The highly anticipated lunar flyby of the Artemis II mission will take four astronauts on a pioneering survey of the moon Monday, including the rarely glimpsed lunar far side, which always faces away from Earth.

The crew, including NASA’s Reid Wiseman, Victor Glover and Christina Koch, and Canadian Space Agency astronaut Jeremy Hansen, will see features on the lunar surface that human eyes have never observed before.

In fact, the astronauts have already experienced what mission control in Houston described as “moon joy” as they’ve drawn closer to the silvery orb over the past few days.

Using a camera equipped with a 400-millimeter lens has enabled them, from a distance of more than two-thirds of the way to the moon, to point out specific lunar craters, features and topography — including the vast Orientale Basin, which had never been seen or photographed by humans prior to this mission. The crater, which is 600 miles (965 kilometers) wide, represents a key transition region between the near and far sides of the moon.

“The moon we are looking at is not the moon you see from Earth whatsoever,” Koch said.

But how much will the astronauts actually be able to observe at an estimated distance of 4,070 miles from the moon — and what could it reveal about enduring lunar mysteries that scientists are eager to solve? Apart from extensive training to observe the moon and its unique features, the astronauts possess one of humanity’s greatest scientific tools: the gift of observation with their own eyes.

“Even from as far as away as 4,000 miles, there are still things that the human eye can pick up with granularity that are important to the science community,” said Judd Frieling, Artemis II ascent flight director.

While Apollo astronauts also circled the moon at even closer distances and spied some of the mysterious far side, the circumstances of the Artemis II flyby differ in a few ways.

Apollo vs. Artemis

The nine Apollo missions that ventured away from Earth orbit were limited as to what parts of the moon they observed based on which regions were illuminated by sunlight during the flight and the trajectories of their capsules.

“When the Apollo missions launched, they prioritized launching into windows where the near side was illuminated because that’s where the missions landed,” said Dr. Kelsey Young, lead for NASA’s Science Mission Directorate, at a news conference on Saturday. “The far side was not illuminated at the time when they were in orbit.”

The Orion capsule, named Integrity by its crew, will also fly by the moon at a greater distance. Apollo command modules flew around the moon from 70 miles (112 kilometers) above its surface and the robotic Lunar Reconnaissance Orbiter comes within 30 miles (48 kilometers) of its cratered face.

Integrity will come within an estimated 4,070 miles during its closest approach, meaning the moon will appear the size of a basketball when held at arm’s length from the perspective of the crew.

Apollo’s closer flybys didn’t allow the astronauts to see lunar poles, but Young said the Artemis II mission has a unique vantage point: the entire lunar disk.

Such a holistic view of the moon can reveal more about its history, especially how space rocks that have slammed into the moon to create craters have exposed the interior and flung lunar rocks across its surface. Erosion and other processes on Earth have essentially wiped away the same history of bombardment on our planet — but not on the moon.

“That period of our planet’s history that we can no longer get here, even if we go to the deepest parts of the ocean, is there on the moon,” Young said.

What human eyes can see

During the Apollo 17 mission, the program’s final crewed venture to the lunar surface in 1972, NASA astronaut and geologist Harrison Schmitt spotted orange soil and collected a sample.

Once back in orbit, the Apollo 17 crew saw the same orange hue on the lunar surface, which later revealed that volcanic processes were active on the moon’s surface for longer than expected, Young said.

“We’re looking for the crew to call out any of those subtle color nuances, especially on the parts of the far side that have never been seen before by human eyes,” Young said. “We’re able to ask more intelligent questions because of what Apollo gave us.”

While the Artemis II crew isn’t landing, their observations of color variances could shed more light on the origin and composition of the moon, as well as why the lunar near side and far side appear so different from each other.

The near side has a thin crust, low topography and extensive evidence of ancient volcanism, while the far side has a thick crust, higher elevations and far fewer signs of previous volcanic activity.

Photometry, or taking advantage of differing illumination conditions throughout the flyby, could also provide unique insights, Young added.

During training, the astronauts did an experiment involving sand. By using different angles of light, they identified texture, color and topography — something that can hint at how the lunar surface has evolved.

“We can’t move the sun in this mission, but we can move Integrity,” Young said. “By looking at the same targets more than once throughout their fly by, they’ll be able to make observations about the same targets in different illumination conditions that would take some spacecraft days, months, weeks, years to build up.”

Apart from simulations, the crew prepared for the historic lunar flyby in a multitude of ways in the months ahead of launch. They attended classes with the scientists, blazing through flashcards to understand lunar geography, handled rocks to get a better grasp of geology and even trained like field scientists in the Icelandic highlands — a great lunar analog on Earth.

The astronauts will spend roughly five hours observing the moon during the flyby. At any given moment, 180 degrees of the moon is illuminated, while the other 180 is not. The Artemis science team began by creating a list of features that may be visible on the moon’s entire surface, then narrowed it based on the trajectory determined by the launch date, Young said.

The crew will use three Nikon cameras equipped with different zoom lenses to capture varied features like impact basins and ancient lava flows. They will also provide live descriptions of what they’re seeing to scientists at Johnson Space Center a few times an hour.

“Just having this crew describe visuals of the moon, visuals of the lunar surface, will give you goosebumps,” Young said. “They are absolutely prepared not just to give really scientifically compelling descriptions, but also to bring that experience verbally to us watching here on Earth.”

NASA’s Artemis program is sending humans into deep space for the first time in more than five decades. Sign up for Countdown newsletter and get updates from CNN Science on out-of-this-world expeditions as they unfold.

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