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Thursday, April 29, 2010

Where on the Moon?

There has been a lot of talk about building a base on the Moon. Where on the Moon would be the best place for it? Real estate on the Moon is pretty cheap right now, but there are other considerations.

If your main purpose is to put a telescope on the Moon, then you probably want to put it on the far side. (Like most moons in the Solar System, our Moon is tidally locked in its orbit, so it always keeps the same side facing the Earth. So the Moon has a near side and a far side, relative to the Earth.) This would shield the telescope from light pollution, coming from Earth. That’s especially important for a radio telescope, because Earth transmits a lot of radio noise.

Unfortunately, for telescopes, the Sun also emits a lot of energy in the visible and radio parts of the spectrum, and since the Moon rotates on its axis every 28 days, your telescope will be in sunlight for a 14-day period every 28 days. You can’t expect to be shielded from both the Earth and the Sun all the time.

The problem with being on the far side is that you don’t have a direct line of communication back to Earth. You would have to rely on a system of lunar satellites. Or perhaps a very long cable, running to an antenna on the near side. We routinely run cables along the ocean floor between continents, so this shouldn’t be impossible. Of course, a cable many kilometers in length would probably be too massive to bring to the Moon from Earth, so it will have to be manufactured on site, from lunar materials. Fortunately, there is plenty of iron on the moon; more in some places than others!

How far from the dividing line between near side and far side do the telescope and the antenna have to be in order to function properly? We need more research on this. One thing to keep in mind is that, due to wobbles in the rotational motion of both the Earth and Moon, this dividing line moves back and forth as much as 8°. That’s about 240 km at the lunar equator.

If you’re going to put a radio telescope on the Moon, you are going to be tempted to use multiple antennas, as far apart as possible. So you are going to need an even more extensive system of cables, or a satellite network in lunar orbit.

There is also the problem of dust. As small as 20 nm (billionths of a meter), the lunar dust is much finer than sand, and it could cause problems for lenses and mirrors, and anything with moving parts. Even though there is no wind to blow the dust around, the dust will be affected by the slightest electrostatic charge on equipment. So you will want to find a location that is as free of dust as possible. We need more reconnaissance on this. Recent studies have shown that the lunar dust settles down during lunar night, but at sunrise, some of the dust particles become ionized, and they are so small that they can become airborne. No, airborne is not the right word – but the dust particles float around and mingle with the very tenuous lunar atmosphere.

Solar collectors and communications/radar antennas won’t be as affected by the dust as optical and infrared telescopes. Light with wavelength significantly less than the dimensions of the dust particles can easily move past individual particles, except in the area that is actually blocked by the particle. A speck of dust doesn’t block a very large area. However, if the dust piles up on the collectors and antennas, it will have to be swept off regularly.

The top layer of the Moon’s surface is called regolith. It is 4 to 10 meters deep, and is made up of partly or mostly pulverized rock. The rock has been bombarded for millions of years by micrometeoroids, cosmic rays and sunlight, causing what is known as space weathering. It is also mixed up occasionally by larger impacts, in a process called gardening. As a result, the lunar regolith contains many substances which may be valuable enough for mining. However, it would not provide much support for underground living quarters. For that, you would need to dig into bedrock. Lunar bedrock is exposed in a few places, such as steep walls of crater rims, and in old lava channels.

Night time on the far side is going to be pretty dark. On the near side, you get plenty of Earthshine. But a base on the far side will have 14 days of bright day and 14 days of pitch black night. Starlight is much too weak to help. Just consider that the nearest Sun-like star, Alpha Centauri, is about 275,000 times as far from us as our Sun is. So its light is 13 trillion times weaker when it gets here. There are about 8,000 stars visible to the naked eye from Earth, and you can see at most half of them at one time, and most of them bring us less light than Alpha Centauri.

The illuminance of the starry sky as seen from Earth is only 0.00005 lux, compared with 110,000 lux for a sunny sky. That's a factor of 2.2 trillion. Even a quarter moon gives you 0.01 lux, still a factor of 2000 more than starlight. On the Moon, you don't have any atmosphere to scatter the light, but you are not going to get a lot more total illuminance than you get on Earth. At lunar night, on the far side, you will be able to see the stars very clearly, but not the regolith you are walking on. The horizon will be a sharp boundary between the starry sky and the pitch black surface. In the daytime, the sky will still be dark, except for the Sun. The shadows will be sharply defined and pitch black. You might want to avoid stepping on shadowed areas, because they might conceal a hole or pit.

Life on the near side would be easier. The sky would still be dark, except for the two light sources. The Earth has 3.67 times the angular size as seen from the Moon as the Moon seen from the Earth, so it gives 13.5 times as much light. And since the light comes from a larger source, the shadows won’t be quite as sharp.

If the base is to serve as a spaceport, or a kind of stepping stone for further space missions, then you want to locate it near a good source of rocket fuel. We know that the lunar soil has plenty of helium-3, which can, theoretically, be extracted and used in a nuclear powered rocket. So we need to find out where the best helium-3 deposits are located.

Another possible fuel source is water. Using electrolysis, you can separate water into hydrogen and oxygen. Then load these on a rocket, in separate containers. Hydrogen and oxygen react very explosively when put together, and the byproduct is water, and lots of energy. Basically, you are storing the energy you put into the electrolysis, to be released later, on the rocket. Water is also nice to have around, for thirsty astronauts, for growing crops in a greenhouse, and an occasional shower.

Finding water on the Moon may not be an easy task. There’s an old saying, that if you had a chunk of concrete on the Moon, it would pay to break it down and extract the water. In other words, the place was thought to be extremely dry! Any water that might be deposited by the occasional comet would evaporate (or sublimate) into space pretty quickly. But recently, some evidence of water ice has been detected in the permanently shadowed craters, near the lunar poles. We should find the largest and most accessible ice deposits, and consider locating our spaceport near these.

Even more recently (Sept 24, 2009), a team of researchers at Brown University announced that the Moon Mineralogy Mapper, and instrument carried into lunar orbit on the Indian spacecraft Chandrayaan-1, had detected trace amounts of water in lunar soil at all latitudes. It might be possible to mine the lunar soil for its water and helium-3 and anything else of value, all at the same time! And in November, 2009, the LCROSS mission found even more evidence of water. We are getting more confident that there is enough water on the Moon to support a Moon base.
Although there seems to be water available on the Moon, most of the lunar regolith is still very dry. It is probably a very poor conductor of electricity. That’s bad because you can’t “ground” electrical systems the way you do on Earth. Also, astronauts and equipment will tend to build up electrostatic charge, which can be dangerous. It might be wise to locate a base near a large metal deposit, which could be used to ground electrical equipment.

NASA’s Lunar Reconnaissance Orbiter is now mapping the lunar resources and environment with a variety of sensors, specifically preparing for future exploration of the moon.

Lunar soil is actually quite rich in oxygen. In fact, it is made of oxides of silicon, potassium, and just about every metal you can think of. Extracting the oxygen from oxidized metals is more difficult than from water, but that’s not why water is important. What is really rare on the Moon is hydrogen. There is also very little carbon and nitrogen. You have to plan to work around these deficiencies if you want to live off the land. It’s ironic that the heavy elements, like gold, silver and platinum, tend to be more valuable on Earth, but on the Moon, it’s the light elements, hydrogen, helium, carbon and nitrogen, that will be of great value.

The lunar soil probably contains trace amounts of hydrogen and helium-3, implanted there by the solar wind. It might be possible to bake the soil by passing a microwave source over it, and then collect the hydrogen and helium and any other volatiles that rise to the surface. According to Lawrence A. Taylor, “lunar soil placed in your kitchen microwave will melt at ~1200°C, BEFORE your tea water will boil at 100°C.” That’s because lunar soil has a lot of iron in it. This can be applied not only to hydrogen and helium extraction, but it might also be used to pave roads, and build other structures.

Humans on the base would require shielding from cosmic rays, solar wind particles, and meteorites of all sizes. It makes more sense to use the lunar soil as a shield than to bring along your own shielding material. But this would involve some excavation. A possible alternative is to live in lava tubes, left over from a time when the Moon was molten rock. We ought to see if we can find some of these lava tubes.

Even if it doesn’t produce rocket fuel, a lunar base will have to have a power source. Building a nuclear plant on the Moon would be difficult, but not impossible. A better idea is to use solar power. With no atmosphere to block the sunlight, and no wind to disturb the collectors, this should work very well. But first, consider the geometry of the Moon relative to the Sun.

The spin axis of the Moon is only 1.54° from the perpendicular to the ecliptic plane (the plane containing Earth’s orbit, and the Sun). This has a couple of interesting consequences for a base near either lunar pole. First, there are many craters near the poles which would contain permanently shadowed areas. I already told you about those. There are also a few hilltops near the lunar poles that are never in shadow. Such a place is the Peak of Eternal Light, near the south pole. A steerable solar collector on this peak would have access to uninterrupted solar power.

However, there is a complication involved with a base located too close to the pole: it may not have direct line-of-sight communication with Earth at all times. It would seem that the simple solution is to locate the base on the near side of the Moon, near the pole. Isn’t the near side always visible from Earth? Well, not quite. The reason is that the Moon’s orbit is inclined 5.15° to the ecliptic. That’s why we don’t have a solar eclipse at every new Moon. The Moon’s orbit precesses (wobbles) a little, relative to the ecliptic, with a period of about 18.6 years. So the Moon’s spin axis varies by as much as 6.69° from the perpendicular to the line-of-sight as viewed from Earth. Any base within 6.69° of either lunar pole will have prolonged periods in which they are out of line-of-sight communication with Earth. To prevent blockage by mountains, a base should probably stay between latitudes +/- 83°. An alternative would be, as mentioned above, to use a long cable between the base and the communications antenna. You would essentially have two lunar bases instead of one.

Speaking of mountains, the Moon’s mountains are not formed by the same tectonic and volcanic processes that form Earth’s mountains. Nor are they eroded by the same processes that erode Earth’s mountains. The Moon’s surface has been subject to bombardment for billions of years, and the high spots are just the places that have not been hit by anything too big, or else, like crater rims, they have been put where they are by nearby impacts. Like a sandbox after children have been playing in it.

Another consideration in choosing a site is temperature variation. Humans are only comfortable in a very narrow range of temperature, about 50°F to 90°F, let’s say. The surface temperature near the lunar equator ranges from a high of about 240°F during the daytime to a low of –280°F at night. An astronaut walking around in a spacesuit wouldn’t immediately be affected by this, because there is no air to transfer heat. However, heat would be transmitted through the boots, or any part of the spacesuit that came into contact with the lunar surface. The spacesuit would have to provide heat or cooling at just the right rate to compensate for this. Cooling is actually harder than heating, if there is no medium to dump heat into, as with an air conditioner. If an astronaut falls down, the area of contact is suddenly increased, and the suit may not be able to maintain adequate temperature. The surface temperature at 85° latitude ranges from a high of about –45°F during the daytime to a low of –330°F at night. The nights are colder, and the days never get anywhere near what you would call warm, but then cold is easier to handle than heat.

A scientific lunar base would naturally be located near places of scientific interest. For example, Earth’s Moon is also the place where we find the largest known impact structure in the Solar System: the South Pole-Aitken Basin. Its diameter of 2,500 km is comparable to the Moon’s diameter of 3,474 km. The aforementioned Peak of Eternal Light is on its rim. Its depth is somewhere between 6 and 13 km, depending on how you measure it, but since it wraps around a good portion of the Moon, the bulge in the middle protrudes far above its rim. So it’s like a skullcap with a very slightly raised edge.

The military aspect of lunar real estate shouldn’t be overlooked. As long as we are in this gravity well, military strategists will always try to claim the “high ground.” Right now, the high ground is LEO (Low Earth Orbit). With only 1/6th of Earth’s gravity, the Moon would be a convenient place for a kind of air force base and/or ballistic missile launchpad. It would be much cheaper to get a spacecraft from the Moon to Leo than from the Earth to LEO. It would also be much easier to bomb any point on Earth from the Moon than from Earth. As for location, you would probably want a ballistic missile launchpad to be on the near side, but for an air force base, the far side might offer more seclusion. The considerations of power availability and communications would be similar to those for any other kind of base.

Where should you place your lunar base? For astronomy, place it on the far side, but not too far on the far side. For power, on a mountain close to one of the poles, but not too close. For rocket fuel, find an ice deposit, or a helium-3 deposit. For science, go where the science leads you, but make sure you have enough power, and a way to communicate with Earth.
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