Alien Hunters Need the Far Side of the Moon to Stay Quiet

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China made history earlier this year when its Chang’e-4 lander became the first spacecraft to land on the far side of the moon. During the two-week-long lunar days, the lander and its small rover, Yutu 2, beam images and other data to an orbiter for relay back to Earth. Together they’ve furnished planetary scientists with unprecedented access to the backside of our Janus-faced neighbor. But not everyone was thrilled that China crossed into this new lunar frontier, and few have been more vocal about their concerns than the scientists involved in the search for extraterrestrial intelligence.

Last month, the SETI permanent committee of the International Astronautical Association hosted its second round of negotiations about the lunar farside in Washington, DC. The exploration of the moon might seem like an issue outside the purview of this group of professional alien hunters, but the far side of the moon is the most radio quiet place in the inner solar system and they want to keep it that way in case ET calls. Indeed, they argue that the fate of the lunar farside may determine whether we ever detect a signal from an extraterrestrial intelligence.

“At the moment, SETI is not doomed, but it might be doomed in the next 50 years and that’s being optimistic,” says Claudio Maccone, an astrophysicist and the chair of the IAA SETI committee. “We must insist on this topic while there is still time to do something.”

On Earth, radio astronomers must contend with interference from television broadcasts, cell phone signals, satellites, and the atmosphere as they scan the cosmos for faint signals from primordial stars, organic molecules, or intelligent life. This makes the lunar farside an attractive site for future radio telescopes because the moon blocks all the radio signals from Earth. It’s like the difference between stargazing in New York City and stargazing in the middle of the desert—in the city light pollution obscures almost all of the good stuff.

As a hedge against the unchecked proliferation of radio frequency interference, a radio astronomer named Jean Heidmann made the case for a SETI radio base on the far side of the moon back in the mid-’90s. Even before cell phones became common, Heidmann realized that radio interference could eventually become so bad that searching for aliens with radio telescopes would be impossible on Earth. Moving radio observatories to the moon wouldn’t require turning the entire lunar farside into a radio quiet zone, but it would guarantee that at least some portions of the moon are preserved for radio astronomy and the hunt for extraterrestrial intelligence.


Here on Earth, governments could create more radio quiet zones like the kind around the National Radio Astronomy Observatory in Green Bank, West Virginia, but even then we might miss a message from ET. The Earth’s atmosphere starts to block out radio frequencies as you move away from a relatively narrow frequency band called the “microwave window,” so unless ET happens to be transmitting on one of those frequencies we’d struggle to hear it. But the lunar atmosphere is virtually nonexistent, which means radio astronomers would have access to frequencies above and below Earth’s microwave window. And, given the moon’s low gravity environment, astronomers would also be able to build massive radio telescopes that dwarf those found on Earth.

After Heidmann’s death in 2000, Maccone took up the cause of preserving the lunar farside for radio astronomy. He has written several papers on the subject and even gave a presentation to the United Nations, but until recently his pleas have fallen on deaf ears. The reason, Maccone says, is that the issue lacked any urgency. Most national space agencies had neither the funding nor the will to launch a mission to the far side of the moon and billionaires were still struggling just to get a rocket to orbit.

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