On the moon's extreme western rim, straddling the border with the lunar far side, a landmark nearly 600 miles wide almost completely escapes Earth's view.
A colossal asteroid-like invader once ripped through the lunar crust, flinging out rings of mountains and scooping out a giant bowl, later flooded and backfilled with dark lava. The bullseye still stands today, scars memorializing that long-ago catastrophe.
From space orbiter cameras, Mare Orientale, which means "Eastern Sea" in Latin, reads clearly as a fossil of violence, geometry, and time. But despite the lunar feature's vastness, its location has forced it to remain one of humanity's white whales.
"Orientale has never been seen by human eyes," said Reid Wiseman, commander of NASA's Artemis II.
On the upcoming historic mission, the first to send people into deep space in 50 years, astronauts Wiseman, Victor Glover, Christina Hammock Koch, and Jeremy Hansen will loop around the moon in the Orion capsule. Depending on final tests and weather conditions, NASA could launch the spaceship as early as Feb. 6. As the crew swing past the far side — the lunar hemisphere that never faces Earth — the astronauts may see landscapes no one has ever seen before in sunlight.
With Apollo 8 astronauts taking the first glimpse of the far side in 1968, Artemis II won't be the first mission to have this vantage point. But this time, the launch period, flight path, and lighting conditions of the lunar terrain could let the crew study parts of the moon humans have never seen directly, revealing subtle surface details that stayed hidden in earlier missions.
The far side of the moon
The far-side hemisphere was once dubbed "the dark side" because people had never seen it. The misnomer has led many to incorrectly assume the far side is shrouded in darkness, a confusion that persists even today. In reality, the hemisphere receives just as much light as the near side.
People only ever see the near side because of one truly astronomical coincidence. It takes about a month for the moon, some 250,000 miles away, to orbit Earth. For the moon to make one full spin on its axis, it takes roughly the same amount of time. Until October 1959, when the Soviet space program swung the robotic Luna 3 probe around the moon and snapped some grainy pictures, no one had any idea what the other side was like.
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Apollo crews only had brief views of the far side as they looped around the moon. Their launches were timed so that the near side, where they landed and explored, would be bathed in sunlight. But that usually meant when the near-side was well-lit, the far side was in shadow or only had a thin crescent illuminated.
By comparison, Artemis II will have a high likelihood of seeing 60 percent of the far side that human eyes have never seen. That matters, mission leaders said, because despite having plenty of robotic spacecraft images of the far side, nothing beats firsthand observations.
After all, having people there, not just machines, is the point.
Artemis II astronauts Jeremy Hansen, left, Christina Koch, center, and Jenni Gibbons, a backup crewmate, get hands-on geology training on a field trip to Iceland.
Credit: Canadian Space Agency
"Most people think that the moon is only gray," said Jacob Bleacher, NASA's chief exploration scientist, "but the human eye can pick out a tremendous amount of detail."
Three full hours of observations
NASA plans to make the most of that opportunity. Even though they won't put boots down on the moon, the Artemis II astronauts have received geology and science training so they know what to look for as the moon fills Orion’s windows.
During the flight, they hope to spend three full hours completely devoted to observations. Their eyes will scan the far-side terrain, studying variations in the grayscale. Those subtle nuances of how bright or dark patches of the surface appear can hint at different rock types and ages.
Artemis II astronauts Christina Koch and Victor Glover, right, practice photography of lunar targets during a training exercise.
Credit: NASA / James Blair
NASA expects the crew to move between looking with their own eyes, taking pictures with cameras, and talking through what they see with flight controllers in Houston. They will carry checklists from science teams on where to look, but they'll also have the freedom to explore as they please.
"The moon will look about like holding a basketball at arm's reach," Bleacher said, "so they'll be able to see a good bit of the moon, if not all of it."
Future Artemis landing sites
Scientists believe a lot could be gained from studying the far side. The near side has large dark patches, called maria, that together resemble the "man in the moon" face. When lava filled in the ancient craters, it erased other craters that recorded some of the moon's geological history. But on the far side, fewer of these dark splotches exist, suggesting that it has a more pristine record of cosmic collisions.
During the NASA and Soviet space race, no one ever landed on this unseen side, even robotically, because of how challenging it is. The moon itself blocks communication between flight controllers on Earth and spacecraft on the other side. But in 2018, China put a communication relay satellite in space about 40,000 miles beyond the moon to allow the exchange of signals. That same year, China succeeded in becoming the first nation to put an uncrewed lander on the far side.
Not only could the Artemis II observations enlighten scientists about solar system history, but the crew could help pick landing sites for future missions and pinpoint compelling science targets. They may just turn our familiar gray saucer into something new again.
"We hope it's not 'the dark side,'" Koch said, referring to its old, misleading nickname. "We hope it's the lit far side."
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