World War II Day by Day

Trinity Test Site near Alamogordo, New Mexico • July 16, 1945

On this date in 1945 the first detonation of an atomic bomb directly led to greater un­imag­in­able destruc­tion when “Little Boy” and “Fat Man” immo­lated Hiro­shima and Naga­saki on August 6 and 9, 1945, respec­tively. Up until July 16, 1945, mili­tary brass and mili­tary and civil­ian scien­tists could only specu­late about the mas­sive explo­sive power of this new type of bomb. The major­ity of scien­tists involved in the Man­hat­tan Pro­ject, the $2 billion U.S. Army’s top-secret nuclear nuclear wea­pons pro­gram, never wanted to build an atomic bomb nor did they want it to be used against civil­ians. Their reser­va­tions were given a cold shoulder, most famously in the case of Hun­garian-born physi­cist Leó Szi­lárd’s peti­tion that was intended for U.S. Presi­dent Harry S. Truman and high-ranking offi­cials and that was even­tu­ally co-signed by over 50 Man­hat­tan Pro­ject per­son­nel. Szi­lard’s co-author of a half-dozen years before, the German-born theo­reti­cal phys­i­cist Albert Ein­stein, in a peti­tion the two men delivered to the White House that sparked the crea­tion of the Man­hat­tan Pro­ject, was barred from working directly for, or con­sulting on, the Man­hat­tan Pro­ject owing to the renown scien­tist’s left-leaning politics and peace acti­vism. Despite having nothing to do with the pro­ject he insem­i­nated, Ein­stein was quoted as saying “Woe is me” upon learning of Hiroshima’s fate.

Both Little Boy and Fat Man had an interesting ges­ta­tion period. The engi­neering design phase was fraught with danger. Little Boy was a rela­tively straight­forward gun-type fission wea­pon while Fat Man was a com­plex implo­sion-type nuclear wea­pon. Little Boy used highly enriched ura­nium‑235, which is fis­sile (capa­ble of being split or divided), meaning it can sus­tain a nuclear chain reaction leading to an explo­sive energy release. Fat Man used pluto­nium‑239 to achieve an explo­sive energy release. A gun-type fis­sion wea­pon earlier than Little Boy was Thin Man, which like Fat Man used pluto­nium as its fis­sile mate­rial. Further develop­ment of Thin Man stopped when the spon­ta­neous fis­sion rate of pluto­nium was dis­covered too high for use in a gun-type design; i.e., the device would under­go a pre­limi­nary chain reaction that destroys the fis­sile mate­rial before it is ready to produce a large explosion.

The gun-type fission weapon of Thin Man found its way into Little Boy when ura­nium instead of pluto­nium was the wea­pon’s fis­sile mate­rial. Fat Boy con­tinued as before as an implo­sion-type nuclear wea­pon with pluto­nium as its fis­sile mate­rial. Implo­sion was tricky. The wea­pon designers created 32 poly­gonal lenses that focused the implo­sion into a sphe­rical shape resembling a soccer ball. Exploding-bridge­wire deto­na­tors, invented at the Man­hat­tan Pro­ject’s Los Alamos Laboratory, got the detonation just right.

The complexity of an implosion-design plutonium bomb called for a full-scale test. J. Robert Oppen­heimer, direc­tor of the Los Alamos Lab­o­ra­tory that designed the two types of nuclear bombs, code­named the iso­lated test site “Trinity.” The test device, or “Gadget,” was of the same design as Fat Man. The weekend Trinity Test attracted some 425 people. When the bomb exploded it released the explo­sive energy of 25 kilo­tons of TNT ± 2 kilo­tons in addit­ion to a huge cloud of radio­active fall­out. Norris Brad­bury, Trinity Test bomb assem­bly group leader, remarked that the explo­sion “did not fit any pre­con­cep­tions pos­sessed by any­body.” Rich­ard Feyn­man, Amer­i­can theo­reti­cal phys­i­cist and Nobel Prize laure­ate, was sta­tioned 20 miles/­32 km from the deto­na­tion point, observed: “A big ball of orange, the center that was so bright, becomes a ball of orange that starts to rise and billow a little bit and get a little black around the edges, and then you see it’s a big ball of smoke with flashes on the inside of the fire going out.” Another wit­ness that day said he “could feel the heat on my face a full 20 miles/­32 km away.” Trinity Test director Kenneth Bain­bridge called the detonation a “foul and awesome display.”

Trinity Test Site, New Mexico, 1945: Detonating World’s First Nuclear Bomb

Left: The 100 ft-/­30 m-tall steel tower constructed at a small Army air­field for the Trinity test. The name Trinity was inspired by the “three-person’d God” in a son­net written by English poet and cleric John Donne. The test site was located on the Alamo­gordo Bombing and Gun­nery Range (now part of the White Sands Missile Range) in a par­tic­u­larly dry stretch of south­ern New Mexico desert called the Jornada del Muerto—Spanish for “Jour­ney of the Dead Man.” Trinity lay 200 miles south of Los Alamos, where the atomic bombs were designed and built.

Right: The explosives of 5-ton Gadget were carefully hoisted to the cor­ru­gated-steel shelter at the top of the tower for final assembly in mid‑July 1945.

Left: Norris Bradbury, bomb assembly group leader, stands next to the part­ially assembled Gadget atop the test tower, July 15, 1945. Among other things, Brad­bury had to ensure that the series of deto­na­tors needed to set off a nuclear chain reaction inside Gadget fired simul­ta­neously within a frac­tion of a mil­lionth of a second, other­wise the plu­to­nium would “fizzle” and not pro­duce a nuclear explo­sion. The July test of the plu­to­nium device turned out to be an unqual­i­fied success. Frank Oppen­heimer watched his brother Robert hardly breathing in the last few seconds before Gadget exploded; after it did his brother’s face relaxed into an expres­sion of tremen­dous relief. In some­thing of an under­statement Robert Oppen­heimer simply said: “I guess it worked.” Oppen­heimer is often called the “father of the atomic bomb.”

Right: Trinity was a test of an implosion-design pluto­nium device, the same con­ceptual design used in the second nuclear device dropped on Japan—Fat Man—which was deto­na­ted over Naga­saki, Japan, on August 9, 1945. This photo was taken at 5:30 a.m., a 0.016 second after test deto­na­tion. The searing light of the explo­sion was more intense than any­thing ever wit­nessed before and could have been seen from space. Its core tem­per­a­ture was four times greater than that at the sun’s core. The awe­some roar of the air blast 30 seconds later “warned of dooms­day,” reported one wit­ness. Oppen­heimer later remarked that the deto­na­tion reminded him of a pas­sage from the second-century BCE Hindu Bhagavad Gita: “Now I am become Death, the destroyer of worlds.”

Left: An aerial photograph of the Trinity crater shortly after the test. The bomb blast left a crater of radio­active glass 10 ft/­3 m deep and 1,100 ft/­335 m wide. Nothing remained of the steel tower. The shock wave was felt over 100 miles/­161 km away, and the mush­room cloud reached 7.5 miles/­12 km in height. Presi­dent Harry S. Truman was posi­tively giddy with new con­fid­ence on a swift victory in the four-year Pacific War. From the Big Three victors’ con­fer­ence in Pots­dam near the former Nazi capital, Berlin, Truman wrote his wife in mid-July 1945: “We’ll end the war a year sooner now, and think of the kids [U.S. service members—ed.] who won’t be killed.

Right: J. Robert Oppenheimer (center, in dark suit and light-colored hat), Brig. Gen. Leslie Groves (mili­tary head of the Man­hat­tan Pro­ject in khaki uni­form to Oppen­heimer’s left), and other scien­tists and mili­tary person­nel inspect the melted remains of the test tower at ground zero after the Trinity blast. The photo was taken in Septem­ber when some parti­ci­pants returned with news reporters. Note men wearing shoe covers to keep from picking up radi­a­tion. The test site was littered with lop­sided marbles and knobbly sheets that later became known as Trini­tite. Trini­tite was primarily quartz and feld­spar, tinted sea green with min­erals in the desert sand, with drop­lets of condensed plutonium sealed into it.

University of California Television: The Manhattan Project