Trinity: The First Atomic Explosion, 16 July 1945

It had rained most of the night on the Jornada del Muerto. In a timber-and-earth bunker 10,000 yards south of a steel tower, J. Robert Oppenheimer waited with a few dozen men while a physicist read out the countdown over the radio and loudspeakers. Ten miles away, at the base camp, scientists lay face down in the sand with their feet toward the tower; twenty miles away, on a low hill, others smeared sunscreen on their faces in the dark and held squares of welding glass. At the top of the tower, in a corrugated-iron shed, sat a sphere of high explosive around a core of Hanford plutonium. At about half past five in the morning of 16 July 1945 the desert was lit, for a few seconds, more brightly than by the noon sun.

Why it mattered

Trinity was the first explosion of a nuclear weapon, and it answered the question on which the plutonium half of the Manhattan Project depended. The uranium gun bomb was considered so certain to work that it was never tested. The plutonium bomb was different. Since July 1944, when Los Alamos had found that reactor-made plutonium could not be used in a gun (Chapter 5), the laboratory had staked everything on implosion, a method that had never been tried at full scale and that many of its own scientists doubted. Hanford was now producing plutonium at an industrial rate (Chapter 4), and that plutonium would be useless unless implosion worked.

The test also gave the President a fact in hand at the moment he needed it. Harry Truman had delayed his meeting with Stalin and Churchill at Potsdam until mid-July in part to know the result, and the news reached him there within hours. And it was the beginning of the nuclear age’s other story: of fallout, secrecy and the people who lived downwind. The families of the Tularosa Basin, who were not warned, not evacuated and not studied, are still seeking recognition eighty years later.

Why test at all?

A full-scale test had been discussed at Los Alamos from early 1944, when implosion first looked like a serious possibility. Groves did not like the idea of exploding a bomb’s worth of plutonium that could otherwise be used against the enemy, and the plutonium was scarce. But the arguments for a test were strong. The implosion design depended on dozens of explosive charges detonating together and on effects that could be calculated only roughly; a weapon that might fizzle could not be dropped on an enemy city, where a failure would hand the Japanese a nearly intact bomb. The laboratory also needed to know what a nuclear explosion would actually do: its yield, its blast and heat, its radiation, and how high above a target a bomb should burst. In March 1944 Oppenheimer gave the planning to Kenneth Bainbridge, a Harvard experimental physicist, and by the spring of 1945 Bainbridge’s organization, known as Project TR, had grown into a division of its own (Hoddeson et al.; Bainbridge, LA-6300).

Fear of losing the plutonium led to one of the strangest objects of the project. If the high explosive detonated but the nuclear reaction failed, the plutonium would be scattered across the desert. The laboratory ordered a huge steel containment vessel, called Jumbo, meant to hold the explosion of the high explosive and allow the plutonium to be recovered. Babcock & Wilcox built it in Barberton, Ohio; it weighed 214 tons and arrived at the site in May 1945 on a specially built trailer, towed some 25 miles from the railroad siding at Pope. By then the explosives work had progressed far enough, and confidence in the device risen high enough, that the scientists decided not to use it. Jumbo was hung from a steel tower 800 yards from ground zero, and it survived (Wikipedia, “Trinity (nuclear test),” citing Hoddeson et al. and Jones).

The Journey of the Dead Man

Bainbridge looked for a site that was flat, so that the ground would not distort the blast; dry and with little wind, so that fallout would not spread far; remote and nearly empty of people; and within reach of Los Alamos. With Robert Henderson, Maj. W. A. Stevens and Maj. Peer de Silva, the Los Alamos security officer, he surveyed eight candidate sites in 1944, from the Tularosa Valley and the lava beds of El Malpais to the San Luis Valley in Colorado, the Army’s desert training area in California, San Nicolas Island off the California coast and Padre Island in Texas. They chose a stretch of the Jornada del Muerto, the “Journey of the Dead Man,” a waterless valley on the old Spanish road from Mexico City to Santa Fe, at the northern end of the Army Air Forces’ Alamogordo Bombing Range. After consulting Maj. Gen. Uzal Ent, commander of the Second Air Force, which controlled the range, the site was settled on 7 September 1944. Ground zero lay about 35 miles southeast of Socorro (Wikipedia, citing Bainbridge and Jones).

The code name Trinity is usually credited to Oppenheimer, who said much later that it came from a sonnet by John Donne, “Batter my heart, three person’d God”; he wrote to Groves in 1962 that he could not be sure why he chose it. The test area was laid out around the tower: three instrument and observation bunkers 10,000 yards north, west and south of it; a base camp about ten miles to the southwest; and the stone McDonald ranch house, abandoned when the Army took over the range, about two miles away, which was turned into an assembly building. A detachment of military police under Lt. Harold Bush arrived on 30 December 1944, twelve men at first and 125 by the end; by July about 250 people were working at the site (Wikipedia; DNA 6028F).

The rehearsal: 100 tons, 7 May 1945

To calibrate the instruments and practice the procedures, Bainbridge’s team fired a rehearsal shot of about 100 tons of high explosive, stacked in wooden crates on a 20-foot wooden platform about 800 yards from the tower site. Holes in the stack were filled with radioactive fission products from Hanford, so that the teams could practice tracking contamination. Scheduled for 5 May and put back two days to install more equipment, it was set for 04:00 on 7 May and fired at 04:37, after a delay to let an observation B-29 get into position; the flash was seen at Alamogordo Army Air Field, 60 miles away. The shot was a success, and its lessons were practical ones: more vehicles, better roads, buried telephone lines and a teletype, and about 20 miles of sealed road so that the convoys would not be lost in their own dust (Wikipedia, citing Bainbridge; DNA 6028F).

Assembling the Gadget, 12–15 July 1945

The test date was set by the arrival of the plutonium and by Washington. Truman wanted the result before the Potsdam Conference, which opened on 17 July, and 16 July was the earliest day for which the parts could be ready (Wikipedia). The plutonium core was driven down from Los Alamos on 12 July and assembled in the clean room set up in the McDonald ranch house, where the work was finished on 13 July. That afternoon the core was carried to the tower and fitted into the high-explosive assembly in a tent at its foot, a delicate operation that stalled for a few anxious minutes when a part would not fit until the heat of the desert had evened out the temperatures. On 14 July the device, known to everyone simply as “the Gadget,” was winched up to a small shed at the top of the 100-foot steel tower, over a stack of Army mattresses piled underneath in case it fell, and its detonators were connected (Hoddeson et al.; Rhodes).

The last days were tense. A test of a dummy explosive assembly at Los Alamos on 14 July suggested that the implosion might fail, a result later traced to the instruments, not the design. Storms built up over the desert on the 15th. Donald Hornig, a young physicist who had worked on the firing unit, was sent up the tower that day to check that all was well and, by later accounts, spent part of the stormy evening on the platform with the device as a guard against sabotage; he was the last man to leave the tower. The arming party, which included Bainbridge, drove out to the tower after 22:00 on 15 July to make the final connections. The shot was set for 04:00 on 16 July.

16 July 1945, 05:29

At 04:00 it was raining, with thunder and lightning, and Bainbridge and Hubbard advised a delay; wind and rain could carry fallout onto people and wet the firing circuits. Groves, who was at the site with Bush and Conant, was furious with the forecasts but agreed to wait. At 04:45 Hubbard forecast a break, and the shot was set for 05:30. At 05:10 the physicist Samuel Allison began reading the countdown in the S-10,000 bunker. The automatic timer took over in the last seconds. The detonation is most often given as 05:29:45 Mountain War Time; recent accounts, working from the test records, give 05:29:21, plus or minus 15 seconds (DOE; Wikipedia, “Trinity (nuclear test)”).

Observers were scattered across the desert. Oppenheimer, Brig. Gen. Thomas Farrell, Bainbridge, Kistiakowsky and the control team were at the S-10,000 bunker; Groves, Bush, Conant, Fermi and Rabi were at the base camp about ten miles away, Groves having left Oppenheimer so that the two would not be killed together if something went badly wrong; a party of scientists and visitors, among them Edward Teller, James Chadwick, Richard Feynman and William Laurence of the New York Times, watched from Compania Hill, about 20 miles to the northwest. Two B-29s circled at a distance with Project Alberta observers aboard. The flash was seen as far away as Albuquerque and El Paso, and the shock rattled windows more than 100 miles away. Within minutes the cloud had climbed to about 7.5 miles (DNA 6028F; Rhodes).

What the observers said and wrote in the following hours and days is better evidence than what they recalled years later. Farrell, standing beside Oppenheimer, wrote a description for Groves’s report two days later that called the effects unprecedented and terrifying. Fermi, at the base camp, tore a sheet of paper into small pieces and dropped them as the blast wave arrived about forty seconds after the flash; from how far they were carried he estimated a yield equal to about 10,000 tons of TNT. Groves, in the report he sent to Stimson on 18 July, wrote that the test had been “successful beyond the most optimistic expectations of anyone.” The famous lines came later. Bainbridge recalled in 1975 telling Oppenheimer that “now we are all sons of bitches.” Oppenheimer, in a television documentary in 1965, recalled a line from the Bhagavad Gita that had come to his mind (Groves to Stimson, 18 July 1945; Bainbridge, 1975; Rhodes).

In their words

The effects could well be called unprecedented, magnificent, beautiful, stupendous and terrifying.

How big was it?

Nobody at Los Alamos knew in advance how big the explosion would be. The Theoretical Division had predicted five to ten kilotons. In a betting pool the night before, Oppenheimer picked 0.3 kilotons, Bethe 8, Kistiakowsky 1.4, Norman Ramsey zero, and Teller 45; Rabi, arriving late and taking one of the few numbers left, picked 18 kilotons and won. The first measurements gave widely different answers. Blast gauges recorded about 9.9 kilotons, plus or minus one; Fermi’s paper scraps about 10. The most reliable method, radiochemical analysis of soil samples scooped from the crater by two lead-lined Sherman tanks, gave about 18.6 kilotons. The figure long used officially was 21 kilotons. In 2021 a team of Los Alamos scientists, re-analyzing samples of trinitite with modern methods, put the yield at 24.8 kilotons, plus or minus 2 (Wikipedia, citing Selby et al., Nuclear Technology, 2021; Bainbridge, LA-6300-H).

By the numbers

The yield of Trinity: predictions, bets and measurements

Kilotons of TNT equivalent. Gray: predictions and bets made before the test. Red: estimates and measurements made afterward.

Oppenheimer’s bet in the pool, 15 July0.3

Theoretical Division prediction (upper end of 5–10)5–10

Rabi’s winning bet18

Blast (box) gauges, 19459.9 ± 1

Fermi’s paper-scrap estimate at base campc. 10

Radiochemistry of crater soil samples, 194518.6

Long-standing official figure21

Reanalysis of trinitite (Selby et al., 2021)24.8 ± 2

The cover story

An explosion seen over hundreds of square miles could not go unnoticed. The Manhattan Project’s press man, William L. Laurence of the New York Times, who had been allowed to follow the project on condition of silence, wrote several versions of a press release in advance, including one for a catastrophe in which people were killed. The version issued on 16 July by the commander at Alamogordo Army Air Field, through the Second Air Force, said that a remote ammunition magazine had exploded, without loss of life, and that weather conditions might require some civilians to be evacuated temporarily. It read in part: “A remotely located ammunition magazine containing a considerable amount of high explosives and pyrotechnics exploded.” Newspapers in New Mexico and Texas ran it, and the story held for three weeks. The truth was released on 6 August, after Hiroshima, and Laurence’s own eyewitness account appeared in the New York Times in September 1945 (Wikipedia; Rhodes).

At Potsdam

In the evening of 16 July, at Babelsberg near Potsdam, Secretary of War Henry Stimson received a coded message from his assistant George L. Harrison in Washington: “Operated on this morning. Diagnosis not yet complete but results seem satisfactory and already exceed expectations.” He took it to Truman and to Secretary of State James Byrnes. A second message on 18 July reported that the “little boy” was “as husky as his big brother.” Groves’s full report, thirteen pages with Farrell’s description, arrived by courier on 21 July; Stimson read it to Truman and Byrnes, and Churchill read it the next day. Truman wrote in his diary on 25 July that the Americans had “discovered the most terrible bomb in the history of the world.” On 24 July, after a plenary session, he walked over to Stalin and mentioned, without detail, that the United States had a new weapon of unusual destructive force. Stalin showed little interest; through Klaus Fuchs and other sources, his intelligence services had known of the test in advance (Wikipedia; Rhodes; Truman diary; Holloway, Stalin and the Bomb).

Two days later, on 26 July, the Potsdam Declaration called on Japan to surrender unconditionally or face “prompt and utter destruction,” without mentioning the bomb. On the same day the cruiser USS Indianapolis delivered the components of the uranium bomb to Tinian (Chapter 7).

Downwind

The site had been chosen as empty, and by the standard of the time it nearly was: but nearly half a million people lived within 150 miles, and there were ranches and Hispanic and Mescalero Apache communities much closer than that (Wikipedia, citing Widner). The cloud drifted northeast. The heaviest fallout outside the test area came down on Chupadera Mesa, about 30 miles from ground zero, where cattle developed beta burns and lost hair in patches; the Army bought 88 head for study. Stafford Warren’s monitors found high readings in places along the roads and at some ranch houses, and the trucks for evacuation stood ready, but no one was moved, warned or told what had happened. Warren afterward recommended that future tests be held at least 150 miles from any populated area (Wikipedia; DNA 6028F; Hacker, The Dragon’s Tail).

The first outsiders to detect the fallout were not doctors but film makers. In August 1945 the Eastman Kodak Company found spots and fogging on sensitive X-ray film; its physicist J. H. Webb traced the cause to radioactive contamination in strawboard packing made at an Indiana paper mill that drew its water from a river on which fallout from the New Mexico cloud had come down, and concluded that a nuclear explosion had taken place in the United States. Kodak kept the finding to itself until 1949 (Wikipedia, “Trinity (nuclear test)”).

The people who lived downwind were never systematically studied in the years after the test. Decades later, a dose reconstruction by the National Cancer Institute, published in 2020, found that the counties most affected were Guadalupe, Lincoln, San Miguel, Socorro and Torrance, and estimated that the fallout caused some additional cancers among people living there at the time, while acknowledging large uncertainties. The Tularosa Basin Downwinders Consortium, led by Tina Cordova, a cancer survivor from Tularosa, campaigned for years to have New Mexico’s downwinders included in the Radiation Exposure Compensation Act of 1990, which had covered people downwind of the Nevada Test Site but not of Trinity. The act lapsed in July 2024. The One Big Beautiful Bill Act, signed on 4 July 2025, reauthorized and broadened it; the Justice Department now lists all of New Mexico as an affected area for downwinders who lived there for at least a year between 24 September 1944 and 6 November 1962 and later developed one of the listed cancers, with claims due by 31 December 2027 (Department of Justice, RECA program).

Timeline

Trinity, 1944–1945

1. Mar 1944Oppenheimer gives the planning of a full-scale test to Kenneth Bainbridge.
2. 7 Sep 1944The Jornada del Muerto site, on the Alamogordo Bombing Range, is chosen.
3. 30 Dec 1944Lt. Harold Bush’s military police detachment arrives at the site.
4. Mar 1945Project TR becomes a separate division of the laboratory.
5. May 1945Jumbo arrives; bomber crews accidentally bomb the base camp twice.
6. 7 May 1945, 04:37The 100-ton rehearsal is fired.
7. 12 Jul 1945The plutonium core is driven from Los Alamos to the McDonald ranch house.
8. 13 Jul 1945Core assembly is completed; the core is fitted into the explosive assembly at the foot of the tower.
9. 14 Jul 1945The Gadget is hoisted to the top of the 100-foot tower.
10. 15 Jul 1945Storms over the site; Donald Hornig checks the device on the tower; the arming party goes out after 22:00.
11. 16 Jul, 04:00Shot postponed for rain and lightning.
12. 16 Jul, 04:45Jack Hubbard forecasts a break; the shot is set for 05:30.
13. 16 Jul, 05:10Samuel Allison begins the countdown.
14. 16 Jul, 05:29:45Detonation (some accounts 05:29:21 ± 15 seconds), Mountain War Time.
15. 16 Jul, morningThe cover story of an exploded ammunition magazine is released at Alamogordo.
16. 16 Jul, eveningHarrison’s “Operated on this morning” message reaches Stimson at Potsdam.
17. 17 Jul 1945The Potsdam Conference opens.
18. 18 Jul 1945Groves signs his report to Stimson, with Farrell’s description.
19. 21 Jul 1945The report reaches Potsdam; Stimson reads it to Truman.
20. 24 Jul 1945Truman tells Stalin of a new weapon of unusual destructive force.
21. 26 Jul 1945The Potsdam Declaration; Indianapolis reaches Tinian with Little Boy’s components.
22. Aug 1945Kodak traces fogged film to fallout.
23. 9 Sep 1945Groves and Oppenheimer bring reporters to ground zero.

Questions readers ask

Was the Trinity device the same as the bomb dropped on Nagasaki?

It was the same type of weapon, a plutonium implosion design, and its core was made of Hanford plutonium like Fat Man’s. The Trinity device was not in a bomb casing and was fired from a tower rather than dropped. The uranium bomb used on Hiroshima, Little Boy, was a different design and was never tested before it was used.

Why is the exact time given differently?

The time most often published, including by the Department of Energy, is 05:29:45 Mountain War Time. Some recent accounts, drawing on the test records, give 05:29:21 with an uncertainty of about 15 seconds. The difference reflects how the time was recorded and reconstructed, not two different events.

What is trinitite?

The greenish glass formed when the fireball melted the desert sand around ground zero, much of which fell back as droplets. Most of it was bulldozed and removed in the early 1950s. Taking it from the site is now prohibited.

Was anyone hurt?

No one at the test was killed or seriously injured by the explosion, and the film badges at the nearest shelters recorded low doses. Some workers who went into the crater area received significant doses. The fallout fell on inhabited areas to the northeast, where cattle were burned; the people living there were not warned, evacuated or followed up, and the National Cancer Institute’s 2020 study estimated that the fallout caused some additional cancers. Many families in the area believe the toll was larger than the official estimates.

Can you visit the Trinity Site?

Yes, but only on the one day a year when White Sands Missile Range holds an open house, currently the third Saturday in October. Visitors can walk to the obelisk at ground zero, see the remains of Jumbo, and visit the restored McDonald ranch house where the core was assembled.

How it connects

Trinity proved the design that Los Alamos had turned to in 1944 (Chapter 5: Los Alamos) using plutonium from the Hanford reactors (Chapter 4: Hanford). Its result reached Truman at Potsdam in the days when the decision to use the bomb and the final ultimatum to Japan were made, and when the 509th Composite Group was preparing on Tinian (Chapter 7: The 509th Composite Group, Tinian and the Decision). The same design was dropped on Nagasaki on 9 August (Chapter 9). The B-29 crews who flew the missions are part of our Army Air Forces collection, and the voyage of the Indianapolis connects this story with our Submarine War collection, since she was sunk by the Japanese submarine I-58 on her way back from Tinian.

Seeing it today

Trinity Site lies inside White Sands Missile Range and is open to the public only on its annual open house, held on the third Saturday of October. The first open house, in September 1953, drew about 650 people; the fiftieth anniversary tour on 16 July 1995 drew about 5,000. The site was declared a National Historic Landmark on 21 December 1965, and the lava-rock obelisk at ground zero was put up that year. Visitors can see the stub of one of the tower’s footings, the remains of Jumbo, whose ends were blown off by an Army ordnance test in April 1946, and the restored McDonald ranch house. Low levels of residual radioactivity remain at ground zero. The National Museum of Nuclear Science & History in Albuquerque and the Bradbury Science Museum in Los Alamos display Trinity material, and the Tularosa Basin Downwinders Consortium holds vigils for the families who lived downwind.

Sources

• Kenneth T. Bainbridge, Trinity, Los Alamos Scientific Laboratory report LA-6300-H (1976), and “‘All in Our Time’: A Foul and Awesome Display,” Bulletin of the Atomic Scientists 31, no. 5 (May 1975).
• Defense Nuclear Agency, Project Trinity, 1945–1946, DNA 6028F (1983).
• Lillian Hoddeson, Paul W. Henriksen, Roger A. Meade and Catherine Westfall, Critical Assembly: A Technical History of Los Alamos during the Oppenheimer Years, 1943–1945 (Cambridge University Press, 1993).
• Vincent C. Jones, Manhattan: The Army and the Atomic Bomb (U.S. Army Center of Military History, 1985); Richard G. Hewlett and Oscar E. Anderson Jr., The New World, 1939–1946 (Pennsylvania State University Press, 1962).
• Leslie R. Groves, memorandum for the Secretary of War, “The Test,” 18 July 1945, with Brig. Gen. Thomas F. Farrell’s statement (National Archives; reprinted in Groves, Now It Can Be Told, Harper & Brothers, 1962).
• Richard Rhodes, The Making of the Atomic Bomb (Simon & Schuster, 1986).
• Ferenc Morton Szasz, The Day the Sun Rose Twice: The Story of the Trinity Site Nuclear Explosion, July 16, 1945 (University of New Mexico Press, 1984).
• Barton C. Hacker, The Dragon’s Tail: Radiation Safety in the Manhattan Project, 1942–1946 (University of California Press, 1987).
• Kai Bird and Martin J. Sherwin, American Prometheus (Knopf, 2005); Ray Monk, Robert Oppenheimer: A Life Inside the Center (Doubleday, 2013).
• David Holloway, Stalin and the Bomb: The Soviet Union and Atomic Energy, 1939–1956 (Yale University Press, 1994).
• Susan D. Selby et al., “A New Yield Assessment for the Trinity Nuclear Test, 75 Years Later,” Nuclear Technology 207 (2021).
• Steven L. Simon, André Bouville et al., Trinity dose and cancer risk reconstruction, Health Physics 119, no. 4 (October 2020), National Cancer Institute.
• Thomas Widner et al., Final Report of the Los Alamos Historical Document Retrieval and Assessment (LAHDRA) Project (Centers for Disease Control and Prevention, 2010).
• U.S. Department of Justice, Radiation Exposure Compensation Program (eligibility after the 2025 amendments, Pub. L. 119-21).
• Harry S. Truman, diary entry of 25 July 1945 (Harry S. Truman Library).
• Wikipedia, “Trinity (nuclear test)” and “Radiation Exposure Compensation Act”, used as guides to the official reports and scholarly works they cite.

Photographs: U.S. government photographs of the Trinity test and its preparations, many reproduced in the Defense Nuclear Agency’s report DNA 6028F and held by the National Nuclear Security Administration and Los Alamos National Laboratory, including high-speed photographs by Berlyn Brixner’s group, and a U.S. Army Signal Corps photograph from Potsdam, all public domain as works of the U.S. government, via Wikimedia Commons. Dates are given as recorded in the official captions; where a caption gives no exact day, the text says so. The unit insignia and the badge illustration are drawn for this collection and are not official emblems.