When Greg Spriggs was 11 years old, his father, a Navy man stationed on Midway Island, took him out one night to watch a nuclear bomb explode in space. The year was 1962 and the nuclear test was Starfish Prime, the largest in a series of high-altitude detonations. A rocket shot the 1.4 megaton nuclear warhead 250 miles above Earth—higher than the International Space Station orbits today.

“It just lit up the sky like day,” recalls Spriggs. The warhead released so much energy it set off an aurora that lasted 15 minutes after the explosion: The sky shimmered white, then red, then purple. “Had I known I would become a weapon physicist,” he says, “I would have paid more attention.”

Half a century later, Spriggs spends a lot of time watching nuclear bombs explode. Not in person of course—atmospheric testing stopped in 19631—but on film. On the original film, even. Over the course of more than 200 nuclear tests in the atmosphere, the US government has amassed thousands of films documenting the tests from every which angle and distance. At Lawrence Livermore National Laboratory in California, Spriggs has begun a program to restore those films in hopes of wringing every last bit of data out of them.

Nuclear weapons physicists have an odd role these days. They can’t actually test anything because, you know, one doesn’t just set off a nuclear weapon anymore (and certainly not in space). But if the US decides to launch a nuclear warhead against another country—or the other way around—weapons physicists still have to anticipate the damage. In practice, that means computer simulations. Lots and lots them.

Spriggs was looking at his computer models of nuclear fallout several years ago when he decided to dive into the original data underlying them. He was amazed to see that the data points—the cloud height for a bomb of a given energy, for example—was scattered all over the place. “We were trying to figure out if there was actually some piece of physics we were missing,” he says. What he found is that the original analysis of those films, done by hand and often in a hurry, was not always accurate. So he started digging.

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Atomic Hollywood

It took Spriggs a year of asking around libraries and archives before he tracked down 7,000 original films at Los Alamos National Labs in New Mexico—where he worked before Livermore. (Los Alamos was, of course, the home of the Manhattan Project.) But the films had laid untouched for so long, people had forgotten their existence. “Los Alamos said, ‘We think we have originals. Nobody’s messed with the films for 40 years but we’ll go dig them up for you,'” Spriggs recalls. The films soon started arriving by mail from New Mexico.

A physicist by training, Spriggs had to get up to speed fast on handling old film. Modern film, made of polyester, is so tough that it’ll break the projector before it itself breaks. But old cellulose acetate film is much more delicate, especially after half a century of rotting. “As it ages, it goes through a decomposition process,” says Spriggs. “It puts off this distinctive odor. They call it vinegar syndrome.” That’s no coincidence: Cellulose acetate breaks down into acetic acid, the same chemical that gives vinegar its sour taste. It also becomes brittle and shrinks. The film only has lifespan of 100 years, and there’s no way to stop vinegar syndrome. The only way to preserve the data on the original prints is to digitally scan them.

To lead the scanning, Spriggs brought in Peter Kuran, a film historian, and Jim Moye, a film preservationist who had worked on the Zapruder film, to handle the old cellulose. They bought a scanner from Sweden, the same kind Hollywood studios would use to digitally preserve their old films. In the early days, the office building they worked in didn’t have AC, and when it got too hot in the California summer, the scanner would automatically shut off. That’s when Spriggs started coming into work late at night and staying until 11am—a schedule he still sticks to.

“It was a lot of pressure,” says Spriggs. “If the scanner chews it up, I’m in big trouble because this is one of a kind, very unique thing.” But Moye, who has worked on preserving old Hollywood films, was an old hand with the films. “It’s like any motion picture film,” he says. The only difference was the diversity of formats over the years: 70 mm, 35 mm, 16 mm, 8 mm, he saw it all. The team has since scanned all the declassified films—3,000 of the 7,000 total. Then came the work of analyzing them.

How to Analyze a Nuclear Bomb

To calculate how much energy a bomb releases, you have to measure the size of the shock wave over time. Plugging that into an equation gets you the bomb’s yield—or the amount of energy it discharges.

So how do you find the front of a shock wave, which is basically air moving faster than the speed of sound? It’s easy at first—the shock wave simply follows the edge of the glowing fireball. But a few milliseconds later, as temperatures cool, the shock wave breaks away from the fireball. To track a shock wave as it moves through empty air, 1950s era physicists actually created an optical illusion. First, they shot rockets at regular intervals into the air. The dense air of the shock wave’s front bends light, so the otherwise straight rocket trails would appear to have hooks when the shock wave passed through. “It’s pretty clever,” says Spriggs.

Back in the 1950s, the physicists measured the edge of the fireball and the shock wave passing rocket trails by hand. They projected the film onto a grid and advanced it frame by frame, noting when the shock wave passed a certain line on the grid.

The manual data, Spriggs has found, is inconsistent, with random variation as high as 20 percent. It was the heyday of the Cold War, and things were moving fast. “They would do like one shot every five days,” he says. “They collected so much film, it was very difficult to have enough manpower to analyze it in detail.”

Now, of course, scientists have computer programs that can analyze every single pixel in a frame over hundreds of frames. What might have taken days by hand takes only minutes. With computer analysis, Spriggs is pinpointing more precise yields. Computer models then use yield to estimate the damage from a bomb in different situations.

The computer models are necessary because an international treaty now prohibits testing nuclear bombs above ground. But the US’s nuclear arsenal is still, for better or worse, an indispensable part of its military might. If the US ever has to use a nuclear weapon, it would want to know, precisely, its yield and the amount of damage it will wreak. Too small a yield and it might not take out a target; too big and it might do unintended damage. And if the US suffers an attack, good yield data underlying computer models of nuclear fallout will help predict and prepare for the damage. All this is theoretical—just computer code for now—but as long as nuclear weapons exist, so does the threat.

Declassified

Spriggs, in his Livermore office, is still going one film at a time. While his team has scanned all 3,000 of the declassified films, they have another 4,000 classified ones to go. The first step is declassifying them all, which is a huge bureaucratic undertaking: Spriggs will sit in a room with another trained declassifier to view and then fill out a form for each and every single film, a process that takes about 10 minutes each. Then someone at the Department of Energy will have to approve each film for declassification. Since the estimated yields for almost all the bombs tested in these films are already public, there’s no good reason to keep them classified, says Spriggs—only that no one’s bothered to fill out all the paperwork until now. “It’s this big bureaucracy that just goes back and forth.”

When the declassification is over, Spriggs and Moye will start scanning the films again. That’s a couple solid years of work ahead. Once the project is over, Spriggs plans to retire. “But I don’t want to retire until the project is over because it’s so fascinating to me,” he says. Besides, he hasn’t seen the original films from Starfish Prime yet.

1UPDATE: Correction 1:00pm ET 12/03/2015 The story misstated the year atmospheric testing of nuclear weapons ended in the United States. Atmospheric testing ended in 1963; all testing, including underground, ended in 1992.