Looking at the Universe’s dark ages from the far side of the Moon

There is a signal, born in the earliest days of the cosmos. It’s weak. It’s faint. It can barely register on even the most sensitive of instruments. But it contains a wealth of information about the formation of the first stars, the first galaxies, and the mysteries of the origins of the largest structures in the Universe. Despite decades of searching for this signal, astronomers have yet to find it. The problem is that our Earth is too noisy, making it nearly impossible to capture this whisper. The solution is to go to the far side of the Moon, using its bulk to shield our sensitive instruments from the cacophony of our planet. Building telescopes on the far side of the Moon would be the greatest astronomical challenge ever considered by humanity. And it would be worth it.

The science

We have been scanning and mapping the wider cosmos for a century now, ever since Edwin Hubble discovered that the Andromeda “nebula” is actually a galaxy sitting 2.5 million light-years away. Our powerful Earth-based observatories have successfully mapped the detailed location to millions of galaxies, and upcoming observatories like the Vera C. Rubin Observatory and Nancy Grace Roman Space Telescope will map millions more. And for all that effort, all that technological might and scientific progress, we have surveyed less than 1 percent of the volume of the observable cosmos. The vast bulk of the Universe will remain forever unobservable to traditional telescopes. The reason is twofold. First, most galaxies will simply be too dim and too far away. Even the James Webb Space Telescope, which is explicitly designed to observe the first generation of galaxies, has such a limited field of view that it can only capture a handful of targets at a time. Second, there was a time, within the first few hundred million years after the Big Bang, before stars and galaxies had even formed. Dubbed the “cosmic dark ages,” this time naturally makes for a challenging astronomical target because there weren’t exactly a lot of bright sources to generate light for us to look at. But there was neutral hydrogen. Most of the Universe is made of hydrogen, making it the most common element in the cosmos. Today, almost all of that hydrogen is ionized, existing in a super-heated plasma state. But before the first stars and galaxies appeared, the cosmic reserves of hydrogen were cool and neutral. Neutral hydrogen is made of a single proton and a single electron. Each of these particles has a quantum property known as spin (which kind of resembles the familiar, macroscopic property of spin, but it’s not quite the same—though that’s a different article). In its lowest-energy state, the proton and electron will have spins oriented in opposite directions. But sometimes, through pure random quantum chance, the electron will spontaneously flip around. Very quickly, the hydrogen notices and gets the electron to flip back to where it belongs. This process releases a small amount of energy in the form of a photon with a wavelength of 21 centimeters. This quantum transition is exceedingly rare, but with enough neutral hydrogen, you can build a substantial signal. Indeed, observations of 21-cm radiation have been used extensively in astronomy, especially to build maps of cold gas reservoirs within the Milky Way. So the cosmic dark ages aren’t entirely dark; those clouds of primordial neutral hydrogen are emitting tremendous amounts of 21-cm radiation. But that radiation was emitted in the distant past, well over 13 billion years ago. As it has traveled through the cosmic distances, all those billions of light-years on its way to our eager telescopes, it has experienced the redshift effects of our expanding Universe. By the time that dark age 21-cm radiation reaches us, it has stretched by a factor of 10, turning the neutral hydrogen signal into radio waves with wavelengths of around 2 meters.

The astronomy

Humans have become rather fond of radio transmissions in the past century. Unfortunately, the peak of this primordial signal from the dark ages sits right below the FM dial of your radio, which pretty much makes it impossible to detect from Earth. Our emissions are simply too loud, too noisy, and too difficult to remove. Teams of astronomers have devised clever ways to reduce or eliminate interference, featuring arrays scattered around the most desolate deserts in the world, but they have not been able to confirm the detection of a signal. So those astronomers have turned in desperation to the quietest desert they can think of: the far side of the Moon. It wasn’t until 1959 when the Soviet Luna 3 probe gave us our first glimpse of the Moon’s far side, and it wasn’t until 2019 when the Chang’e 4 mission made the first soft landing. Compared to the near side, and especially low-Earth orbit, there is very little human activity there. We’ve had more active missions on the surface of Mars than on the lunar far side.

And that makes the far side of the Moon the ideal location for a dark-age-hunting radio telescope, free from human interference and noise. Ideas abound to make this a possibility. The first serious attempt was DARE, the Dark Ages Radio Explorer. Rather than attempting the audacious goal of building an actual telescope on the surface, DARE was a NASA-funded concept to develop an observatory (and when it comes to radio astronomy, “observatory” can be as a simple as a single antenna) to orbit the Moon and take data when it’s on the opposite side as the Earth. For various bureaucratic reasons, NASA didn’t develop the DARE concept further. But creative astronomers have put forward even bolder proposals. The FarView concept, for example, is a proposed radio telescope array that would dwarf anything on the Earth. It would be sensitive to frequency ranges between 5 and 40 MHz, allowing it to target the dark ages and the birth of the first stars. The proposed design contains 100,000 individual elements, with each element consisting of a single, simple dipole antenna, dispersed over a staggering 200 square kilometers. It would be infeasible to deliver that many antennae directly to the surface of the Moon. Instead, we’d have to build them, mining lunar regolith and turning it into the necessary components. The design of this array is what’s called an interferometer. Instead of a single big dish, the individual antennae collect data on their own and then correlate all their signals together later. The effective resolution of an interferometer is the same as a single dish as big as the widest distance among the elements. The downside of an interferometer is that most of the incoming radiation just hits dirt (or in this case, lunar regolith), so the interferometer has to collect a lot of data to build up a decent signal. Attempting these kinds of observations on the Earth requires constant maintenance and cleaning to remove radio interference and have essentially sunk all attempts to measure the dark ages. But a lunar-based interferometer will have all the time in the world it needs, providing a much cleaner and easier-to-analyze stream of data. If you’re not in the mood for building 100,000 antennae on the Moon’s surface, then another proposal seeks to use the Moon’s natural features—namely, its craters. If you squint hard enough, they kind of look like radio dishes already. The idea behind the project, named the Lunar Crater Radio Telescope, is to find a suitable crater and use it as the support structure for a gigantic, kilometer-wide telescope.

This idea isn’t without precedent. Both the beloved Arecibo and the newcomer FAST observatories used depressions in the natural landscape of Puerto Rico and China, respectively, to take most of the load off of the engineering to make their giant dishes. The Lunar Telescope would be larger than both of those combined, and it would be tuned to hunt for dark ages radio signals that we can’t observe using Earth-based observatories because they simply bounce off the Earth’s ionosphere (even before we have to worry about any additional human interference). Essentially, the only way that humanity can access those wavelengths is by going beyond our ionosphere, and the far side of the Moon is the best place to park an observatory.

The engineering

The engineering challenges we need to overcome to achieve these scientific dreams are not small. So far, humanity has only placed a single soft-landed mission on the distant side of the Moon, and both of these proposals require an immense upgrade to our capabilities. That’s exactly why both far-side concepts were funded by NIAC, NASA’s Innovative Advanced Concepts program, which gives grants to researchers who need time to flesh out high-risk, high-reward ideas. With NIAC funds, the designers of the Lunar Crater Radio Telescope, led by Saptarshi Bandyopadhyay at the Jet Propulsion Laboratory, have already thought of the challenges they will need to overcome to make the mission a success. Their mission leans heavily on another JPL concept, the DuAxel, which consists of a rover that can split into two single-axel rovers connected by a tether. To build the telescope, several DuAxels are sent to the crater. One of each pair “sits” to anchor itself on the crater wall, while another one crawls down the slope. At the center, they are met with a telescope lander that has deployed guide wires and the wire mesh frame of the telescope (again, it helps for assembling purposes that radio dishes are just strings of metal in various arrangements). The pairs on the crater rim then hoist their companions back up, unfolding the mesh and lofting the receiver above the dish. The FarView observatory is a much more capable instrument—if deployed, it would be the largest radio interferometer ever built—but it’s also much more challenging. Led by Ronald Polidan of Lunar Resources, Inc., it relies on in-situ manufacturing processes. Autonomous vehicles would dig up regolith, process and refine it, and spit out all the components that make an interferometer work: the 100,000 individual antennae, the kilometers of cabling to run among them, the solar arrays to power everything during lunar daylight, and batteries to store energy for round-the-lunar-clock observing. If that sounds intense, it’s because it is, and it doesn’t stop there. An astronomical telescope is more than a data collection device. It also needs to crunch some numbers and get that precious information back to a human to actually study it. That means that any kind of far side observing platform, especially the kinds that will ingest truly massive amounts of data such as these proposals, would need to make one of two choices. Choice one is to perform most of the data correlation and processing on the lunar surface, sending back only highly refined products to Earth for further analysis. Achieving that would require landing, installing, and running what is essentially a supercomputer on the Moon, which comes with its own weight, robustness, and power requirements. The other choice is to keep the installation as lightweight as possible and send the raw data back to Earthbound machines to handle the bulk of the processing and analysis tasks. This kind of data throughput is outright impossible with current technology but could be achieved with experimental laser-based communication strategies.

The future

Astronomical observatories on the far side of the Moon face a bit of a catch-22. To deploy and run a world-class facility, either embedded in a crater or strung out over the landscape, we need some serious lunar manufacturing capabilities. But those same capabilities come with all the annoying radio fuzz that already bedevil Earth-based radio astronomy. Perhaps the best solution is to open up the Moon to commercial exploitation but maintain the far side as a sort of out-world nature preserve, owned by no company or nation, left to scientists to study and use as a platform for pristine observations of all kinds. It will take humanity several generations, if not more, to develop the capabilities needed to finally build far-side observatories. But it will be worth it, as those facilities will open up the unseen Universe for our hungry eyes, allowing us to pierce the ancient fog of our Universe’s past, revealing the machinations of hydrogen in the dark ages, the birth of the first stars, and the emergence of the first galaxies. It will be a fountain of cosmological and astrophysical data, the richest possible source of information about the history of the Universe. Ever since Galileo ground and polished his first lenses and through the innovations that led to the explosion of digital cameras, astronomy has a storied tradition of turning the technological triumphs needed to achieve science goals into the foundations of various everyday devices that make life on Earth much better. If we're looking for reasons to industrialize and inhabit the Moon, the noble goal of pursuing a better understanding of the Universe makes for a fine motivation. And we’ll all be better off for it.