacher, Dr. Samuel Allison, Dr. Cyril Smith, and Dr. Kenneth Bainbridge.  
   
  Monday, March 12, 1945  
  K-25 gaseous diffusion plant began production at Oak Ridge.  
   
  Thursday, March 15, 1945  
  All 21 racks at the S-50 thermal diffusion plant at Oak Ridge were finally in operation.  
   
  Tuesday, April 3, 1945  
  Preparations began at Tinian Island, Marianas Islands to support the 509th Composite Group and to assembly the atomic bombs.  
   
  Wednesday, April 11, 1945  
  Oppenheimer reported that Kistiakowsky had achieved optimal performance with implosion compression in sub-scale tests.  
   
  Thursday, April 12, 1945  
  Dr. Otto R. Frisch completed criticality and "zero-yield" experiments with U-235 at Los Alamos, New Mexico.  
   
  Friday, April 13, 1945  
  U.S. President Harry S. Truman learned of the existence of the atomic bomb development from U.S. Secretary of War Henry L. Stimson.  
   
  Wednesday, April 25, 1945  
  U.S. Secretary of War Henry L. Stimson and Brigadier General Leslie R. Groves briefed U.S. President Harry S. Truman on the Manhattan Project.  
   
  Friday, April 27, 1945  
  The first meeting of the Target Committee to select targets for atomic bombing was held. Seventeen targets were 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).  
   
  Monday, April 30, 1945  
  The Initiator Committee (consisting of Dr. Hans Bethe, Dr. Enrico Fermi and Dr. Robert Christy) selected the most promising design for fission initiator (neutron generator) to be used in the implosion bomb. The "Urchin" design was favored, and work on initiator fabrication began.  
   
  The first batch of supplies for the atomic bomb deployment left for Tinian from Wendover Field, Utah.  
   
  May 31 - June 1, 1945  
  The Interim Committee met to make recommendations on wartime use of atomic weapons, international regulation of atomic information, and legislation regarding domestic control of the atomic enterprise. The Committee's draft legislation becomes the basis for the May- Johnson bill. The May- Johnson bill would put the atomic program under control of a nine-member commission of military men and civilians and would legislate severe penalties for security violations. Many scientists were opposed to the bill because they felt it would stifle the free exchange of information and would lead to a military monopoly over nuclear research.  
   
  Wednesday, June 6, 1945  
  Stimson informed Truman that the Interim Committee recommended keeping the atomic bomb a secret and using it as soon as possible without warning.  
   
  Tuesday, June 12, 1945  
Physicist James Franck delivered the Franck Report to officials in Washington. The report was produced by scientists at the Metallurgical Laboratory at the University of Chicago and advocated international control of atomic research and proposed a demonstration of the atomic bomb prior to its use in combat. The report spoke about the impossibility to keep the United States atomic discoveries secret indefinitely. It predicted a nuclear arms race, forcing the United States to develop nuclear armaments at such a pace that no other nation would think of attacking first from fear of overwhelming retaliation. Truman appointed a separate committee that would meet on June 21 to reexamine the use of the atomic bomb.
   
  Thursday, June 14, 1945  
  Groves submitted the target selection group's recommendation to Marshall. This committee of scientists and U.S. Army Air Force officers listed Kokura Arsenal, Hiroshima, Niigata, and Kyoto as the four best targets, believing that attacks on these cities - none of which had been bombed by Major General Curtis LeMay’s Twentieth Air Force - would make a profound psychological impression on the Japanese and weaken military resistance. Stimson would veto Kyoto, Japan’s most cherished cultural center, and Nagasaki would replace it in the directive issued to the U.S. Army Air Force on July 25.  
   
  Thursday, June 21, 1945  
  A committee appointed by Truman held a meeting to discuss the Franck Report and to reexamine the use of the atomic bomb. This committee rejected the Franck Report and reaffirmed the Interim Committee's decision that there was no alternative but to use the bomb.  
   
  Monday, July 2, 1945  
  Stimson sent a memorandum to Truman that outlined the Interim Committee's deliberations, reasoning, and outlined a draft proposal concerning warning Japan about the atomic bomb. Read the text of the memorandum.  
   
Tuesday, July 3, 1945
  The casting of the U-235 projectile for “Little Boy” was completed. "Little Boy" was the codename for the atomic bomb that was detonated over Hiroshima, Japan on August 6.  
   
  Friday, July 6, 1945  
  The machining of the uranium reflector for the “Trinity” test was completed. “Trinity” was the code name of the first nuclear weapons test of an atomic bomb at the White Sands Proving Ground near Alamogordo, New Mexico.  
   
  Saturday, July 7, 1945  
  Explosives lens casting for Trinity completed.  
   
  Tuesday, July 10, 1945  
  The best available lens castings were selected for the “Trinity” test.  
   
  Thursday, July 12, 1945  
  The plutonium core and the Gadget components left Los Alamos for the test site separately.  
   
  Friday, July 13, 1945  
  Assembly of Gadget's explosive lens, uranium reflector, and plutonium core was completed at Ground Zero at 1745 hours.  
   
  Saturday, July 14, 1945  
  Little Boy bomb units, accompanied by the U-235 projectile, were shipped out of San Francisco on the heavy cruiser USS Indianapolis (CA 35) for Tinian, Marianas Islands.  
   
  Gadget was hoisted to the top of the 100 foot test tower, and the detonators were installed and connected. Final test preparations began.  
   
  The only full scale test of the implosion lens system (before Gadget) was conducted. Initial analysis indicates failure. Bethe later corrected mistaken calculations and found that the measurements were consistent with optimum performance.  
   
  Monday, July 16, 1945  
  At 5:29:45 a.m. scientists from Los Alamos successfully detonated the Gadget device creating a a plutonium implosion blast in the first atomic explosion in history in the Trinity shot at the White Sands Proving Ground near Alamogordo, New Mexico.  
  Thursday, July 19, 1945  
  Oppenheimer suggested to Groves that the U-235 from "Little Boy" be reworked into uranium/plutonium composite cores for making more implosion bombs (4 implosion bombs could be made from "Little Boy's" uranium). Groves rejected the idea since it would delay combat use.  
   
  Friday, July 20, 1945  
  The 509th Composite Group began flying practice missions over Japan.  
   
  Saturday, July 21, 1945  
  Groves sent Stimson a report on the successful Trinity test.  
   
  Monday, July 23, 1945  
  U.S. Secretary of War Henry L. Stimson, in Potsdam, Germany for at the between Truman, Stalin, and Churchill, received the current target list. It listed in order of choice: Hiroshima, Kokura, and Niigata. He also received an estimate of atomic bomb availability: The "Fat Man" bomb should be ready for use on August 6, a second Fat Man-type by August 24, 3 should be available in September, and more each month - reaching 7 or more in December.  
   
  A B-29 Superfortress piloted by Tibbets dropped a dummy “Little Boy” assembly off Tinian, Marianas Islands to test radar altimeter detonators.  
   
  Combat hemispheres for the "Fat Man" bomb were fabricated.  "Fat Man" was the codename for the atomic bomb that was detonated over Nagasaki, Japan on August 9.  
   
  Tuesday, July 24, 1945  
  At the Potsdam Conference U.S. President Harry S. Truman informed Soviet Marshal Josef Stalin that the U.S. possessed "a new weapon of unusual destructive force." Stalin had already been informed about it by his spies.  
   
  The War Department was given orders by Truman to instruct General Carl A. Spaatz, commander of the U.S. Strategic Air Forces in the Pacific, that first atomic bomb would be dropped as soon after August 3, 1945 as weather permitted. Truman told Stimson that the order to Spaatz would stand unless Truman notified him of favorable Japanese reply.  
   
  Groves drafted the directive authorizing the use of the atomic bombs as soon as bomb availability and weather permit. It lists the following targets in order of priority: Hiroshima, Kokura, Niigata, and Nagasaki. This directive constituted final authorization for atomic attack, no further orders are issued.  
   
  Wednesday, July 25, 1945  
  Official orders were issued to the 509th Composite Group to attack Japan with an atomic bomb when it would “deliver its first special bomb as soon as weather will permit visual bombing after about 3 August 1945 on one of the targets. . . . Additional bombs will be delivered on the above targets as soon as made ready by project staff.”  
   
  Peer de Silva, the official courier for the “Fat Man” core, signed for 6.1 kg of plutonium at Los Alamos.  
   
  Thursday, July 26, 1945  
  Five C-54 transport planes left Kirtland Air Force Base, Albuquerque, New Mexico with the "Little Boy" U-235 target (its final component), the Fat Man plutonium core, and its initiator.  
   
  The heavy cruiser USS Indianapolis (CA 35) arrived at the American base on Tinian, Marianas Islands with its cargo of uranium and major components for the “Little Boy” bomb.  
   
  Saturday, July 28, 1945  
  Five C-54 transport planes arrived at Tinian with the “Little Boy” U-235 target, the "Fat Man" plutonium core, and its initiator. All the components for "Little Boy" were now on site. No “Fat Man” bomb assemblies had yet arrived.  
   
  Monday, July 30, 1945  
  The nuclear components (target, projectile, and 4 initiators) were inserted into bomb unit number L11 ("Little Boy").  
   
  Tuesday, July 31, 1945  
  The assembly of “Little Boy” was completed. It was ready for use the next day.  
   
  Wednesday, August 1, 1945  
  A typhoon approaching Japan prevented launching an attack with “Little Boy.“ Several days were required for weather to clear.  
   
  Thursday, August 2, 1945  
  The “Fat Man” bomb cases F-31 and F-32 arrive on Tinian allowing “Fat Man” assembly to begin. The bombing date set for “Fat Man” was August 11.  
   
  Saturday, August 4, 1945  
  Tibbets briefed the 509th Composite Group about the impending attack. He revealed that they will drop immensely powerful bombs, but the nature of the weapons was not revealed.  
   
  Sunday, August 5, 1945  
  Major General Curtis LeMay officially confirmed the mission to drop the first atomic bomb on Japan for the next day. Tibbets would take over as pilot of the B-29 No. 82. Captain William S. Parsons was assigned to fly the mission as weaponeer. Tibbets named B-29 Superfortress No. 82 the "Enola Gay" after his mother, over the objections of its normal pilot Captain Robert A. Lewis. Lewis was assigned to fly the mission as co-pilot.  
   
  The “Little Boy” bomb was on the “Enola Gay.” is loaded on the plane.  
   
  The dummy "Fat Man" unit F33 (complete except for plutonium core) was prepared for a practice bombing run.  
   
  Monday, August 6, 1945  
  At midnight the final briefing for the mission to drop the first atomic bomb took place. The target of choice is Hiroshima. Tibbets was pilot, Parsons was was weaponeer, and Lewis was co-pilot.  
   
  At 2:45 a.m. the the "Enola Gay" began its takeoff roll.  
   
  At 8:50 a.m., flying at 31,000 feet, the “Enola Gay” crossed Shikoku, Japan due east of Hiroshima. Bombing conditions were good, the aimpoint was easily visible, and no opposition from Japanese aircraft was encountered.  
   
  At 9:15:17 a.m. the “Little Boy” bomb unit was released at 31,060 feet.  
   
  At 9:16:02 a.m. (8:16:02 a.m. Hiroshima time) the “Little Boy” bomb unit exploded at an altitude of 1,850 feet, 550 feet from the aim point, the Aioi Bridge, with a yield of 12.5-18 Kilotons of TNT (the best estimate was 15 Kilotons of TNT).  
   
  Tuesday, August 7, 1945  
  In the absence of an immediate surrender after the dropping of the first atomic bomb on Hiroshima, a crash effort began to print and distribute millions of leaflets to major Japanese cities warning of future atomic attacks.  
   
  The date for dropping the “Fat Man” bomb was moved up to August 10, then to August 9, to avoid a projected 5 days of bad weather. This required skipping many check out procedures during assembly.  
   
  Wednesday, August 8, 1945  
  Leaflet dropping and warnings to Japan by Radio Saipan began. Nagasaki did not receive warning leaflets until August 10, one day after the second atomic bomb was dropped on the city.  
   
  The dummy “Fat Man” unit F33 was dropped in a practice bomb run.  
   
  Assembly of the “Fat Man” bomb unit F31 with the plutonium core was completed in the early morning and loaded onto the B-29 Superfortress named "Bock's Car" at 10 p.m.  
   
  Thursday, August 9, 1945  
  At 3:47 a.m., "Bock's Car" took off from Tinian, Marshall Islands carrying the “Fat Man” bomb. The target of choice was the Imperial Japanese Arsenal at Kokura. Major Charles Sweeney was the pilot. Soon after takeoff he discovered that the fuel system would not pump from the 600 gallon reserve tank.  
   
  At 10:44 a.m. “Bock's Car” arrived at Kokura but found it covered by haze preventing the aimpoint from being be seen. Flak and fighters appeared, forcing the plane to stop searching for it. Sweeney turned toward Nagasaki, the only secondary target in range.  
   
  Upon arriving at Nagasaki, Bock's Car had enough fuel for only one pass over the city even with an emergency landing at Okinawa. Nagasaki was covered with clouds, but one gap allowed a drop several miles from the intended aimpoint.  
   
  At 11:02 a.m. “Fat Man” exploded at 1950 feet near the perimeter of the city, scoring a direct hit on the Mitsubishi Steel and Arms Works. The yield of this second atomic weapon used wa 19-23 kilotons with the best estimate being 21 kilotons.  
   
  Oppenheimer cabled Groves with the following shipping schedule: 11 Aug. first quality HE unit; 12 Aug. next plutonium core; 14 Aug. another first quality HE unit.  
   
  Saturday, August 11, 1945  
  Groves reported that the second plutonium core would be ready for shipment on August 12 or 13, with a bombing possible on August 17 or 18.  
   
  Truman ordered a halt to further atomic bombing until further orders were issued.  
   
  Groves decided to delay shipping the second Pu core and contacted Dr. Robert F. Bacher just after he had signed receipt for shipping the core to Tinian. The core was retrieved from the car before it left Los Alamos.  
   
  Monday, August 13, 1945  
  Stimson recommended shipping the second plutonium core to Tinian, Mariana Islands.  
   
  Friday, August 17, 1945  
  Oppenheimer warned Stimson that: 1) atomic weapons would improve qualitatively and quantitatively over coming years, 2) adequate defenses against nuclear weapons would not be developed 3)the US would not retain hegemony over nuclear weapons, and 4) wars could not be prevented even if better nuclear weapons were developed.  
   
  September, 1945  
  The shutdown of the Y-12 electromagnetic plant at Oak Ridge began.  
   
  Sunday, September 9, 1945  
  The S-50 thermal diffusion plant at Oak Ridge shut down.  
     
  Click here for information on the German Atomic Program  
   
     
   
 

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