•             US scientists tested an atomic bomb in the New Mexico desert exactly 75 years ago.

•             The launch, part of the Manhattan Project, marked the development of the deadliest and most powerful weapon in history.

•             In an excerpt published by the Bulletin of the Atomic Scientists, physicist Robert Wilson suggested that scientists didn't grapple with the moral consequences of their work until after the explosion.

Scientists describe setting off the world's first nuclear bomb 75 years ago: No one 'even raised the possibility that what we were doing might be morally wrong'

Seventy-five years ago, a group of scientists and soldiers camped out at a previously abandoned ranch in New Mexico, waiting for history to be made. Their mission — to produce the world's first nuclear bomb — was so top-secret, many of their wives and children weren't privy to what they were working on. Even their mail and phone calls were monitored.

The world would later know their work as the Manhattan Project. The code name for their first nuclear test, conducted on July 16, 1945, was "Trinity." It marked the development of the deadliest and most powerful weapon in history — and the beginning of the end of World War II.

But its success was never guaranteed.

Before the test was approved, scientists debated whether the explosion could ignite the atmosphere and destroy life on Earth. The project went ahead after Nobel Laureate Arthur Compton determined the odds of that doomsday scenario were "slightly less" than one-in-3 million.

In 1943, the team assembled a secret laboratory in Los Alamos, New Mexico. The testing site, about 200 miles south, had to be remote to limit exposure to dangerous radioactive fallout. Scientists and soldiers slept on cots in humble barracks in the middle of a sweltering desert surrounded by scorpions and venomous lizards, Emilio Segrè, the physicist in charge of radioactivity research, recalled in an excerpt published by the Bulletin of the Atomic Scientists. (Segrè died in 1989.)

About a year into the Los Alamos project, researchers gathered at small location known as "Building X" to discuss again how the bomb — nicknamed "The Gadget" — might impact civilization.

"At that time, we were perhaps overly obsessed by what we regarded as the evil of military security," Robert Wilson, the youngest research leader at Los Alamos, said, according to the Bulletin. (Wilson died in 2000.) "We feared that the military would keep nuclear energy a secret were the bomb not revealed by an actual explosion."

The scientists had become even more zealous in their mission by then, Wilson added.

"It is significant that no one at that meeting in Building X even raised the possibility that what we were doing might be morally wrong," he said. "No one suggested that we should pack our bags and leave."

A rush to complete the test

In the days leading up to the launch, scientists recalled burying their heads in their work.

"Perhaps events were moving just too incredibly fast," Wilson said. "We were at the climax of the project — just on the verge of exploding the test bomb in the desert. Every faculty, every thought, every effort was directed toward making that a success."

He added that "there was an absolutely Faustian fascination about whether the bomb would really work."

George Kistiakowsky, a Harvard physical chemist who led the explosives division at Los Alamos, recalled scientists placing bets on how big the explosion would be.

Kistiakowsky, who died in 1982, predicted that it might produce 100 tons of TNT (a measure of the weapon's force), according to the Bulletin. But the real-life explosion was 200 times more powerful than his estimate.

"We did not know just how big the explosion would be or what its effects would be," Lieutenant General Leslie Richard Groves said, according to the Bulletin. (Groves died in 1970.) "Like too many things in the Manhattan Project — we were dealing with unknowns outside the realm of man's experience, and we simply had to try to imagine everything that might happen."

'The full awful magnitude of what we had done came over me'

Scientists were forced to make even more tough guesses when a storm broke out the morning of the launch. By then, the bomb had already been loaded on top of a 100-foot steel tower.

The test was originally scheduled for as early as 2 a.m. on July 16. But the inclement weather pushed the launch back to 5:30 in the morning. Before his death in 1996, Kenneth Bainbridge, a Harvard physicist who oversaw the test, reported that the weather still wasn't ideal then, but scientists weren't willing to wait another half day. TE

"To my distress, I found an air of excitement in the base camp instead of the calmness essential to sound decision-making," Groves said of the moments before the test.

At around 5:20 a.m., people stationed at the barracks crouched on the ground, wearing dark glasses to shield their eyes. Ten minutes later, the explosion produced a burst of light, followed by a giant orange fireball and mushroom cloud.

Residents in the nearby region noticed a brilliant flash in the sky, Segrè said. A few glass windows cracked in Silver City, New Mexico, about 180 miles away. But for the most part, the details of the test remained secret until shortly after the Hiroshima bombing in August 1945.

In the years that followed, Manhattan Project scientists grappled with consequences of their work.

"That which had been an intellectual reality to me for some three years had suddenly become a factual, an existential reality," Wilson said. "My technical work was done, the race was run, and the full awful magnitude of what we had done came over me."/ Insider