Authors: Brian Van DeMark
Time had passed, problems had been solved, and work had progressed with increasing speed. Things were moving fast now. Two types of bombs neared completion. To obtain a powerful explosion, sufficient fissionable material had to be brought together quickly, then kept together long enough to release a lot of energy. The most obvious and direct assembly method was a uranium bomb, using a simple and tested “gun” design in which a cannon fired a subcritical slug of U-235 into a subcritical core of U-235, bringing them to a critical mass that generated a tremendous radioactive explosion. Because U-235 was in extremely short supply and because Oppenheimer and his colleagues felt highly confident that the straightforward design of the gun would work, the decision was made not to test it.
Not so for the second type: a plutonium bomb. The gun-assembly method could not be used with plutonium, since this new element exploded too
easily
. Plutonium emitted alpha rays and would be a source of background neutrons. Calculations strongly suggested that these background neutrons would be strong enough to predetonate a gun-type weapon; that is, even at the highest possible muzzle velocities, the chain reaction would start before the two pieces of the core had come close enough together to become a critical mass. In that case the reaction would fizzle and the weapon would release only a minute fraction of its explosive energy. It would be a dud.
An alternative to the gun method, called implosion, was proposed by Caltech physicist Seth Neddermeyer and developed by Harvard chemist George Kistiakowsky. Instead of shooting two subcritical masses of plutonium together, implosion involved taking a subcritical mass and compressing it to criticality in millionths of a second. If a sphere could be uniformly squeezed tight enough over its entire surface, the plutonium molecules would be compressed to a higher density at which the subcritical mass would become critical. The great speed of compression would also eliminate the danger of predetonation.
Plutonium was considerably easier to produce than U-235, but there was also considerably more uncertainty whether the implosion design would work. Oppenheimer and most of his division leaders, including Bethe, were skeptical at first. The technical problems of implosion were tremendous. If one point of the plutonium sphere were detonated more than one-millionth of a second later than another point, implosion would not work. More than six hundred Los Alamos scientists ended up working on the problem and devising feasible solutions, but without a test, there was no way to tell if the implosion bomb would work.
Nagging doubts persisted as Oppenheimer and his colleagues went about their last weeks of preparations, and a gloomy parody repeated up and down the halls of the Tech Area: “From this crude lab that spawned a dud / Their necks to Truman’s axe uncurled / Lo, the embattled savants stood / And fired the flop heard ‘round the world.”
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Each scientist on the Hill gave a different estimate of the chance of success, but all agreed that the answer was uncertain. They were reaching into the unknown.
Oppenheimer argued that delays in development, as well as the tight schedule in production of fissionable material, made it necessary to postpone the test date. But Groves was unrelenting. He insisted that the bomb be tested before July seventeenth (when the final wartime conference between Truman, Churchill, and Stalin was scheduled to begin at Potsdam). On July second, Groves phoned Oppenheimer about the test date. Oppenheimer requested a delay; the implosion device, he said, would not be ready before July seventeenth. Groves insisted that the target date of July sixteenth be kept, and explained why. Oppenheimer, who had few reserves left, said, according to Groves’s notes of the call, that “it went against his own feeling, but if the general wanted it that way, they would do it.”
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As they had been for the past two years, Oppenheimer and the other scientists at Los Alamos would be pushed to the limit.
At the beginning of July, large numbers of scientists began making mysterious trips off the mesa to a destination in southern New Mexico. Nearby canyons echoed with explosions as test work on implosion reached a climax. Then the explosions suddenly stopped. Excitement was at fever pitch. It seemed to everyone that something big was about to happen.
On July fifth, just six days after enough plutonium had arrived from Hanford, Oppenheimer wired Lawrence: “Anytime after the 15th would be a good time for our fishing trip. Because we are not certain of the weather we may be delayed several days. As we do not have enough sleeping bags to go around, we ask you please not to bring any one with you.”
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The test site, codenamed Trinity—it had been named by Oppenheimer while he was reading seventeenth-century English poet John Donne: “Batter my heart, three-person’d God.”—lay in a vast, empty, and forbidding sweep of desolate land fifty miles northwest of Alamogordo, New Mexico. It was bounded on the west by the Rio Grande River and on the east by the Sierra Oscura Mountains, which rose low and broken on the distant desert horizon. Dotted with mesquite, yucca, and cactus and inhabited by a menagerie of desert insects and reptiles, the site was originally part of the royal road north from Mexico City to the farthermost regions of Spain’s New World empire. The area had been known ominously from Spanish times as Jornada del Muerto—the dry and dangerous Dead Man’s Trail, the Journey of Death.
Apaches roamed here originally, but during the nineteenth century it became ranch land where cattle and sheep grazed as best they could. The state of New Mexico owned most of the land, but leased it to a handful of homesteaders at minimal cost. After Pearl Harbor the Jornada became the northwest sector of the Alamogordo Air Base. The Army Air Corps leased a ranch in the middle of the Jornada, and ten miles north of the ranch house marked out Ground Zero. At points 10,000 yards north, west, and south from Ground Zero, contractors built observation bunkers with concrete slab roofs supported by sturdy oak beams. South-10,000 would serve as the control bunker for the test. Another five miles beyond South-10,000 was a base camp of tents and barracks. Behind the camp, desert brush ran all the way to the Oscura Mountains. Here—on the endless stark desert, empty almost all the way to Mexico—was the Trinity site in all its strangeness, an oasis willed into existence overnight.
The convoy carrying the precious plutonium core from Los Alamos arrived at Alamogordo in the early evening of July twelfth. The core, transported in a shock-mounted case, was carried into a room at the ranch house, which was guarded throughout the night by heavily armed MPs. The next morning—a Friday the thirteenth—a team of physicists gathered in white labcoats at the ranch house to begin the assembly. On a table in the room, the assemblers spread brown wrapping paper and laid out two hemispheres of plutonium warm to the touch and plated in nickel to make them safe to handle, a shiny beryllium/polonium initiator and, to confine these elements, several pieces of plum-colored natural uranium. With Oppenheimer walking in and out like an expectant father, adding to the already considerable tension in the room, the assemblers carefully nestled the small cylinder with the initiator between the plutonium hemispheres, then the nickel ball in the hollowed tamper of uranium.
Late that afternoon, the eighty-pound bomb core, readied at last, was placed in a sedan and carefully driven to Ground Zero. The following day, a team of physicists working under Oppenheimer’s close and constant attention gingerly completed the final assembly. The team, many of them shirtless, worked inside a muslin tent to protect them from the scorching rays, a soft light guiding their work. Everyone in the tent seemed outwardly calm, but each felt the tension in the air. The plutonium core, suspended by a chain from a hoist, was slowly lowered into the center of a sphere of high explosives, like the pit in a peach. The team finished its work under lights in the late evening.
At 8:00 the next morning, the tent was removed, and the plutonium bomb started its slow ascent to the top of the steel tower. Several mattresses were stacked under the bomb in the hope of softening the impact of an accidental drop. Powered by a motor, the hoist rose slowly, only a foot a minute. Workers climbed just ahead of the bomb, guiding its passage from platform to platform until they finally placed it in a sheet-steel cage on top of the tower. Dozens of fragile detonators, arranged symmetrically on the outside surface to be triggered simultaneously by an electric circuit, were then delicately attached. Late that night, the work was finished. Finally everything was ready.
The steel tower at Ground Zero looked like an oil rig without the pipes, a spindly structure of steel beams and tie braces. Concrete footings supported its four legs, spaced thirty-five feet apart. Braced with crossed struts, the tower rose one hundred feet to a wooden platform that was roofed and sheltered on three sides with sheets of corrugated iron. The bomb atop the tower was five feet around and weighed five tons, but the plutonium core was not much bigger than a grapefruit. Cables connected the bomb’s sixty-four detonator ports to electrical boxes. Out of the boxes’ switchboard backs an equal number of cables hung down to the firing unit, a padlocked aluminum case. Below on the desert floor, hundreds of six-foot wooden poles strung with five hundred miles of wire fanned out from Zero to measuring instruments anchored miles away. Zero Hour was fixed for Monday morning, July sixteenth.
The early morning of July 16, 1945, was dark—not a star could be seen. A tropical air mass thick with humidity hung over the Jornada, and winds gusted to thirty miles an hour. A low, unbroken belly of clouds stretched from one end of the valley to the other. Lightning was striking everywhere. In every arc of the horizon a bolt was hitting. One report of thunder overlapped and muffled another. As lightning flashed, everyone, tense and nervous, glanced apprehensively toward Zero. A bomb of similar size, but filled with ordinary explosives, had been hauled up the tower a few days before to test the measuring instruments. That bomb had been struck by lightning and exploded with a thunderous bang. If a bolt were to strike now, who knew what might happen? The desert was damp, and the waiting scientists shivered.
Oppenheimer, even more nervous and tense than usual, paced back and forth in the doorway of South-10,000, peering at the weather outside. It was miserable. A colossal storm had raged all night long. The bunker was damp and wet. Oppenheimer had passed the night stirring restlessly on his bunk, racked with coughing from chain-smoking. “He was very tense,” recalled a young scientist who was in the bunker that night. “I tried to be funny, to lighten the situation. He seriously debated firing me on the spot.”
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Oppenheimer spent the final, harrowing hours before the test with Groves. The physicist seemed to totter on the verge of a nervous collapse. Groves made an effort to keep him away from the mounting tension in the dugout. Each time that Oppenheimer seemed on the point of breaking down, Groves would take him out and walk with him in the rain, reassuring him that everything would be all right. Oppenheimer wanted the test to succeed. Yet just two nights before, in a mood of foreboding, he had recited to a friend a stanza from the sacred Hindu epic poem the Bhagavad Gita: “In battle, in forest, at the precipice in the mountains / On the dark great sea, in the midst of javelins and arrows / In sleep, in confusion, in the depths of shame / The good deeds a man has done before defend him.”
Always thin, Oppenheimer now looked emaciated. Fatigue had worn away his cheeks, and his crystal blue eyes recessed deep in their sockets, an image of implosion. His voice, dry and scratchy, seemed to cry out for rest, but instead his body was in constant motion, pacing edgily. He vibrated with tension, his cigarette hand moving to his mouth in tiny jerks. Groves feared he might come apart at the last minute. Oppenheimer scarcely breathed and held on to a post to steady himself. “Lord, these affairs are hard on the heart,” he whispered.
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Twenty miles northwest of Zero, Teller stood atop Compañia Hill where he had hiked with a heavy, uneven step because of his artificial foot to view the long-awaited test. Other scientists who also had been bused in to view the test stood with him in the dark atop the sandy desert ridge. They talked in groups like guests at a tailgate party. Most had been there all night and were stiff with cold and waiting. The wind had died down to a still hush before dawn. Now the crowd members grew quiet, stamping their feet to keep warm.
Although all had been instructed to lie down on the sand and turn their faces away from the blast and bury their heads in their arms, Teller had no intention of doing so. Suspecting the flash might be even bigger than expected, in the pitch black of this early morning, he hurriedly smeared suntan lotion on his face and hands. He put on dark glasses and pulled on a pair of heavy gloves. With both hands, he pressed a piece of dark welder’s glass to his face so that the light would not damage his eyes. “A hundred-to-one it’s not needed, but what do we know?” he said nervously to those around him.
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Teller then looked straight toward the distant tower.
Bethe moved into position with Teller on Compañia Hill. Bethe had addressed “T” Division personnel the night before in Los Alamos’ biggest community hall, concluding: “Human calculation indicates that the experiment must succeed. But will nature act in conformity with our calculations?”
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Bethe and his audience had then boarded buses camouflaged with paint and set off on the four-hour journey through thunderstorms and hail to Trinity. Like Teller, Bethe now rubbed suntan lotion on his face and hands for protection against damaging ultraviolet rays from an expected flash twenty miles away. The one-minute warning rocket fired.
Lawrence was also on Compañia Hill, having made the three-hour drive down from Albuquerque in the middle of the night. Lawrence had wagered other scientists on the test’s success, but his usual assurance was missing. “Our tenseness grew as zero hour approached,” he wrote later that day.
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Lawrence had planned to watch the shot through the windshield of his olive-drab Plymouth, allowing the tinted glass to filter out the ultraviolet rays, but at the last minute, as the final countdown began, he decided to get out and look toward distant Zero with just sunglasses.