The far side of the moon is poised to become our newest and best window on the hidden history of the cosmos. Over the course of the next decade, astronomers are planning to perform unprecedented observations of the early universe from that unique lunar perch using radio telescopes deployed on a new generation of orbiters and robotic rovers.
These instruments will study the universe’s initial half-billion years—the first few hundred million or so of which make up the so-called cosmic “dark ages,” when stars and galaxies had yet to form. Bereft of starlight, this era is invisible to optical observations. Radio telescopes, however, can tune in to long-wavelength, low-frequency radio emissions produced by the gigantic clouds of neutral hydrogen that then filled the universe. But these emissions are difficult, if not downright impossible, to detect from Earth because they are either blocked or distorted by our planet’s atmosphere or swamped by human-generated radio noise.
Scientists have dreamed for decades of such studies that could take place on the moon’s far side, where they would be shielded from earthly transmissions and untroubled by any significant atmosphere to impede cosmic views. Now, with multiple space agencies pursuing lunar missions, those dreams are set to become reality.
“If I were to design an ideal place to do low-frequency radio astronomy, I would have to build the moon,” says astrophysicist Jack Burns of the University of Colorado Boulder. “We are just now finally getting to the place where we’re actually going to be putting these telescopes down on the moon in the next few years.”
The Hydrogen Heartbeat
The idea that telescopes could even detect neutral hydrogen goes back to the 1940s, when Dutch astronomer Hendrik Christoffel van de Hulst predicted that hydrogen atoms can spontaneously emit pulses of electromagnetic radiation. This happens because each atom of hydrogen can flip between two energy states, emitting or absorbing radiation at a wavelength of 21 centimeters (or a frequency of 1,420 megahertz). Such emissions are the “heartbeat” of hydrogen and can add up to detectable signals when clouds of the gas accumulate on cosmic scales.
Such signals should have first emerged about 380,000 years after the big bang, when the universe cooled enough for protons and electrons that previously filled space to coalesce into atoms of hydrogen. Besides forming the raw material from which all subsequent objects would arise, this event had the added benefit of making the universe essentially transparent rather than opaque—liberating the fossil radiation produced by the big bang to stream through the cosmos. We now see this radiation—the big bang’s afterglow—as the cosmic microwave background (CMB). Thereafter, neutral hydrogen pervaded the dark universe for perhaps 100 million years until the break of cosmic dawn, when the first stars and galaxies began to shine.
Cosmologists are particularly interested in the dark ages because they offer a glimpse of the universe when it was relatively pristine, free of confounding astrophysical effects. Back then, the distribution of neutral hydrogen still carried the imprints of primordial quantum fluctuations that had been profoundly magnified by the universe’s rapid expansion in the first fractions of a second of its history—unsullied by the emergence of stars, galaxies and galaxy clusters. Presumably, the 21-centimeter signals from the dark ages could carry indications of new physics or deviations from the standard model of cosmology. “It's a playground for testing cosmology,” Burns says.
The very first radio telescopes on and around the far side of the moon will be simple. They will gather little more than vague hints of this shadowy slice of otherwise unseen cosmic time. But as more sophisticated instrumentation comes online, the 21-cm signals will emerge in richer detail, allowing astronomers to create dynamic, high-resolution maps of hydrogen clouds.
“The nice thing about neutral hydrogen is that it’s not just a snapshot in time like the CMB,” says Kristian Zarb Adami of the University of Oxford. By tracking the fluctuating 21-cm signal over cosmic time, telescopes can chart the evolution of the early universe through the dark ages all the way up to the cosmic dawn and even beyond. After the dawn comes the epoch of reionization, when the radiation from the first massive stars and other violent astrophysical phenomena sufficiently heated the remaining neutral hydrogen to transform it back to plasma. That event ultimately extinguished the 21-cm signals.
Some pathfinder instruments are already in operation as part of the Chinese Chang’e-4 lander on the moon’s far side and a lunar orbiter named Queqiao (“Magpie Bridge”), which relays signals from the lander to Earth. Queqiao was launched in May 2018, and Chang’e-4 reached the lunar surface in January 2019. “This was the first time there was a soft landing on the far side of the moon,” says Bernard Foing, executive director of the International Lunar Exploration Working Group and a planetary scientist at VU Amsterdam. “It was a great success.”
Radio antennas will be carried by both Chang’e-4 and Queqiao. But those on Queqiao are only partially deployed, while Chang’e-4’s single antenna is hindered by radio-frequency interference (RFI) coming from the lander’s electronics. Future dark-age-surveying lunar spacecraft could include additional shielding to minimize RFI—and could also deploy multiple antennas across tens or even hundreds of kilometers of lunar soil.
The next preparatory phase for far-side astronomy is set to begin with the launch of ROLSES (Radiowave Observations at the Lunar Surface of the Photoelectron Sheath) in October 2021. ROLSES will travel to the moon within a privately developed lander licensed by NASA as part of the space agency’s Commercial Lunar Payload Services program. Although it will touch down in the Oceanus Procellarum region on the moon’s near side, ROLSES’s task of characterizing the RFI generated by lunar soil is crucial for future work on the far side. “This is real,” says Burns, who is a member of the ROLSES team. “I have been working on this for 35 years. It’s actually happening.”
Yet another mission to characterize the RFI on the moon, the Lunar Surface Electromagnetics Experiment (LuSEE), is slated to launch as early as 2024. “LuSEE is going to the far side,” Burns says. “It’s going to go to the Schrödinger impact basin.”
The lander carrying LuSEE may also have another payload: DAPPER (Dark Ages Polarimeter Pathfinder), a telescope for detecting the 21-cm signal from the cosmic dark ages. “DAPPER was originally designed to be an orbiter around the moon, but it may go on this lander,” Burns says. “NASA has funded us to work on the mission concept for DAPPER. We’ll be ready to go.”
Whether in orbit or on the lunar surface, DAPPER will be limited to a set of dipole antennas in one location. But more ambitious plans exist for deploying arrays of antennas on the moon. Such arrays, which combine signals from individual antennas spread over large distances, act as telescopes with resolutions far greater than would be possible with a single antenna and can effectively pinpoint sources in the sky.
The Era of Arrays
Xuelei Chen of the National Astronomical Observatories at the Chinese Academy of Sciences thinks lunar orbit is the best near-term site for creating dark-age-mapping lunar arrays. Antennas on a number of satellites could be configured into an array that carries out observations when the satellites are all on the far side. “This is a small experiment with moderate cost, and we could accomplish it with current technology,” Chen says.
The tentative plan calls for a fleet of five to eight satellites flying in carefully choreographed formation to form an array. One of the satellites would be a larger “mothership” to host most of the electronics for receiving and combining the signals from other satellites and then relaying the results to Earth. “We want to have them launched as an assemblage, and then they will be released one by one,” Chen says.
Putting such an array on the far side’s surface will be far more challenging for many reasons, among them the moon’s rugged terrain and the spacecraft-threatening-chill of the 14-day-long lunar night. To begin preparing for such an eventuality, Foing’s team is planning to test the deployment of radio antennas using robotic rovers designed by the German Aerospace Center. The test will occur in June on the flanks of Mount Etna, an active volcano in Sicily meant as a proxy for the lunar surface. The rovers will be controlled remotely, and they will carry four boxes of antennas. “We will position them in different configurations to show that we will be able to do that in the future on the moon,” Foing says.
Another way of deploying a radio array on the moon’s far side would be to simply drop antennas from an orbiter to land and unfurl where they may. Adami and his colleagues are working on one such idea: a low-frequency interferometer design optimized for picking up radio emissions in a wide range of frequencies that involves 128 fractal-like “mini stations.” Each station has eight arms, and each arm combines 16 spiral antennas. “My idea would be that these fall off from the satellite and all land in different parts on the moon’s surface,” Adami says.
To make the process as robust as possible, the team has figured out how to print these antennas. “You could print antennas as fast as you print newspapers. We’ve been testing this technology for the past four or five years,” Adami says. “We are in the process of prototyping these spiral antennas.” The next step, he says, is for the scientists to design a mini station and drop it from a drone in remote regions, such as an arid region of Western Australia, to see if it unfurls.
Meanwhile Burns is also leading a NASA-funded concept study for building another lunar radio telescope, aptly called FARSIDE (Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets). To design FARSIDE, Burns and co-principal investigator Gregg Hallinan of the California Institute of Technology have teamed up with NASA’s Jet Propulsion Laboratory. The scientists are looking to land a payload of four rovers and 256 antennas, totaling about 1.5 metric tons, using lunar landers funded by NASA. The rovers would deploy the antennas connected by tethers, spreading them in four flowerlike petals over a region that is 10 kilometers in diameter. “We can do this with current technology,” Burns says. “So this all looks very plausible [for] later in the decade.”