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 farthest flung objects in the universe from the far side of the Moon. So, forgive me if I am a bit skeptical about all the recent talk about lunar-based observatories.
These are not new ideas. But even so, I’m heartened to see that renowned astrophysicist Joseph Silk has written a book imploring us to use the Moon as a new base for observational astronomy.
In “Back to the Moon: The Next Giant Leap for Humankind,” just out from Princeton University Press, Silk, a 2019 Gruber Prize in Cosmology winner, spends the first part of his book reviewing plans for lunar exploration. For those who don’t follow astronomy and space it will be helpful. But for those who read the daily science pages of the major media outlets, it may seem a bit repetitious.
Yet Silk, a Professor of Astrophysics at the Institut d’Astrophysics de Paris (IAP), hits paydirt when he writes: “The last frontier in astronomy is exploring the dark ages, before the faint glimmerings of the first light in the Universe. Pristine clouds of hydrogen are building blocks of the future as well as direct witnesses of the past.”
Silk notes that the Moon’s low gravity will enable the engineering of enormous telescope structures that should outperform their terrestrial and space-based counterparts.
This may be true, but to implement the kind of lunar hardware necessary to make it happen would require a sea change in current lunar utilization.
As I noted in a 2003 article in Discover magazine, “NASA has been enticing scientists with talk about [crewed] observatories on the Moon for nearly 40 years.” Some were even willing to make a one way trip to set them up. Today, such lunar observatories would be set up robotically or by using a combination of human astronauts and robotic rovers.
With this week’s successful launch of Artemis 1, lunar astronomy advocates do have renewed hope.
Today, the commercial backdrop opens a highly subsidized window for doing science on the Moon, Silk writes in his book. We can make a compelling case for leveraging telescope construction, which is relatively cheap and opens up new vistas of human exploration, he notes.
One of Silk’s most powerful arguments for astronomy from the Moon is in tapping into a vast low-frequency array from a far side crater, where there is no Earth-related radio interference and no Earth ionosphere. It’s here, he writes, that such a low-frequency telescope would target “the dark ages,” —- the early cosmos’ dim shadows where there was no light and no stars, but vast numbers of cold gas clouds were everywhere.
Using millions of simple radio antennas, deployed over an area that is 100 kilometers wide, the idea is to map remote hydrogen clouds against the backdrop of the cosmic microwave background in hopes of finding what Silk terms tiny distortions in their distribution.
“We need to study these feeble signals to learn how inflation proceeded in the first trillionth of a trillionth of a trillionth of a second after the beginning,” Silk writes. “A radio array able to capture these data would use millions of simple radio antennas deployed over an area that is 100 kilometers or more across on the Moon’s far side.”
Another option, Silk writes, is to build a monolithic radio telescope in a large far side crater basin and fill it with wire mesh. He notes that would still enable astronomers to probe the universe at wavelengths of only tens of meters which are optimized to tune into the dark ages.
For high-resolution infrared astronomy, Silk notes that well-known French astronomer Antoine Labeyrie has already advocated filling an entire permanently dark, extraordinarily cold, polar crater “with an array of mounted five-meter infrared mirrors to develop a parabolic bowl configuration.” The idea, as Silk describes, would be to focus an infrared signal from a given target to produce a single image.
Such a 10-kilometer aperture lunar hyperscope could image extrasolar oceans and continents that are 1,000 kilometers across, Silk notes. In his book, he also argues that only a megatelescope on the Moon could provide the kind of high-resolution infrared imagery needed to characterize the potentially millions of habitable exoplanets out there.
“Instead of surveying a handful of planets, a sample so small that no epidemiologist would ever consider this size appropriate for a statistical sample of a rare disease, a lunar telescope opens up the horizons,” Silk writes. “The target volume for a 300-meter telescope is billions of cubic light-years. This means that up to one million habitable exoplanet targets would now be accessible.”
Indeed, Silk does a great job of summing up why the Moon should be important to humanity as a platform for science of all sorts. But the crux of “Back to the Moon” is really Silk’s clarion call for humanity to actualize its astronomical potential.
Not an hour passes without millions on this planet looking up and wondering what it’s all about. Tentative answers to many of our civilization’s oldest philosophical questions are within our reach. They include details about how the cosmos began and grew; the birth and death of our own solar system; and whether our planet and life itself is rare or ubiquitous.
As a platform for astronomy and planetary science, the Moon is ready and waiting. But we continue to squander the opportunities it offers us all. In his timely book, Silk makes a strong and important case for a robust return to the Moon for science. But a half century after the last Apollo moonwalkers left the Taurus Littrow Valley, we have yet to return. And despite the best laid plans for future lunar exploration, I remain doubtful that we will ever build the astronomical far side wonderlands that Silk so artfully describes in “Back to the Moon.”
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