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A team of astronomers is reviving an idea that NASA put aside ten years ago, in which a huge observatory on the Moon would have been installed. Dubbed the Ultimately Large Telescope, the facility would easily outperform any other telescope in its class and spot objects predicted by theory but never seen.
A large liquid mirror telescope installed on the lunar surface could perform a task that no other telescope can: search for signs of the first stars in the universe. Even the very powerful James Webb Space Telescope, which is due to launch on October 31, 2021, will not be able to see the first stars.
Such is the thesis of astronomers from the University of Texas at Austin, who have detailed their topic in an article to be published in a future issue of the Astrophysical Journal (a pre-press is currently available at arXiv).
Conceptual image of the ultimately large telescope. The version proposed by the authors of the new study would be five times larger than this one. (Image: Credit: Roger Angel et al./University of Arizona)
The concept dates back to 2008, when a team of astronomers from the University of Arizona proposed the liquid mirror lunar telescope. NASA flirted with the idea shortly thereafter, but eventually dropped the concept due to the paucity of relevant science having to do with Population III stars, the first stars to appear in the universe. A telescope on the moon would be able to peer into space without being hindered by atmospheric effects and light pollution.
“Over the course of the history of astronomy, telescopes have become more powerful, allowing us to probe sources from later cosmic epochs, ever closer to the Big Bang,” Volker Bromm, a co-author of the article, said in a statement from University of Texas McDonald Observatory. “The next James Webb Space Telescope will reach the point where galaxies first formed.”
The problem is that JWST – however powerful it may be – will not be able to detect smaller and fainter objects that existed prior to the formation of galaxies, namely Pop III stars. The “very first light” moment is beyond the capabilities of even the mighty JWST, and instead it needs a ‘definitive’ telescope, “said Bromm.
Pop III stars appeared a few hundred million years after the Big Bang, after being generated from a mixture of hydrogen and helium. The theory suggests they were anywhere from tens to hundreds of times larger than our Sun, but even then, it falls short of the size and brightness of an entire galaxy. As such, Pop III stars have escaped detection.
That said, the authors of the new study, led by NASA colleague Hubble Anna Schauer, argue that Pop III stars should be detectable. We just need to identify their spawning sites, which should take the form of “minihalos”. The first stars were born within tiny proto-galaxies, but the brightness of these objects is “too dim to be detected by even the longest exposure times,” as the authors write in their paper.
“Our neighboring galaxy Andromeda has about a trillion stars and we can only see it with the naked eye in very dark places on Earth,” Schauer explained in an email. “These first tiny galaxies have 10 to 1,000 stars and are much farther away: it took more than 13 billion years for light to reach Earth. Both factors play together and we expect minihalos to be around 100 trillion times weaker than Andromeda. “
By studying Pop III stars, we can study conditions in the early universe, he added.
“In the early universe, before those first stars emerged, visible matter was composed only of hydrogen and helium. Stars are needed to “reproduce” higher elements, such as oxygen and carbon, which are fundamental to life, “said Schauer.” We are running computer simulations to better understand Pop III stars, but we are not yet sure how massive and large were these first stars and whether they formed in larger or smaller clusters. These questions could be answered with observations “.
This is where a lunar observatory can help. And indeed, Schauer and his colleagues checked the numbers, finding that a sufficiently large mirror telescope on the lunar surface should do the trick. Niv Drory, a co-author and senior researcher with the McDonald Observatory, said the proposed Ultimately Large Telescope is “perfect” for the challenge.
Located at the north or south pole of the moon, the stationary mirror would measure 328 feet (100 meters) in diameter. The telescope would be autonomous and powered by a nearby solar power plant. The observatory would transmit the data to a satellite placed in lunar orbit.
The telescope mirror would be made of liquid, as opposed to glass (this solution is lighter and cheaper in terms of transportation). The mirror should continuously rotate to keep the liquid surface in a parabolic shape. A metallic liquid would comprise the top layer of the mirror to provide the necessary reflectivity. To prevent excessive heat from ruining the show, the telescope would have been built inside an impact crater and placed inside a perpetual shadow.
As the authors write, however, “it is not clear what effect moon dust would have on the instrument and observations.”
Peering into the cosmos, the Ultimately Large Telescope would fix on a single portion of the sky to absorb as much light as possible, as it searches for minihalos in the near infrared and extreme redshifts (objects whose light is very red shifted, i.e. wavelengths have been stretched by the expansion of space – they are further away, and the farther we look into space, the deeper in time we will see). As the authors note in their article, minihalos should produce a distinctive signature, so they should be “uniquely identified”.
Sure, we wouldn’t be looking directly at Pop III stars, but we would be looking at their training sites – a kind of smoking gun for their existence, and certainly for the best of it.
Looking ahead, Schauer is thrilled with the launch of JWST, which will allow scientists to study the early universe, including the first generation of stars to appear after Pop III star formation.
“For the future, I hope that both theorists and observers will work together to further develop the technology for this lunar telescope,” he said. “I also hope humans return to the moon, to potentially create a site where the ULT could be built.”
This is not the only proposal to build a large telescope on the moon. NASA roboticist JPL Saptarshi Bandyopadhyay outlined his vision for a lunar observatory, which would be built inside a large impact crater. Unlike the Ultimately Large Telescope (which will search for infrared light sources), the Lunar Crater Radio Telescope would be an ultra-long wavelength radio telescope capable of detecting some of the fainter and more distant signals traveling through space. The Bandyopadhyay project is currently in the first phase of NASA’s Innovative Advanced Concepts (NIAC) program.
Maybe someday we will see a NASA NIAC phase one for the Ultimately Large Telescope. The first stars are getting impatient.
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