The moon’s far side marks the final frontier for astronomy, says cosmologist at the Big Bang

At least two decades ago, I found myself in the office of one of NASA’s leading optical engineers talking about the prospect of observing the most distant objects in the universe from the far side of the moon. So, forgive me if I’m a little skeptical about all the recent talk about lunar-based observatories.

These are not new ideas. But even so, I am encouraged to see that the famous astrophysicist Joseph Silk has written a book demanding that we use the Moon as a new base for observational astronomy.

In “Returning to the Moon: Mankind’s Next Giant Leap,” just out of Princeton University Press, Silke, winner of the 2019 Gruber Prize in cosmology, spends the first part of his book reviewing plans for lunar exploration. For those who do not follow astronomy and space, it will be useful. But for those who read the daily science pages of the mainstream media, it may seem a bit repetitive.

Nevertheless, Silk, professor of astrophysics at the Astrophysical Institute in Paris (IAP), smacks of warmth when he writes: “The last frontier of astronomy is the exploration of the dark ages, before the faint flashes of the first light in the universe. The original hydrogen clouds are the building blocks of the future in addition to being direct witnesses to the past.”

Silk points out that the moon’s lower gravity will enable the engineering of massive telescope structures that should outperform their terrestrial and space counterparts.

This may be true, but to implement the kind of lunar hardware needed to achieve this would require a radical change in the current use of the moon.

As I noted in a 2003 article in Discover magazine, “NASA has been tempting scientists to talk about it [crewed] Observatories have been on the Moon for nearly 40 years.” Some were willing to take a one-way trip to prepare them. Today, such lunar observatories will be set up automatically or using a combination of human astronauts and robots.

With this week’s successful launch of Artemis 1, advocates of lunar astronomy have renewed hope.

Today, a commercial background opens a heavily subsidized window for doing science on the moon, Silk writes in his book. We can make a convincing case for the benefit of building a telescope, which is relatively cheap and opens up new avenues for human exploration, he notes.

One of Silk’s strongest arguments for lunar astronomy is to take advantage of a broadband, low-frequency array from a distant crater, where there is no Earth-bound radio interference and no Earth’s ionosphere. Here, he writes, such a low-frequency telescope would target the “dark ages”—the dim shadows of the early universe where there was neither light nor stars, but vast numbers of clouds of cold gas everywhere.

Using millions of simple radio antennas, spread over an area 100 kilometers wide, the idea is to map distant hydrogen clouds against the cosmic microwave background in hopes of finding what he calls silks of tiny distortions in their distribution.

“We need to study these weak signals to see how inflation proceeded in the first trillionth of a trillionth of a trillionth of a second after the onset,” Silk wrote. “A radio array capable of capturing this data would use millions of simple radio antennas deployed over an area 100 kilometers or more across on the far side of the moon.”

Another option, Silk writes, is to build a monolithic radio telescope in a large, off-side crater trough and fill it with wire mesh. He points out that this would still enable astronomers to probe the universe at wavelengths of only tens of metres, which are optimized for the dark ages.

For high-resolution infrared astronomy, Silk notes, well-known French astronomer Antoine Lapérie has already advocated filling a very dark and cold polar crater “with an array of five-meter infrared mirrors to develop a parabolic bowl configuration.” The idea, Silk describes it, is to focus an infrared signal from a specific target to produce a single image.

Silk notes that such a lunar telescope with a 10-kilometer aperture could image oceans and extrasolar continents 1,000 kilometers across. In his book, he also argues that only the Massive Telescope on the Moon can provide the kind of high-resolution infrared imagery needed to characterize the millions of habitable exoplanets out there.

“Instead of surveying a handful of planets, a sample so small that no epidemiologist would consider this size appropriate for a statistical sample of a rare disease, the Lunar Telescope opens horizons,” Silk wrote. “The target size for the 300-meter telescope is billions of cubic light-years. This means that it will now be possible to hit up to 1 million targets for habitable exoplanets.”

Indeed, Silk does a great job of encapsulating why the Moon is so important to humanity as a platform for science of all kinds. But the essence of “Back to the Moon” is really Silk’s unmistakable call for humanity to realize its astronomical potential.

Not an hour goes by without millions on the planet looking up and wondering what it is all about. Principled answers to many of our civilization’s oldest philosophical questions are within our grasp. They include details about how the universe originated and grew; the birth and death of our solar system; and whether our planet and life itself is rare or ubiquitous.

As a platform for astronomy and planetary sciences, The Moon is ready and waiting. But we keep squandering the opportunities it offers us all. In his timely book, Silk makes a strong and important argument for a solid scientific return to the moon. But half a century after the last Apollo moonwalkers left Taurus Valley, we’re not back yet. And despite the best laid plans for future lunar exploration, I remain skeptical that we’ll ever build the astronomical wonders on the far side that Silk so skillfully describes in “Return to the Moon.”

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