A leading hope of the Manhattan Project from the earliest days was that if a U-235 bomb could be built, then so could a plutonium weapon. Since the initial calculations that Pu-239 would probably have an explosive effect 170 times stronger than U-235, planning for a plutonium bomb had proceeded along the same lines as the U-235 bomb. The gun method of firing a small sub-critical mass into a larger sub-critical mass should result in a nuclear detonation, but it would require the uranium slug to be fired at a speed of 3,000ft per second. As research continued through 1944, however, it became clear to the atomic scientists at Los Alamos that the gun method would not work on plutonium.
At the start of the project, in early 1943, plutonium existed only at a microscopic level. To make a weapon, plutonium had to be transformed into a metal, refined to a high level of purity to ensure that it would reach critical mass and simply not melt down (a process known as pre-detonation), and then be produced in large quantities. Chemists assigned to the project examined various methods of manufacturing purified plutonium and eventually settled on a multi-staged process of precipitation, or using a chemical reaction in a liquid solution to form a precipitate of solid metal. As the initial production of plutonium began, Emilio Segre, a former student of Fermi’s and a member of Fermi’s team in Italy before the war, began experiments to determine the spontaneous fission rate. Segre, a brilliant physicist and former director of the Physics Laboratory of the University of Palermo, was another Jewish refugee who had stayed in the US after the Fascist government banned Jews from university positions in 1938. After working for Ernest Lawrence and lecturing at the University of California, Segre joined the Manhattan Project to take a leading role in the Metallurgical Lab.
In June 1943, Segre’s initial experiments showed that plutonium should detonate inside a gun bomb. More plutonium became available after November when the Metallurgical Lab was finally able to create plutonium in a metallic form. After further experiments in March 1944 demonstrated an even higher rate of fissioning, and potential problems with pre-detonation, further analysis became essential. In April, analysis of the first samples of plutonium produced at Oak Ridge revealed that the plutonium was likely not to work inside a gun bomb. It was not pure enough, and it would fizzle out in a pre-detonation. The results, however, had come from a limited sample, and so Segre and Oppenheimer kept the report quiet. Segre continued to examine additional plutonium as it became available for testing, to make sure that the results were accurate.
By July, it was clear that plutonium would definitely not work in a gun bomb. On the 4th, Oppenheimer told the Los Alamos staff the bad news. The plans for a plutonium gun bomb were immediately scrapped, and a back-burner effort, implosion, was quickly moved into the lead position. While Parsons had favored the gun method, as sufficiently refined U-235 would work in a gun-based bomb, he now agreed to step up implosion research. Oppenheimer and Parsons quickly reorganized Los Alamos to tackle the engineering and development problems of an implosion weapon. One critical aspect of the shift, in addition to research and development, was the need for a test of the bomb to make sure it actually worked.
The shift to implosion was both a vindication for Seth Neddermeyer, the scientist who had first proposed it as a method of detonation at the beginning of the project, and Oppenheimer’s management of the lab. A back-burner project had been allowed to continue, which was exactly what Oppenheimer had planned for when he and Groves had established the unique working atmosphere of Los Alamos. The race was now on, with only a year to go before the deadline for a deliverable atomic bomb.
The concept of implosion called for explosives to compress a ball of plutonium from a sub-critical to a critical mass to start the chain reaction and detonate the bomb. During the first meetings of laboratory staff in April 1943, Neddermeyer, a young scientist transferred from the National Bureau of Standards, suggested the concept. It met with cynical reactions from most of the other scientists, who feared that explosive charges could not be made to perform properly to compress simultaneously a plutonium or U-235 hollow core into a solid ball. They feared that all that would result was a fizzle, or the bomb blowing apart and scattering highly radioactive fragments. Nonetheless, Oppenheimer supported Neddermeyer’s pursuit of the problem, and assigned him to work under Parson’s ordnance group.
Neddermeyer’s first experiments began with test explosions in the back canyons of Los Alamos. Rather than try to blast the complex geometry of a sphere, Neddermeyer encased thick steel pipe in explosives and attempted to crush the pipe equally. He quickly discovered that the pipe was always thrown out of the blast as a twisted, badly warped piece of metal. To compress the pipe thoroughly would also require enough explosives that would rip it apart, ruining the experiments.
As Neddermeyer continued to grapple with the problem, Oppenheimer and Parsons began to push for more results, especially as the first inklings of a Pu-239 gun bomb problem began to surface. When visiting mathematician John von Neumann looked at Neddermeyer’s results and at calculations by physicist Edward Teller of the pressures generated in the blasts, he quickly determined that there could only be a very narrow margin for error in the symmetry of the shock waves generated by blasts in order to successfully compress plutonium. That narrow margin meant a variation in the symmetry would shatter the plutonium core, and the bomb would fail. Neddermeyer added six more team members to tackle the extra work required to meet such exact specifications.
At the same time, von Neumann’s calculations also showed that if the high-velocity shock waves were generated, a plutonium bomb could be made with an even smaller amount of plutonium than originally believed. Groves and Oppenheimer were by now worried that the failure of the gun method for a plutonium bomb meant that the United States would have only one lower-yield U-235 bomb by the summer of 1945. Even if used in combat, a single bomb would be an insufficient demonstration of America’s new atomic power, and it might not only not end the war but also actually spur the Japanese, who were known to have their own nuclear program, into a rushed effort to attack with a bomb of their own.
Oppenheimer asked the theoretical physics team, headed by Hans Bethe, to examine implosion physics more closely, and Teller began to work on more calculations. In January 1944, Teller assumed more responsibility as head of a new implosion theory group. However, he was unhappy with his assignment, believing that Oppenheimer was relegating him to a less important task when his research was centered on a fusion bomb. While the implosion concept was sound, and would ultimately lead to the development of the hydrogen bomb, Oppenheimer believed it would not be developed in time to affect the war, and had placed Teller under Bethe to tackle what Teller saw at best as sidetracking and at worst a breach of faith. In time, Oppenheimer would shift Teller entirely into an effort, not completed at war’s end, to create his “super-bomb.”
Oppenheimer and Parsons were also having problems with Neddermeyer. The young scientist was taking too long, and he needed more help, a fact that he did not seem to realize. In September 1943, Oppenheimer turned to explosives expert George Kistiakowsky, who arrived at Los Alamos in January 1943. By mid-February, Kistiakowsky had replaced Neddermeyer as the head of the implosion team. Under “Kisty,” the implosion team, known as the X Division, focused on making implosion work, while Bethe, still in charge of the theoretical group, assisted. Robert Bacher headed the G Division responsible for building the new weapon (G stood for “Gadget,” the nickname for the plutonium bomb) and Parsons, while skeptical of the ultimate success of implosion, oversaw the merging of all aspects into a bomb that could be manufactured, sent off to war, and delivered to a target.
To accentuate the push, more supplies and more bodies were needed. With Groves’ approval and the US Army’s assistance, enlisted personnel with relevant experience were reassigned to Los Alamos. Known as the Special Engineering Detachment (SED), the draftee scientists and technicians eased a critical shortage in people power. The Manhattan Project was now costing the United States $100 million a month. Results were essential, and obstacles were quickly dealt with, no expense spared.
A key shift in the implosion project came through a suggestion from British scientist James Tuck. After arriving in April 1944, Tuck had suggested using an example from British work on shaped explosive charges designed to crack armor. Tuck believed that high-powered, three-dimensionally shaped charges, used to create “explosive lenses,” could generate the powerful series of shock waves necessary for successful implosion. A problem that required solving, however, was achieving simultaneous detonation of the charges. The possible solution was the use of exploding wire detonators in the charges, and the first tests of the detonators took place at the end of May.
The detonation sequence of the Little Boy weapon was simpler than a plutonium core implosion. The bomb was essentially a powerful gun that fired a uranium projectile into a “target” of uranium-235, causing a chain reaction and a nuclear explosion.
The development of the new methods did not necessarily mean success. Over the next few months, an exhaustive program of developing molds and casting explosive charges with fragile explosive compounds discarded tens of thousands of imperfect castings. Over 20,000 lenses that passed quality control were detonated in experiments to develop the simultaneous ignition system and to produce sufficient and equal compression of a plutonium core. As theory and practical application continued at a rushed pace in tandem, the work environment was later described as one in which “in their cubicles the theoretical scientists would sit for many hours working with pieces of colored chalk on a blackboard or with pencil on pads of paper. At frequent intervals one would hear the boom of great explosions on the various proving grounds in the distant canyons.” This, said the project’s official chronicler, New York Times reporter William L. Laurence, represented “in the true sense the explosion of ideas in the minds of men.”1
One of the explosive ideas that proved to be a further breakthrough came when Robert Christy, another of the British scientists, suggested the use of a smaller, solid core approximately the size of a grapefruit. A smaller, solid mass would theoretically be easier to compress, but it would still require a precise burst to achieve criticality. Squeezed to twice its density, the plutonium would react as the nuclei were shoved closer to each other and free neutrons would punch through the nuclei, releasing more neutrons that in turn would strike more nuclei and release the incredible energy of a nuclear explosion. By the end of December, 1944, the tests of the lenses were progressing sufficiently to suggest that success was around the corner.
To test what constituted the critical mass for an explosion in the uranium bomb, other experiments in another “distant canyon” used a device known as the “guillotine.” A wooden frame supported two steel rods that were set into two small blocks of “active material.” A smaller block, suspended between the rods, was dropped to come into contact with the other blocks, and then slide past them. For a brief moment criticality, measured by neutron flux, would occur. While this test was dangerous because of the release of harmful radiation for that fraction of a second, another test, conducted under the supervision of Otto Frisch, used a small pile, pushing it to the brink of criticality, essentially modeling the “conditions prevailing in the bomb.”
These tests showed that without a doubt the uranium gun bomb would work, and 1944 came to an end, it seemed that a successful method of implosion would be developed, even if by trial and error. To ensure success, however, a test of the Gadget was necessary. In March 1944, Kenneth Bainbridge, a member of Kistiakowsky’s explosives group, was placed in charge of group X-2 to “make preparations for a field test in which blast, earth shock, neutron and gamma radiation” were to be measured and studied and to also make “complete photographic records … of the explosion and any atmospheric phenomena connected with the explosion.”2 Bainbridge, a Harvard University physics professor, had previously been in charge of high-explosive development. Described as “quiet and competent” by General Groves,3 Bainbridge worked closely with the SED and a team of other scientists as he started the massive preparations for the test.
As Bainbridge began his preparations, the issues facing the Manhattan Project other than the questions of implosion were steadily being resolved. The first was both the quantity and the purity of processed plutonium. At the start of the year, Hanford began to produce more plutonium, while at the same time Oak Ridge’s gaseous diffusion facilities, built in a rush, went on line. The final tests with U-235 were made, and in February 1945 Oppenheimer ordered the team designing the uranium gun bomb to finalize the design. Parsons turned his focus to planning for the delivery and use of the weapon, which was now codenamed “Little Boy.”
At the end of February, Groves and Oppenheimer met with George Kistiakowsky, Hans Bethe, Charles C. Lauritsen, Arthur Conant, and Richard Tolman to examine the best design options for the plutonium weapon. The meeting resulted in a series of decisions. Even through Christy’s theory of solid core compression had not yet been proved in a test, the team decided that the use of multiple explosive lenses with a modulated initiator and electric detonators would be used to create simultaneous, converging shock waves to compress the core. With that, all work on other designs for implosion compression was dropped and the design of the plutonium bomb was frozen. The decision was another gamble, because all of the components had problems. Groves had set a deadline of August 1 for a bomb to be ready for combat use, and so the team agreed on a series of deadlines to solve the problems and test an implosion weapon, which was codenamed “Fat Man.”
The goals and deadlines were:
| March 15–April 15: | The detonators were to have their problems resolved and be ready for mass production. |
| April 2: | A full-scale mold for the explosive lenses was to be completed |
| and ready for use to cast the lenses out of the explosive compounds selected for the best results. | |
| April 15: | Less than two weeks later, enough lenses were to be ready for multi-point electrical detonation. |
| April 25: | “Hemisphere tests” were to begin and would measure how the shock waves converged. |
| May 15: | A full-scale test of implosion would successfully compress a solid core of metal. |
| May 15–June 15: | Enough plutonium would be on hand and full-scale spheres would be manufactured and tested for criticality. |
| June 4: | Molding lenses and assembling the detonators for the field test of the bomb would begin. |
| July 4: | The field test of the bomb would successfully detonate a plutonium-core weapon. |
To meet the deadlines, Oppenheimer established a committee to see the various phases through, including delivery of sufficient plutonium from Hanford, the work at Los Alamos, and the construction of a massive new base to test the bomb. Known as the “Cowpuncher Committee” because in western parlance they were to “ride herd” on the various scientists and technicians and their projects, the committee was Captain Parsons, Charles Lauritsen, Samuel K. Allison, Robert Bacher, and George Kistiakowsky. Parsons had advocated the committee, writing to Oppenheimer that “ruthless, brutal people must band together to force the Fat Man components to dovetail in time and space.”4 Allison, a former member of the University of Chicago’s Metallurgical Laboratory and a highly respected member of the Los Alamos team, chaired the cowpunchers. With the committee in place, Oppenheimer then moved into what would be the final phases of the Manhattan Project.
In March 1945, Oppenheimer split Los Alamos’s efforts into two separate projects – “Project Alberta” and “Project Trinity.” The same month, another gamble paid off when the first evidence of implosion-produced compression of a solid sphere was observed in a test. By mid-April, Kistiakowsky was sure he had at last achieved optimal results in his sub-scale tests. Another hurdle had passed. To continue the tests with larger-scale cores, in early May the team introduced a Raytheon Mark II X-Unit, protected from the blasts in closely spaced concrete blockhouses, to shoot fast X-rays every ten-millionth of a second to measure the blasts.
Project Trinity was Bainbridge’s all-out drive to ready a site to test the Fat Man plutonium-core bomb. Oppenheimer later explained that he had selected the name “Trinity,” a choice inspired by poet John Donne – the fourteenth of Donne’s Holy Sonnets starts: “Batter my heart, three person’d God.” The preparations for the test took place at a site selected by Bainbridge after an exhaustive tour of the New Mexico wilderness and approved by Oppenheimer and Groves in the late fall of 1944. The best site would be within a day’s drive of Los Alamos, have good weather, and be in as flat an area as possible. The choice was an isolated spot in an area known since Spanish colonial days as the Jornada del Muerto (Journey of Death). The test site was an 18 by 24-mile section inside a bombing range of the 2,000-square-mile Alamagordo Air Base. Located 125 miles south of Albuquerque and 30 miles east of the nearest settlement, Carrizozo, then a town of 1,500 inhabitants, the Trinity Site was some 210 miles from Los Alamos. It would be a long day’s drive.
To detonate the plutonium core “Fat Man,” shaped charges of fast and slow explosives focused a spherical shock wave to compress the inner components - a beryllium–polonium-210 “initiator” known as the “urchin” and a “pit” of plutonium-239/240. As the metals came into contact, they fissioned and a chain reaction that was controlled and focused by a uranium tamper. About 20% of the pit fissioned, and released energy in the form of a nuclear explosion.
There, as a Los Alamos technical history later recounted, Bainbridge raced to “establish under conditions of extreme secrecy and great pressure a complex scientific laboratory in a barren desert.”5 Preparations for the test had languished in late 1944 and early 1945 until the implosion experiments showed promise and the cowpunchers had formed. Spurred on in February 1945, Bainbridge had five months to complete his task before the scheduled date of the test on July 4. Fortunately, preparation of the site had started in December 1944 as workers began to build a series of wood and concrete slab bunkers covered by earth to protect test instruments, motor generators, cameras, and test personnel. A base camp, erected 9 miles southwest of the test site, became the administrative center for the Project Trinity team, with labs, maintenance, repair and support facilities, and living quarters for the military personnel and scientists who worked at the site. By July, the base camp’s population had grown to 125 souls who contended with the isolation, heat, alkali-laden water (which forced them to shower with Navy-issue saltwater soap), and an even larger population of scorpions. Boots and shoes needed shaking every morning to liberate unwelcome guests who had crawled into them during the night.
At the test site, Bainbridge selected a flat area termed the “Zero Point” to erect a tower on which the bomb would be detonated. Clustered around Zero were various bunkers. The instrument bunkers were erected 800 yards west and 800 yards north of the Zero Point. Three personnel bunkers were built 10,000 yards from Zero to the north, west, and south. Generator bunkers were built alongside them, and camera bunkers were built next to the north and west personnel bunkers. Later, two more instrument bunkers were built 600 yards northwest and 1,000 yards north, and a small firing bunker was built 500 yards west of Zero Point.
Workers strung nearly 500 miles of communications and signal cables on poles or buried them in trenches to link the various bunkers and instruments. The instruments for measuring the blast’s heat, flash, radiation, and shock effects were a diverse assembly of equipment, some of it specially developed at Los Alamos. They included condenser gauges, piston gauges, quartz piezo gauges, crusher gauges, excess velocity gauges, impulse meters, ionization chambers, sulfur threshold detectors, gold foil detectors, gamma ray recorders, electron multiplier chambers, oscilloscopes, coil loudspeaker pickups, geophones, seismographs, spectographs, tracking radar, highspeed cameras, motion picture cameras, and cellophane catcher cameras. While many instruments were placed on the ground at the time of the test, others were suspended from weather balloons or dropped by parachute from observer aircraft.
In May, work on the cross-braced, four-legged steel tower to hold the bomb began. When completed in mid-June, it was 100ft high. At the top was a metal shed to keep the bomb out of the weather. A heavy-duty electrical winch to hoist the bomb up was also installed in the shed. Close by, another tower rose, this one to support a massive steel chamber shaped like a jug. When the test was first proposed, Groves had expressed concern that a pre-detonation would lose the only plutonium the United States had on hand. A variety of methods to save the plutonium (itself valued in the hundreds of millions of dollars) were studied, among them testing the bomb in water, or burying it beneath a large mound of sand that could then be mined to recover the precious metal in the event of a fizzle. Ultimately, the idea of a massive steel container to hold in the blast prevailed.
The problem was that a steel container big enough to hold the bomb and withstand the detonation of the 5,300lb of high-explosive charges inside the Gadget had never been built before, and contractors approached declined the job until the boiler-making firm of Babcock and Wilcox, in Baberton, Ohio, agreed to take the job. When completed at a cost of over a half million dollars (some accounts claim the actual cost was $12 million), the huge steel jug, nicknamed “Jumbo,” was 25ft long and 12ft in diameter. With 14in.-thick steel walls, it weighed 214 tons. Shipped by rail to Pope, New Mexico, crews loaded Jumbo onto a specially designed, 64-wheel trailer that transported it to the Trinity Site. By the time Jumbo arrived, however, confidence in the Gadget was higher, and fear that the atomic blast would vaporize the jug and add its 214 tons of steel to the cloud of nuclear fallout that would follow the blast led to a decision to sidetrack Jumbo. Hoisted halfway up a steel tower 800ft from the Zero Point, Jumbo would bear silent witness to the test.
A third tower, built 800 yards from the Zero Point, was a heavily reinforced wooden platform 20ft high for a pre-atomic test. Known as the “100-ton test,” the experiment was intended to test both the detonation of the Gadget and to calibrate the instruments. It used 100 tons of high explosive, packed in cases and stacked atop the tower. To measure the dispersion of radiation, 1,000 curies of radioactive material, produced from a slug sent from Hanford was dispersed in cylinders laced into the stack. The 100-ton test, at the time the world’s most powerful explosion, lit up the predawn sky at 04:37:05hrs on May 7. The fireball expanded into an oval before dissipating, and a mushroom cloud of smoke and dust climbed 15,000ft into the desert sky.
The 100-ton test was successful, especially in pointing out logistical and organizational problems such as the need for more people, better communications, and the improvement of the test site’s dirt roads, which bogged down vehicles – and hence progress. As a result, 20 miles of roads were paved with asphalt, additional telephone lines and radios were ordered, additional staff were brought in, and a “town hall” was built to better house meetings as the July 4 deadline for the atomic test approached.
By the end of May, enough plutonium had arrived to allow for the final tests of critical mass. Within three weeks, Frisch was able to report that the implosion design would work, and soon after, the lab decided that the bomb would generate a blast somewhere between 4 and 13 kilotons. (A kiloton is the force generated when a thousand metric tons of explosive detonates.) A pool was established to take bets on the actual yield, with some scientists betting zero, and others, like an optimistic Edward Teller, suggesting that the yield would be higher at 45 kilotons.
Trinity, where the Manhattan Project first tested the atomic bomb, is in a relatively isolated spot in the New Mexico desert. This map shows the layout of the test site’s landmarks. (Artist info)
The pool was one means of dealing with the tension everyone felt as the test approached. At that stage, nearly $2 billion had been spent, and reputations were on the line. The war in Europe was over, but the ongoing struggle with Japan continued. A new president, Harry Truman, was in office following Franklin Roosevelt’s death on April 12. This change inspired a satirical bit of doggerel that made the rounds of Los Alamos:
From this crude lab that spawned a dud.
Their necks to Truman’s ax uncurled
Lo, the embattled savants stood,
and fired the flop heard round the world.6
By June, it was clear that the initial deadline of July 4 could not be met, and a new date, July 16, was set.
At Trinity, final preparations for the test continued up to the day of the shot. After delays caused by inferior castings of high explosive, mostly caused by air bubbles forming inside the explosive as it set, there were not enough lenses ready by July 9. Kistiakowsky, guided by X-ray images, borrowed a dental drill, bored into the castings, and filled the air voids with “molten explosive slurry” to make them acceptable. Enough castings of high explosive were ready on July 10 for the Los Alamos team to begin assembly of two packages, one of which would be fired in a test with a non-fissionable core to see how it worked.
Lieutenant Commander Norris Bradbury, US Navy, one of Parson’s team, took on the task of assembling the charges while the plutonium cores were cast. The charges were nestled into place, but despite the careful attention to detail, none fitted tightly. A suggestion to fill the voids with grease was rejected, and instead Bradbury and a team of SED technicians filled the spaces with facial tissue paper and then used Scotch tape to hold the charges together. Equally ingenious and last-minute thinking was also needed with the cores. Coated with nickel to absorb alpha particles and avoid corrosion, the cores soon blistered because plating solution was trapped beneath the nickel and against the plutonium, which gave off heat. The cores required a perfect fit inside the bomb, but stripping the cores was out of the question, as it would expose the plutonium. The blisters were ground down and the irregular surface of the cores was filled with gold foil, leaving each core a brilliant reflective surface.
The core for the Trinity test left Los Alamos on the afternoon of July 12, nestled in the back seat of a US Army Plymouth sedan with scientist Philip Morrison for the long drive to the test site. On arrival, the core was unloaded into the abandoned ranch house of George McDonald, which had lain vacant since the creation of the bombing range in 1942 and the McDonald family’s evacuation. The master bedroom, turned into a “clean room” for the bomb’s assembly, had its windows sealed with plastic, and now held the workbenches for the scientists and technicians. At Los Alamos, Bradbury’s team finished the assembly of the charges into the hemispherical casing of the Gadget, and then lowered a tamper sphere of uranium (which would act as a neutron reflector) inside to fit, as historian Richard Rhodes would later describe it, into the “cavity like the pit in an avocado.”7
On July 13, just past midnight, an Army 5-ton truck loaded with the high-explosive assembly for the bomb left Los Alamos and drove for eight hours through the night and into the dawn, arriving at Trinity an hour before Bainbridge’s team assembled in the McDonald ranch house to start the final assembly. As the cores were laid out, scientist Robert Bacher, the senior advisor, asked the Army for a receipt for the multi-million dollar plutonium that was about to be destroyed. Brigadier General Thomas Farrell, Groves’ deputy, signed the receipt:
I recall that I asked them if I was going to sign for it shouldn’t I take it and handle it. So I took this heavy ball in my hand and I felt it growing warm, I got a certain sense of its hidden power. It wasn’t a cold piece of metal, but it was really a piece of metal that seemed to be working inside. Then maybe for the first time I began to believe some of the fantastic tales the scientists had told about this nuclear power.8
The assembly was ready by the afternoon and the scientists took it by car to the Zero Point, arriving at 03:18hrs to meet up with Bradbury and the rest of the bomb team. Winched off its truck and on skids below the tower, the Gadget was missing one lens, a gap through which the cylindrical plug with the core and its initiator were to fit, tightly, into the heart of the bomb. Everything had been machined to fit perfectly, but as the two assemblies were mated, the plug jammed. As one scientist recalled, consternation reigned until it was pointed out that the core had slightly expanded in the heat of the ranch house overnight, while the rest of the bomb, kept under cover, was cooler. After taking a break to allow the temperatures to equalize, the team found that the pieces fitted together. By late evening, Bradbury had completed the assembly of the bomb, stopping short of inserting the detonators, which would happen the following day after the bomb was winched atop the tower.
On July 14, the team slowly winched the Gadget 100ft up and into the tower, pausing while soldiers piled a stack of mattresses beneath it to cushion the bomb if the winch failed and it fell. Once up, the Gadget’s assembly resumed as the detonators were inserted into place, covering the Gadget with an array of wires and plugs. As the assembly progressed, however, bad news came from Los Alamos. The test firing of the other, non-fissionable assembly had produced results that suggested the Trinity bomb would be a dud. Intense discussion, complaints, and several grillings of Kistiakowsky ensued until Hans Bethe reported that a review of the data could not be taken at face value and that a working bomb was still possible if not probable.
By the 15th, everything was at last ready. Tempers were high, tension was perceptible, and Oppenheimer alternately paced, chain-smoked and read as he tried to stay calm. The difficult situation grew worse at 02:00hrs, as a storm hit complete with thunder, lightning, and heavy rain. As the storm continued, Groves postponed the test to 05:30hrs in the morning when his weather forecaster suggested the storm would end. “You’d better be right on this, or I will hang you,” Groves barked.9
As test personnel made the last-minute preparations, observers gathered in the bunkers and at a VIP lookout atop Compañia Hill, 20 miles northwest of the Zero Point. At Los Alamos, a group of distant observers hiked and spent the night on a mountain, watching in the distance. Aloft, aircraft stood by with instruments and observers, while at distant places, including a Carrizozo motel room, others waited with instruments. The rain finally stopped and as rockets arced into the darkness to signal the impending shot, and a warning siren wailed, the various groups stood waiting, some hastily slathering on sunblock and donning thick dark glasses to shield them.
Then, at 05:29:45hrs, the Gadget detonated. The world’s first nuclear explosion was described by William L. Laurence, the only reporter present:
…there rose from the bowels of the earth a light not of this world, the light of many suns in one. It was a sunrise such as the world had never seen, a great green super-sun climbing in a fraction of a second to a height of more than eight thousand feet, rising even higher until it reached the clouds, lighting up earth and sky all around with a dazzling luminosity.10
Hans Bethe described the detonation as looking “like a giant magnesium flare which kept on for what seemed a whole minute but was actually one or two seconds. The white ball grew and after a few seconds became clouded with dust whipped up by the explosion from the ground and rose and left behind a black trail of dust particles.”11
The fireball continued to expand, changing colors, followed by a cloud that climbed to 41,000ft as it punched through the clouds above. A “mighty thunder,” as Laurence termed it, followed, the ground trembled, and a blast of hot wind swept over the desert, and then came silence punctuated by the exclamations of the assembled scientists and military officials who had watched from a safe distance. The flash and the blast were seen and heard hundreds of miles away.
Determining the force of the bomb was a key aspect of the test. Watching the blast, Enrico Fermi decided to try an informal experiment:
About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during, and after the passage of the blast wave. Since, at the time, there was no wind, I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast wave was passing. The shift was about 2½ meters, which at the time, I estimated to correspond to the blast that would be produced by ten thousand tons of TNT.12
Later, the test instruments showed that the Trinity blast was equivalent to 18.6 kilotons.
As the blast faded, two lead-lined Sherman tanks rumbled to life and drove into the heart of the test area, finding a mile-wide area devoid of life, scorched and swept clean. The tower was gone, leaving only the stubs of its concrete supports. A crater 400 yards in diameter, 25ft deep at the center and tapering up to a 10ft deep depression at the edges, was lined with melted sand that the heat of the explosion had turned into a greenish-gray, highly radioactive glass. Lifted into the fireball and heated to over 14,710°F (8,430°C), the molten glass had rained back into the depression formed by the blast. Termed “Atomsite” and later “Trinitite,” the atomic slag was one of many new phenomena observed that morning.
As the bright light faded and the blast echoed, the tension melted. After the cheers stopped, a grim reality set in. “Now we’re all sons of bitches,” Bainbridge said.13 Oppenheimer later recalled, “we knew the world would not be the same. A few people laughed, a few people cried. Most people were silent.” He went on:
I remembered the line from the Hindu scripture, the Bhagavad-Gita: Vishnu is trying to persuade the Prince that he should do his duty and to impress him he takes on his multi-armed formed and says, “Now I am become Death, the destroyer of worlds.” I suppose we all thought of that, one way or another.14
The incredible display was not lost on the military. General Farrell approached Groves and said, “The war is over.” Groves quickly replied, “Yes, after we drop two bombs on Japan.”15 Two days later, over a meal, President Truman and Prime Minister Churchill discussed the success of the joint American-British effort, and agreed to proceed with the combat deployment of the weapon by the Americans as quickly as possible. The bomb was on its way to Japan.