It happened on December 7, 1972, sometime between 4:59:05 and 5:08:14 hours after launch. The three astronauts aboard the Apollo 17 aircraft—Eugene Cernan, Ron Evans, and Harrison Schmitt—watched Earth recede below them as they traveled at up to 25,000 miles per hour.

Cernan spoke to Mission Control’s Robert Parker, a capsule communicator stationed at Johnson Space Center in Houston. “Bob, I know we’re not the first to discover this, but we’d like to confirm, from the crew of America, that the world is round.”

“Roger,” Parker said. “That’s a good data point.”

In the next seven minutes, one of the astronauts picked up a 70-millimeter Hasselblad Data Camera and fired off four shots of the whole Earth as the craft ascended high enough for the globe to fill the cockpit windows. NASA plucked the second and clearest of the photos and presented it to the public 16 days later, on December 23; it was on most newspaper covers by Christmas. The Blue Marble shot, as it came to be known, wasn’t the first photo of Earth in its entirety—satellites had already done that. Still, riding the wave of public wonder of humans in space, and after being picked up by the environmental movement as a symbol of global consciousness, it remains the most famous photo of Earth ever taken. It’s still the most requested photo from the NASA archives.

This year marks the 45th anniversary of the Apollo 17 mission. Evans died in 1990, and Cernan died this year on January 16; Schmitt, now 82, has retired from a political career in New Mexico; and the Blue Marble shot has become an iconic legacy of humanity’s last mission to the moon, plastered on T-shirts and on the cover of the first issue of the Whole Earth Catalog. But to this day, we don’t know who took the photo. All three men have always claimed it as their own.

Schmitt told me in January, “How honored and privileged I feel to have participated in Apollo and to have the chance to take this picture that so many people find pleasing.” (I had reached out to Cernan in January as well, but he died a couple weeks after I did so.) Evans and Cernan used to answer the same way when they were asked about the photo. Curious NASA employees and an obsessive subculture of volunteer space historians have played photo detective in the decades since Apollo 17. Their search takes us as close as we’ll ever be to sitting in the cockpit at the moment the Blue Marble was taken, and a little closer to understanding how the minutia of a photo credit can really be a love letter to space exploration.

The official Blue Marble photo credit reads “NASA.” It’s the agency’s default; a specific astronaut only gets credit when it’s unmistakably obvious who took the photo. The photos of Buzz Aldrin on the moon in 1969, for instance, could only have been taken by Neil Armstrong because we know the third astronaut, Michael Collins, remained inside the command module and never walked on the moon. Even in cases like that, many within NASA would argue it still doesn’t matter who took the shot. “The astronauts work for all of us as taxpayers,” says Bill Barry, chief historian at NASA. “They don’t have any ownership or financial rights in the photography that they do as part of their official work.”

But Blue Marble is such a genuine mystery that NASA has officially entertained the credit question. Photo experts at the Johnson Space Center have revisited the time frame in which the photo was taken, using transcribed conversations between Mission Control and the cockpit—a task made all the more difficult because photos from the mission did not have time stamps. Maybe one of the astronauts said something to indicate he was taking photos when Blue Marble was shot. Instead, the transcripts made clear that all three were passing the Hasselblad around during the seven minutes in question.

“I don’t know what to take a picture of,” Evans says. A few minutes later, he hands the camera to Schmitt, who went by the name Jack: “Here, Jack, can you see him good? Check the settings there. I took an f/22 stop.” In the minutes of silence throughout this conversation are occasional shutter sounds. It’s rarely clear who’s holding the camera.

“There were comments of things the crew were taking pictures of, but nothing that could have been referring to the Blue Marble shot,” Barry says. NASA’s official conclusion was that the photo experts “could not confirm or deny” any of the three astronauts’ claim to the photo. The credit remains with NASA and the entire crew of Apollo 17, and NASA has no plans to revisit the question.

“NASA’s got a pretty good-sized budget, but most of that goes to building things and doing scientific research,” says Barry. “History’s got a tiny budget.”

Today, anyone who wants to Nancy Drew the Blue Marble question can revisit those seven minutes, hear the astronauts and the sounds of the camera shutter, and see the series as it was taken, and decide for themselves who may have taken the photo. But that’s not entirely thanks to NASA.

NASA’s Apollo program, which spanned from 1961 to 1972, has attracted a particularly fervent group of armchair historians. These amateur archivists and detectives gather astounding amounts of primary source material and reconstruct missions down to the minute. Sometimes, they even do so in a secret developer fort on a French island around a coal fire. (See: Spacelog.org, where volunteers have logged complete transcripts and images from ten NASA missions. The initial work happened at an off-the-grid developer camp.) “As a historian, I’m impressed with the quality and tenacity of some of these folks,” Barry says.

To understand how good they are, you have to see their work, which is conveniently located online, on webpages that look like they were built in the 1990s. (Most of them were.) NASA even hosts one site on its servers—the Apollo Lunar Surface Journal, which is arguably the origin of all the rest.

Eric Jones, a scientist with a CalTech astronomy degree who spent 30 years working at Los Alamos National Laboratory in New Mexico, visited Johnson Space Center out of curiosity in the late 1980s. He discovered that the transcripts from all the Apollo Missions were gathering dust, and when he had a chance to meet Schmitt in 1989, Jones proposed making them available online.

Over two decades, Jones and Ken Glover, who owns a metalworking shop in Ottawa, Ontario, edited the site with help from hundreds of volunteers around the world. Ranging from a former musician to a German professor of dentistry who collects “flown” items, the volunteers helped gather, organize, and discuss thousands of primary sources from Apollo 11, 12, 14, 15, 16, and 17. There are indexed image libraries, press kits that NASA distributed at the time, flight plans, training plans, catalogs of every sample taken by the astronauts, transcripts and video clips and deep dives on crater names. There are photos of Schmitt’s worn spacesuit, with notes on wear and tear. (“The scratches undoubtedly resulted when Jack used the fingers on his dust-impregnated right glove to clean dust off the gauge.”) There’s an esoteric 44-page commentary from an engineer who worked on television cameras used on the surface of the moon. “Astronauts have gone to the website and seen stuff and said, ‘You got this wrong!’” Barry says.

Jones is a fan of Captain James Cook’s journals from his 18th-century Antarctic explorations, which helped inform later such expeditions. “Over the years, there’s been a lot of talk among people inside and outside of NASA about going back to the moon,” Jones says. Part of his motivation was to present such a thorough log of the Apollo missions as to serve as a planning resource for future lunar endeavors. “My continuing goal is to get it right.”

Around 1997, a developer at an advertising agency stumbled across the ALSJ. Ben Feist (actual brother of that Feist) remained just a fan for a few years. “I couldn’t believe the volume and depth of the information,” he writes on his website. But in 2001, Feist decided to start experimenting with a multimedia presentation of the media and transcripts. He publicly released the Apollo 17 Real-Time Mission Experience in May 2015.

He worked in earnest for six years to create a 305-hour real-time recreation of the entire mission from liftoff to landing back on Earth. The upper-left corner of the screen shows videos and simulations that sync exactly with the time they would have happened on the mission. Below that is a transcript of all communications between the astronauts and Mission Control (there’s audio, too), and to the right, photos taken by the astronauts, also timed to exactly when they would have been shot.

In the painstaking process, Feist contributed the first complete transcript of the 13-day journey, corrected and properly time-stamped from NASA’s initial transcript, which was riddled with errors.

Feist, like Jones, Glover, and the 33 developers at Spacelog.org, has a full-time job. But if you love space enough, you want to spend hours of free time listening to astronauts updating Houston on orbital stabilization. You want to gather every scrap of the original thing that remains, decades later, and make it publicly available. The Apollo program represents a time when humans watched breathlessly, on newfangled televised updates direct from space, as two humans set foot on a celestial body for the first time. And then Americans started losing interest—TV networks stopped broadcasting much beyond the launches following Apollo 11. “Some people didn’t see the point after the moon landing,” Jones says. “We beat the Russians, and for those sorts of folks, that was what mattered.”

Maybe these volunteer space historians want the rest of us to feel what they felt on July 20, 1969. “The NASA employees I talked to look back on that time as the best in their career. I think they were living their dreams,” Jones says. “One of my reasons for doing the journal is it gives me an opportunity to participate in the missions even at this late date.”

They may not have anticipated this side effect, but here’s one more. The people behind the Apollo 17 Real-Time Mission Experience and the ALSJ, with their exacting reproduction of NASA’s last journey to the moon, have given laypeople everything we could possibly use to figure out who took the most famous photograph of Earth. You don’t have to spend days triangulating transcripts and frame count readings in order to reach an informed guess.

It should be said that no one who’s actually spent days (or years) with this information will claim they absolutely know who took the photo. But most of the people who care enough about this one moment do have a guess, and most of them guess the same person.

One armchair historian named Eric Hartwell seems to have answered the question nine years before Feist’s website made it fairly easy to watch the moment when Blue Marble was taken. Hartwell, who appears to work in software development, among many other technological skills, and has been looking into Apollo 17 since at least the early 2000s, didn’t respond to requests for an interview—and his website no longer exists, though it would be unfair not to share a cached version. Even Barry points to him as someone who’s “done the work.” Hartwell details various credit claims, including when Time inexplicably credited Evans for the photo. He details where each man was sitting during the flight from Earth, what they said in the minutes the photo was taken, and the nitty-gritty of the camera settings and window positions in the spacecraft.

Hartwell concluded in January 2006: “When I started this project, I expected to prove that the ‘Blue Marble’ photo was taken by Jack Schmitt as commonly accepted. More than a year later, the answer is still up in the air.” In March 2006, he updated: “I still believe Schmitt took the picture.”

Still, Hartwell seems to conclude that we’ll never be satisfied using the evidence from the cockpit, so it’s up to what the astronauts themselves say. Did they truly each believe they’d taken the photo? Do they remember at all?

I could ask one person who’s spoken to all three astronauts—and has asked each of them specifically about the Blue Marble photo.

Mike Gentry started working at Johnson Space Center as a photo archivist in July 1969, the same month Apollo 11 landed on the moon. “All the newsmen were still hanging around, although the crew had landed and come back to Houston,” he says. Gentry helped add photo information, including credits, to everything the crews brought back. He spoke with astronauts—“I’d ask them who took that picture and what was the purpose of that?”—and he helped the public find photos in the archives. (John Denver once called Gentry about turning the Blue Marble photo into a hologram for his performances.) He casually throws out lengthy photo index numbers but says when he can’t remember the last digit, “My brain is getting clogged up—I need to put some Brillo in it.”

When asked to theorize on Blue Marble, Gentry is game. “I was always curious, because I even had to change my caption a couple times,” he says. “This is just my theory, and I will state is as such: I think the crew got together and said, ‘Let’s don’t really tell them, and when they ask, you just say you took it.’ I asked all three of them face to face. They all said, ‘I did.’ ‘I did.’ ‘I did.’”

But did any of the three convince him? “I feel in my heart that Jack Schmitt took it,” he says. “In retrospect, I think now of the looks on their faces. You know how you can pick up sarcasm from just the look on somebody’s face sometimes? I recall both Cernan and Ron Evans having the ‘well of course I took it’ look, kind of like if you found a $20 bill on the ground and you asked, ‘Whose $20 bill is this?’ ‘Oh, mine!’ Who’s gonna say, ‘Not me’?”

It’s a satisfying answer—the most human logic applied to a question that can go deep in the weeds on PDFs and triangulation. Or maybe the most satisfying answer is that it doesn’t matter who took it. “It’s one of those things where as a historian you look back and say I wish I knew the answer to this. But does it matter in the end?” Barry says. “The image has had a huge impact on our culture and society. Do we need to know whose finger pushed the shutter button? Probably not. And I’m fine with that.” It’s enough that all of us on Earth got to see the Blue Marble, and that it reminds us how small we are.

As Schmitt said to Mission Control nearly an hour after the Blue Marble shot was taken, “I’ll tell you, if there ever was a fragile-appearing piece of blue in space, it’s the Earth right now.”