A 100 meter rotating liquid mirror telescope on the moon? Sure, please.

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In the years to come, some really great next-generation telescopes will be collecting their first light. Between space telescopes like James Webb and Nancy Grace Roman and ground-based telescopes like the Extrem Large Telescope (ELT) and the Giant Magellan Telescope (GMT), astronomers can study aspects of the universe that were previously inaccessible.

For example, there are Population III stars, which are the first stars to form in the universe. These stars cannot be seen in visible light, and even next generation devices (such as those mentioned above) cannot see them. According to a team led by NASA Hubble Fellow Anna Schauer, the solution could be to build what she called the “Ultimate Large Telescope” (ULT) on the moon.

This idea, put on hold by NASA a decade ago, was introduced by Schauer and her colleagues at the University of Texas at Austin in a recent article due for publication in an upcoming issue of the Astrophysical Journal. It requires a liquid mirror telescope with a diameter of 100 m (~ 330 ft) and a solar system that works autonomously on the lunar surface and forwards data to a satellite in orbit.

Artist's impression of the Lunar Liquid-Mirror Telescope (LLMT) Photo credit: Steward Observatory / University of Arizona

Professor Volker Bromm, theoretician at UT Austin and co-author of the paper, has been studying the first stars to form in our universe for decades. As he recently stated in a recent press release from the McDonald Observatory (which is overseen by UT Austin):

“In the course of the history of astronomy, telescopes have become more and more powerful, so that we can examine sources from ever earlier cosmic times – ever closer to the Big Bang. The upcoming James Webb Space Telescope (JWST) will arrive at the time when galaxies first formed. "

“The theory, however, predicts that there was an even earlier time when galaxies didn't exist, but when individual stars formed – the elusive Population III stars. This "very first light" moment is beyond the capabilities of the powerful JWST and instead requires an "ultimate" telescope. "

The current scientific consensus is that Population III stars formed a few hundred million years after the Big Bang (over 13 billion years ago). These stars were different from today's, they consisted of hydrogen and helium and were comparatively short-lived (several million years). Heavier elements formed in the cores of these giant stars and became part of their outer layers, which were then blown off when these stars died.

Artist's impression of a spaceship taking off from a moon base. Image Credit: SpaceX

This process enabled the formation of heavy metals and silicates that would enable the planets to form. This also meant that all subsequent generations of stars had higher levels of metal (also known as metallicity), which astronomers use to determine the age of the stars. By studying Population III stars, astronomers would reveal a lot about the evolution of our universe.

Unfortunately, these stars formed during what is known as the cosmic “dark age,” when the universe was filled with clouds of gas that obscured visible and infrared light. This makes Population III stars invisible except for portions of the near-infrared and radio spectrum that are currently inaccessible to even our most advanced instruments.

Fortunately, calculations by Schauer and her colleagues show that a liquid mirror telescope that works from the surface of the moon can examine these stars. The concept, originally known as the Lunar Liquid-Mirror Telescope (LLMT), was first proposed in 2008 by a team led by Roger Angel, the regents professor of astronomy and optical sciences at the University of Arizona.

After NASA examined this proposal shortly afterwards, it decided not to pursue the project any further. According to Niv Drory, a senior scientist at the McDonald Observatory and co-author of the paper, supporting science about the earliest stars did not exist at the time. However, later studies on stars of Population III and NASA's plans to return to the moon (Artemis project) make this proposal possible again.

https://royalsociety.org/-/media/events/2020/03/astronomy-moon/Observatory_on_the_moon-web.jpg?w=400Artistic concept of astronomy on the lunar surface. Photo credit: NASA

Similar to the LLMT, the ULT would rely on liquids rather than coated glass (which makes transportation to the moon much cheaper. One type of liquid would be placed in a rotating vat while a second metallic liquid (such as mercury, which reflects) The tub would rotate continuously to keep the surface of the liquid in the correct parabolic shape and act as a mirror.

Similar to what NASA, ESA, China and other space agencies are planning – building a lunar base in the South Pole Aitken Basin – the telescope would be stationed in the polar regions of the moon (north or south). In one of the many permanently shadowed craters in these regions, the ULT would be free of radio or atmospheric interference.

In addition, it could continuously stare at the same patch of sky and collect as much light as possible in the near infrared spectrum. As Bromm summarized:

“We live in a universe of stars. How star formation began early in cosmic history is a key question. The appearance of the first stars marks a crucial transition in the history of the universe as the original conditions of the Big Bang gave way to increasing cosmic complexity and finally breathed life into planets, life and intelligent beings like us.

“This moment of first light is beyond the capabilities of current or near future telescopes. It is therefore important to think about the “ultimate” telescope capable of directly observing these elusive first stars at the edge of time. "

In this illustration, an astronaut carefully descends the ladder and carefully places her foot on the moon. Photo credit: NASA

The Ultimate Light Telescope is one of many proposals for a lunar observatory. For example, several recommendations have been made for the stationing of radio observatories on the other side of the moon. The absence of interference from terrestrial sources would be ideal not only for observing the invisible parts of the cosmos, but also for finding extraterrestrial intelligence.

In addition, Dr. Karan Jani and Prof. Abraham Loeb from the Laser Interferometer Gravitational Observatory (LIGO) and the Harvard Smithsonian Center for Astrophysics (CfA) recently suggested that the moon should also be an ideal place for gravitation -wave moon observatory for cosmology (GLOC ).

NASA also plans to launch a small satellite called the Dark Age Polarimetry Pathfinder (DAPPER) into lunar orbit in the coming years. This joint project by UC Boulder and the National Radio Astronomy Observatory (NRAO) will build on previous work by the Wilkinson Microwave Anisotropy Probe (WMAP) to explore the early universe.

These and other suggestions will be implemented in the years to come. In addition to returning astronauts to the moon by 2024, the long-term goal of the Artemis project is to set up a program for “sustainable lunar exploration”. Once this infrastructure is in place, permanent facilities can be built to help advance the science of space exploration!

Further reading: McDonald Observatory, arXiv

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