Are telescopes on the moon doomed before they’re even built?

For radio astronomers, the far side of the moon may be the last untouched refuge in the solar system. Planet Earth—and all the man-made electromagnetic noise it spews into space—remains permanently below the horizon, so any radio observatories located there will be free to observe the cosmos without interference.

But a coming boom in lunar exploration could jeopardize that. Over the next ten years or so, the Moon will be the target of hundreds of orbiters and landers, each of which can create radio noise. Scientists voiced their concerns last month at a conference called Astronomy from the Moon: The Next Decades, which took place at the Royal Society in London.

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“This is probably the most radio-quiet place in the solar system, and we need to preserve it,” said Marc Klein Wolt, an astronomer at Radboud University Nijmegen in the Netherlands.

“Will the other side remain dark? You should already be nervous about me asking the question, Joseph Lazio, a scientist 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 it holds unique promise for cosmology. Astronomers have mapped the sky using much of the spectrum of electromagnetic waves, from microwaves to visible light and γ-rays. But cosmic radio waves at frequencies below about 100 megahertz are extremely challenging to measure from Earth, due to the planet’s noise. And anything below 30 megahertz is completely banned because it is absorbed in the ionosphere – the zone where the Earth’s atmosphere meets space. However, these low-frequency waves hold a treasure trove 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 location from which to explore these eras.

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If all goes according to plan, a small American lander called the Lunar Surface Electromagnetic Experiment (LuSee) Night in 2026 will be the first dedicated cosmological mission to take advantage of these conditions — and it is being designed with this goal in mind. (Chang’e-4, the historic Chinese mission that landed on the other side in 2019, carried a simple radio astronomy antenna. But the mission was not optimized for cosmological observations, so the experiment was ruined by radio frequency interference from the lander itself. .)

With funding from both NASA and the US Department of Energy, LuSee-Night will be carried to the other side by a private contractor as part of NASA’s nascent Commercial Lunar Payload Services program. The four 3-meter-long antennas, arranged in a cross shape, will attempt to measure “cosmic dawn,” a feature believed to be detectable in the radio spectrum that would reveal the appearance of the universe’s very first stars.

Noise limitation

Even from the peaceful solitude of the moon’s far side, however, LuSee’s measurement of cosmic dawn will be a challenge: the early universe’s signature is 100,000 times weaker than the noise produced by the Galaxy in the same frequency range. It will be crucial 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 through the two-week 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 computers running, must be designed to “fence” any emissions into a limited part of the spectrum, says Bale. “We require that all oscillators 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 can take, including commercial ones, says Bale. If spacecraft are designed to contain radio frequency interference, it could greatly reduce the chances of harming future science experiments.

Melanie Johnston-Hollitt, former director of the Murchison Widefield Array radio observatory in Western Australia, agrees. At Murchison, which will be the Australian site for the giant Square Kilometer Array radio telescope, she helped establish what is probably the world’s largest radio-quiet zone, more than 500 kilometers wide.

Permits are required to bring electronic devices into the site, and “any equipment you bring into that area goes through an additional electromagnetic testing 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 certainty that you can do that with a cubesat,” she says, referring to the small satellites that scientists fear could swarm around the Moon, creating a source of noise.

Still, “suppressing interference to the level necessary to perform precision radio astronomy is incredibly difficult,” says astronomer Andrew Siemion, who leads the Breakthrough Listen search for extraterrestrial intelligence project at UC Berkeley. This work involves looking for signals across 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 experiments will also make the environment more crowded. More than 250 lunar missions are expected over the coming decade from 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 private companies. That would amount to a US$100 billion “moon economy,” according to Northern Sky Research, a consulting firm in Cambridge, Massachusetts. There are also plans to install a lunar satellite navigation system, which could be a source of noise.

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Alanna Krolikowski, a political scientist at the Missouri University of Science and Technology in Rolla, believes scientists should push for international treaties to protect the moon. “There is now widespread recognition that we need governance for this coming lunar renaissance,” she told last month’s conference.

The Artemis Agreement, an international agreement led by NASA, attempts to provide some guidance to help the agencies involved avoid interfering with each other’s missions. But it is mainly designed to serve the needs of the signatory countries; a better way to regulate the moon might be to have rules drawn up by the United Nations Committee on the Peaceful Uses of Outer Space, Krolikowski said. “The window to do that is small — and shrinking.”