This image from the report shows permanently shadowed regions on the lunar South Pole. These regions could be desirable locations for an optical interferometer, though they present their own problems. Image Credit: W. M. Keck Institute for Space Studies (KISS)
Earth’s atmosphere is an impediment to astronomical observations. Not only is cloudy weather a problem, but temperature fluctuations in the atmosphere mean that ground-based telescopes require sophisticated adaptive optics systems to see clearly. Radio telescopes aren’t bothered by clouds, but need to be built in ‘radio quiet’ locations to do their job best.
The desire to get beyond Earth’s atmospheric and other limitations has led to a fleet of space telescopes, and collectively, they’ve advanced our understanding of the cosmos in profound ways. But they’re expensive and complicated, and for telescopes at L1, beyond the reach of maintenance missions.
For decades, thinkers have been talking about building telescopes on the moon. Lunar telescopes share many of the same benefits as space telescopes, outperform them in some ways, and could be much less expensive, and potentially easier to maintain.
It’s a natural location for observing, since the moon has no atmosphere to foul up observations. And when it comes to radio telescopes, the lunar far side is an excellent location because it’s isolated from Earthly radio noise and is nearly radio-silent.
As different space agencies and private companies work to establish a presence on the moon, the idea of a lunar telescope is inching forward from daydream to a possible, eventual practical reality. The latest study for a lunar telescope comes from Keck Institute for Space Studies (KISS) and concerns an optical interferometer. Interferometers are powerful and allow multiple telescopes to work together as a single virtual telescope.
Lunar telescopes have clear benefits, but also some drawbacks. The new report digs into not only the benefits, but the drawbacks, too. They also debunk some obstacles as not that big of a deal.
The report’s title is “Astronomical Optical Interferometry From The Lunar Surface.” It’s the result of a study workshop held on November 18–22, 2024. The study leads are from the Lowell Observatory, JPL, and the California Institute of Technology. The participants come from NASA, JAXA, Caltech, as well as from multiple other universities and private space companies. The work is published on the arXiv preprint server.
Interferometry is well understood and already delivers exceptional results. The issue is the lunar environment itself. What makes it beneficial? What makes it problematic?
Start with the lunar surface. It’s covered in dusty regolith, and is inherently unstable. The Apollo missions demonstrated how abrasive and corrosive the dust can be. Lunar dust has sharper edges than Earth dust, and can eat away at important components like seals. In the lunar vacuum, dust particles can attach themselves electrostatically to equipment. The dust is difficult to remove and can hasten the wear on moving parts.
But the report points out that this problem is well-known and people are working on a solution. “Technologies are being developed to effectively remove the regolith from surfaces that may be applicable to various interferometry designs,” the authors write.
They point out that the Firefly Blue Ghost Mission 1 took the Electrodynamic Dust Shield on its mission, and the Shield effectively removed lunar dust on some of the spacecraft’s surfaces. They also point out that China’s Chang’e 3 lander carried an ultraviolet telescope that operated for three years without problems stemming from lunar dust.
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The moon’s wild temperature swings from day to night are another impediment. “Large temperature swings can be a challenge to electronics and optics that require thermal stability for operation and computation,” the report states. The lunar regolith itself, which is highly insulating, could provide a solution. The authors explain that some of the lunar interferometers could be buried in the regolith to avoid the large temperature swings. However, that could also contribute to overheating.
The report says that a system of active and passive cooling systems could be tailored to a specific site. It might be best to build an interferometer in a shaded region that remains cold, and to manage that rather than attempt to manage pronounced temperature swings. Regardless of the telescope’s site, it would have to be built to accommodate some degree of heating, cooling, and expansion and contraction.
The moon’s permanently shadowed regions (PSR) are desirable locations because they have the smallest temperature fluctuations. These are also good locations for infrared telescopes because they need to be kept cooled. But PSRs are cold traps that accumulate water ice, and there are concerns that ice and frost could impair a telescope’s function.
The lunar surface is a challenging environment, and there’s no glossing over it. But the challenges don’t seem insurmountable, and there are very likely technological solutions to all of them. It comes down to selecting a site that limits the challenges while also allowing for solutions and the achievement of science goals.
Despite the challenges of building and maintaining a telescope on the moon, the idea is alluring. The scientific results would be worth it, according to the authors.
They explain that two paramount aspects of telescope performance can be improved by a lunar telescope. A lunar telescope will have improved spatial resolution compared to a single-aperture space-based system. It will also be more sensitive than a terrestrial optical interferometer. “The combination of high spatial resolution and high sensitivity carve out unique areas of discovery space,” the authors write.
One of the benefits of building an optical interferometer on the moon concerns apertures. “Telescopes will likely have much smaller apertures (by 10–50×) than terrestrial facilities due to the lack of atmosphere on the moon, which allows for much longer coherent integration times, unconstrained by atmospheric seeing,” the report states. “The lack of atmospheric turbulence means small lunar telescopes outperform even the largest terrestrial telescopes.”
All of these factors mean that questions that have stumped astronomers and astrophysicists may finally be amenable to answers.
One concerns the ‘final parsec problem’ in black hole mergers. Astrophysicists understand the forces that cause supermassive black hole to spiral towards one another, but they don’t understand how they finally merge. With its improved performance, a lunar optical interferometer may be able to see into the inner regions of a galaxy and find an answer.
Another concerns planet formation. ” Given the ubiquity of exoplanets, planet formation must be a highly efficient process, but theories that describe the formation and evolution of planets from protoplanetary disks around pre-main sequence stars have been poorly constrained because of a lack of sensitivity and resolution at the scales of planet formation,” the report states. Understanding how habitable planets form is a huge goal in astronomy, and a lunar optical interferometer could help.
A lunar interferometer could also help study the surfaces of brown dwarfs, the physics of stellar explosions, the extragalactic distance scale, and will of course help in studying our own solar system. It can also help with astrometry, and “would be a transformative capability in the search for true Earth-analog exoplanets,” the authors explain, by improving radial velocity measurements.
An optical interferometer on the surface of the moon will help the Habitable Worlds Observatory do its job. “Astrometric single-measurement precision at the 0.1µas level, enabled by >100 m lunar interferometric baselines, would guide HWO observations and provide masses for atmospheric retrievals from HWO spectra,” the report states.
Astronomers know that optical interferometry works. The Very Large Telescope Interferometer (VLTI), and the Center for High Angular Resolution Astronomy (CHARA) have demonstrated that unequivocally. The question is, can it be done on the moon?
In their report’s conclusion, the researchers point out that some milestones have already been reached. There are more and more opportunities for flights to the moon as both government agencies and private entities ramp up their activities. We already know that optical interferometry can work. We also know that the lunar conditions, including the omnipresent dust, can be dealt with, as shown by the Chang’e 3 lander and its telescope. So we’re not starting from scratch.
So what are we waiting for?
We still need to develop the right lunar infrastructure and technologies to support a lunar telescope. “This includes surface mobility, communications / data infrastructure, and nighttime survival capability,” the authors write. Include a power source in there.
Beyond that, what is needed is specific mission proposals that can build up to a lunar telescope. That boils down to funding, and in the U.S. at least, science funding is anything but secure under the current regime.
One proposal is the Moon Lightweight Interior and Telecoms (MoonLITE) experiment. It would consist of a lander, a rover, and two small, 50 cm (2 inch) interferometer elements. The lander holds one element, and the rover would deliver the second element to the right location 100 meters away. It would only work for a short time, but would demonstrate milli-arcsecond-sized measurements of faint objects.
But the overarching finding in the report seems clear: “The intersection of mature optical interferometry technology, and rapidly maturing lunar surface access and survival technology, presents an opportunity to achieve optical imaging systems with angular resolutions orders of magnitude greater than currently possible with current space observatories, at sensitivity levels at orders of magnitude greater than terrestrial interferometric facilities,” the authors write in their conclusion.
More information:
Gerard van Belle et al, Astronomical Optical Interferometry from the Lunar Surface, arXiv (2025). DOI: 10.48550/arxiv.2510.24901
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