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This Day in History: The top-secret Manhattan Project

On this day in 1945, the world’s first atomic bomb is detonated in New Mexico. The top-secret Manhattan Project had succeeded.

Researchers had been working for decades to understand the nature of the atom. But that research took a new tone in the late 1930s, when radiochemists in Nazi Germany made a startling discovery: The nucleus of a uranium atom could be divided in such a way that it released energy. Some physicists began to worry that Germany would be the first to turn this knowledge into the development of a new, more powerful bomb.

You’ll recognize the name of one man who was worried: Albert Einstein wrote a letter to Franklin D. Roosevelt, urging the President to “speed up the experimental work” for atomic energy through government support. FDR was cautious in his approach to the issue at first, but he got more serious after the attack on Pearl Harbor.

Within a matter of weeks, FDR had tentatively authorized Vannevar Bush, head of the U.S. Office of Scientific Research and Development, to pursue atomic research. The Manhattan Project was soon established under the Army Corps of Engineers.

Can you believe that 120,000 Americans were employed on the project—in secrecy? Most of these employees had no idea what they were really working on. Even Vice President Harry Truman did not know about the project . . . until he unexpectedly became President.

Can you imagine? Truman had to decide whether to use an atomic bomb within months of learning about it.

Scientists pursued two sources of power for the atomic bomb: plutonium and uranium. At first, no one was quite sure which would be better, but by July 1945, the plutonium bomb was ready for a test. A remote site in New Mexico was chosen as a detonation site.

No one was quite sure what would happen. Would it work? How strong would the explosion be?

The test device, “Gadget,” was detonated at 5:30 a.m. on July 16. Not only did it work, but the explosion was even bigger than expected.

Brigadier General Thomas F. Farrell watched the explosion from a nearby control shelter, and he described the “tremendous burst of light followed shortly thereafter by the deep growling roar of the explosion.” One nuclear physicist said that he was “enveloped with a warm brilliant yellow white light—from darkness to brilliant sunshine in an instant . . . . there was a gigantic ball of fire rising rapidly from the earth—at first brilliant as the sun, growing less brilliant as it grew boiling and swirling into the heavens.”

The blinding light could be seen for nearly 200 miles. A mushroom cloud appeared on the horizon. A shockwave blew out some windows in houses 100 miles away. When the explosion came to an end, it left a crater half a mile wide.

Farrell spoke of the mood as the explosion came to an end: “[All] felt their profound responsibility to help in guiding into right channels the tremendous forces which had been unlocked for the first time in history. As to the present war, there was a feeling that no matter what else might happen, we now had the means to insure its speedy conclusion and save thousands of American lives.”

One surprising result of this test? Even though a plutonium bomb was tested in New Mexico, a different type of bomb was dropped on Hiroshima a few weeks later. “Little Boy” was a uranium bomb. No one had wanted to test it, in part because uranium was harder to come by. Three days later, a plutonium bomb, “Fat Man,” was dropped on Nagasaki.

The Japanese emperor would soon surrender, bringing World War II to an end.