by For radio astronomers, the far side of the Moon could be the last pristine refuge in the Solar System. Planet Earth—and all the man-made electromagnetic noise it spews into space—remains permanently below the horizon, so that any radio observatories located there are free to observe the universe without interference.
But an upcoming boom in lunar exploration could jeopardize that. Over the next ten years or so, the Moon will be targeted by hundreds of orbiters and landers, each of which could generate radio noise. The researchers raised their concerns last month at a conference entitled Astronomy from the Moon: The Next Decades, which took place at the Royal Society in London.
“This is probably the quietest place in the Solar System and we have to preserve it,” said Marc Klein Wolt, an astronomer at Radboud University Nijmegen in the Netherlands.
“Will the far side remain dark? You should already be nervous that I’m asking the question,” Joseph Lazio, a researcher at NASA’s Jet Propulsion Laboratory in Pasadena, California, told the conference.
Quiet zone
The far side of the Moon has enormous potential for many fields, but holds unique promise for cosmology. Astronomers have mapped the sky using much of the electromagnetic wave spectrum, from microwaves to visible light and gamma rays. But cosmic radio waves at frequencies below about 100 megahertz are extremely difficult to measure from Earth because of the planet’s noise. And anything below 30 megahertz is completely off limits because it is absorbed in the ionosphere – the zone where Earth’s atmosphere meets space. These low-frequency waves, however, carry a wealth of information about the first billion years or so of the Universe’s history.
The far side of the Moon is shielded from radio emissions from Earth, and with almost no atmosphere and long, cold nights, it offers an almost ideal spot to explore these seasons.
If all goes according to plan, a small US spacecraft called the Lunar Surface Electromagnetic Experiment (LuSee) Night in 2026 will be the first dedicated cosmology mission to take advantage of these conditions—and it’s being designed with that goal in mind. (Chang’e-4, the historic Chinese mission that landed on the far side in 2019, carried a simple radio astronomy antenna. But the mission was not optimized for cosmological observations, so the experiment was marred by radio frequency interference from the craft itself.)
Funded by both NASA and the US Department of Energy, LuSee-Night will be flown to the far side by a private contractor as part of NASA’s nascent Commercial Lunar Payload Services program. Its four 3-meter-long antennas, arranged in a cross shape, will try to measure the “cosmic dawn,” a feature believed to be detectable in the radio spectrum that would reveal the appearance of the Universe’s first stars.
Noise reduction
However, even from the peaceful solitude of the lunar far side, measuring LuSee’s cosmic dawn will be a challenge: the signature of the early Universe is 100,000 times weaker than the noise produced by the Milky Way in the same frequency range. It will be critical to limit noise from the spacecraft itself. “The only way to do it is to turn off the lander completely” and pack enough batteries to last the radio receiver for two weeks’ nights, says Stuart Bale, an astrophysicist at the University of California (UC), Berkeley. who is the mission’s principal investigator for NASA. The receiver’s electronics, including the clocks that keep the computers running, must be designed to “fence” any emissions in a limited part of the spectrum, Bale says. “We require all oscillators to operate at known frequencies and with certified frequency stability.” A known, predictable source of noise is easier for experimenters to remove during data processing.
These are relatively simple precautions that all lunar missions, including commercial ones, could take, Bale says. If spacecraft are designed to contain radio frequency interference, it could greatly reduce the chances of them harming future science experiments.
Melanie Johnston-Hollitt, former director of the Murchison Widefield Array radio observatory in Western Australia, agrees. At Murchison, which is to be the Australian site of the giant Square Kilometer Array radio telescope, he has helped create what is perhaps the world’s largest radio-quiet zone, more than 500 kilometers wide.
Permits are required to bring electronics into space, and “all the equipment you take into that area goes through an additional electromagnetic test process,” to check for unwanted radio emissions, says Johnston-Hollitt, currently a radio astronomer at Curtin University in Perth. Australia. “I can tell you with confidence that you can do that with a cubesat,” she says, referring to the tiny satellites that researchers fear could swarm around the Moon, creating a source of noise.
Even so, “suppressing interference to the level necessary to do precision radio astronomy is incredibly difficult,” says astronomer Andrew Siemion, who leads the Breakthrough Listen research on extraterrestrial intelligence at UC Berkeley. This work involves searching for signals in a wide range of radio waves – including the gigahertz frequencies at which satellites communicate.
Lunar economy
Astronomers face an uphill battle. The same technological advances that promise to make the Moon more accessible for their experiments will also make the environment more crowded. More than 250 missions to the Moon are expected over the next decade by the space agencies of the United States, Europe, Russia, South Korea, China, Japan, India, Canada and the United Arab Emirates — as well as a number of by private companies. That would add up to a $100 billion “lunar economy,” according to Northern Sky Research, a consulting firm in Cambridge, Massachusetts. There are also plans to install a lunar navigation satellite system, which could be a source of noise.
Alanna Krolikowski, a political scientist at Missouri University of Science and Technology in Rolla, believes researchers should push for international treaties to protect the Moon. “There is now widespread recognition that we need governance for this coming lunar rebirth,” he told last month’s conference.
The Artemis Accords, an international agreement led by NASA, is trying to provide some guidance to help the agencies involved avoid disrupting each other’s missions. But it is primarily designed to serve the needs of signatory countries. A better way to regulate the Moon could be for the United Nations Committee on the Peaceful Uses of Outer Space to write rules, Krolikowski said. “The window in which this can be done is small – and shrinking.”
This article is reproduced with permission and was first published on March 3, 2023.
