The first nuclear weapon test, Trinity, was July 16, 1945

The first nuclear weapon was detonated at 5:30 a.m. on July 16, 1945, in the Trinity test on the Alamogordo Bombing Range in New Mexico, about 210 miles south of Los Alamos. The plutonium implosion device, nicknamed the “Gadget”, sat atop a 100-foot steel tower; the blast vaporized the tower and fused the desert sand into a green glass later called trinitite.

Nobody knew in advance how big it would be. Minutes after the shot, General Leslie Groves phoned in the scientists’ first guesses: a minimum of 21 kilotons from J. Robert Oppenheimer, 18 from Isidor Rabi, 10 from Enrico Fermi, and 5 from Groves himself. His formal report settled on 15 to 20 kilotons. The Department of Energy long quoted 18.6; its official estimate is now about 21 kilotons, and one unofficial reassessment puts it near 25.

The fallout nobody followed up

• 1945, weeks before the shot: Los Alamos physicians warn that radioactive dust could fall on nearby towns. Secrecy comes first, and Groves accepts monitoring and an evacuation plan mainly to avoid legal trouble.
• July 16, 1945: fallout contaminates more than 1,100 square miles of New Mexico, with traces reported as far away as Canada.
• 1990: Congress passes the Radiation Exposure Compensation Act, and New Mexico’s downwinders are left out.
• July 4, 2025: the law is reauthorized. People who lived in New Mexico for at least a year between September 1944 and November 1962 and later developed a listed cancer can now claim a one-time $100,000.

The human cost was real. One ranch family near a “Hot Canyon”, the Ratliffs, received an estimated 47 roentgens in two weeks against an allowable limit of 1.4, and apparently were never told of the risk. Army data showed signs of beta-ray damage in up to 600 cattle, and National Cancer Institute researchers later estimated that the fallout caused excess cancers, chiefly of the thyroid.

The test site, now inside White Sands Missile Range, opens to the public only on rare designated days.