In July 1945, as J. Robert Oppenheimer and the other researchers of the Manhattan Project prepared to test their brand new atomic bomb in the New Mexico desert, they knew relatively little about how this mega-weapon would behave.

On July 16, when the plutonium implosion device was set off atop a hundred-foot metal tower in a test codenamed “Trinity,” the resulting explosion was much stronger than anticipated. The irradiated mushroom cloud also went many times higher into the atmosphere than expected: about 50,000 to 70,000 feet. Where it would ultimately go was anyone’s guess.

A a new study, published Thursday before submission to a scientific journal for peer review, shows that the cloud and its fallout went further than anyone in the Manhattan Project imagined in 1945. Using state-of-the-art modeling software and newly discovered historical weather data, the study’s authors say that radioactive fallout from the Trinity test reached 46 states, Canada and Mexico within 10 days.

“It’s a huge finding and, at the same time, it shouldn’t surprise anyone,” said the study’s lead author, Sébastien Phillippe, a researcher and scientist at Princeton University’s Program on Science and Global Security.

The study also reanalyzed fallout from all 93 above-ground US nuclear tests in Nevada and created a map depicting a composite deposit of radioactive material across the contiguous US (The team also hopes to study US tests over the Pacific in the future).

How much of Trinity’s fallout still remains at original repositories around the country is difficult to calculate, said Susan Alzner, an author of the study and the co-founder of shift7, an organization that coordinated the study’s research. The study documents testimony when it originally hit the ground in 1945.

“It’s a frozen in time image,” she said.

The findings could be cited by advocates seeking to increase the number of people eligible for compensation from the federal government for potential exposure to radiation from atmospheric nuclear explosions.

The drift of the Trinity cloud was monitored by physicists and doctors of the Manhattan Project, but they underestimated its reach.

“They were aware that there were radioactive dangers, but they thought about acute risk in the areas around the immediate detonation site,” said Alex Wellerstein, a nuclear historian at the Stevens Institute of Technology in New Jersey. They had little understanding, he said, of how the radioactive materials could become embedded in ecosystems, near and far. “They didn’t really think about effects of low doses on large populations, which is exactly the fallout problem.”

At the time, Dr. Stafford L. Warren, a Manhattan Project physician specializing in nuclear medicine, reported to Lt. Gen. Leslie Groves, leader of the Manhattan Project, that the Trinity cloud “remained over the northeast corner of the site for several hours.” Soon, he added, “various levels were seen moving in different directions.” Dr. Warren assured General Groves that an assessment of the extent of the fallout could be undertaken later on horseback.

In the decades that followed, a lack of conclusive data stymied estimates and empirical studies of the fallout from the Trinity test. The United States had no national monitoring stations in 1945 to track the fallout, Dr. Phillippe said. Additionally, key historical weather and atmospheric data were available only from 1948 onwards. Remodeling a fallout from tests in Nevada—starting in 1951—was easier, but Trinity remained frustratingly difficult to reanalyze.

“The data sets for the Nevada tests and the available data that we could potentially find for Trinity were not comparable,” Ms. Alzner said. “You couldn’t put them on the same map. We decided to keep pushing.”

Determined to fill in the gaps, the team began the study about 18 months ago. Dr. Phillippe has an extensive background in residue modeling and was author of a similar project in 2021 that documented the effects of French nuclear tests.

A breakthrough came in March, when Ms Alzner and Megan Smith, another co-founder of shift7 and a former US chief technology officer in the Obama administration, contacted the National Oceanic and Atmospheric Administration. There, Gilbert P. Compo, a senior researcher at the University of Colorado and NOAA’s Physical Sciences Laboratory, told the team that the European Center for Medium-Range Weather Forecasting had just a week earlier released historical data that mapped weather patterns extending 30,000 feet or more above the Earth’s surface.

“For the first time, we had the most accurate hourly reconstruction of the weather back to 1940, around the world,” said Dr. Compo, who co-authored the study. “Every event that puts something on the air, no matter what it is, can now be tracked, hourly.”

Using the new data and software built by NOAA, Dr. Phillippe then reanalyzed the Trinity fallout. And while the study’s authors acknowledge limitations and uncertainties within their calculations, they assert that “our estimates likely remain conservatively low.”

“It’s a very comprehensive, well-executed study,” said MV Ramana, professor and Simons Chair in disarmament, global and human security at the University of British Columbia, who was not involved in the study. Dr. Ramana was unsurprised by the findings of the Trinity study. “I expected the old estimates to be underestimating what was actually deposited,” he said.

The results show that New Mexico was heavily affected by Trinity’s fallout. Computations by Dr. Phillippe and his colleagues show the trajectory of the cloud mainly spreading over northeastern New Mexico and part of the cloud circling south and west of zero during the following days. The researchers wrote that there are “places in New Mexico where radionuclide deposition has reached levels equal to Nevada.”

Trinity’s tailings, Dr. Phillippe says, account for 87 percent of the total deposit found across New Mexico, which also received testimony from Nevada’s surface tests. The study also found that Socorro County—where the Trinity trial took place—has the fifth highest deposition per county of all counties in the United States.

Trinity test “downwinders”—a term describing people who lived near nuclear test sites and may have been exposed to deadly radioactive fallout—were never eligible for compensation under the 1990s. Radiation Exposure Compensation Act (RECA). It provided more than $2.5 billion in payments to nuclear workers in much of the western United States and to downwinders who were located near the Nevada test site and may have developed cancer or other illnesses as a result of radiation exposure.

“Despite the Trinity test taking place in New Mexico, many New Mexicans were left out of the original RECA legislation and no one has ever been able to explain why,” said Sen. Ben Ray Luján, New Mexico Democrat. He helped lead efforts in Congress to expand and extend the legislation, currently due to sunset in 2024.

Census data from 1940 shows that up to 500,000 people were living within a 150-mile radius from the test site. Some families lived as close as 12 miles away, according to the Tularosa Basin Downwinders Consortium. However, no civilians were warned about the test beforehand, and they were not evacuated before or after the test.

“This new information about the Trinity bomb is monumental and a long time coming,” said Tina Cordova, co-founder of the consortium. “We expected confirmation of the stories told by generations of people from Tularosa who witnessed the Trinity bomb and talked about how the ash fell from the sky for days afterward.”

The study also documents significant deposition in Nevada, Utah, Wyoming, Colorado, Arizona and Idaho, as well as dozens of federally-recognized tribal lands, potentially strengthening the case for people seeking expanded compensation in those areas.

Although Dr. Wellerstein said he treats such reanalyses of historical fallout with a certain amount of uncertainty, in part because of the age of the data, he said there is value in such studies keeping nuclear history and its legacy in the public discourse.

“The extent to which America has nuked itself is not fully appreciated yet, to this day, by most Americans, especially younger Americans,” he said.

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