Their goal is to lay the foundation for what may become the most ambitious alien-hunting mission in history: a dedicated SETI observatory on the farside of the moon.

Although it will be years before a SETI radio telescope lands on the moon, the three-day conference that began on May 22nd in Australia is an important first step in that direction and will mark the first time that the requirements for a SETI lunar observatory mission have been rigorously defined. The goal of the attendees is to hash out the technical specifications for this mission, which will inform a formal Phase A study that the group hopes to publish by the end of next year.

The study grew from a paper submitted by researchers from Breakthrough Listen, the world’s largest SETI program, to the National Academy of Sciences’ 2020 Planetary Science and Astrobiology Decadal Survey that made the case for a SETI observatory on the lunar farside. Now, with additional support from Breakthrough Listen, an international coalition of SETI researchers will take the rough sketch presented to the Decadal Survey and use it to create a detailed plan for what this mission would actually look like. 

“The benefits of a technosignature observatory on the lunar farside were identified early on in SETI, but it was just not thought to be a credible prospect because of the very large costs involved with putting anything on the moon,” said Andrew Siemion, the Director of the Berkeley SETI Research Center and a leader of the Phase A study. “That recently changed for a number of reasons, but I think most importantly the advent of commercial spaceflight dramatically reduced the cost of bringing a payload to the lunar surface, which has fueled something of a renaissance in lunar astronomy.”

There are two main reasons that SETI researchers are interested in building an observatory on the lunar farside. The first is that the Earth is a very loud place, electromagnetically speaking. Satellites, cell phones, TV towers, military radars, and other modern technologies all produce high-frequency radio signals in the same part of the spectrum where SETI scientists are looking for messages from ET. The second is that the Earth is enveloped in a thick atmosphere that blocks about a quarter of the high-frequency radio spectrum from ever reaching the Earth’s surface.

Terrestrial interference has been a challenge since the very first SETI experiment in 1960 when the planetary astronomer Frank Drake mistook a radio signal from a passing airplane as an extraterrestrial greeting. This led SETI researchers to start contemplating a truly “extra-terrestrial” solution to their problem: a dedicated observatory on the farside of the moon. 

During the two-week-long lunar night, the farside is the most radio-quiet place in the solar system because the moon acts like a giant electromagnetic shield. And because the moon has virtually no atmosphere, a SETI observatory would also be able to access swathes of the electromagnetic spectrum that have never been previously observed during a SETI campaign. 

“The lunar farside is the most pristine place in our solar system because it’s the only place that always points away from Earth,” says David DeBoer, a research astronomer at the University of California, Berkeley, and the Breakthrough Listen project manager for the Berkeley SETI Research Center. “This would be the first time ever that we’d be able to do an observation that’s free of radio frequency interference. The expectation is that we’re mostly going to see blank static, which means that any signal we pick up is going to be amazing because there’s nothing that can mimic the signals we’re looking for like there is on Earth.” 

The idea for a lunar observatory has been floating around the SETI research community for decades, but it wasn’t until very recently that it became a feasible goal. The first successful mission to the surface on the farside of the moon didn’t occur until 2019, and even this mission required marshaling the resources of one of the world's most powerful countries. But as the cost of space access continues to plummet thanks to reusable heavy launch vehicles and the commoditization of space-grade hardware, the once far-fetched idea of a SETI observatory on the lunar farside no longer seems so outlandish.

The only thing that was missing was a solid plan. 

“We really needed to undertake a formal Phase A study to figure out what this observatory could look like and what we could do for a reasonable amount of money—say $150 or $200 million,” said Siemion. The study is funded by a grant from Breakthrough Listen, a SETI initiative bankrolled by the Russian-Israeli billionaire Yuri Milner, and Siemion says their primary goal is to “identify a credible path forward for a lunar farside radio technosignature mission that could be accomplished in the next five to seven years.” Although Siemion believes that technology has progressed to the point where this ambitious mission is feasible, the devil is in the details. 

On Earth, SETI mostly boils down to a data analysis challenge. Researchers use powerful computers to sift through massive amounts of data collected by radio telescopes in search of signals that look unnatural. These are so-called “technosignatures,” radio signals that could only be produced by an artificial source of extraterrestrial origin. But searching for these faint signals among the overpowering noise from radio sources on Earth requires a lot of equipment and a lot of power. Any mission to the lunar farside, however, will be inherently resource-constrained. Figuring out how to efficiently crunch data on the lunar farside while providing adequate power to the radio telescope is a major challenge that the SETI researchers will need to iron out as part of their Phase A study. As an added challenge, this equipment will have to contend with an incredibly harsh environment that will see temperatures swing from 250 F to -200 F over the course of the lunar day-night cycle.

“The overall philosophy of the project is we want to use as much off-the-shelf technology as we can—and there’s a lot of it,” Siemion said. “But we’re not used to operating in this highly power-constrained environment so we need to identify the scientific opportunities and then the salient elements of the digital signal processing pipeline that are absolutely necessary to achieve those objectives.”

Fortunately, the SETI researchers aren’t the first ones to attempt to tackle this problem. NASA also has plans to put radio telescopes on the lunar surface as part of the Lunar Surface Electromagnetics Experiment mission. The LuSEE mission will place two landers on the moon’s surface, one on the near side and one on the far side, which are expected to launch as soon as 2025. Although the farside LuSEE lander is designed to observe low-frequency radio waves to study the early universe—making it unsuitable for SETI work—the technical challenges faced by the mission are analogous to those of the SETI observatory. 

“We're the only people who have actually spent any time thinking about how to do radio astronomy on the moon,” said Stuart Bale, the LuSEE principal investigator and a collaborator on the Breakthrough Phase A study. “We know how to do the system engineering and we know how to do the thermal engineering. None of it has flown yet, but we have a big head start on what [Breakthrough] wants to do so we’re going to try to help work their concept into something similar to LuSEE and put together a fundable proposal.” 

Although the exact design of the SETI observatory is still very much an open question, Siemion says it is unlikely to look anything like a typical radio telescope on Earth. For starters, the observatory will have to be small enough to fit in on a lander weighing less than 500 pounds. The lander will also need assistance from an orbiter that can relay data back to Earth, although it’s unlikely that the SETI mission will launch its own orbiter and instead piggyback on existing lunar satellites. 

“The challenge with the farside of the moon is that you need a way to get signals back to Earth and that requires some sort of relay,” DeBoer said. “So the first thing is to make sure we have a way to get the data back because without that we can’t do anything. Fortunately, that infrastructure is being developed as part of all these NASA commercial and lunar programs.”

Another big challenge is power. There are various options available to the team, but each comes with significant tradeoffs. A radioisotope thermoelectric generator—also known as a “nuclear battery”—is one option, but these systems are heavy and expensive. Solar power is another option but would mean that the lander would lose power for weeks on end during the lunar night. Regardless of what solution the team picks, however, the entire system will likely have to operate using the equivalent power of a 100-watt lightbulb. 

The technical challenges that will have to be ironed out by the Breakthrough team during their Phase A study are immense, but not insurmountable. Siemion hopes that they will be able to conclude the study by the end of next year and if the project goes forward the world could see its first SETI lunar observatory by the end of the decade. And it can’t come soon enough. Now that NASA has committed to establishing a permanent presence on and around the moon through the Artemis missions, the lunar radio environment is expected to become increasingly noisy. The window of silence on the lunar farside may be short-lived and endows the project with a palpable sense of urgency. China, now operating the largest radio observatory on Earth, is also quickly expanding its footprint on the moon.

“It’s only going to get louder. the sooner we can do this, the better.” 

But even as the moon gets more crowded and noisy, the SETI observatory would still have to contend with far less interference than it would on Earth, and would open up vast portions of the radio spectrum that used to be completely inaccessible. It’s a brave new frontier in the search for extraterrestrial life and with any luck may be the giant leap we need to finally make first contact.