On Monday, a group of researchers sponsored by Breakthrough Listen, the world’s largest SETI program, submitted a paper to National Academy of Sciences’ Planetary Science and Astrobiology Decadal Survey that makes the case for establishing a SETI radio observatory on the farside of the moon. The decadal survey establishes scientific priorities for the next ten years and the new paper addresses one of the biggest problems facing the search for extraterrestrial intelligence today: The overwhelming amount of radio interference. 

Our planet has become so “loud” in the part of the radio spectrum observed by SETI that it threatens to drown out any signal sent from an intelligent civilization. Not only would a lunar radio telescope not have to deal with terrestrial radio interference, it could also significantly increase our chances of hearing from ET by opening up parts of the radio spectrum that are blocked by Earth's atmosphere. While the idea of using the moon for radio astronomy is decades old, the researchers make the case that technological advancements have finally made a lunar SETI observatory truly feasible.

Radio interference has been a problem for SETI from the very beginning. In the spring of 1960, the planetary scientist Frank Drake trained the massive radio telescope at Green Bank Observatory in West Virginia on Tau Ceti and Epsilon Eridani, two stars a mere 12 light years from Earth. That summer, Drake spent his days studying the signals picked up by Green Bank’s giant mechanical ear in the hopes of receiving a message broadcast by an alien civilization orbiting those stars. Known as Project Ozma, Drake’s experiment marked the beginning of SETI, the scientific search for extraterrestrial intelligence. 

Shortly after Drake started his observations, he was surprised to find what appeared to be a signal of intelligent origin. After days of watching a needle drift lazily over a spool of paper recording the random undulations of cosmic static, Drake and his colleagues were jolted awake when the machine started recording the frantic pulses of a strong radio signal picked up by the telescope. The timing and magnitude of the pulses clearly marked them as artificial; there was nothing in the natural world that could produce such a frenetic radio profile. It would have been an astounding stroke of luck to pick up an alien message after only a few hours of observation, but it was hard to argue with the data. 

“None of us had ever seen anything like it,” Drake recalled in Is Anyone Out There?, his autobiographical book about the early days of SETI. “We looked at each other wide-eyed. Could discovery be this easy?”

Earth was never a particularly great place to do any kind of radio astronomy due to our thick atmosphere blocking a large portion of the radio spectrum. The proliferation of radio communication technologies has only made things harder. The moon, by comparison, has no atmosphere and its nights last for weeks on end, which limits radio noise from the sun. And as NASA discovered through a spate of lunar orbiter missions in the late 1960s, the moon also acts as a natural shield that blocks radio signals emanating from Earth. As the planetary astronomer Phillipe Zarka has put it, “the farside of the moon during the lunar night is the most radio-quiet place in our local universe.” It’s exactly the sort of peace and quiet you want if you’re searching for faint radio signals from solar systems that might be hundreds of light years away. 

The new Breakthrough Listen paper proposed two main approaches to a lunar SETI observatory: an orbiter and a telescope on the surface. The basic idea behind a SETI lunar orbiter would be to scan for signals as it passed over the lunar farside and relay data back to Earth as it passed over the near side. One of the main advantages of an orbiter is cost. The proliferation of small satellites that are capable of accurate tracking combined with low-cost small launch providers like Rocket Lab means that a SETI orbiter could conceivably be sent to the moon for less than $20 million. This would be a valuable pathfinder mission that could pave the way for a more ambitious observatory on the surface, but without the risk and cost.  As the ill-fated Israeli Beresheet lander mission reminded us, landing on the moon is extremely challenging even when the mission is backed by $100 million. 

But a SETI lunar orbiter would also come with a lot of compromises. It would only be able to conduct observations during the brief stretches when it was on the lunar farside, which would make a sustained observation campaign more challenging. The upshot is that an orbiter would have access to the full sky, whereas a telescope on the surface would be constrained by the moon’s rotation. The biggest downside of an orbiter is that it might lose a lot of the shielding benefits of the moon and be more vulnerable to radio interference from Earth since it would be orbiting high above the lunar surface. 

“The first SETI observations that are done from the lunar farside will be done from orbit, there’s no question about that,” says Andrew Siemion, the director of the Berkeley SETI Research Center and the second author on the paper. “I think eventually we absolutely want to do something on the surface because we want to build a very large aperture telescope, but even when we’re at that point I don’t think that would negate the utility of doing things from orbit as well.” 

So what would a SETI observatory on the moon look like? One idea is to use the naturally parabolic lunar crater as a radio dish, much like the Arecibo telescope in Puerto Rico and the FAST telescope in China, which are built into natural depressions in the land. This idea was first considered back in the late 1970s by a group of scientists at the radio physics lab at the Stanford Research Institute. Their idea was to recreate Arecibo on the moon by suspending an antenna from the lip of a crater and using the basin as a reflector. The reduced gravity on the moon would allow for a radio telescope far larger than any on Earth, which could significantly enhance the sensitivity of SETI searches. Ultimately the researchers concluded that a lunar radio observatory was too expensive compared to SETI telescopes that could be built on Earth. 

But 40 years later, Michaud says that building a radio dish in a lunar crater may finally be cheap enough to pull off. One of the main drivers of this cost reduction is the advent of commercial launch providers like SpaceX and Rocket Lab, which have dramatically lowered the cost of space access. Another driver is NASA’s push to establish a permanent human presence on the moon, which has subsidized the development of a fleet of commercial lunar exploration vehicles. “There’s so much interest in going back to the moon,” says Michaud, who cited Blue Origin’s lunar lander and Rocket Lab’s Photon Lunar satellite as examples of technologies enabled by NASA’s Artemis program. 

A crux of the original vision for lunar SETI observatories was that it would require a human settlement on the moon to build and operate the radio dish. But robotic systems have improved enough that it may be possible to take humans out of the equation. This was clearly demonstrated in 2019 when China’s Chang’e 4 rover landed autonomously on the farside of the moon. These advancements in autonomous navigation have laid the foundation for a lunar radio observatory that is built entirely by robots. 

It sounds like science fiction, but earlier this year NASA’s Advanced Innovative Concepts program awarded one of it’s prestigious grants to Saptarshi Bandyopadhyay, a researcher at the Jet Propulsion Laboratory, to figure out a way to make it happen. His idea is to use rovers to deploy wire mesh in a crater on the lunar farside and suspend a receiver over the dish. NIAC is all about funding high risk, high reward missions, and there’s no guarantee that Bandyopadhyay’s proposal will ever come to fruition. Still, addressing the technical problems associated with building a radio receiver on the farside of the moon is an important first step.

And Bandyopadhyay isn’t the only NASA-backed researcher contemplating a lunar radio observatory. Jack Burns, a radio astronomer at the University of Colorado, has also received a grant to study a mission concept for a radio telescope array called FARSIDE. Instead of using a crater as a dish, FARSIDE would deploy several smaller antennas across the lunar surface that would collectively form a large radio telescope. Both NASA studies are focused on radio astronomy rather than SETI, but Siemion sees the two disciplines as natural allies in the quest to establish an observatory on the lunar farside. SETI has piggybacked on other radio astronomy projects in the past—SERENDIP, for instance, opportunistically searched for ET signals during radio observation campaigns at a variety of telescopes—and it seems plausible that a similar arrangement could be made with an observatory on the moon. 

Siemion acknowledged that there were certain technical challenges that would arise in a collaboration on a lunar radio observatory. The biggest issue, he says, is that a lot of radio astronomy is done at frequencies that don’t really require an observatory on the moon. “Radio frequency interference is far less of a problem for conventional radio astronomy than it is for SETI,” Siemion says. “ Putting radiometers that operate at centimeter wavelengths on the lunar farside is really kind of a niche activity for SETI because radio frequency interference is such a singular challenge for us.”

And as the Breakthrough Listen team points out in their new paper, building an observatory on the lunar surface comes with significant engineering and operational challenges. The two-week-long nights on the lunar surface means that an observatory would have to find a way to get power that doesn’t rely on solar panels, and the energy requirements of an Arecibo-sized observatory would be substantial to run the telescope and do on-site data analysis. NASA has been developing small nuclear reactors that would deliver enough power to keep a moonbase running and these could also conceivably be deployed for a lunar radio telescope. But so far NASA researchers haven’t tested one beyond Earth. 

A farside crater observatory would also depend on an orbiter to relay data back to Earth, which could create radio interference for the dish, the very thing it was trying to avoid. It may not be necessary to start on the farside, however. In a separate paper submitted to NASA’s Artemis III science definition team, the Breakthrough Listen team suggested it might be possible to create a SETI observatory at the lunar south pole, the purported destination for NASA’s first crewed return to the moon. While the south pole isn’t as radio-quiet as the farside, if an observatory was placed on the back of Malapert Mountain near the pole, it would also be protected from a lot of Earth’s radio interference. 

But in both cases, the observatory would still be exposed to radio interference from existing orbiters such as China’s Queqiao satellite that serves as a relay for its farside rover. And the moon is only going to get noisier the next few years. NASA and other national space agencies have a number of lunar missions on the books—both crewed and robotic—that may contribute to radio interference. Claudio Maccone, the chair of the International Astronautical Association’s SETI committee, has advocated for the creation of a radio-quiet preserve on the lunar farside to avoid this problem. The concept is similar to radio-quiet zones around telescopes on Earth but is complicated by the fact that establishing a similar zone on the moon would require cooperation from every country and company with the technological means to get there. 

So will we ever see a SETI observatory on or around the moon? Both Michaud and Siemion are confident that we will. “The future of astronomy is going to be in space for the most part,” says Siemion. “As access to space becomes cheaper and more democratized, astronomy will move there and I think much of that will be done from the lunar surface.” There are a number of technical, political, and financial challenges that will have to be addressed before it happens, but the Breakthrough Listen team is optimistic that a small orbital pathfinder mission is feasible within the next few years. As they note in the conclusion of their paper, “a lunar SETI mission would mark the beginning of a new era in the history of SETI.” And after decades of fruitless searching, it may be exactly the type of innovation that leads to first contact.