What's on the far side of the moon? Well, not darkness.
The far side of the moon is the hemisphere Earthlings never see. Though it's not shrouded in darkness, many mysteries persist.
It took less than a week for NASA Administrator Bill Nelson's gaffe to make it to social media.
Over an hour into a budget hearing for the U.S. space agency, a congressman asked Nelson why China is sending spacecraft to the "backside" of the moon.
"They are going to have a lander on the far side of the moon, which is the side that's always in dark," said Nelson, a former senator and astronaut, during the April 17 hearing. "We're not planning to go there."
He proceeded to tell the lawmaker that "We don't know what's on the backside of the moon."
On two counts, that was untrue: The so-called far side of the moon is not dark, despite popular belief, nor is it completely unknown to American scientists.
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What is the dark side of the moon?
The far side is the hemisphere of the moon facing away from Earth. Because people never see it, that portion was once dubbed "the dark side." The confusing misnomer has led many to incorrectly assume the far side is shrouded in darkness. In reality, it receives just as much light as the near side.
Perhaps the actual "dark" location on the moon is where NASA plans to send Artemis astronauts in the coming years: the polar south region, where scientists believe ice water is buried. The south is pockmarked with frigid craters cloaked in shadow.
Why do we only see one side of the moon?
It takes about a month for the moon, some 250,000 miles away, to orbit Earth. It also takes the same amount of time for the moon to make one full rotation on its axis. This coincidence is why Earthlings always see the same lunar side.
Prior to space exploration, many speculated the far side was a frozen and ominous region, a myth perpetuated by Pink Floyd's trippy "Dark Side of the Moon" album in 1973.
Indeed the far side remained an enigma to humans, but only until October 1959. That's when the Soviet space program swung the Luna 3 probe around the moon. The spacecraft returned several grainy images that revealed a curiously different surface than that on the familiar near side.
About a half-century later, NASA launched the Lunar Reconnaissance Orbiter to explore the region from space. Since then, the spacecraft has sent home loads of data, allowing scientists to map the far side in detail.
What does the far side look like?
What scientists have learned is that the side humans can't see is vastly different.
Though the near side has large dark patches that together resemble a face, known as the "man in the moon," the far side has few of these spots. The spots are called maria, dark areas formed when meteoroids slammed into the moon, causing lava to emerge. Not only did the lava make the surface darker, but it erased previous craters that recorded some of the moon's geological history.
The far side, on the other hand, is blanketed in more craters of various sizes and depths, including the enormous South Pole-Aitken basin.
During the NASA and Soviet space race era, spacecraft never landed on the unseen side. Part of that was because of how difficult it is. The moon itself blocks communication between mission controllers on Earth and the far side. But in 2018, China put a communication relay satellite in space about 40,000 miles beyond the moon that could exchange the signals.
Why is China going to the far side of the moon?
Getting back to that House Appropriations Committee hearing on April 17: U.S. Rep. David Trone asked Nelson what made China so interested in the far side, rather than the south pole.
"I have no idea," Nelson said. He did, however, explain that the U.S. is going to the moon's south pole because that's where NASA suspects water is preserved in dark craters. It's a vital economic resource for future lunar endeavors.
But scientists say there's actually a lot that could be gained from studying the far side, which seems to have a more pristine record of earlier cosmic collisions and impacts.
On Dec. 8, 2018, China launched the Chang'e-4 mission and became the first nation to land on the reverse side of the moon. It touched down at the Von Karman crater, a site within the South Pole-Aitken basin.
The basin is a depression probably formed by an ancient asteroid collision so cataclysmic that it excavated some of the lunar mantle — material between the core and the crust — and brought it to the surface. The advantage of going to this location is the potential to learn more about the interior composition of the moon.
What China found, according to a paper published in the journal Science Advances, is that the layer of soil there is much thicker than on the near side. It was about 130 feet deep.
If the crust were a lot thinner on the side facing Earth, that might explain why lava was able to break through and form the maria spots. But why one hemisphere's crust would be so drastically different in thickness from the other is unclear. China's future missions may help answer that question.
Solving such mysteries could not only tell us more about the moon's evolution, but provide further insight into the history of the solar system.