When interviewed for a later (1980) documentary, The Day After Trinity, Oppenheimer’s brother, Frank, stated that he thought all Robert had said was ‘It worked!’.
Trinity’s fireball fused some quarter-million square meters (70 acres; figure 5.11) of desert sand to a depth of about half an inch into a greenish, glassy material that came to be known as Trinitite; a small sample owned by this author is still about as radioactive as a modern household smoke detector. The common unit for measuring radioactivity is the Curie (Ci), which corresponds to 37 billion radioactive decays per second. A single Ci is a lot of radioactivity; smoke detectors contain about a microcurie of radioactive material that serves as an air-ionizing source to aid in the detection of smoke particles. Trinity released over a trillion Curies of ‘prompt’ radioactive fission products. Because wind patterns at the time of the test were favorable, there was no serious fallout from Trinity, although some farm animals suffered discoloring ‘burns’ from radioactive dust landing on them.
Figure 5.11. Aerial view of the aftermath of the Trinity test. The 0.1 kt test crater is from the 100 ton TNT test. The area covered by this image is about 1550 meters wide by 1400 meters tall [8].
President Truman was promptly informed of the results of the test. In his diary for July 18, he wrote that at lunch that day with Churchill, he ‘Discussed Manhattan (it is a success)’. He also wrote that ‘Believe Japs will fold up before Russia comes in. I am sure they will when Manhattan appears over their homeland. I shall inform Stalin about it at an opportune time’. Stalin, however, was already well aware of the Manhattan Project due to the efforts of agents at Los Alamos.
Various estimates of the yield of the Trinity test have been published over the years; a Department of Energy report on all United States nuclear tests published in late 2000 lists an official yield of 21 kilotons. The site of the test is now open to tourists two days per year, normally the first Saturdays of April and October, depending on security conditions at the White Sands Missile Range (figure 5.12).
Figure 5.12. Left: The author, second from right, at the Trinity ground-zero monument. October, 2004. Right: Monument plaque. The inscription reads: Trinity site, where the world’s first nuclear device was exploded on July 16, 1945.
With the successful completion of the Trinity test, the stage was set for combat use of nuclear weapons.
5.6 Hiroshima and Nagasaki
Preparations for the use of atomic bombs in combat began well before the designs of the bombs were finalized. In the fall of 1944, a special Army Air Force unit known as the 509th Composite Group was formed, and was placed under the command of Colonel Paul Tibbets, a veteran of numerous European combat missions. Air Force units were normally single-purpose entities for tasks such as maintenance, bombardment, engineering, transport, and the like, but the 509th was unique in that it was entirely self-sustaining, including even its own Military Police detachment. Two of the group’s B-29 bombers would go down in history as the planes that carried the atomic bombs dropped on Japan. The Enola Gay, chosen personally by Tibbets as the bomber that he would pilot for the strike against Hiroshima, was named after his mother (figure 5.13). The Nagasaki bomber, Bockscar, was named after its usual pilot, Captain Frederick Bock. Enola Gay and Bockscar now reside at museums in Washington, DC, and Dayton, Ohio, respectively.
Figure 5.13. Left: Enola Gay on Tinian. Right: Hiroshima mushroom cloud. From [9].
The 509th initially trained at Wendover Army Air Base in Utah, one of the largest bombing ranges in the world. In addition to this, the group required an advance base at which the bombs would undergo final assembly and be loaded onto aircraft. The site selected for this was on Tinian Island in the Pacific (figure 5.14). Only about 12 miles long, Tinian had been conquered by the Marines in July, 1944. Its six runways, each 8500 feet long, served as launching points for round-the-clock bombing raids against the Japanese home islands. The first members of the 509th arrived on Tinian on May 19, 1945, and practice bombing runs began soon thereafter. Technically, the 509th lay in the theatre of operations of General Curtis LeMay, who had taken command of the Twenty-First Bomber Command of the Twentieth Air Force in January, 1945.
Figure 5.14. Left: map of Japan showing main islands and major cities. Adapted from http://www.hist-geo.co.uk/japan/outline/japan-cities-1.php. Right: the Hiroshima and Nagasaki bombing missions. The distance from Tinian to Hiroshima is about 2740 km (1700 miles). From [9].
In April, 1945, General Groves turned his attention to the issue of selecting targets for the new weapons, establishing a Target Committee comprising three scientists from Los Alamos and representatives from the office of General Henry Arnold, Commanding General of the Army Air Forces. The committee was charged with developing a list of previously unbombed cities, chosen so that three could be available for each mission. Over the course of three meetings between late April and late May, the list of potential target cities was narrowed to four: Hiroshima, Kokura, Niigata, and Nagasaki (figure 5.14). Hiroshima was the largest target not on the Twenty-First Bomber Command’s priority list, and was the site of Japanese Second Army Headquarters, from which the defense of the southern island of Kyushu would be directed. Kokura was the site of one of the largest armaments production complexes in Japan; Niigata was a large naval port and the site of machine-tool and oil refining installations; and Nagasaki was the site of a number of Mitsubishi arms-production factories, including those where the torpedoes used by the Japanese at Pearl Harbor had been made. Tokyo was not a priority as it had been largely bombed out, and had been the site of a devastating fire-bombing raid in March which had killed some 84 000 inhabitants.
The decision to use the bombs still engenders passionate debate. Any assessment of this decision must, however, be considered in the context of the war at the time. By early 1945, some 5000 Americans and many more Japanese were dying each week as American forces advanced through Japanese-held islands. To conquer the islands of Leyte, Luzon, Iwo Jima, and Okinawa had cost the United States some 110 000 casualties (killed and wounded); some 300 000 Japanese had been killed or taken prisoner. If an invasion of the home islands of Japan went ahead, casualties on both sides could be astronomical. Planning for an invasion was well-advanced by the summer of 1945, and involved two main components. First, the southern island of Kyushu was to be the target of Operation Olympic, scheduled to begin on November 1, 1945. This would involve some 760 000 ground forces, more than four times the number landed during the June 6, 1944, invasion of Normandy; three naval fleets would provide support. The plan was for the invasion force to advance about one-third of the way along the island, setting up air bases in support of a planned invasion of the area around Tokyo, which was scheduled for March 1, 1946. This second invasion would involve just over one million ground forces. In the meantime, LeMay’s bombers would continue laying waste to Japanese cities and industries. Given this situation and the momentum that the Manhattan Project had acquired by the summer of 1945, it is difficult to imagine events unfolding differently than they did. Also, had the consequences of nuclear weapons not been so dramatically demonstrated to the world, what much worse horrors might have unfolded in a subsequent war? It is arguable that Hiroshima and Nagasaki have been responsible for decades of ‘nuclear inoculation’.
The first few days of August in the Pacific were overcast and rainy, but by Saturday, August 4, the forecast began improving. The first atomic mission was scheduled for the morning of the 6th. In order of priority, the targets were Hiroshima, Kokura, and Nagasaki; Niigata had been scratched for being too far away from the others. Tibbets began briefing his men on the afternoon of the 4th, telling them that what they had trained for was approaching, but he did not reveal the nature of their payload. The mission profile called for sorties by seven aircraft. The Enola Gay, to be piloted by Tibbets, was the ‘strike’ plane–the one which carried the bomb. Weather reconnaissance planes would be directed toward the target cities, and would depart an hour before the strike plane. Two ot
her aircraft carried blast-measurement instruments and cameras, and the seventh was deployed to Iwo Jima as a backup. Little Boy was wheeled out of its assembly building on Sunday afternoon and loaded into the Enola Gay. Final briefings began at 11:00 p.m.
The three weather planes began departing at 1:37 a.m. Monday morning. Tibbets began Enola Gay’s takeoff roll at 2:45 a.m., using almost every yard of the two-mile runway to get airborne. Table 5.2 lists some of the parameters of the Hiroshima and Nagasaki missions (Washington was 14 h behind Tinian).
Table 5.2. Some parameters of the Hiroshima and Nagasaki missions. Mission time for Bockscar includes three-hour stop at Okinawa.
Parameter Hiroshima Nagasaki
Strike aircraft Enola Gay Bockscar
Takeoff (Tinian time) 02:45 Aug 6 03:48 Aug 9
Takeoff (Washington time) 12:45 Aug 5 13:48 Aug 8
Bombing (Japan time) 08:15 Aug 6 11:08 Aug 9
Bombing (Washington time) 19:15 Aug 5 22:08 Aug 8
Landing (Tinian time) 14:58 Aug 6 23:06 Aug 9
Landing (Washington time) 00:58 Aug 6 09:06 Aug 9
Mission duration 12 h 13 min 19 h 18 min
Drop height (ft/m) 31 600/9630 28 900/8810
Bomb detonation height (ft/m) 1900/580 1650/503
Bomb yield (kt) ∼15 ∼21
After a flight of over six hours, Enola Gay arrived over Hiroshima within a minute of its scheduled time. Hiroshima is located on the Ota River in the southern part of Honshu, the main island of Japan; before the war it had been the seventh-largest city in Japan, with a population of about 340 000. Its population in August, 1945, has been estimated at some 280 000 civilians plus approximately 43 000 soldiers. Flat and unbroken by hills, the city was a perfect target for determining the effects of the new weapon. Bombardier Thomas Ferebee’s aiming point was the distinctive T-shaped Aioi bridge in the heart of the city; he missed by only a few hundred feet. The bomb was programmed to detonate at an altitude of about 1900 feet in order to minimize fallout associated with debris being rendered radioactive by neutron capture. Figure 5.13 shows the Hiroshima mushroom cloud. Figure 5.15 shows post-strike photos; the Aioi bridge survived the blast and is clearly visible in the center of the image on the left.
Exercise
Assuming free-fall with no air resistance, calculate the time required for Little Boy to fall from its release height to its detonation height.
Answer
About 43 s.
Figure 5.15. Left: Aerial view of Hiroshima, post-bombing. The Aioi bridge is in the center of the image. Right: General view of damage at Hiroshima. From [9].
The world learned of the bombing when a statement authorized by President Truman was released in Washington at 11:00 a.m. Monday morning; Truman was still at sea on his way home from Potsdam. In part, the release read as
Sixteen hours ago an American airplane dropped one bomb on Hiroshima, an important Japanese Army base. That bomb had more power than 20 000 tons of TNT… It is an atomic bomb. It is a harnessing of the basic power of the Universe. The force from which the Sun draws its power has been loosed against those who brought war to the Far East…
Truman’s 20 000 tons was an overestimate, probably caused by confusing Little Boy with the Trinity test.
Despite the bombing, the Japanese government could not yet bring itself to agree to surrender, but its situation was becoming dire. At 5:00 p.m. local time on the afternoon of August 8, the Japanese ambassador in Moscow was informed that the Soviet Union would consider itself in a state of war with Japan as of August 9. The Japanese had been hoping to use Russia as a go-between in surrender negotiations, but that hope was no longer viable.
The second nuclear strike was originally scheduled for August 20, but by late July enough time had been made up to permit advancing the date to August 11. In anticipation of bad weather, preparations were further pressed to facilitate a mission on the 9th. From its start, however, the Nagasaki mission suffered a number of misfortunes. The front and rear halves of Fat Man’s armor-plated casing were out of round, and bolt holes for securing the casing to the high-explosive assembly did not align properly; an ordinary steel casing was substituted. The bomb was ready by 10:00 p.m. on the evening of August 8, and loaded into Bockscar, which would be piloted by Major Charles Sweeney. But then another problem arose: Flight Engineer John Kuharek discovered that a pump for transferring fuel from tanks located in the rear bomb-bay was malfunctioning. The fuel would not only be inaccessible, but would represent almost two tons of dead weight to be carried through the mission. To empty the tanks, replace the pump, or transfer the bomb to another plane were not options; the window of good weather was narrowing. Sweeney decided to proceed, and Bockscar departed at 03:48 Tinian time, Thursday, August 9. The primary and secondary targets for the second atomic mission were Kokura and Nagasaki.
Bockcar’s bad luck continued in the air. Sweeney was to rendezvous with camera and instrument planes off the southern coast of Kyushu, but the camera plane missed the rendezvous as it was flying at the wrong altitude. This caused Sweeney to burn 45 minutes of fuel before he decided to head for Kokura. The flight to Kokura took another 50 minutes, but the city was obscured by smoke and haze caused by bombing of the nearby city of Yawata the previous day. Sweeney made three passes over Kokura, but the smoke made a visual bombing run impossible; beginning to run low on fuel, he decided to proceed to Nagasaki. This was only a 20 minute flight, but the city was obscured by clouds. A radar-guided bomb run was authorized, but about 30 s before the drop, a hole opened in the clouds and bombardier Kermit Beahan was able to secure a visual aiming point. Fat Man was released at 11:08 a.m. Nagasaki time (10:08 p.m. Washington time, August 8). The bomb detonated over the Mitsubishi arms complex; because of the reflective hilly geography, the crew felt five shock waves. Sweeney promptly set course for Okinawa, the nearest friendly base, where he executed an emergency landing just as he was running out of fuel. After refueling, Bockscar made its way back to Tinian, the entire mission having occupied over 19 h.
In Japan, the barrier to surrender was concerned with the fate of Emperor Hirohito. On the morning of August 9, a meeting of the government’s Supreme War Council was underway when word was received of the bombing of Nagasaki. The meeting continued into the late evening; unable to reach a consensus, the Council requested a meeting with the Emperor, who made it known that he was in favor of ending the war. On the morning of the 10th, the Japanese signaled that they were ready to surrender, provided that terms would not include ‘any demand for modification of the prerogatives of His Majesty as a sovereign ruler’. This reached President Truman early on the morning of the 10th, and a response was soon developed which stipulated that ‘the authority of the Emperor and the Japanese Government to rule the state shall be subject to the Supreme Commander of the Allied Powers who will take such steps as he deems proper to effectuate the surrender terms’. While the Japanese considered this, President Truman ordered a halt to any further atomic bombings. The Japanese announced on the 14th that they would accept the surrender terms, bringing the war to an end.
Also on August 10, General Groves informed Chief of Staff of the Army General George Marshall as to the delivery schedule of the next bomb:
The next bomb of the implosion type had been scheduled to be ready for delivery on the target on the first good weather after 24 August 1945. We have gained 4 days in manufacture and expect to ship from New Mexico on 12 or 13 August the final components. Providing there are no unforeseen difficulties in manufacture, in transportation to the theatre or after arrival in the theatre, the bomb should be ready for delivery on the first suitable weather after 17 or 18 August.
The fissile material for this third bomb never left the United States.
What were the effects of the bombings? Groves dispatched survey teams to Japan which included a number of Los Alamos scientists, and the United States Strategic Bombing Survey conducted a separate analysis. A selection of statistics drawn from the reports prepared
by these groups gives an indication of the power of the bombs. ‘Point X’ is ground zero, the location on the ground below the point of explosion of the bomb:
At Hiroshima:
Estimated 66 000 dead and 69 000 injured of estimated pre-raid population of 255 000. 60% of deaths were attributed to burns, and 30% to falling debris.
60 000 of 90 000 buildings destroyed or severely damaged.
Firestorm burnt out about 4.4 square miles around X. Heavy fire damage covered a circular area of about 6000 feet radius to a maximum radius of about 11 000 feet. Fires started by heat radiation in both cities extended to about 15 000 feet.
Almost everything out to about one mile from X was completely destroyed except for heavily-reinforced concrete buildings. All homes seriously damaged to 6500 feet; most to 8000 feet.
Dry combustible materials were ignited to 6400 feet; telephone poles were charred to 9500 feet.
At Nagasaki:
Estimated 39 000 dead and 25 000 injured of estimated pre-raid population of 195 000; 95% of deaths attributed to burns. People suffered burns to almost 14 000 feet, telephone poles were charred to 11 000 feet, and fire was noted as far as 10 000 feet to the south of X, where it was stopped by a river.
About 20 000 of 50 000 buildings and houses destroyed. Total destruction area about 3 square miles. The extreme range of building collapse was 23 000 feet. All homes seriously damaged to 8000 feet; most to 10 500 feet.
Dry combustible materials were ignited to 10 000 feet.
By far, blast and burn effects were the greatest causes of death and injury. Manhattan Project medical teams estimated that some 7% of deaths resulted primarily from radiation, although some estimates of radiation-induced deaths ran as high as 20%. Radiation effects included depressed blood counts, loss of hair, bleeding into the skin, vomiting, diarrhea, and fever. Deaths from radiation began about a week after exposure, peaked in about 3–4 weeks, and ceased by 7–8 weeks. Radiation units and effects are discussed in the following chapter.
The Manhattan Project Page 9
