The four-person crew of NASA's Artemis II mission saw parts of the moon no human has ever seen before on Monday and described their findings in intricate detail to scientists on Earth.
Monday's lunar flyby — where the astronauts circled the moon — marked the farthest humans have ever traveled into space. At 1:57 p.m. Monday, the astronauts surpassed the Apollo 13 mission's distance record of 248,655 miles.
Jeremy Hansen, Artemis II mission specialist and a member of the Canadian Space Agency, honored the efforts of earlier space explorers as the crew broke the record and implored future generations to carry on their mission.
"We will continue our journey even further into space before Mother Earth succeeds in pulling us back to everything that we hold dear," Hansen said on Monday. "But we most importantly choose this moment to challenge this generation and the next to make sure this record is not long-lived."
While NASA does have photos of the moon from satellites such as the Lunar Reconnaissance Orbiter, experts told NPR that the ability for humans to observe parts of the moon for the first time — and relay details in their own words — is vital to lunar understanding.
On Monday, the astronauts described the moon's topography and meteoroids hitting the far side of the moon, allowing some people to hear a human observing the moon from space for the first time ever.
"It's really important for the astronauts to have an opportunity to make observations with the human eye and to describe them in a human voice with the wonder of the human heart," said David Kring, a planetary geologist with the Universities Space Research Association, a nonprofit research group working to advance space science and technology.
Astronauts also can locate "unusual things" on the moon's surface that may not have been seen otherwise, as they did during the Apollo missions, Paul Hayne, a planetary scientist at the University of Colorado Boulder, told NPR's Morning Edition on Tuesday.
"The human eye is able to pick up details of the lunar surface that cameras sometimes can't," Hayne said.
Here's what the astronauts saw on a mission that sent humans to the moon for the first time in more than 50 years.
Lunar surface targets
The astronauts received a final list of 30 lunar surface targets for the flyby.
One of these targets was the 3.8-billion-year-old Orientale basin. The nearly 600-mile-wide crater, which resembles a bullseye, sits between the moon's near and far sides and formed when a large object struck the lunar surface. NASA said the Artemis II crew was the first to see the entire basin, known as the "Grand Canyon" of the moon.
Up close, Mission Commander Reid Wiseman described its annular ring, one of the basin's key aspects: "The annular ring, which I think everybody describes as a pair of lips or a kiss on the far side of the moon, from here is very circular in nature."
"The northern part of it is wider, darker. The southern part is much lighter," he continued. "It is very neat-looking … far more circular than I remember it looking in our training."
During their flyby, the crew took time to suggest names for two additional craters. For one, they chose "Integrity" after the name of their spacecraft, and for the second, "Carroll," in honor of Wiseman's late wife, Carroll Taylor Wiseman.
In addition to these targets, mission specialist Christina Koch described the moon's craters more generally.
"When you look at the moon … all the really bright new craters, some of them are super tiny, most of them are pretty small, and there's a couple that really stand out," Koch radioed. "What it really looks like is like a lampshade with tiny pinprick holes and the light shining through. They're so bright compared to the rest of the moon."
The moon versus the earth
The astronauts compared the moon to the Earth from their unprecedented vantage point.
"The moon is about three to four times the size of the earth and it's almost full and it is just a small crescent out there," Wiseman said. "It's magnificent. I got a picture of it with the wide lens. Such a majestic view out here."
At one point, two of the astronauts could see the Earth and the moon at the same time.
"It's interesting because the Earth looks way brighter," Koch said. "So the Earth itself seems to have a much higher albedo [reflectivity] than the moon."
Planetary geologist Kring, who is currently a fellow at the Harvard Radcliffe Institute, said it wasn't the size differences the astronauts commented on that he found most interesting.
"It was that you could see them in substantial detail at the same time," he said. "Exploration, like many things, is a step-by-step process. … You have to look at this as the first step in a comprehensive lunar science and exploration program."
Mission pilot Victor Glover also described the terminator — the boundary between the moon's night and day.
"Wow, I wish I had some more time to just sit here and describe what I'm seeing," he said. "But the terminator right now is just fantastic. It is the most rugged that I've seen it from a lighting perspective."
Kelsey Young, the Artemis II lunar science lead, responded: "Oh my gosh, that was an amazing picture you just painted. … Those types of observations are things that humans are uniquely able to contribute, and you just really brought us along with you."
Kring, who gave basic training to all four astronauts, said he has been working for years to explain the "dramatic" elevation of features on the moon's south pole. Pictures on flat pieces of paper don't capture the topography, he said.
"The elevation changes are greater than the elevation of Mount Everest on Earth," Kring said. "I could hear Victor [Glover] finally getting it."
Color nuances
One of the scientific objectives of the flyby was to observe color variations on the lunar surface.
"The human eye, especially when it's connected to a well-trained brain, which I assure you these four people have, are capable of just in literally the blink of an eye, making nuanced color observations," Young said at a news conference over the weekend.
Young used the analogy of a sandbox: At the beach, the sand is not perfectly flat. It has texture and the grains are different particle sizes. Using a flashlight, Young described two angles of looking at the sandbox.
Shining a light directly on top of the sandbox, "you'll see the shades of color and albedo [reflectivity] on that surface." But using the same flashlight and moving it to the side of the sandbox, "you'll lose all the color nuance, but you will see topography and morphology," she said.
The astronauts will be able to look at the same locations more than once with different angles of illumination, according to Young. This analogy came to life during the flyby, as the astronauts pointed out shades of browns and blues that can help reveal the mineral composition of a feature and its age, NASA said.
Mission specialist Hansen described one plateau as having "unique," "greenish hues."
"I didn't see anything like that anywhere else on this side of the moon," he said.
Kring said he is cautious about attributing a lot of weight to the colors since eyes "can be fooled by color," Instead, he said future missions will collect samples that will allow scientists to have an up close look at the color nuances of the moon.
Glover described one crater basin in detail, saying it had rings and ridges that looked like they'd been dusted with chalk. "Or maybe dusted with snow," Glover said. "If this was the Earth, I would say there was snow dumped on some of the ridges on the interior of the rings."
Seeing a solar eclipse
The astronauts had a rare opportunity to observe a solar eclipse from the opposite vantage point, watching the sun disappear behind the moon. The roughly hourlong phenomenon allowed the crew to study the solar corona, the sun's outermost atmosphere, as it peeked around the edge of the moon, NASA said
"It's truly hard to describe," Glover said of the eclipse. "It's a wicked view."
To which Wiseman added: "No matter how long we look at this, our brains are not processing this image in front of us. It is absolutely spectacular, surreal, there's no adjectives."
Glover also described Earthshine -- sunlight reflected off the earth's surface -- illuminating the moon's surface during the eclipse: "After all of the amazing sites that we saw earlier, we just went sci-fi. ... You can actually see a majority of the moon. It is the strangest looking thing."
During the eclipse, the astronauts also saw several flashes of small meteors hitting the lunar surface, to the delight of the Mission Control room.
"There was a little bit of giddiness," Wiseman said. "Undoubtedly quick impact flashes. ... It was definitely impact flashes on the moon."
Kring said while the space community has photos of geological events like impact flashes, it's "just cool" to see it happen in real time. He said it was one of several moments when he could tell the crew was "startled" or "amazed."
"That's important not only for themselves, but for the other astronauts who are going to be walking on the lunar surface," Kring said. "They are going to carry back that energy. They're going to carry back that sense of wonder."
An Earthset and Earthrise
The crew saw both an Earthset, the moment the Earth drops below the lunar horizon, and an Earthrise, when the Earth peeks out above the lunar surface.
"On the International Space Station, we're 250 nautical miles up and that's the most beautiful view I think a human could ever experience," Wiseman said, adding that the Artemis crew was 250,000 miles away. "And every time, Mission Control points this vehicle either at the moon or at Earth, it reminds me every day that humans have to go."
"We've got to explore," he said.
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