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Chronology For The Origin Of Atomic Weapons

 I have included here a chronological listing of events and milestones leading up to the use of atomic weapons against Japan. Brief explanatory notes are inserted to provide some context and interpretation. The interested reader is directed to several excellent books available (see bibliography), particularly the Pulitzer Prize winning book by Rhodes, and Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years 1943-1945, from whom the bulk of the material for this timeline was extracted.

The timeline is divided into several epochs which seem to me to be naturally separated by critical events. Each epoch begins with a short summary of the key themes that characterize it. Although this is a strict chronology which list events that are more or less datable, occasional paragraphs are interspersed summarizing the thrust of events.

Racing Against Victory - The Final Year

*** From January 1, 1945 to VJ Day ***

At the start of 1945 the Manhattan Project has Ôturned the corner'. The uranium bombs seem assured of success in a matter of months. The prospects for the plutonium bomb are looking up although meeting an August 1 deadline imposed by Groves is far from certain. However, allied military successes against Germany and Japan make it a horse race to see whether it will matter to the war effort.

January, 1945 -

• Y-12 output reaches an average of 204 grams of 80% U-235 a day; projected production of sufficient material for a bomb (~40 kg) is July 1.
• Usable barrier tubes begin arriving at the K-25 plant.
• 160 g of plutonium from the X-Pile is on hand at Los Alamos. The first shipment from Hanford has not yet arrived.
• Substantial production of ~0.85% enriched uranium begins at S-50, with ten of 21 racks going in to operation.

January 18, 1945 - The Dragon experiment, conducted by Frisch in which a U-235 hydride slug is dropped through a barely subcritical U-235 hydride assembly creates the world's first assembly critical through prompt neutrons alone (prompt critical). The largest energy production for a drop is 20 megawatts for 3 milliseconds (the temperature rises 6 degrees C in that time).

January 20, 1945 -

• Curtis LeMay takes command of the Twentieth Air Force in Marianas. Fleet contains 345 aircraft, but in three months of bombing none of the nine top priority targets have been destroyed.
• The first stage of the K-25 plant is charged with uranium hexafluoride and begins operation.

January 31, 1945 - Robert Bacher reports to Oppenheimer that a Po-210/Be-9 implosion initiator (still to be designed) is possible.

February, 1945 -

• The F reactor goes on-line at Hanford, raising theoretical production capacity to 21 kg/month.
• Uranium gun design is completed and frozen. Only planning for deployment and combat use once the U-235 is delivered is now required (although studies of an improved gun design, begun on Dec. 7 and later abandoned, are underway).
• Planning for an implosion bomb test begins in earnest.
• Initiator tests begin. Demand for polonium rises to 100 curies/month.
• Plutonium begins arriving from Hanford.
• Admiral Nimitz, Commander in Chief, Pacific Ocean Areas, is notified of the nature of the atomic bomb project.
• Tinian Island is selected as the base of operations for atomic attack.

February 13, 1945 - Dresden, Germany is burned down in an incendiary raid killing 50,000.

February 19, 1945 - Marines land on Iwo Jima, a Japanese observation post for the B-29 raids. Over the next two months 6,281 marines are killed, and 21,865 are wounded in capturing the island from 20,000 defenders.

February 20, 1945 - First stage of K-25 begins operating.

February 23, 1945 - A fire bomb test raid on Tokyo with 172 planes burns one square mile, the most destructive raid on Japan to date.

February 28, 1945 - A meeting between Oppenheimer, Groves, Kistiakowsky, Conant, Tolman, Bethe, and Charles Lauritsen is held to fix the design approach for the plutonium bomb. It is agreed that work will 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 have been proved yet. Solid core compression has not been demonstrated at this time. A schedule for completing research, development, engineering, and testing is also established. The (partial) goals are:

15 April Solve detonator timing problem
15 April Have detonators in full production
15 April Begin large-scale lens production
25 April Begin hemisphere shots to measure shock wave convergence
15 May Demonstrate implosion compression in full scale test
4 June Begin lens fabrication for Trinity test
4 July Begin assembly of Trinity test gadget

March 1, 1945 - The powerful Cowpuncher Committee is organized to "ride herd" on implosion bomb development.

March 5, 1945 - Oppenheimer officially freezes explosive lens design.

March 9-10, 1945 - LeMay launches an all-out low altitude fire bomb raid on Tokyo with 334 B-29s, stripped of guns for greater bomb load, carrying 2000 tons of incendiaries. 15.8 square miles of Tokyo burn, killing at least 100,000 people, injuring 1,000,000 (41,000 seriously).

March 11-18, 1945 - During these eight days fire raids with similar tactics are launched on Nagoya, Osaka, and Kobe; the second, third, and fourth largest cities in Japan. An additional 16 square miles of city are burned, killing more than 50,000 people.

March 15, 1945 - All 21 racks at the S-50 thermal diffusion plant finally in operation.

Mid-March, 1945 - The first evidence of solid compression from implosion is observed (5%).

April 3, 1945 - Preparations begin at Tinian Island to support the 509th Composite Group, and to assembly the atomic bombs.

April 11, 1945 - Oppenheimer reports that Kistiakowsky has achieved optimal performance with implosion compression in sub-scale tests.

April 12, 1945 -

• Otto Frisch completes criticality and "zero-yield" experiments with U-235 at Los Alamos.
• Pres. Roosevelt dies of a brain hemorrhage.

April 13, 1945 - Pres. Truman learns of the existence of atomic bomb development from Secretary of War Henry Stimson.

April 25, 1945 - Truman receives first in-depth briefing on the Manhattan Project from Stimson and Groves.

Although no atomic bombs yet exist, there is no longer any doubt about their imminent availability. Firm production schedules are in hand, and technical effort has already shifted to improving designs and production techniques. Producing reliable detonators in the required quantities (thousands each week) remains a problem. The dominant events in this final epoch of the war is the suspense of the first atomic test, and the political and military preparations for actual use. At this point, about 25 kg of U-235, and 6.5 kg of Pu-239 are on hand.

April 27, 1945 - 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).

April 30, 1945 -

• Initiator Committee (Bethe, Fermi and Christy) selects the most promising design for fission initiator (neutron generator) to be used in the implosion bomb. The "Urchin" design is favored, and work on initiator fabrication begins.
• First batch of supplies for the atomic bomb deployment leaves for Tinian from Wendover Field, UT.

May 2, 1945 - The first Raytheon Mark II X-Unit arrives for detonation testing.

May 7, 1945 - The 100-ton test is conducted. 108 tons of TNT, laced with 1000 curies of reactor fission products, are exploded 800 yards from Trinity ground zero to test instrumentation for Trinity. This is the largest instrumented explosion conducted up to this date.

May 8, 1945 - V-E Day. Germany formally capitulates to the allies.

May 9, 1945 - General procedures for atomic bombing are completed by D.M. Dennison, under Parsons.

May 10-11, 1945 - Target Committee reconvenes. On the committee now are Oppenheimer, Von Neumann, Parsons, and Bethe. Meeting discusses issues combat employment of atomic bombs (e.g. proper burst height, etc.). Target list is shortened to Kyoto, Hiroshima, Yokohama, and Kokura Arsenal (Niigata is considered).

Mid-May, 1945 - Little Boy is ready for combat use, except for the U-235 core. It is estimated sufficient material will be available by 1 August.

May 25, 1945 -

• 464 B-29s raid Tokyo again, burning out nearly 16 square miles of the remaining city. Only a few thousand are killed, urban inhabitants have learned to flee fire bomb attacks quickly and escape the flames.
• Operation OLYMPIC, the invasion of Kyushu (the southern Japanese island), is set for November 1.

May 28, 1945 - Target Committee meets with Lt. Col. 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.

May 30, 1945 - Sec. of War Stimson rules out Kyoto, the ancient capital of Japan, as a target for atomic attack.

May 31, 1945 - Critical mass tests with plutonium begin at Los Alamos.

June 1, 1945 - The Interim Committee, organized to guide the final conduct of the war and the post-war reconstruction and lead by Secretary of State Designate James Byrnes, issues the recommendations that the atomic bomb be dropped as soon as possible, that a urban area be the target, and that no prior warning be given.

June 10, 1945 - 509th Composite Group crews begin arriving on Tinian with their modified B-29s.

June 21, 1945 - The first implosion initiator is ready.

June 24, 1945 - Frisch confirms that the implosion core design is satisfactory after criticality tests.

June 27, 1945 - Groves meets with Oppenheimer and Parsons to plan delivery of atomic bombs to the Pacific theater.

Late June, 1945 -

• LeMay estimates that the Twentieth Air Force will finish destroying the 60 most important cities in Japan by Oct. 1.
• The T-5 group in the Los Alamos T (Theory) Division estimates the Trinity explosion yield at 4-13 Kt.

July, 1945 - Final preparations begin at the New Mexico test site, the Jornada del Muerto at the Alamagordo Bombing Range, for the first atomic bomb test, code named Trinity. The date is set for July 16. Jumbo is not used in the test, since plutonium delivery schedules make recovery of active material (in the event of a fizzle) less important.

July 3, 1945 - Casting of the U-235 projectile for Little Boy is completed.

July 6, 1945 - Machining of the uranium reflector for the Trinity test completed.

July 7, 1945 - Explosives lens casting for Trinity completed.

July 10, 1945 - The best available lens castings are selected for Trinity.

July 11, 1945 -

• Assembly of Gadget, the first atomic bomb begins.
• Japanese Foreign Minister Shigenori Togo cables Ambassador Naotake Sato in Moscow advising him to explore using the USSR as an intermediary in surrender negotiations.

July 12-13, 1945 - The plutonium core and the Gadget components leave Los Alamos for the test site separately. Assembly of Gadget begins at 1300 hours on July 13. Assembly of Gadget's explosive lens, uranium reflector, and plutonium core is completed at Ground Zero at 1745 hours.

July 14, 1945 -

• Gadget is hoisted to the top of the 100 foot test tower, and the detonators are installed and connected. Final test preparations begin.
• Little Boy bomb units, accompanied by the U-235 projectile, are shipped out of San Francisco on the USS. Indianapolis for Tinian.
• The only full scale test of the implosion lens system (before Gadget) is conducted. Initial analysis indicates failure. Bethe later corrects mistaken calculations and finds that the measurements are consistent with optimum performance.

July 16, 1945 - At 5:29:45 a.m. Gadget is detonated in the first atomic explosion in history. The explosive yield is 20-22 Kt (initially estimated at 18.9 Kt), vaporizing the steel tower.

July 19, 1945 - Oppenheimer suggest 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 pit). Groves rejects the idea since it would delay combat use.

July 20, 1945 - The 509th begins flying practice missions over Japan.

July 23, 1945 - Stimson, in Potsdam for meeting between Truman and Stalin, receives current target list. In order of choice it is: Hiroshima, Kokura, and Niigata. He also receives an estimate of atomic bomb availability: Fat Man 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.

• First A-bomb test unit dropped by 509th at Tinian.
• Combat hemispheres for Fatman are fabricated.

July 24, 1945 -

• Truman discloses the existence of the atomic bomb to Stalin (who had already been informed about it by his spies).
• Groves drafts 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 constitutes final authorization for atomic attack, no further orders are issued.
• The U-235 target for Little Boy is cast at Los Alamos.

July 25, 1945 - Peer de Silva, the official courier for the Fatman core, signs for 6.1 kg of plutonium at Los Alamos.

July 26, 1945 -

• Truman issues the Potsdam Declaration, requiring unconditional surrender of the Japanese armed forces.
• The Indianapolis delivers Little Boy bomb units, and the U-235 projectile to Tinian.
• 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.

July 28, 1945 -

• The Japanese government rejects the Potsdam surrender demand.
• The five C-54 transports arrive at Tinian. All components for Little Boy are now on site, but no Fat Man bomb assemblies have yet arrived.

July 30, 1945 - The nuclear components (target, projectile, and 4 initiators) are inserted into bomb unit number L11.

July 31, 1945 - The assembly of Little Boy is completed. It is ready for use the next day.

August 1, 1945 - A typhoon approaching Japan prevents launching an attack with Little Boy. Several days are required for weather to clear.

August 2, 1945 - Fat Man bomb cases F-31 and F-32 arrive on Tinian. Fat Man assembly begins. Bombing date is set for August 11.

August 4, 1945 - Tibbets briefs the 509th Composite Group about the impending attack. He reveals that they will drop immensely powerful bombs, but the nature of the weapons are not revealed.

August 5, 1945 -

• At 1500 Gen. LeMay officially confirms the mission for the next day. Tibbets will take over as pilot, Parsons will fly as weaponeer.
• Tibbets names B-29 No. 82 the "Enola Gay" after his mother, over the objections of its pilot Robert Lewis.
• Little Boy is loaded on the plane.
• Dummy Fat Man unit F33 (complete except for plutonium core) is prepared for practice bombing run.

August 6, 1945 -

• 0000, final briefing, the target of choice is Hiroshima. Tibbets is pilot, 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 aimpoint is easily visible, no opposition is encountered.
• 0915:17 Little Boy is released at 31060 feet.
• 0916:02 (8:16:02 Hiroshima time) 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).

August 7, 1945 -

• In the absence of an immediate surrender, a crash effort begins to print and distribute millions of leaflets to major Japanese cities warning of future atomic attacks.
• The date for dropping Fat Man is moved up to August 10, then to August 9, to avoid a projected 5 days of bad weather. This requires skipping many check out procedures during assembly.

August 8, 1945 -

• At Foreign Minister Togo's request Ambassador Sato tries to persuade the Soviets to mediate surrender negotiations. Molotov cancel's the meeting, then announces that the Soviet Union is at war with Japan effective the next day.
• Leaflet dropping, and warnings to Japan by Radio Saipan begin (Nagasaki does not receive warning leaflets until August 10).
• Fat Man unit F33 is dropped in practice bomb run.
• Assembly of Fat Man unit F31 with the plutonium core completed in the early morning.
• 2200, Fat Man is loaded on B-29 "Bock's Car".

August 9, 1945 -

• 0347, Bock's Car takes off from Tinian, the target of choice is Kokura Arsenal. Charles Sweeney is pilot. Soon after takeoff he discovers that the fuel system will not pump from the 600 gallon reserve tank.
• 1044, Bock's Car arrives at Kokura but finds it covered by haze, the aimpoint cannot be seen. Flak and fighters appear, forcing the plane to stop searching for it. Sweeney turns toward Nagasaki, the only secondary target in range.
• Upon arriving at Nagasaki, Bock's Car has enough fuel for only one pass over the city even with an emergency landing at Okinawa. Nagasaki is covered with clouds, but one gap allows a drop several miles from the intended aimpoint.
• 11:02 (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).
• Oppenheimer cable Groves with the following shipping schedule: 11 Aug. first quality HE unit; 12 Aug. next plutonium core; 14 Aug. another first quality HE unit.

August 10, 1945 - Japanese civilian and military leaders are still unable to agree on accepting the Potsdam Decree's surrender terms. Emperor Hirohito instead breaks the tradition of imperial non-intervention in government and orders that surrender be accepted, provided that the Emperor be allowed to retain his position.

August 11, 1945 -

• Truman and Sec. of State Byrnes reply with an amended form of the Potsdam Decree that acknowledges the Emperor, but still refuses to guarantee his position.
• Groves reports 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 orders a halt to further atomic bombing until further orders are issued.
• Groves decides to delay shipping the second Pu core and contacts Bacher just after he had signed receipt for shipping the core to Tinian. The core is retrieved from car before it leaves Los Alamos.
• Strategic Air Forces Carl Spaatz halts area fire bombing.

August 13, 1945 -

• Stimson recommends shipping the second plutonium core to Tinian.
• Truman orders area fire bombing resumed. Gen. Henry Arnold, US Army Air Force, launches the largest raid on Japan of the war with over 1000 B-29s and other aircraft, carrying 6000 tons of bombs.

August 14, 1945 -

• Following leaflet bombing of Tokyo with surrender terms, Hirohito orders that an Imperial Edict accepting surrender be issued.
• 2:49 p.m. (1:49 a.m. Washington time), Japanese news agency announces surrender.

August 17, 1945 - Oppenheimer warns 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;
4. wars could not be prevented even if better nuclear weapons were developed.

September 9, 1945 - S-50 plant completely shut down.

October 16, 1945 - Oppenheimer resigns as director of Los Alamos, accepting a post at Caltech.

October 17, 1945 - Norris Bradbury takes over as director of Los Alamos (a position he would hold for 25 years).