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LUNAR OBSERVATORY

If you want to take the most pristine, unpolluted images of the Universe, your best bet is to leave the Earth behind. Here on our planet, there are all sorts of effects that interfere with our imaging capabilities. Light pollution limits how deep we can see; the atmosphere harms our resolving power and our ability to observe clearly; clouds and weather interfere with our light-collecting goals; the Sun and the Earth itself block our view of large portions of the sky from all terrestrial locations.

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Yet observatories like Hubble, Chandra, Fermi, Spitzer, and more have showcased how remarkably effective a space telescope can be. The views and data they've returned to Earth have taught us more than any similar observatory could have revealed from the ground. So why not put a telescope on the Moon, then? Believe it or not, it's a terrible idea in all ways except one. Here's why.

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The transmittance or opacity of the electromagnetic spectrum through the atmosphere. Note all the absorption features in gamma rays, X-rays, and the infrared, which is why they are best viewed from space. Over many wavelengths, such as on the radio, the ground is just as good, while others are simply impossible.

The Moon, at first glance, seems like the ideal location for a telescope. There's practically no atmosphere at all, which removes all the light pollution concerns. It's far away from the Earth, which should greatly reduce the interference from any signals that humans produce. The ultra-long nights mean that you can observe the same target, continuously, for as long as 14 days at a time with no interruptions. And because you have solid ground to brace yourself against, you don't need to rely on gyroscopes or reaction wheels for pointing. It sounds like a really good deal.

 

But if you start thinking about the way the Moon orbits the Earth, with the entire Moon-Earth system orbiting the Sun, you might start to realize some of the problems that a setup like this would inevitably encounter.

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The Moon takes a little over 27 days to orbit 360º around Earth, a little over 29 days to go from new Moon to new Moon again, but a total of 14 lunar cycles, or 411 days, to go from a full Perigee Moon to a Full Perigee Moon again due to the motion of its elliptical orbit around the Sun. The Earth-Moon-Sun configuration is essential for understanding the implications of building a lunar observatory.

If you place your telescope on the near (Earth-facing) side of the Moon, you will always have a view of the Earth. This means you can send-and-receive signals, control your telescope, and download-upload data in nearly real-time, with only the light-travel-time of signals across space limiting you. But it also means that Earth-produced interference, like radio broadcast signals, will always be a problem you need to shield yourself from.

 

On the other hand, if you're on the far side of the Moon, you shield yourself from everything coming from Earth quite effectively, but you also have no direct path for data transfer or signal transmittance. There would have to be an additional mechanism set up, like a lunar orbiter or a link to a transmitter/receiver on the near side, just to operate it.

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THREE ISSUES

ISSUE 1: LUNAR DUST

The optimum localization for this lunar infrared observatory would be not far from one of the lunar poles. The ecliptic poles are localizations optimized for deep infrared because the infrared sky background from zodiacal dust is minimized.

It is well known that low-level dust can coat the optics, reduce throughput, and damage components. On the other side, high-level dust will elevate the infrared sky background and reduce the projected sensitivity of any instrument optimized for high dynamic observations. Several studies were based on observations done by Apollo 15, 17, and Lunokhod-2 missions, which observed excess of light observing the solar corona just after (5sec) orbital sunset, as shown in Figure 5.

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Figure 5: The ecliptic pole image in UV, the circle shows the six-degree diameter field accessible to the zenith pointing telescope at the lunar south pole. Image recorded on the Moon by Apollo astronauts John Young and Charles Duke (Page, T, and Carruthers, G. R., 1981)

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Figure 6: A sketch of the lunar sunrise seen from orbit by Apollo 17 astronaut Eugene Cernan. On the right, the sketch is highlighted to show the sources of the scattered light: red indicates Coronal and Zodiacal Glow, blue is the Lunar Horizon Glow, perhaps caused by exospheric dust, and green indicates possible "streamers" of light (crepuscular rays) formed by shadowing and scattered light. Credit: NASA [Nasa Ref;].

A detection in-situ measurement of sky brightness was done by Lunokhod-2, a Soviet lunar lander. The Lunokhod-2 measured an unusually high sky background that depended on the zenith angle of the Sun, which is a characteristic signature for a scattering atmosphere. Severny et al. 1975. Any form of lunar dust atmosphere low-level that may coat the optics and plus the high-altitude dust that may produce a strong infrared thermal background that would undermine the quality of the dedicated high dynamic observations goals. In such a way lunar dust atmosphere is a crucial issue that should drive the baseline lunar observatory configuration to facilitate possible dust mitigation. Though the design of each piece of equipment (mechanisms, optics, and detector) sent to the Moon must consider the effects of lunar dust, particular issues arise in the context of an infrared telescope.

ISSUE 2: THERMAL STRAIN AND MASS MINIMIZATION

One of the driver issues to the construction on the Moon is its temperature range. The equator represents the largest temperature variation, up to 280K variations on the Moon. The coldest and constant temperatures occur in the permanently shadowed parts at the poles at about 40K (Aulesa et al, 2000). Meanwhile the “peaks of eternal light” would be considered as useful locations where the temperature is relatively constant at about -50°C, +/-10°C (Bussey et al., 2005). Also, these peaks are located near permanently shadowed craters making available solar power for the most part of the time, and plus it may contain ice water.

The mass is also a major asset for a telescope(s) configuration(s) considering that it must be shipped to and assembled on the moon. The mass is a major cost driver for space expeditions and the fully robotic assembly of a telescope structure system is beyond the present state of the art of robotics.

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The Earth, as seen rising over the lunar limb in a location where the Sun is just barely incident on the Moon's surface. You can tell that this is a photo of the lunar nearside, otherwise, Earth would not be visible at all.

While a space-based telescope can control its temperature through either active or passive cooling (or a combination of both), a telescope must cool down below the temperature of the wavelengths it's trying to observe, or noise will swamp your intended signal. This would be a tremendous drawback for ultraviolet, optical, or infrared astronomy, all of which would have severe problems on the Moon for anything other than the goal of Earth (or Sun) observing.

Engineering a telescope that can survive those temperature extremes and still function optimally is an extraordinary challenge. It's no wonder that the only lunar-based telescope we have, at present, is a UV-telescope on the Moon's near side, at wavelengths where the Earth's atmosphere absorbs almost all of the light.

The concept design of the LUVOIR space telescope would place it at the L2 Lagrange point, where a 15.1-meter primary mirror would unfold and begin observing the Universe, bringing us untold scientific and astronomical riches. Note the plan to shield itself from the Sun, to better isolate it from a broad spectrum of electromagnetic signals. This is far superior to using the Moon as a base. 

NASA / LUVOIR CONCEPT TEAM; SERGE BRUNNER (BACKGROUND)

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For most applications, going to space is going to be a superior option for going to the Moon. The lunar surface, in terms of temperature extremes and difficulties communicating with Earth, offers more drawbacks than having a surface to push against/build on offers.

But there is one very specific application that the Moon offers an unprecedented advantage over any other environment: radio telescopes. The Earth is an incredibly "radio-loud" source, due to both natural and human-made causes. Even in space, the signals that emanate from Earth pervade throughout the Solar System. But the Moon provides a stunning environment for immunity to Earth's radio signals: the far side literally uses the Moon itself as a shield.

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A small section of the Karl Jansky Very Large Array, one of the world's largest and most powerful arrays of radio telescopes. The Moon's far side would be even more isolated, but far more expensive.  JOHN FOWLER

AS COSMOLOGIST JOE SILK WROTE EARLIER THIS YEAR:

The far side of the Moon is the best place in the inner Solar System to monitor low-frequency radio waves — the only way of detecting certain faint ‘fingerprints’ that the Big Bang left on the cosmos. Earth-bound radio telescopes encounter too much interference from electromagnetic pollution caused by human activity, such as maritime communication and short-wave broadcasting, to get a clear signal, and Earth’s ionosphere blocks the longest wavelengths from reaching these scopes in the first place.

We could detect signals of inflation, the early stages of the Big Bang, and the formation of the Universe's very first stars with a lunar radio telescope. While there are hopes for doing this either on Earth or in space, the lunar far side offers more sensitivity, due to being shielded from Earth, than any other option.

Currently, whenever any spacecraft travels behind the Moon as seen from Earth's perspective, it causes what we call a radio blackout. The fact that radio waves cannot pass through the Moon means that no signals can be sent or received during that time period. 

 

Orbiting satellites, any far-side stations or rovers, and even Apollo astronauts all have no means of communicating with Earth with the Moon in the way.

But this also means that they were shielded from all sorts of contaminating radio signals that occur on Earth. GPS communications, microwave ovens, radar, cell phone, and WiFi signals, and even digital cameras are among the many terrestrial sources that contaminate radio observatories. But on the far side of the Moon, all of humanity's sources of interference are 100% blocked. It's the most pristine environment for radio astronomy we could ask for.

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With no atmosphere, no visible views of Earth, and even no Venus, a night on the moon's far side is darker than any night on Earth.  JAY TANNER

As Dr. Jillian Scudder once noted, though, there are drawbacks, too. Data transmission requires something like an orbiter that can link with both the Earth and the telescope. A telescope or array of radio telescopes must be constructed and deployed on the Moon and linked together if we go the array route. (Which is greatly preferred.) Alternatively, cables could be run to the near side for transmission back to Earth.

 

But perhaps the greatest prohibitive element is cost. Transporting material to the Moon, landing on the lunar surface, deploying it and more is a tremendous undertaking. Even the most modest proposal, a Lunar Array for Radio Cosmology (LARC), consists of more than a hundred simple-design antennas spread out over a two-kilometer range. It would come with a price tag, just for that, in excess of $1 billion, comparable to the most expensive radio arrays ever built on Earth.

This shows a particular antenna design LUNAR is investigating. The black X's on the arms of the antennae are the photon-collecting dipoles. The yellow arm is made of an extremely thin sheet of Kapton film. The dipoles are connected by an electric transmission line to the central hub, shown in purple. This hub transmits the data back to Earth.  NASA / LUNAR UNIVERSITY NETWORK FOR ASTROPHYSICS RESEARCH / UC BOULDER

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For almost every conceivable application to astronomy, going to the Moon is a vastly inferior location than simply being above the Earth's atmosphere. The temperature extremes experienced everywhere on the Moon are an extraordinary challenge over and above any benefit you get from being on the Moon's surface. Only in radio frequencies do the benefits of being on the lunar far side offer an opportunity for observing that we cannot get from either terrestrial or space-based observing.

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