The Trinity test

The mushroom cloud produced by the Trinity test.

The Trinity test

By Virginia Grant| July 13, 2020

Seventy-five years ago, Los Alamos scientists detonated the Gadget—the world’s first atomic bomb.

On July 16, 1945, a predawn thunderstorm moved through the desert near Alamogordo, New Mexico, about 120 miles south of Albuquerque. After it passed, at 5:29:45 a.m., detonators ignited explosives around a large, steel, globe-shaped device on a 100-foot tower. The explosion prompted a fission chain reaction in the plutonium inside the globe. The resulting nuclear blast from the Gadget, as the device was called, released an explosive force of 21 kilotons (equivalent to 21 thousands tons of TNT), more than had been predicted. It created a blinding flash of light, a thunderous sound, and a mushroom cloud 38,000 feet tall. “Some people claim to have wondered at the time about the future of mankind,” remembered physicist Norris Bradbury of witnessing the event. “I didn’t. We were at war, and the damned thing worked.” This was the Trinity test, the culmination of 27 months of work at Project Y—a secret laboratory in Los Alamos—to create the world’s first atomic bomb.

The necessity of testing

Project Y covertly developed two types of nuclear weapons. The first type—exemplified by Little Boy, the bomb eventually dropped over Hiroshima, Japan—achieved detonation by firing a subcritical piece of uranium at another subcritical piece of uranium, thus starting a supercritical reaction. Scientists were so certain that this gun-type design would work that full-scale testing was deemed unnecessary.

The Gadget and Fat Man (the bomb dropped over Nagasaki) were of a different type; they were implosion bombs, with cores of subcritical plutonium that reached a supercritical mass because of pressure from the explosives surrounding them. Scientists weren’t quite as confident in this design and felt the need to test the technology before its use in war.

Workers stand on top of stacks of TNT.
In preparation for the Trinity test, an experiment using ordinary high explosives took place on May 7, 1945. The experiment, which detonated more than 100 tons of TNT stacked on a tower, allowed scientists to practice timing and calibrate instruments that would detect energy release, thermal radiation, and other factors. Los Alamos National Laboratory

“The set of problems connected with implosion was the most difficult,” said Laboratory Director Robert Oppenheimer of the development of the Gadget. “It required very new experimental techniques, and it was not a branch of physics which anyone was very familiar with.”

Preparing to (hopefully) make history

Trinity Tower

The Trinity test, directed by physicist Kenneth Bainbridge, would take place at the U.S. Air Force’s Alamogordo Bombing and Gunnery Range in the Jornada del Muerto (“Journey of Death”) desert of New Mexico. The site, which was selected in September 1944, provided isolation and also proximity to Los Alamos, which was about 210 miles away. The area was flat, with little wind, providing better conditions for studying the explosion and its aftermath.

A great deal of construction commenced that fall. A base camp was built for the approximately 250 people who worked on preparations, roads were paved for the transport of materials to the site, and electricity was run to the test tower for the detonators. Three shelters—one each at 10,000 yards north, west, and south of ground zero—were constructed to protect people (scientists and soldiers) and equipment (for observation and radiation detection) during the test. More than 52 cameras were used to photograph and video record the test, including special high-speed cameras developed by members of the Weapons Physics Division for the purpose of recording nuclear explosions.

Gadget mounted on tower.

To prevent the waste of the plutonium in the Gadget’s core, scientists considered building a giant concrete bowl filled with water to catch plutonium. This idea never materialized at the Trinity site, although a smaller version was built at Los Alamos. Scientists also contemplated detonating the Gadget inside Jumbo, a 216-ton steel cylinder that would contain plutonium if the detonation failed. Jumbo was built but never used. A quarter mile from ground zero, the vessel was unharmed by the blast. After World War II, eight 500-pound bombs were exploded inside Jumbo, but much of the cylinder stayed intact and remains today at the Trinity site.

Another important construction project was the 100-foot steel tower atop which the Gadget was detonated. The tower was used to get good images of the early fireball expanding and to reduce fallout—the radioactive dust and ash created when a nuclear weapon explodes. (Ultimately the tower was too short for a 21-kiloton test, and the blast produced a lot of fallout.)

The Gadget was hauled up the tower by an electric winch. During the process, part of the device became unhinged. For just this circumstance, a truckload of mattresses had been brought in and placed around the tower. Despite some onlooker panic, the Gadget was stabilized before it could fall.

A group of men surrounding the Gadget, which is about to be lifted out of a truckbed and up into the tower.

In another mishap, Kenneth Greisen, whose group was in charge of the Trinity detonators, was pulled over for speeding in Albuquerque just a few days before the test. Luckily, the officers didn’t check his trunk, as it contained the detonators themselves, which he was delivering to the site. Such delivery methods were the norm for Trinity—the plutonium core of the Gadget was delivered to the site in an Army sedan.

Raising the Gadget to the top of the tower.

Preparations also included two explosives tests. The first, on May 7, was a 100-ton shot of TNT, used to make sure the test timing was right and to calibrate the detection instruments. The second took place near Los Alamos just two days before the Trinity test. The plan was to practice for Trinity using a model Gadget that did not contain plutonium. Although the faux Gadget did detonate, the initial interpretation of the data indicated the implosion of the actual Gadget would not be fast enough to initiate a full-scale nuclear detonation. However, physicist and Nobel Laureate Hans Bethe recalculated the data and concluded there would be adequate force for a successful implosion and that the Trinity test would be successful.

The Gadget

The Gadget was “from a theoretical, from an observational, and from a practical point of view quite an adventure,” Oppenheimer explained in a 1965 interview with journalist Stephane Groueff. “It was still a very reasonable opinion that one of the many things that we needed to make it work was not completely in order on July 16.”

Perhaps that’s why, among the numerous provisions made to protect the test’s secrecy, false press releases were created for distribution after the blast. (For example, one New Mexico paper, the Clovis News-Journal, would later report that “a remotely located ammunition magazine containing a considerable amount of high explosive and pyrotechnics exploded” at the Alamogordo Army Air Base. “There was no loss of life or injury to anyone, and the property damage outside of the explosive magazine itself was negligible.”)

“We knew the world would not be the same”

The Trinity test was originally scheduled to take place at 4 a.m. on Monday, July 16, but a thunderstorm caused a delay. Meteorologist Jack Hubbard predicted a narrow window of favorable weather between 5 and 6 a.m.; the rain stopped at 4, and the test was conducted seconds before 5:30.

The first visible product of the blast was an incredibly intense flash of light that could be seen for 160 miles. After the light came the fireball, then a column of smoke that flattened into a mushroom cloud. The Trinity test created a crater a half-mile wide and melted the asphalt and sand around ground zero, leaving behind a green glassy substance now called trinitite.

Greisen recalled that “between the appearance of light and the arrival of the sound, there was loud cheering in the group around us. After the noise was over, we all went about congratulating each other and shaking hands.”

Years later, Oppenheimer would remember about Trinity, “We knew the world would not be the same. A few people laughed, a few people cried, most people were silent.” But at the time of the test, according to his brother, he simply said, “It worked.”

The culmination of a “Herculean project”

After the detonation of the Gadget, General Leslie Groves, director of the Manhattan Project, was “struck by [a] feeling that the faith of those who had been responsible for the initiation and the carrying on of this Herculean project had been justified.”

The Trinity test was important not only for its world-changing historical significance, but also because of the confidence it gave the United States that the Fat Man device would work and could help end World War II. Less than a month after Trinity, Fat Man was detonated over Nagasaki, Japan; less than a week after that, Emperor Hirohito announced Japan’s surrender.

In the decades since the Trinity test, Los Alamos has continued the work of nuclear weapons design and stewardship, with the added responsibility of nonproliferation. When scientists waited in the desert before dawn 75 years ago, they knew the significance of what they had accomplished, just as current Los Alamos personnel know how important it is to continue and guard that work in the 21st century.