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.
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)
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.
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)
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.
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.
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
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.