When the arming party, composed of Kenneth Bainbridge, George Kistiakowsky, and physicist Joseph McKibben arrived at the base of the bomb tower at just before midnight, Donald Hornig was more than ready to return to South-10,000, the control shelter 10,000 yards from ground zero. He had spent an uneasy few hours in the tower during that night's thunderstorm, baby-sitting the atom bomb.
Monday, July 16
Truman awaits the news
With zero hour only two hours away, Los Alamos lab director J. Robert Oppenheimer and Manhattan Project leader Maj. Gen Leslie Groves drove from base camp to South-10,000, then phoned the tower for a conference with Professor Bainbridge and weatherman Jack Hubbard, who also had gone to the base of the tower. It was still raining, the winds over ground zero were unfavorable, and Hubbard could give no promise that the front would break up before dawn. Radio contact with Albuquerque revealed that two B-29s, scheduled to simulate a bombing run, were grounded because of the weather. Test director Bainbridge decided the test could not possibly be held at the scheduled zero hour of 4 a.m.
What to do? Postpone the test until a later date, or wait out the weather a little longer. The heavy rains might already have shorted out some of the circuitry. And a delay until dawn might hamper some vital photographic measurements. Weighing against these factors was the knowledge that President Truman was waiting at Potsdam, Germany, for the results from Trinity.
Oppenheimer, Groves, and Bainbridge agreed to hold everything in readiness and hope for a break in the weather. It would be possible to fire the bomb up until 5:30 a.m., they decided. After that, daylight would nullify essential measurements.
Oppenheimer and Groves paced the desert near South-10,000. At the tower, Hubbard and two Army sergeants sent weather balloons aloft every 15 minutes to check high-altitude wind velocities. Shortly after 3 o'clock the rain stopped. At 4 a.m., the cloud cover began breaking up, and the winds died down somewhat. In Albuquerque, the weather cleared, and the B-29s took off and headed south.
At 4:45 a.m., the last possible moment for a decision, Hubbard's weather instruments indicated all high-altitude winds were favorable. Bainbridge phoned South-10,000 with the eagerly awaited order: ''Prepare to fire at 5:30.''
Bainbridge unlocked a coffinlike box at the base of the tower and, with McKibben checking every movement, threw the switch to close the tower end of the firing circuit. The bomb was now fully armed. Before heading for South-10,000, Bainbridge turned on a floodlight to serve as an aiming point for the B-29s.
It was a few minutes after five when the arming party arrived at South-10,000. Bainbridge, who held the only key, unlocked the cover of the all-important firing switch. McKibben took up his position in front of the automatic timer. Behind him, facing two microphones, stood University of Chicago physicist Samuel Allison, who had been elected to perform the countdown. At 10 minutes after 5, Bainbridge nodded to Allison. Across the desert and over the radio to the two B-29s came the announcement: ''It is now zero minus 20 minutes.''
Twenty minutes to zero
Standing by to evacuate
At Harry Miller's Tourist Court in Carrizozo, N.M., on the other side of the Oscura Mountains, Alvin Graves and his wife Elizabeth (Diz) gave a final check to their portable seismograph and Geiger counter and peered out of the west window of cabin No. 4. The Graveses, both Los Alamos scientists, were stationed in Carrizozo to measure shock and radiation effects.
At Albuquerque's Hilton Hotel, Army security officers Capt. Thomas Jones and Lt. Phil Belcher were trying to grab a few moments of rest after being on the telephone all night to the more than 20 military-intelligence agents scattered throughout New Mexico. Another phone was kept open to Trinity base camp.
Twenty miles northwest of ground zero, at their hilltop observation post, scientists started emerging from the buses. Welders' glasses were distributed. One man passed around a bottle of sunburn lotion, cautioning that even at this distance the flash might be strong enough to burn.
Not far from the hilltop observers, an Army unit with 150 trucks waited near highway 380, its leaders briefed on the location of all residents within 50 miles should evacuation be necessary. Along roads near Trinity, Army military-police patrols maintained a constant vigil.
At zero minus 20 minutes, General Groves left South-10,000 and returned to base camp. Above Trinity, the B-29s flew back and forth, unable to locate the light near the base of the tower. Fearful that they might be directly over the tower at zero hour, the pilots abandoned their mission.
At minus 10 minutes, Hornig went to a position alongside McKibben at the control panel. After the automatic timer had gone into action, it would be Hornig's job to hold his hand on or near a stop switch. Up to the last second, should anything go wrong, Hornig could prevent the bomb from going off by flipping up this knife switch.
''It is now minus five minutes.''
Oppenheimer, his face ashen, came in off the desert and stood in the doorway of the control shelter. The uncertainties and responsibilities of the past three years flooded over him. The bomb would work only if everything functioned perfectly - and there were so many things that could go wrong. Was there some unknown physical law that had not been taken into account?
Young Don Hornig, standing next to Oppenheimer, tried to break the tension. ''What's likely to happen, Oppie,'' Hornig joked, ''is that at minus five seconds I'll panic and say, 'Gentlemen, this can't go on,' and then pull the switch.''
Oppenheimer was in no mood for humor. For a moment he seriously questioned Hornig to see if he really might break under the tension. Satisfied that Hornig had only been joking, Oppenheimer moved silently away. The minutes ticked on.
''It is now minus two minutes.''
Two minutes to zero
'I must remain conscious'
On the hill 20 miles away, the scientist-observers readied their welders' glasses. At base camp, in a shallow trench, Marshall Holloway lay still, wondering, for the hundredth time that night, if he had assembled the core exactly right. A few yards away, Conant and Groves were stretched out on the ground facing away from zero, according to instructions. The always-confident Groves - he had told President Roosevelt before Yalta that it was a 90-per-cent certainty the bomb would work - now began to worry that the gadget might work too well and endanger the lives of the test crew and observers.
At minus 45 seconds, McKibben threw the switch to the automatic timing device. The timer started activating the circuits. At minus one second it would send a signal to a high-voltage circuit that would supply the last-second surge of power needed to fire the gadget. There was nothing more man could do.
Oppenheimer leaned against a wooden pillar for support. ''I must remain conscious,'' he reminded himself. Hornig watched the voltmeter on the panel in front of him, his hand poised on the stop switch. Four small lights at his eye level must turn red when the firing unit was energized - and at minus 30 seconds, all four blinked on. The needle on the voltmeter swung far to the right. The condensers on the firing unit at the tower were fully charged.
''Minus 30 seconds,'' intoned Allison over the network. ''Twenty-five ... 20 ... 15.'' At 10 he began calling off each second: ''Nine ... eight ... seven ... six ... five....''
* Part I ran Monday; Part III will appear Thursday. Robert Cahn won a Pulitzer Prize for a Christian Science Monitor series on America's national parks. This article is adapted from one he wrote for The Saturday Evening Post that ran July 16, 1960.
The always-confident General Groves now began to worry that the gadget might work too well and endanger the lives of the test crew and observers.