Timeline of the Manhattan Project

The Manhattan Project was a research project that made the first atomic bombs during World War II. The United States led it, with help from the United Kingdom and Canada. It was directed by Major General Leslie Groves of the US Army Corps of Engineers. It began in 1939 and grew to include over 130,000 people. It cost about US$2 billion at the time.

Two main types of atomic bombs were made. One used uranium-235, a rare isotope that was hard to separate from more common uranium-238. Three ways were used for this separation: electromagnetic, gaseous, and thermal, mostly at Oak Ridge, Tennessee. The other bomb used plutonium, made in reactors at Oak Ridge and Hanford, Washington. Because of this, a more complex implosion-type nuclear weapon was built at the main laboratory in Los Alamos, New Mexico.

This timeline shows important events from before the Manhattan Project began through the atomic bombings of Hiroshima and Nagasaki, until the project was replaced by the Atomic Energy Commission in 1947. It shows how science and engineering worked together to change history.

1939

In August 1939, physicist Leó Szilárd wrote a letter. Albert Einstein signed the letter and it was sent to President Franklin D. Roosevelt. The letter warned that Nazi Germany might make a very powerful weapon using nuclear fission. Roosevelt agreed to pay for research on this idea.

Later, on September 3, Great Britain and France declared war on Nazi Germany after its invasion of Poland. This started World War II. In that same month, economist Alexander Sachs gave the letter to President Roosevelt. He started a special group called the Advisory Committee on Uranium. The group met for the first time on October 21. They decided to spend money on early tests with neutron science.

1940

In 1940, scientists took important steps to learn about atomic energy. In March, scientists at Columbia University found that uranium 235 could cause atomic fission. Around the same time, scientists in Britain learned that a tiny amount of special uranium could make an atomic bomb.

Later that year, new groups were created to study this idea. Money was given for uranium research. A lot of uranium was secretly brought to the United States from a mine in Africa.

1941

In 1941, important steps were taken toward developing nuclear weapons. On February 25, scientists Glenn Seaborg and Arthur Wahl at the University of California, Berkeley discovered plutonium.

Later, in May, a report by Arthur Compton and the National Academy of Sciences showed that creating nuclear power for military use might be possible.

After Japan attacked Pearl Harbor on December 7, 1941, the United States entered World War II. This made the nuclear weapons project even more urgent. The Office of Scientific Research and Development was created to manage this important research.

1942

In January 1942, President Roosevelt approved the atomic bomb project. During the year, important choices were made to organize and move the work forward. Scientists like Compton picked the University of Chicago to study plutonium, and leaders formed the S-1 Executive Committee to guide the project.

Key events included choosing Stone & Webster to build buildings in Tennessee, and Robert Oppenheimer organizing talks on bomb design at the University of California, Berkeley. General Leslie Groves took control of the project in September, and uranium ore was taken from Congo for use. By December, the first nuclear reactor, Chicago Pile-1, had a reaction that kept going on its own, a big science success.

1943

In 1943, important steps were taken to build the first atomic bombs. In January, General Groves approved the building of the Hanford Site. Construction started on big buildings for separating and enriching uranium at Oak Ridge. The Los Alamos laboratory was set up in April to bring scientists together to design the bombs.

Important meetings happened all year. In July, Los Alamos, Oak Ridge, and Hanford became military districts. In August, the first sample of plutonium arrived at Los Alamos, and the Medical Section of the Manhattan Project was created. In September, the Quebec Agreement was signed, joining the British Tube Alloys project with the Manhattan Project. This formed the Combined Policy Committee to help the United States, United Kingdom, and Canada work together. Construction also began on the first reactor at the Hanford Site, and the X-10 Graphite Reactor at Oak Ridge started working in November.

1944

In 1944, important things happened in the Manhattan Project. On January 11, a special group led by Edward Teller started studying a new way to create explosions at Los Alamos. By April 5, scientist Emilio Segrè found that a material from Oak Ridge wasn’t right for one kind of bomb.

Later that year, work changed to focus on a new bomb design called "Fat Man". On September 26, a big nuclear reactor started working at the Hanford Site. By December, a group was formed to get ready to use the new bombs, showing how close scientists were to making history.

1945

In January, General Thomas Farrell joined General Groves to help with the project. In July, there was a big test explosion in New Mexico called the Trinity nuclear test.

Later, two atomic bombs were dropped on Japan. On August 6, a plane called the Enola Gay dropped a bomb named Little Boy on the city of Hiroshima. Then, on August 9, another plane called Bockscar dropped a bomb named Fat Man on the city of Nagasaki. After these events, Japan surrendered on August 14, ending World War II.

1946

In 1946, important events helped shape the future of atomic energy. In February, news about a Russian spy ring in Canada caused concern in the United States about controlling atomic research after the war. Later that year, on July 1 and July 25, two tests called Able and Baker were conducted at Bikini Atoll as part of Operation Crossroads. Finally, on August 1, President Truman signed the Atomic Energy Act of 1946, which made new rules for managing atomic research in the United States.

1947

In 1947, big changes happened to the Manhattan Project. On January 1, the Atomic Energy Act of 1946, also called the McMahon Act, started. This act moved the Manhattan Project to the United States Atomic Energy Commission. Later, on August 15, the Manhattan District, which was part of the project, was closed down.