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Chapter 25: Weapon Development and Testing

A watershed in the development of nuclear science was the Army’s building and testing of the atomic bomb. In early 1943, with America engaged in what was believed to be a desperate race with Germany, American and foreign-born physicists, chemists, metallurgists, and engineers, as well as military technical experts, came together at Los Alamos to devise a weapon with a power hitherto unmatched by man. This practical objective melded with the larger scientific challenge of turning atomic theory into a material reality and resulted in a unity of purpose that sustained the assembled scientists in their unique atomic adventure.

Organized by Oppenheimer into specialized research and technical divisions and groups, the Los Alamos scientists divided their efforts between two fundamental tasks: solving the theoretical and experimental problems of a fission bomb,1 and working out the complex ordnance and engineering problems of weapon design and fabrication. Their concentrated activity over a two-year period, from 1943 to 1945, transformed the laboratory, for all intents and purposes, into a weapon assembly and test plant. The climax was Project Trinity, the crucial test of their creation: the first atomic bomb.

Building the Bomb

By the fall of 1943, with the laboratory’s administrative organization largely worked out and the scientists’ talents and energies channeled into various research programs, Oppenheimer, Groves, Conant, and the other project leaders turned their attention to the problem of determining the most suitable design of an atomic device.2 During inspection

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visits to Los Alamos, Groves found that some of the scientific staff members, including Captain Parsons, strongly favored the gun rather than the implosion principle as more feasible for developing a usable fission weapon. They pointed out that the well-established mechanical techniques of the gun made this weapon type almost certain to work if properly designed and that the design and engineering of the outer configuration and mechanics of the gun were already well advanced. Furthermore, once the physicists, chemists, and metallurgists could provide the precise nuclear specifications for the active material – whether U-235, Pu-239, or even U-233 from thorium – development of a workable gun-type weapon would be only a matter of time.

Assessment of precise nuclear specifications for a fission weapon was the responsibility of the laboratory’s experimental physics division. Through intensive research, the division’s physicists gathered considerable data on the effect of cosmic rays on fissioning, on measurement of nuclear cross sections, on scattering phenomena, and on other aspects of the fission process that related to bomb specifications and efficiency. With this data they were able to calculate by the summer of 1944 that the destructive effect of either an implosion- or gun-type bomb would justify the effort required to fabricate it. They still lacked an answer, however, to the question on which the success of the entire project hinged: How much fissionable material would be needed for an effective weapon? Whether or not atomic weapons would be available for use in the war depended on the answer to that question.3

One way to increase the efficiency of a fission bomb was to achieve maximum purity in the active materials. Hence, a major program of the laboratory’s chemistry and metallurgy division was to improve the methods for purifying U-235 and Pu-239. Because purity requirements for uranium were about one-third less than those for plutonium and because, until early 1944, there was not enough Pu-239 available to permit effective work on its purification, the chemists experimented with uranium but with the purpose of developing techniques that might also be used with plutonium. When sufficient amounts of Pu-239 arrived from the Clinton pile, the chemists developed both wet and dry purification processes. Subsequently, they employed the more satisfactory wet process in final purification of most plutonium for the bomb.

Before U-235 or Pu-239 could be used in a fission bomb, they had to be converted into metal of the proper configuration and purity. Metallurgists at Los Alamos faced a number of problems in making uranium or plutonium metal of the desired quality, including the tendency of uranium to catch fire during processing and the difficulty of handling the highly reactive and poisonous plutonium. For forming uranium into metal, they experimented with electrolytic and centrifuge processes but finally settled upon a modification of the stationary

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Technical area at Los 
Alamos, built around Ashley Pond and along Trinity Avenue

Technical area at Los Alamos, built around Ashley Pond and along Trinity Avenue

bomb method, devised earlier at Iowa State. For plutonium, the metallurgists were as handicapped as the chemists, with only microscopic quantities available. Fortunately, many of the methods they developed for uranium proved adaptable to plutonium. Again like the chemists, the metallurgists had to devote considerable effort to devising improved recovery methods so that virtually none of the precious metal would be lost in processing it for use in a weapon.4

While awaiting the physical and nuclear specifications for the active materials, the laboratory’s ordnance division worked on the development and proving of the mechanical components for the first experimental guns. First priority was design and fabrication of a plutonium-projectile gun. This gun type posed more problems than a uranium gun, because of Pu-239’s higher propensity to predetonation, but the division’s theory that a gun with sufficient muzzle velocity to avoid predetonation with Pu-239 was certain to be suitable for U-235 justified the concentration of effort.

Using standard ordnance and interior ballistics data obtained from the National Defense Research Committee (NDRC), the ordnance division had its design engineers complete the drawings for a high-velocity gun and, with subsequent approval from the Navy’s Bureau of Ordnance, ordered forgings for two guns from the Naval

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Gun Factory in Washington, D.C. In the meantime, while the guns were being manufactured, Captain Parsons arranged for construction of the Anchor Ranch Proving Ground, some 8 miles east of the central laboratory facilities, where, by September 1943, the division’s proving ground group began testing and perfecting gun performance techniques on a limited and then increased basis.

By early 1944, gun research was advancing smoothly, despite a constant shortage of experienced personnel and difficulties in materials procurement. The division’s design engineers had established the exact specifications of a low-velocity gun, to be used with U-235. Hence, because these specifications were considerably less stringent than previously anticipated for a U-235 gun, the engineers were able to reduce the original muzzle velocity requirements. This achievement made it possible for the division to place a March order with the Naval Gun Factory for three of these uranium guns, which was much earlier than expected and just days after the factory had delivered the first two plutonium prototypes to Los Alamos.5

Primarily because of the undeveloped state of the art, interest in implosion research for a time ranked second to that in gun assembly research. Since April 1943, physicist Seth H. Neddermeyer from the California Institute of Technology had been conducting laboratory experiments with high explosives, designed to test the feasibility of the implosion principle. Handicapped by the shortage of experienced personnel and by the general lack of enthusiasm for implosion among his colleagues, Neddermeyer’s project had definitely remained a “dark horse” in the race for completion of a workable atomic device.

But all of this changed with the arrival of John von Neumann in midsummer 1943. The widely respected Hungarian-born mathematician from the Institute for Advanced Study at Princeton had been carrying out work on shock waves for the NDRC. Applying knowledge of explosives gained in his work with shaped charges, he theorized the likely effects of increasing the velocity of convergingly focused active material in the implosion bomb. His calculations convinced him that if the mechanical problems of achieving higher velocity could be solved, an implosion bomb would attain criticality using less active material of a considerably lower level of purity than hitherto believed possible. If he were correct, implosion offered a means to save precious months in developing a weapon – provided, of course, that ways could be devised to avoid predetonation and achieve symmetry in the imploding shock wave inside the bomb.

By early fall Oppenheimer, Groves, Conant, and the other project leaders were reevaluating implosion. Groves conferred with George B. Kistiakowsky, the distinguished Harvard chemist who was an expert on explosives, and with Oppenheimer and members of the laboratory’s implosion study group. This led to a decision by Oppenheimer and the laboratory’s governing board to expand the implosion

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program immediately, beginning with construction of an on-site plant for casting and trimming test components and installation of the unusual facilities required for testing implosion devices. In early November, Groves and Conant outlined the advantages of implosion to the Military Policy Committee. The following February, the committee informed the President that “there is a chance, and a fair one, if a process involving the use of a minimum amount of material proves feasible, that the first bomb can be produced in the late fall of 1944.”6

Once project leaders had approved undertaking a major developmental program for the implosion bomb, General Groves began a full and objective analysis of the laboratory’s organization, personnel, and facilities for carrying it out. Consulting with von Neumann and Parsons in Washington, D.C., he arranged to have Tolman visit Los Alamos for an extended period to investigate the program. Giving special attention to the laboratory’s ordnance division, Tolman prepared a detailed analysis of its organization and activities, including estimates of the additional personnel that he believed the division would require to complete the implosion program. Tolman found that the laboratory had indeed made considerable progress toward shifting priority to implosion, although Oppenheimer was not yet prepared to abandon some further efforts on the almost certain-to-work plutonium gun.7

By the time of Tolman’s visit, the inevitable shift in emphasis from research and experimentation to engineering, fabrication, and testing was already well under way. Construction crews, under direction of Maj. Wilber A. Stevens and partially comprised of men from the Provisional Engineer Detachment, had completed or were at work on a number of essential test areas (eventually there would be more than thirty of these). They had built a facility for casting containers for explosive charges at the Anchor Ranch Proving Ground and, less than a mile to the south, were well advanced on a much larger and more elaborately equipped area – designated S (for Sawmill) Site – with a laboratory, shops, powder magazines, and even a dining hall. In addition, Major Stevens’s crews had begun work on several outlying sites required especially for testing various implosion devices. Special Engineer Detachment (SED) troops provided a considerable part of the manpower operating these test sites.

Ordnance teams from Los Alamos also assembled and tested bomb components at test sites at Wendover Field (Utah), Inyokern (California), and Alamogordo Army Air Field (New Mexico). (See Map 2.) For these tests, the laboratory procured normal weapon components and high explosives from a variety of government and private suppliers – the Naval Gun

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Factory in Washington, D.C.; the Naval Ordnance Plant in Centerline, Michigan; the Naval Depot in Yorktown, Virginia; the Expert Tool and Die Company in Detroit; the Hercules Powder Company in Wilmington, Delaware; the Monsanto Chemical Company in Dayton, Ohio, to name only a few. But for special parts and materials that were unobtainable, the laboratory itself had to function as an ordnance manufacturing plant. Best illustrating this concentration of effort was the major task of converting U-235 and Pu-239 into metal bomb components.8

In early 1944, the laboratory intensified procurement efforts for specialized equipment for implosion testing. In April, the IBM machines needed to speed up analysis of useful data from implosion tests arrived. And in July, the Military Policy Committee approved procurement of a huge solid steel receptacle for testing the first implosion device, thus ensuring recovery of the active material in the event of a fizzle. By then, implosion development had made giant strides, but still unknown were the relative efficiency of such a design and how long it would take to build a moderately effective implosion device.9

Despite frequent changes in the general specifications for an atomic weapon, the laboratory’s ordnance division had worked out the design of two basic bomb models by the summer of 1944. The gun-type model, the “Thin Man,” was about 10 feet in length, with a varying diameter of 1.5 to 2.5 feet, and had an estimated weight (when loaded) of 5 tons. The implosion-type model, the “Fat Man,” was almost as long (9 feet) but thicker, tapering down from a hemispherical nose measuring 5 feet in diameter to a tailend of about 3 feet, and had an estimated weight (when loaded) of 6 tons. Captain Parsons had the models constructed at the Applied Physics Laboratory in Silver Spring, Maryland, and tested at the Naval Proving Ground on the Potomac River at Dahlgren, Virginia. The laboratory’s delivery group then conducted in-flight tests in a modified B-29, dropping dummy models of both types of bombs, at the Muroc Army Air Field near San Francisco. The ballistical characteristics of Thin Man were satisfactory, but Fat Man displayed serious instability, fortunately soon overcome by a relatively simple modification in the tail assembly.10

But the sense of having achieved substantial progress in weapon design and fabrication was marred by a number of uncertainties. The feasibility of implosion had yet to be demonstrated and the rate at which U-235 and Pu-239 could be produced by the Clinton and Hanford plants remained very much in question. And in July,

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Los Alamos scientists furnished disquieting new data on the plutonium that would be produced in the Hanford piles, indicating the composition of its neutron background would cause predetonation in the plutonium gun.

Project scientists had known for some time that in the process of irradiating uranium in the pile some of the Pu-239 was likely to pick up an extra neutron, forming Pu-240. When plutonium from the Clinton pilot pile became available in the spring of 1944, the radioactivity group at Los Alamos ran a series of tests that confirmed the presence of Pu-240 and indicated it would be present in far larger amounts in plutonium from the Hanford piles. Hence, the neutron background of the active material for the bombs would be several hundred times greater than was permissible. While the Pu-240 could be separated from the Pu-239 by the electromagnetic process, construction of a plant to do so would delay production of a plutonium weapon for many months.

Oppenheimer informed Conant of the 240 problem in early July. To decide how best to deal with it, Conant took immediate steps to assemble project leaders for a conference at the Metallurgical Laboratory on the seventeenth. Besides Conant, the following were in attendance: Oppenheimer, Compton, Charles A. Thomas, in his capacity as coordinator of active material purification research, Fermi, Groves, and Nichols. After some deliberation, the group decided that the predetonation threat posed by 240 made the use of plutonium in the gun-type bomb impracticable and work on this system should be suspended immediately. With this decision, even greater urgency was placed on the development of a workable implosion weapon, in which the 240, because of the higher velocities involved, would be unlikely to cause predetonation.11

Abandonment of the plutonium gun compelled General Groves to revise his predictions on when an atomic weapon would be ready for employment against the enemy. In a progress report to General Marshall in early August, he presented a revised timetable of weapon production: five to eleven implosion bombs in the period from March through June 1945, with an additional twenty to forty implosion bombs of the same size by the end of the year. He cautioned, however, that this schedule would not apply “if experiments yet to be conducted with an implosion type bomb do not fulfill expectations and we are required to rely on the gun type alone” and suggested that, if this delay should occur, the first bomb would not be ready until 1 August 1945, with one or two more by the year’s end. In Groves’ opinion, any delay virtually guaranteed that the bomb would not be used against Germany, which by the late summer of 1944 appeared likely to be defeated within a few months. And to many, even the bomb’s use against Japan seemed doubtful.12

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Through the remaining months of 1944 and the first half of 1945, programs to perfect the uranium gun and implosion principle absorbed the major energies and resources of the reorganized laboratory. As predicted by the Los Alamos scientists, development of the gun moved ahead smoothly with few serious problems. Experiments by the laboratory’s physicists proved the correctness of earlier estimates of the critical mass of the U-235 metal required for the gun and the gun group conducted successful firing tests, using a full-sized tube and substituting U-238 for U-235.

Implosion, by way of contrast, continued to be afflicted with doubts and uncertainties. Progress toward achieving sufficient symmetry in implosion was discouragingly slow. Of the various implosion bomb designs, that proposing the use of explosive “lenses” appeared most feasible.13 A more accurate assessment was achieved with the first tests: Results were so unpromising that in December 1944 Groves and Conant concluded that U-235 should not be used in an implosion bomb but be conserved for the certain-to-work gun.14

As the new year opened, surprising developments dispelled the lingering air of discouragement. In February, when Groves, Tolman, and Conant visited Los Alamos, they found far more reasons for optimism. A few days before their arrival on the twenty-seventh, the gun group finally had frozen design on the U-235 weapon, indicating a usable model would be ready by July. Implosion also had made notable progress, and laboratory leaders decided, in a conference that Groves attended, to manufacture the implosion model favored by Oppenheimer. And to ensure at least one implosion bomb test with active material by 4 July, Oppenheimer also decided to use the California Institute of Technology’s Project Camel facilities for construction of a second model with alternate design features. At this juncture, with data from Hanford indicating that shipments of plutonium in quantity would begin to arrive at Los Alamos in May, with experiments on accurate establishment of the critical measurements on Pu-239 in progress at the Metallurgical Laboratory, and with construction of a much larger plant for final purification of plutonium at

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Los Alamos well under way, the Trinity test date now appeared feasible.15

Project Trinity: The Test of the Bomb

Project Trinity was the final step of the Los Alamos weapon program, the culmination of the laboratory’s reorientation from research and experimentation to engineering, fabrication, and testing of an atomic device. Without Trinity, without the test of the bomb, the feasibility of employing the new weapon appeared to be much more questionable. “If we do not have accurate test data from Trinity,” Oppenheimer and Kistiakowsky had warned, “the planning of the use of the gadget over the enemy territory will have to be done substantially blindly.” As 1945 unfolded, the Trinity mission became the central focus for the scientists at Los Alamos. With the bomb test now first priority, the tempo and intensity of Trinity preparations increased dramatically.16

In the critical months of early 1945, making the gadget work consumed the energies of both the bomb builders and Army leaders. While the scientists worked at perfecting implosion assembly and field teams prepared the remote Trinity test site at Alamogordo, General Groves and his new deputy commander, Brig. Gen. Thomas F. Farrell, devoted much time to overseeing Trinity preparations. Because of pressures of other responsibilities, including planning for use of the bomb against Japan and for the postwar control of atomic energy, Groves managed only three hurried visits to Los Alamos during the months of full-scale preparations (April to July), but he was able to maintain day-to-day contact with bomb test developments through timely observation reports from Farrell, who made several extended tours to the Trinity site.

As Trinity preparations began, Groves had advised Colonel Tyler, the Los Alamos post commander, that he must carefully coordinate plans for development of the bomb test with the laboratory staff and with Farrell “so that every part of it fits into a time schedule.” As procurement crises built up in April and May, Groves personally intervened in expediting requisition of lenses for the implosion bomb and globe-shaped container shells (“pumpkins”) for imploring test devices. In May, with a special report by Farrell on means to improve the procurement situation at the New Mexico installation to guide him, the Manhattan commander contributed

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Brig-Gen. Thomas A. Farrell (right) with General Groves

to the agreement with the University of California to hire more procurement personnel. Finally, in the weeks immediately preceding the test, Groves and Farrell devoted special attention to shipment and receipt of active materials from Hanford and Clinton.17

General Farrell represented the Army at Trinity’s first major event on 7 May – a rehearsal shot of 100 tons of high explosives combined with a very small amount of radioactive fission materials atop a 20-foot platform. Observers, including Tolman and Oppenheimer, judged it a successful trial run for the final implosion test. It gave the various Project Trinity teams practical experience in performing their assignments under difficult field conditions, demonstrated a need for improvements in the transportation and communications facilities, helped calibrate instruments, and provided a likely indication of the amount of radioactive materials needed for the final test.18

In early June, “Jumbo,” the huge steel container to be used in exploding the first atomic device, arrived at Trinity. General Groves had maintained a special interest in the design, procurement, and shipment of the vessel, which was moved in early April on a special railroad car from Barberton, Ohio, via a carefully planned route to a railroad siding at Pope, New Mexico. There, Trinity workers loaded it on a massive trailer pulled by two tractors for the 25-mile trip to the test site. When the vessel finally came to rest some 800 yards from the final test tower, there it remained never to be used. For by the time of Jumbo’s arrival, Los Alamos scientists had decided to dispense with the container, concluding that its use would interfere with obtaining adequate data on the nature of the atomic explosion – the primary reason for conducting the Trinity test.19

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Although 4 July had been set as the target date for the test, few scientists at Los Alamos were convinced it could be met. Precise scheduling depended upon bringing a tremendous number of factors into proper juxtaposition, including weather, procurement of key components and equipment, production and shipment of active material, preparation of many experiments, and arrangement of security and safety measures. In mid-June, Oppenheimer announced to the laboratory’s group leaders that 13 July was the earliest possible date, with up to ten days later not unreasonable. He based his estimate upon information provided by the laboratory’s cowpuncher committee, which had primary responsibility for coordination and scheduling of Trinity.

Following another review of developments on 30 June, this committee advanced the test date to 16 July to permit inclusion of certain additional vital experiments. Two days later, Oppenheimer indicated to Groves that the laboratory leaders finally had agreed on the seventeenth. Groves, however, objected to the later date, pointing out that the situation in Washington required an earlier date. With the end of the war in Europe, Secretary Stimson was scheduled to depart in early July for the Potsdam Conference, with sessions starting on the sixteenth. The Manhattan commander undoubtedly had conferred with Conant, Tolman, and Stimson’s assistants, George L. Harrison and Harvey Bundy, all of whom favored carrying out the test on the fourteenth. Again Oppenheimer consulted with the bomb test team, which reported continued difficulties with the implosion device, wiring at Trinity, and uncertainty concerning receipt of active material. On that basis he informed Groves on 3 July that the test date of the seventeenth must stand. But final preparations advanced more rapidly than expected, and Oppenheimer called Groves on the seventh to announce that the test might take place after all on the sixteenth.20

In the final days before the test, the Army had the major responsibility for completing security and safety arrangements. To meet the eventuality that the people living in towns and on ranches in the immediate vicinity might have to be evacuated to avoid radioactive fallout, the Army stationed a detachment of 160 enlisted men with vehicles at Socorro (New Mexico) and other strategic points along main highways a few miles north of the site. (See Map 6.) To supplement this detachment and also to increase security, the Army detailed about 25 CIC (Counterintelligence Corps) members to towns and cities up to 100 miles from the Trinity site, with instructions to summon evacuation

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troops if they were needed and to help circulate the Manhattan Project’s cover story about an ammunition dump explosion. An officer from Groves’ headquarters had already taken this story to the commander of the Alamogordo base, to be issued as soon as the test took place. Another project officer took up a station in the Associated Press office in Albuquerque to suppress any stories that might alarm the public unduly. Earlier, Groves had arranged with the Office of Censorship in Washington, D.C., to keep news of the explosion from getting into newspapers in other parts of the country. Finally, the Alamogordo commander had reluctantly acceded to the Army’s request to suspend all flights during the test.21

Meanwhile, scientists and technicians at the Trinity site were completing preparations. On 12 July, two scientists from Los Alamos arrived in an Army sedan with the Pu-239 core for the implosion device. The next day a convoy came from the Hill with the nonnuclear components, including the high explosives. Before the test device assembly team moved the plutonium core to the tent at the base of the 100-foot steel shot tower, General Farrell signed a receipt for the active material, thus formally completing transfer of the Pu-239 from the scientists to the Army for use in the test. With all components in place except the detonating system, workers removed the tent and a hoist lifted the device to a metal shed on a platform at the top of the tower. The detonator group then completed the firing circuit and other technicians added apparatus for experiments. By five in the afternoon of the fourteenth, the device was ready for the test.22

The next day, a Sunday, Trinity crews carried out last-minute inspections and observers checked into the base camp, about 10 miles south of the test tower. OSRD Director Vannevar Bush and Conant arrived from Pasadena with General Groves; Army sedans brought Charles Thomas from Santa Fe and Ernest Lawrence, Sir James Chadwick, and New York Times science reporter William L. Laurence, as well as others, from Albuquerque. Compton had decided not to come. Tolman and General Farrell were already on hand. The large contingent from Los Alamos, aboard three buses, did not reach Trinity until shortly before three in the morning of 16 July, barely in time for the originally scheduled zero hour, 4:00 A.M. They stepped out into blustery and rainy weather with occasional flashes of lightning – not the clear skies and moderate winds the Trinity meteorologists had predicted.23

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Trinity control dugout and 
observation post, located six miles from the detonation point

Trinity control dugout and observation post, located six miles from the detonation point

Oppenheimer and Groves had reviewed the weather situation at midnight and then had gone forward from the base camp some 7,000 yards to the control dugout (10,000 yards from the test tower) to wait with Farrell, physicist Kenneth Bainbridge, who was the leader of the bomb test team, and chief meteorologist Jack M. Hubbard, who with Oppenheimer had responsibility for making the final decision on whether to carry out the test as scheduled. As four o’clock approached and the rain continued, Groves and Oppenheimer weighed the risks of going ahead – the likelihood of heavier radioactive fallout at some points, electrical failures from dampened circuits, and poor visibility for the observation airplanes. They decided to delay the shot an hour and a half. The rain stopped at four and shortly before five, with wind still blowing in the right direction, they gave the go-ahead signal for the test.24

As the final countdown began, Groves left Oppenheimer and Farrell in the control dugout and returned to the base camp, a better point of observation and in compliance with the Manhattan chief’s rule that he and Farrell must not be together in situations where there was an element of danger. At approximately the same time, the five Trinity scientists who had been guarding the test device drove away in their jeeps as bright

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The atomic explosion at 
Trinity, 16 July 1945

The atomic explosion at Trinity, 16 July 1945

lights illuminated the tower to foil any would-be saboteurs. Precisely at 5:30 A.M., an automatic firing mechanism actuated the implosion device.

Data from hundreds of instruments recorded what occurred in that desolate stretch of the Jornada del Muerto valley: the dawn of the atomic age. It began with a brilliant yellow light that suffused the remotest recesses of the Trinity site and was seen as far away as Albuquerque and Los Alamos to the north, Silver City (New Mexico) to the west, and El Paso (Texas) to the south. With the light came a sensation of heat that persisted even as a huge ball of fire – like a rising sun – took shape, then transformed quickly into a moving orange and red column. Out of this broad spectrum of colors rose a narrower column that rapidly spilled over to form a giant white mushroom cloud surrounded by a blue glow. Only as the glow began to fade did observers at the base camp feel the pressure of the shock wave, but its rumble reverberated for more than five minutes in the surrounding hills.25

The effects of this explosion on eyewitnesses were as varied as the observers themselves. What General Farrell, for example, saw and heard from the control dugout was “unprecedented, magnificent, beautiful, stupendous and terrifying. … The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a beauty … the great poets dream about. … Thirty seconds after, the explosion came … followed almost immediately by the strong, sustained, awesome roar which warned of doomsday. … What General Groves recalled was that “Drs. Conant and Bush and myself were struck by an even stronger feeling that the faith of those who had been responsible for the initiation and the carrying-on of the Herculean project had been justified. I personally thought of Blondin crossing Niagara Falls on his tightrope, only to me this tightrope had lasted almost three years, and of my repeated, confident-appearing assurances that such a

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thing was possible and that we would do it.”26

But the Manhattan commander permitted himself only a fleeting moment of relaxation. Less than half an hour after the test shot he called his secretary in Washington, D.C., to inform George Harrison so that he could pass on word of the test to Stimson in Potsdam. Groves’ two main concerns were the explosive strength of the implosion device and the impact of the test on project security. There were strong indications, Groves reported, that the strength of the explosion was at least “satisfactory plus” and perhaps far greater than estimated. As to the effects of the test on project security, he would take the necessary measures as soon as its impact on the public had become apparent. By late morning there was evidence that the explosion had aroused considerable excitement throughout New Mexico and in west Texas, near El Paso. Groves gave permission to the Associated Press at Albuquerque to release the previously prepared cover story with such changes as were necessary to fit the exact circumstances of the test:–

Alamogordo, N.M., July 16

The commanding officer of the Alamogordo Army Air Base made the following statement today:

Several inquiries have been received concerning a heavy explosion which occurred on the Alamogordo Air Base reservation this morning.

A remotely located ammunition magazine containing a considerable amount of high explosives and pyrotechnics exploded.

There was no loss of life or injury to anyone, and the property damage outside of the explosive magazine itself was negligible.

Weather conditions affecting the content of gas shells exploded by the blast may make it desirable for the Army to evacuate temporarily a few civilians from their homes.27

That same afternoon, news of the momentous event reached Secretary Stimson in Potsdam:

Operated on this morning. Diagnosis not yet complete but results seem satisfactory and already exceed expectations. Local press release necessary as interest extends great distance. Dr. Groves pleased. He returns tomorrow. I will keep you posted.28

A follow-up cable from Harrison confirmed the success, tentatively implied in the first message:

Doctor has just returned most enthusiastic and confident that the little boy is as husky as his big brother. The light in his eyes discernible from here to High Hold and I could have heard his screams from here to my farm.29

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Stimson passed on this second cable to Truman at once, explaining to the President that Groves (“Doctor”) was convinced that the implosion bomb (“little boy”) was as powerful as the gun-type bomb (“big brother”). Proof of its power was the fact that the light of the explosion was visible for 250 miles (the distance from Washington to Stimson’s summer home at High Hold on Long Island) and its sound was audible for 50 miles (the distance from Washington to Harrison’s farm near Upperville, Virginia). Stimson, Truman, Churchill, and other Allied leaders at Potsdam were quick to realize that this preliminary evidence of the enormous power of the Trinity explosion, followed soon by more detailed substantiating data from General Groves, had introduced a new factor that would profoundly affect not only their own deliberations on how to end the war with Japan but also the whole course of international relations in the postwar world.30