Roosevelt died on April 12, 1945, and within 24 hours Pres. Harry S. Truman had been briefed about the atomic bomb program by Stimson. Germany surrendered in May 1945, thus ending the war in Europe, but combat raged on in the Pacific. Sanguinary battles at Iwo Jima (February–March 1945) and Okinawa (April–June 1945) offered a preview of what an invasion of the Japanese home islands might look like, and there remained a strong impetus to see the Manhattan Project through to its conclusion. By the summer of 1945, the production plants had delivered a sufficient amount of fissionable material to produce a nuclear explosion, and bomb development had advanced to a point that an actual field test of a nuclear weapon could be conducted. Such a test would obviously be no simple affair. A vast array of complex equipment had to be assembled so that the success or failure of the test could be analyzed.
The bomb development teams at Los Alamos had settled on two possible designs. One, fueled by uranium-235, would utilize a “gun assembly” that used high explosives to shoot two subcritical slugs of fissionable material together in a hollow tube. The violent collision of the two slugs would cause the uranium-235 to reach critical mass, thus triggering a chain reaction and explosion. Engineers were confident that this comparatively simple design would work, but a sufficient quantity of uranium-235 would not be available until about August 1, 1945. The Hanford site would be able to deliver enough plutonium-239 for testing by early July, but Los Alamos scientists had determined that the gun assembly model would not be compatible with plutonium as a fuel source. An alternative design had been proposed, one that would use concentric layers of high explosives to implode the fissionable material under enormous pressures into a denser mass that would immediately achieve criticality. It was believed that this “implosion” design would be the most efficient way to weaponize the meagre amount of plutonium that had been produced thus far.
For the test, Oppenheimer selected a location on the Alamogordo Bombing Range (now White Sands Missile Range), 120 miles (193 km) south of Albuquerque, New Mexico.He called the site “Trinity” in reference to one of John Donne’s Holy Sonnets. The first atomic bomb—a plutonium implosion device called “Gadget”—was raised to the top of a 100-foot (30-metre) steel tower that was designated “Zero.” The area at the base of the tower was marked as “Ground Zero,” a term that would pass into common parlance to describe the centre of an (often catastrophic) event. Military officials and scientists occupied observation posts at distances ranging from 10,000 to 17,000 yards (9 to 15.5 km). They had been instructed to lie down with their feet toward the tower and to protect their eyes from the blinding flash of the explosion.
On the morning of the test, the skies were dark and it was raining, with occasional lightning. “Gadget” was detonated at 5:29:45 AM on July 16, 1945. The explosion caused a flash that illuminated the mountain peaks 10 miles (16 km) away. Soon there followed a tremendous sustained roar accompanied by a tornado-like burst of wind. Where the tower had stood, there was a great surging ball of fire, followed by a mushroom cloud that rose some 40,000 feet (12,200 metres) into the sky. The heat of the explosion had completely vaporized the tower; in its place was a saucer-shaped crater about a half mile (800 metres) in diameter and 25 feet (almost 8 metres) deep. The floor of the crater was fused into a glassy jade-coloured mineral subsequently dubbed trinitite. The bomb had generated an explosive power equivalent to approximately 21,000 tons of trinitrotoluene (TNT). The blast was visible from a distance of 50 miles (80 km), and it shattered windows 125 miles (200 km) away. Residents of Gallup, New Mexico, more than 180 miles (290 km) from Ground Zero, reported feeling the ground shake. In an attempt to head off questions about the world-changing event that had occurred at Trinity, the army issued a brief statement to the press: “A remotely located ammunition magazine containing a considerable amount of high explosives and pyrotechnics exploded, but there was no loss of life or limb to anyone.”