Manhattan Project Science at Los Alamos

Black and white photo of a checkpoint with 1940s car at front.
Los Alamos main gate, 1943.


On April 20, 1943, the University of California signed a contract with the United States Army Corps of Engineers to operate a secret laboratory hidden away in the mountains of northern New Mexico. This laboratory soon became home to some of the most revolutionary science in US history. Led by scientist J. Robert Oppenheimer, the staff at this secret Manhattan Project location called Los Alamos was responsible for the development and testing of the nuclear weapons dropped on Hiroshima and Nagasaki Japan in August, 1945. The development and testing culminated with the successful detonation of the first atomic bomb device- nicknamed the “Gadget”- in New Mexico's Tularosa Basin on July 16, 1945.
Black and white photo of Oppenheimer in a suit.
J. Robert Oppenheimer.


When General Leslie Groves was appointed to lead the Manhattan Project in September, 1942, he saw a need to recruit top scientists throughout the country to staff the new secret site in New Mexico for atomic weapons testing and development. Physicist J. Robert Oppenheimer, already a well-respected instructor at both the University of California at Berkeley and the California Institute of Technology, proved to be the ideal candidate to lead the newly-formed Los Alamos Laboratory. Although Groves and Oppenheimer had distinctly different managerial styles, their partnership proved ideal in the development and successful test of the world's first atomic weapons.

Oppenheimer recruited the majority of the Los Alamos staff personally, traveling to universities throughout the country including Cornell, Princeton, MIT, the University of Chicago, and Berkeley. The majority of scientists that Oppenheimer contacted were willing and eager to join the new project. By spring of 1943, most of the Los Alamos staff had arrived at the isolated location on a mesa in New Mexico, the former site of an all-boys preparatory school.

While Los Alamos was under construction, Oppenheimer developed four different divisions within the new laboratory, with each one having a specific purpose. Hans Bethe, a Cornell professor, was tasked to lead the Theoretical Division. Robert Bacher, also a Cornell professor, was assigned the head of the Experimental Physics Division. Joseph Kennedy from Berkeley led the Chemical Division, and Navy Captain William S. Parsons was assigned to lead the Ordinance Division. The majority of research equipment used at Los Alamos either arrived with or was shipped to Los Alamos by the scientists themselves.
Black and white photo of woman hanging laundry in front of small one- story trailers
Laundry day at Los Alamos.


While secrecy was a top priority at Los Alamos, a semblance of normalcy often occurred there as well. Husbands and wives and their children ate family meals, enjoyed hikes throughout the area, and attended parties which were often hosted on weekends at scientists’ residences in the gated facility. All the while, however, research and assembly was taking place on the world’s first nuclear device, the Gadget.

The Gadget, an implosion-type nuclear weapon, held a core of subcritical plutonium which would reach criticality when high explosives surrounding the core detonated causing the core to compress instantly. Selected in September 1944, the test location for the Gadget was located in New Mexico’s Alamogordo Bombing and Gunnery Range, a flat, calm desert area approximately 200 miles southeast of the Los Alamos Laboratory. Construction at the Trinity site, the name given to both the test location and overall development program, began that fall, with over 250 workers transporting materials to construct three observation shelters, run electricity, provide over 50 cameras to record the test, and construct a 100-foot-tall steel tower to attach the Gadget.
Black and white photo of several scientists assembling a large sphere
J. Robert Oppenheimer (second from left) oversees final assembly of the Gadget.


At 5:29 am Mountain War Time on July 16, 1945, after the Gadget was hauled to the top of the tower via electric winch, the world’s first successful nuclear explosion took place. A blinding flash of light, visible over 160 miles away, occurred followed by a huge fireball and mushroom cloud. The explosion left a crater over one-half mile wide and turned the surrounding sand at ground zero into glass, or trinitite. Oppenheimer recalled: “We knew the world would not be the same. A few people laughed, a few people cried, most people were silent.”
Black and white photo of nuclear explosion, a large dome-shaped blob
The Trinity Test. 5:29am MST.


The success of the Trinity Test proved that an implosion-type, plutonium fueled nuclear weapon could work. On August 9, 1945, three days after the uranium-fueled Little Boy atomic bomb detonated over Hiroshima, Japan, Fat Man, the plutonium-fueled implosion-type atomic weapon developed at Los Alamos and based on the success of the Gadget, exploded over Nagasaki. It was the first, and so far only, time atomic weapons were ever used in war.

Manhattan Project National Historical Park

Last updated: April 4, 2023