The Eyes of Diana
Throughout the last century, astronomers have been building bigger and bigger telescopes, allowing them to see the heavens in ever greater detail. But even as their instruments have become capable of finer and finer resolution, they have been hampered by the steady increase in light pollution of the Earth's night sky. Cities have grown larger to the point where they make the sky glow for miles around. Air travel has become so widespread that a long exposure in image of a distant galaxy can be ruined by passing airliner. And satellites have become so numerous that they, too, have become a nuisance.
As we reach for the stars, we are creating a few of our own. With its solar panels fully deployed, the international space station is already one of the brightest objects in the sky. As we build other structures, we will gradually obscure more and more of the sky.
But we need not feared the prospect of armed bands of astronomers banging down our doors. Our upward aspirations put us in an ideal position to help the Earth's astronomers.
Earth-based astronomy is limited, even with recently developed technology to "subtract" the atmosphere from the telescope's image mathematically (especially when such technologies are used to the very limits of their resolution to prop up an ailing theory, even though the phenomena observed are millions of times too small to have any effect on the theory). The Hubble space telescope is a huge improvement over ground-based astronomy, but even it has its limitations. Hubble orbits at an altitude of about 600km, and makes a complete orbit every 97 minutes (NSSDC). It is therefore unable to focus on any object near the plane of its orbit for more than about 45 minutes before experiencing atmospheric interference. The procession of its orbit means that most of the sky can be observed in a long exposure, but for phenomenon like distant supernovae, you don't want to have to wait a few months for a favorable observation position.
Hubble's other problem is that, in its low orbit, it is technically within Earth's atmosphere. Though it is capable of much finer resolution than any land-based telescope, observations of extremely distant objects, or small objects, like planets around nearby stars, are limited. Being a reflecting telescope, Hubble's resolution is also limited by the reflectiveness of its mirrors. That is why it is unfortunate that its mirrors are covered with an anti-reflection coating. Because it was assembled on Earth, and because, in its orbit, it still needs to withstand exposure to the tenuous atmosphere, Hubble's mirrors are coated with magnesium fluoride, used as an anti-reflection coating in many optical applications.
The Hubble telescope is also low enough to have its vision obscured by the Van Allen radiation belts. Earth's magnetic field carries a steady electrical current, flowing along the magnetic field lines. When the charged particles of the solar wind, or a solar flare, hit Earth's magnetic field, the electrons carrying the current become excited, and release radiation. This radiation, while quite nearly transparent, is enough to interfere with the limits of Hubble's resolution.
Even with all of these limits, the advent of the Hubble Space Telescope era has been described by various scientists as "having a cataract removed." The next step will be like opening our eyes for the first time. For this great observational lead, we will enlist the help of Gaia's daughter, Diana.
Earth's daughter world orbits about 385,000 kilometers away, oscillating 5.5 degrees either side of the ecliptic. This distance puts it far above the Earth's atmosphere and magnetic field. What passes for an atmosphere on the moon is a barely measurable sheath of gas one trillionth as dense as Earth's atmosphere. This gives the moon a tremendous advantage over low Earth orbit, where the Hubble telescope must contend with atmospheric gases, atomic oxygen, and any number of chemical, particle, and radiation effects.
Luna's greatest advantage over Earth is the length of its day. Earthbound telescopes have a very limited amount of time in which they can observe a particular object, several hours at most. The Hubble telescope is able to see objects toward the poles of its orbit for longer periods, but it relies on attitude rockets to adjust its viewing angle, and while these move the scope quite slowly, their sudden thrusts would destroy the sharp focus of a long exposure. On the moon, the day lasts a little over 27 Earth days (the same length as a month, as the moon always keeps one face toward Earth). An object rising on one lunar horizon will be in view of moon-based telescopes for 13 days before it sets. But we needn't be limited to a paltry 13 day exposure, either. At the lunar poles, objects never set. Anything now visible from the lunar poles will always be visible.
The ideal location for lunar telescopes is in craters at the poles. The crater rims will shield the telescope from any visible light from the Earth or the sun, as long as the rim is at least 5.5 degrees above the telescope mirrors. This placement, of course, will leave parts of the sky invisible, specifically an expanding disk 11 degrees wide in the mid-latitudes of the lunar sky. Placement of three more telescopes spaced evenly around each 80th parallel will provide complete coverage of the celestial sphere, with only minor interference to the lower latitude telescopes.
Other lunar locations serve other purposes. Telescopes on the far side, completely shielded from Earth's interference, would be ideal for observing the planets, although observatories spread around the equator would be needed to eliminate the obvious blind spot. The near side, of course, would be an ideal location for observing Earth and its magnetic field.
The moon's advantages are such that we need not build large telescopes to realize good results. In fact, the first telescopes on the moon will be based on the small Cassegrain reflectors used by backyard astronomers. The obvious additions to them will be a digital camera, a tracking system, and antennas for remote operation. With the moon's various advantages, an eight-inch Cassegrain could probably take a better long exposure image of the distant galaxy than even King Keck (the largest telescope on Earth, in Hawaii). These first small telescopes will probably not be taken to the moon on special missions, but will be piggybacked on larger missions, possibly within a decade or so.
Our large telescopes can have an open reflector format with CCDs both at the prime focus, above the scope, and Cassegrain focus, below the center the primary mirror. On Earth, telescope mirrors are constructed of glass, because it sags under its own weight less than most metals do. On the moon, with its lower gravity, we may be able to build our mirrors from aluminum; after polishing, no tricky coating process would be necessary.
Even if we do need to coat the mirror, that process would be much easier than it is on the Earth. When Hubble telescope's primary mirror was built, a special vacuum chamber was constructed. The coating process had to happen very precisely. Once the aluminum vapor was on the mirror, a layer of magnesium fluoride was immediately added to prevent the atmosphere in the vacuum chamber, however rarefied, from damaging the aluminum finish. Since the moon's "atmosphere" is many times thinner than that in the best Earth-based vacuum chamber, a mirror-coating facility would be much easier to construct, and we might not even need a magnesium fluoride coating, with its antireflection properties.
Our telescopes will gradually increase in size, from the first self-contained reflectors, through large, open-construction reflectors, until we build huge, multi-mirror reflectors, covering entire crater floors, the prime focus suspended from towers on the rim, as with the Arecibo radio telescope in Puerto Rico.
The placement of the first telescopes will be affected by lunar infrastructure, but as time goes by, lunar infrastructure will be affected by the placement of our large telescopes. The first communities on the moon will be mining settlements, which will be joined by rover routes. It will not be ideal to place telescopes close to the mines, as the minor seismic activity caused by mining operations will jiggle them, destroying their sharp focus. Therefore the first telescopes will be placed a certain distance away, near the rover routes.
By the time we build large crater-based telescopes, a lunar rail system will be well under development, and rail links will connect our observatories with other facilities. Laws will be established to insure that mines must be dug far enough from the observatories to prevent seismic disturbances. Soon enough, rails built specially to provide transportation to observatories will see development at favorable locations beside him. Of course, anyone will be able to download images from the telescopes from anywhere on the moon or the Earth, one of the features of the Inter(planetary)net.
Even with all of their advantages, lunar telescopes will not be without their problems. While the moon does not have an atmosphere, is covered with extremely fine dust that is easily thrown into the air by footsteps, micro-meteorites, even bright sunlight. Our telescopes must be mounted high enough to be above most of this dust, and should have shields on their sides to protect against obliquely thrown dust. When dust does get on a mirror surface, the mirror should be raised to its highest angle, and the dust blown off with the gentle puff of gas. Even with these precautions, the dust will eventually scratch the mirrors to the point where they need to be re-polished. This is another advantage to the technology of building the mirrors from aluminum. When it becomes scratched, in need only be re-polished, and need not undergo a far more complex re-coating job.
The moon's lack of atmosphere actually causes a problem. With no atmosphere to distribute heat, the temperature of anything on the lunar surface varies by several hundred degrees from day to night. The expansion caused by this temperature change could seriously damage our telescopes. To minimize this expansion, our telescopes should be made highly reflective on all surfaces, reducing the amount of absorbed radiation. In addition, a heat sink, made of a highly heat conductive metal (such as aluminum), should be inserted deep underground, where it can give the telescope's absorbed heat to the lunar soil.
The moon's greatest advantage, in terms of the light-gathering ability of our telescopes, also gives us are greatest technical obstacle. Earth-based telescopes use extremely fine reduction gears to track the sky at a rate of one revolution every 24 hours. Lunar telescopes will have to track sky at a rate of one revolution every 30,026 hours, requiring the design of even finer reduction gears. To avoid the necessity of microscopic gears to move a huge telescope, the final gear ring could be made the size of the entire telescope array, but this would only work for large telescopes. Alternatively (or additionally), the telescope could be floated on liquid helium. Once it is started moving on such a super-fluid base, it would keep moving very smoothly at the same speed almost indefinitely. Of course, this would complicate the problem of heat dissipation to an extreme.
Whenever problems arise, the eyes of Diana will be the most powerful telescopes we have ever used, and will be a similar (or possibly greater) observational advance to when Galileo turned his first telescope skyward. In fact, one would be tempted to say that lunar telescopes will allow us to see the very fringes of the universe. After reading my article "Big Bang: You're Dead!" However, one should instead look forward to seeing beyond the edge of the presently visible universe.
We should not expect crystal-clear seeing to Jupiter and beyond the infinite, though. The farther we look into space, the more fog we have to see through. Even without the Earth's magnetic field, extremely close observations of other planets, especially Jupiter, will be hampered by their magnetic fields. Beyond the planets, we will be looking through the solar magnetic field, than that of our galaxy. The celestial Rayleigh scattering caused by all of these magnetic fields will limit even Diana's vision.
Diana's observations of the edge of the universe will be spectacular, but of greater immediate importance will be close observations of nearby star systems. When we look at the planetary region of a Sun-like star, perhaps only 30 light years or so from us, and see the unmistakable spectrum of an oxygen atmosphere, even the most guarded skeptic will moderate his position regarding our potential neighbors.
(no author) NSSDC Master Catalog Display: Spacecraft Launch: HST. NASA.
The Eyes of Diana, copyright 1996 by George Beckingham