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Astronomie - Telescopes on the Moon

21.10.2022

Our future in space relies on settling the Moon and using it as a base to probe the deepest questions in the cosmos

moon-telescope

s it happens, driven by a desire for extraterrestrial tourism and a new frontier for resources, we are returning to the Moon in force. Several countries are involved, largely motivated by commercial prospects. The resulting lunar infrastructure will open the way to building powerful telescopes that will provide new vistas into key questions that have long obsessed humanity. Where did we come from? And are we alone in this vast Universe?

Space exploration is our destiny, but we can only fulfil it, only discover the deepest mysteries of our Universe, by first returning to the Moon. Many commercial activities are already on the drawing board, spearheaded by space agencies and the private sector alike. Mining rare earth elements. Rocket fuel production. Low-gravity manufacturing. And tourism.

The Moon offers dazzling new horizons for leisure and sports activities. Transport for the first decades of human lunar travel will be expensive. But there is a pent-up demand for luxury tourism. Today’s overwhelming demand will be addressed initially with orbital trips around the Moon. Tickets are already being sold by the likes of SpaceX for launches planned within five years. Tourist lunar landings may now seem pure fantasy, but the will is there.

Such a hugely commercial activity, with vast potential returns, is attracting some of the wealthiest entrepreneurs on our planet. Imagine resort complexes. Lunar golf, with mile-long drives. Buggy rides on the lunar soil, or regolith. Vistas of Earth rising at lunar dawn.

Lunar mining may provide an effectively limitless supply of rare earth elements

The Moon will initially be a playground for the super-rich. Their appetite for new forms of tourism seems insatiable. However, access is certain to change over time once low-cost space transport systems are developed. We would establish giant lunar parks for leisure and relaxation. Low-cost housing would be designed to host the necessary support personnel. Mass tourism will have its day. Commercial backing will certainly fund these activities.

That’s just the beginning. As reserves of rare earth elements are depleted on Earth, lunar resources will step up to the task. Lunar mining may provide an effectively limitless supply of them. Rare earth elements are central to present and future technologies. Mining companies will race to address the challenge of lunar extraction. The potential rewards are enormous.

Here on Earth, scientists project that we will exhaust rare earth elements in less than 1,000 years. Yet bombardment of the Moon by asteroids over billions of years has deposited trillions of tons of rare earths on the lunar surface, based on analysis of the Apollo lunar samples. This amounts to 10,000 times the terrestrial reserves.

Rare earth elements are mined on Earth through environmentally polluting operations. This is such a toxic process that extraction is highly restricted. We can limit the inevitable pollution with robotically aided extraction, and lunar launch sites will facilitate ejection of toxic debris into space. Rare earth elements are key to present and future technologies. It will be difficult for mining companies to resist the challenges of lunar extraction. The potential rewards are enormous.

Because they are ideal for production of rocket fuel, lunar and cislunar (between Earth and Moon) environments will serve as launch sites for interplanetary space probes. The needed fuel, in the form of liquid hydrogen and oxygen, would be sourced from ice deposits in cold polar craters. Rocket fuel depots and spaceports are the future. Lunar fuel resources are a key component of interplanetary travel. We will make use of low lunar gravity to launch spacecraft throughout the solar system. Lunar spaceports will eventually serve as gateways to the stars.

Perhaps the most important outcome from our return to the Moon will be an explosion of pure science. We can build huge telescopes on the Moon to peer further back in time than we could ever do from Earth, or even in space. We must look beyond the compelling goals of lunar and even interplanetary exploration along with commercially driven projects to seek answers to the most fundamental questions ever posed by humanity: where did we come from? Are we alone in this vast Universe? Telescopes will eventually provide the answers, but on a scale beyond our current dreams.

Giant telescopes can be constructed in dark lunar craters near the lunar poles, where the Sun never rises. There’s no atmosphere to limit our view. Stars don’t twinkle, they shine as brilliant points of light. Such clarity is crucial if we are to search for distant planetary systems. There are sites with unlimited solar power on the tall crater rims to power our instruments. Here the Sun never sets. Yet there is extreme cold in the deep crater basins that remain in permanent shadow.

From the Moon, we can search throughout the infrared spectrum for the elusive molecular signatures of life. These might involve oxygen and carbon dioxide in planetary atmospheres, and more complex molecular tracers. Photosynthesis generates a characteristic signature, microbes produce phosphine, cows belch methane. Human activity generates pollutants. Nuclear explosions generate radioactivity.

We need to search huge numbers of exoplanets for the elusive signatures of life

Such signatures may be very rare. The conditions for the origin of life are unknown. Based on what we know of the solar system, life is a rare phenomenon. We have no idea how complex organisms might evolve. There are many random evolutionary directions that lead nowhere. Darwinian evolution is often invoked. It seems to have worked on Earth but, for all we know, it could have been an immensely improbable fluke. We might even be alone in the Universe.

The ultimate challenge is seeking signs of intelligent life. Distant civilisations could exist. There are billions of exoplanets in our galaxy. Even if life was incredibly rare, there might well be candidates. Most of these are billions of years older than Earth. Such exoplanets, if inhabited, would inevitably be thousands or even millions of years ahead of us in evolution. And in technology. They would have had so much more time to evolve.

The comforting thought is that any such advanced civilisations could accomplish interstellar travel. However, intelligent life is likely to be rare and relatively short-lived; this we know because we haven’t yet encountered any advanced civilisations and because of the potential existential catastrophes that await us. These span global epidemics to nuclear wars and major asteroid impacts. Since the number of likely targets with signs of life is small, we need to search huge numbers of exoplanets for the elusive signatures of life.

To seek out signatures of life in nearby exoplanets, future space telescopes will focus on spectral features in their atmospheres. But we will need more than spectral coverage if we are to seek robust indicators of life in the Universe.

Above all, we need many targets. Exoplanets with rocky cores and Earth-like masses are preferred. Such relatively low-mass exoplanets are hard to detect. It’s the larger ones we find most easily. And these are gas giants like Jupiters or Neptunes, hardly congenial to life. Exoplanets need to have rocky cores and be in habitable zones around Sun-like stars for conditions required for life as we know it. After all, that’s the only known criterion for life. No guarantees, but we have just the one example, our Earth.

We have detected only a handful of exoplanet Earth twins so far. Unfortunately, we have little idea of how terrestrial life originated. So we need to greatly boost the statistics. Numbers count. And we will need light-gathering power to examine the atmospheric spectrum of our targets.

Is there oxygen? Carbon dioxide? Methane? The list of biological tracers goes on. This means digging deeply into the infrared region of the electromagnetic spectrum. And that requires a really large telescope if we are to see far away.

We can’t do this from Earth, where the available spectrum is highly constrained by our atmosphere. The world’s largest telescope, now in construction in the high Atacama Desert in Chile, has a diameter of 39 metres. But even this won’t get us far into the infrared region where our prime signatures of extraterrestrial biology are to be found.

Space telescopes are one option. However, current plans for space telescopes in the next decades envisage apertures of just tens of metres. It’s simply too expensive to launch larger free-flyers.

We need to build much larger telescopes still.

The Moon offers the only solution. We will be there. Transporters and infrastructure will be in place. Incremental costs to such a megaproject should be tolerable. We will build telescopes hundreds of metres in aperture in permanently dark lunar craters. With no winds and low gravity, there is no technological limit. And there are futuristic ideas on how to build crater-spanning telescopes that are kilometres in diameter.

Huge lunar telescopes will explore the first galaxies and stars in unprecedented detail

With these, we could image the nearest exoplanets, such as those around Alpha Centauri, our nearest stellar neighbour some four light-years away. We could detect oceans. We could detect the night glow of any large cities.

The deeper we can search, the more likely we are to sample varied life-friendly environments. We don’t know what to expect, and we are certain that life is fragile and that life tracers are rare. We need many targets. And this requires monstrous megatelescopes, with enormous light-collecting apertures.

Only by detecting unprecedented numbers of Earth-like planets can we hope to optimise our chances of finding signs of extraterrestrial life. We may finally answer one of humanity’s ultimate questions: are we alone?

Huge lunar telescopes will also explore the first galaxies and stars in the Universe in unprecedented detail. They will investigate the first massive black holes that we see shining as quasars. Such black holes are monstrous objects, weighing millions or even billions of solar masses. They are found in the centres of galaxies. Even our Milky Way galaxy hosts such a monster in its centre.

Which formed first, the galaxy or the black hole itself? Amazingly, we don’t know. We see massive galaxies and black holes appear in the distant Universe, as far back as we can see. A huge black hole can form directly from collapse of a massive cloud of gas. Perhaps the black holes seeded the galaxies as their violent activity triggered star formation. Or did the massive black holes grow only by gathering vast amounts of densely packed stars in the hearts of galaxies?

We don’t know. With large lunar telescopes, we can learn about the dawn of the Universe. We will see the end of the dark ages, before there were stars and the first starlight that heralded a new cosmic age.

The most intimate secrets of the dark ages are best probed with a very different type of telescope, a radio telescope capable of detecting the hydrogen clouds that were the raw material of galaxies. Specifically, we will need a special type of radio telescope operating at very low radio frequencies. And the far side of the Moon is a unique site for a low-frequency radio observatory.

At low radio frequencies, we can look far into the early Universe, answering the fundamental question: where did we come from? Indeed, nearby clouds of hydrogen gas are observed at the easy-to-detect-frequency of 1,420 megahertz (MHz).

As the clouds become more distant, their frequencies decrease dramatically. That is because the expansion of the Universe stretches the wavelength of the light received from distant galaxies toward the red end of the visible spectrum. (Hence we say the light is redshifted.) Those longer, redshifted waves with their lower frequency are too ‘dim’ for the telescopes of today. At such low radio frequencies, the terrestrial ionosphere simply scatters low-frequency radio waves from deep space. Terrestrial radio noise created by marine radars, radio and TV broadcasting, and cellphones all get in the way.

But we won’t have this problem on the far side of the Moon, the most radio-quiet spot in the inner solar system, and the perfect place for low-frequency radio astronomy.

We must go to the lowest radio frequencies. Remember, the Universe is expanding and the energy of photons decreases with time. So detecting hydrogen at low frequency takes us back in time. The expansion of space lowers the frequency of these radio waves to the limits of what is observable. By searching for hydrogen clouds at a frequency of, say, 30 MHz, we peer back to a time long before there were any galaxies. Using telescopes on the Moon, we’ll map their radio shadows and finally start to answer the fundamental question: where did we come from?

The dark ages are our only hope. They are completely unexplored territory

Our astronomers have already explored the fossil radiation from the Big Bang and the cosmic microwave background that has existed from the beginning of time. Sure, we have uncovered the seeds of creation, the fluctuations that seeded galaxies. Yet the data is limited. Ultimately, the signals we tap come from a few million independent points in the sky. We are striving to do better. But we risk running out of information in the microwave sky.

Much more accuracy will be needed to test the greatest question of all – the cosmic origin hypothesis. Did we begin in a Big Bang, causing the Universe to inflate? To test this, we will need to look more deeply and sensitively into the past. This is where exploration of the dark ages can be a game changer – provided that we can extract more information from the sky than current approaches allow.

Already there are planned surveys of galaxies to be conducted by the Nancy Grace Roman Space Telescope and others over the next decade. But we are limited by the number of detectable galaxies.

There is only one way to beef up the precision of cosmology: get more information. With millions of clouds required to form a typical galaxy, the dark ages are our only hope. They are completely unexplored territory. They certainly present an enormous challenge, but they also offer a unique glimpse of the beginning.

The ultimate precision will come when we optimise the information content of our signal. Typical future surveys will target billions of galaxies. But there are so many more remote clouds of hydrogen, the building blocks of massive galaxies. Probing the dark ages will open up a trillion bits of information and allow a huge increase in precision over surveys of all the galaxies in the visible Universe. By moving to the dark ages and focusing on low-frequency radio signals, we can take a giant step forward.

As current theory holds, inflation of the early Universe occurred in the first 10-36 second. Then the Universe was teeming with particles and antiparticles that come and go over immeasurably small times. This creates a sort of effervescent quantum foam in what otherwise is a vacuum. In other words, the vacuum has energy. It is this energy that drives inflation. This phase does not last long; the quantum fluctuations are over as soon as space continues to expand and the matter cools slightly, though a trace is left behind in infinitesimal seeds of future structure. The end of inflation is where the cosmic journey begins.

It’s a compelling story. Yet we have little direct evidence of the beginning. To make progress, we need to probe the final unexplored frontier of the early Universe, the dark ages. This epoch provides our clearest glimpse of the Universe before any stars formed. The gaseous building blocks of galaxies were all that existed in terms of structure. These are seen as fossil shadows against the primordial radio glow from the Big Bang. They are ghosts from the past. And with better telescopes capable of detecting very low radio frequencies, these ghosts can help us probe the end of inflation. This is achievable via telescopes on the lunar far side.

Here is the link that will help us attack the ultimate mystery of inflation. The fluctuations in the hydrogen absorption signal are not totally random. They have some slight asymmetry in their distribution of strengths. The asymmetry amounts to a primordial deviation from the usual bell-shaped curve that describes any random distribution. Inflation predicts tiny deviations from randomness in the primordial density fluctuations. This effect has yet to be measured. But it is a robust prediction, true for all inflation models.

Piggybacking on lunar infrastructure meant for industry and tourism opens up new options for science

In short, we seek a very small radio-wave effect in those ancient clouds. We’ll need a huge increase in our current experimental sensitivity to detect it. Remember, elemental hydrogen is comprised of a single electron orbiting a single proton. The radio waves in question are generated when electrons orbiting their protons flip their spin due to a collision with neighbouring atoms. When in the original position – when electron and proton are aligned – hydrogen has a lower energy signal. When the electron has been knocked out of alignment by the impinging light of the Universe, its spin-flips and the radio frequency is higher. By seeking out the lower frequency of the original cloud, we can detect the shadow of a remote cloud of hydrogen, against the cosmic background radiation. In short, studying the extremely faint shadows imprinted on the radio sky from early times can be done only at very low radio frequencies. A telescope on the far side of the Moon is by far our best bet for achieving that goal.

The discovery potential is vast, from the radio to the infrared and optical domains, and even beyond. As yet, little attention has been given to the unique advantages of a lunar platform for studying the Universe. A standalone giant telescope project is inconceivable for budgetary reasons. Instead, to cover the cost, lunar telescopes should be a key component of future lunar settlements. Piggybacking on lunar infrastructure meant for industry and tourism opens up new options for science. Telescopes built alongside other megaprojects will be a minor overhead, all in all.

Rewards include advances in planetary science and a deep understanding of the origin of the Moon. Astronomers will be able to image distant exoplanets and the first stars. The most extraordinary new frontier will be probing the dark ages of the Universe, just as a geologist studies the origins of progressively older layers of rocks here on Earth. In space, our rocks and fossils are the hydrogen clouds from which the galaxies formed.

In order to achieve this vision, it must be integral to lunar projects from the earliest stages of planning. There is an irrefutable case to be made for science-driven projects integrated into commercial activities. The scale for all these ventures is decades or more, but the time to embrace the intent is now.

What can we achieve with such megatelescopes? We will find out if there are remote planets conducive to life. We will look back to our origins. We will see the primeval dawn of the stars. We will seek out the first monster black holes in the Universe.

We hope to answer humanity’s most fundamental questions: where did we come from? Are we alone? There is a compelling scientific case to be made now for a new era of unparalleled exploration to visualise the edge of the Universe from the surface of the Moon.

Quelle: aeon

 

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