. CASE FOR MOON FIRST - 05 Where to build our first lunar base for humans
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WHERE TO BUILD OUR FIRST LUNAR BASE FOR HUMANS
We have two choices for a lunar base, well on the basis of what we know so far: the poles, or lunar caves.
LUNAR POLES
The lunar poles seem very attractive for our first base, after discovery of volatiles at the poles, as well as confirmation of the peaks of (almost) eternal light As we've seen, the temperature at the poles hardly varies, and it gets solar power nearly 24/7. Our base needs to be protected from solar storms and cosmic radiation, but that should be easy, with regolith piled over the habitats. See for instance Lunar Station: The Next Logical Step in Space Development (from 2015)
This is the the choice for both ESA and ROSCOSMOS, as well as many others.
I've covered some of the details here already in
Earth length day on Mars versus advantages of close to 24/7 solar power at the lunar poles
Long shadows at the lunar poles and early and late in the lunar day elsewhere
LUNAR CAVES AS A SITE FOR A HUMAN BASE
The poles aren't our only option by any means. One other place to build a base is in the lunar caves. See for instance, Technologies Enabling Exploration of Skylights, Lava Tubes and Caves (from 2011). Advantages of the caves include:
Natural protection from cosmic radiation and solar storms.
Temperature is lower, but constant and the base would be easy to insulate and keep warm.
Easy to make a low maintenance airtight enclosure, for instance, by covering the interior of the caves with a layer of glass formed from the regolith dust
Sunlight can be brought in from the surface through the roof using light pipes / collectors
However, a base in a cave only has access to nearby sunlight and solar power for 14 days of a lunar month. Some power storage would also be needed for the lunar poles, to deal with the short periods of night they get, including lunar eclipses.
Everything else is great, but the long 14 day lunar night is an issue, unless the cave happens to be right at the poles of the Moon close to the peaks of eternal light - or we dig a huge artificial cave there. However it is easier to cope with a 14 day lunar night than you might think. I cover this in the section on Power during the night
SCOUTING OUT CAVES WITH ROBOTS
The basic idea is to send robots first to scout it out before you send humans - for science reasons and for safety reasons. First, a lander flies over the cave entrance before it lands, to image it close up. It then drops a line into the cave entrance which is used to supply power to the bots underground, and for them to communicate to the surface.
The bots themselves can be lowered down on tethers, or just be dropped in, in the low lunar gravity and drive, hop or crawl. or move like a snake, or like a spider crawling over the walls - or they can have little rocket engines and fly about inside.
For more on this, see the various cave bots on page 21 of the paper. The bots discussed include:
Spherical hopping microbots - an idea from Penelope Boston - launch many into the cave, each with a small instrument package. Many will not survive but the ones that do can be used to explore, for communications links to the surface and data return.
Multi-segment tethered robot - the tether is used to lower it down, and then for communication with the surface and recharging.
Legged tethered robot - the same idea but with legs so it can navigate rough terrain more easily.
Snake tethered robot - the same idea but it moves like a snake
Cave hopper - combines hopping with wheels
Climbot - future robot able to climb walls with a high level of autonomy
Elevator - large tethered platform used to lower multiple robots at once into the cave, with wheels to allow it to maneuver over debris once down.
Propulsive flying bot - uses small thrusters to navigate, so it can get to parts that may be unreachable by the other bots
Telescoping ball robot - has two modes: enclosed in a ball, for launching into the cave and rolling down slopes inside the cave, and then the two halves of the sphere can separate to be used as wheels, with extending tail for balance.
Prismbot - the bot can move by tipping from one side to another.
Rope climbing - a fixed tether and the robots can climb up and down it.
See also the sections of this booklet: Lunar caves and Example lunar cave skylights - Lacus Mortis, Marius pit and the King-y natural bridge
Team Hakuto plan to send a simple tethered bot, with two wheels, to explore the Lacus Mortis pit, in 2017, see Robotic missions to the Moon, already planned, or near future, from 2017 onwards, below.
ROBOTS FIRST
In either of these cases, a base at the poles, or in lunar caves, we would explore using robots first.
They need to check conditions on the ground to find the safest place to set up a base for the humans
They need to do scientific study before the humans arrive - because of the possibility of contaminating scientifically important sites such as volatiles in the dark areas close to the proposed sites at the lunar poles or perhaps deep underground in permanently shadowed regions at lower latitudes.
The robots can also be used to start construction of the bases via telerobotics from Earth, so that there are shelters for the humans already ready for them when they arrive.
YOUTUBE VIDEO
Resource Prospector Sand-Crawl Time Lapse
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The Resource Prospector Ground Test Unit demonstrates the rover's skills and adaptive wheels as it navigates through tough, sandy terrain. The flight unit will experience similar conditions on the moon in the early 2020s as it searches for a spot in a lunar polar region to mine for water and other volatiles.
00:07
https://www.youtube.com/watch?v=T456Fr8z3rk&feature=youtu.be
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NASA's lunar resource prospector, in "pre formulation" stage proposed for launch in mid 2020s. It would be controlled in real time via the Deep Space Network. This is not designed for low latency control - and from the LADEE and LCROSS, then they expect " typical two- way signal transmission durations of 6 - 10 seconds with edge cases of 25 seconds and irregular bursts of 15 seconds". For the resource prospector with higher bandwidth they expect latency of 10 - 30 seconds. As a result the rover will be controlled by using waypoints - they select a point for it to travel to, say 8 meters ahead. As it arrives there it takes stereo images which the operators use to choose the next waypoint and so on. It uses solar power and so will make short visits to permanently shadowed regions for a few hours at a time. For details see this paper.
For more about this, see Robotic scouting phase - need to go back and find out what's there first
ORIGAMI ROVER
This is an idea for an “origami rover” the size of a smartphone which spaceships and rovers could take with them to deploy to scout out places where it’s too dangerous for the main rover to go, or get to places they can’t reach (because they are much smaller). A larger rover could also deploy several of them, to scout its neighbourhood, to see which is the most interesting place to go to next.
It can climb 45 degree slopes, crawl into overhangs, and drop into pits and craters. Because it can be folded flat, you can stack them one on top of each other a bit like a pack of cards, and they can do parallel science increasing the amount you can do in a day. It can survive a fall of three meters on Mars so that would be about 6.86 meters on the Moon.
This is what it is like in the tests:
YOUTUBE VIDEO
See NASA's origami robot in action
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•Mar 15, 2017
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PUFFER is a tiny, foldable robot that will help NASA researchers explore rough terrains in outer space.
01:03
https://www.youtube.com/watch?v=rxupKyB7iAg&feature=youtu.be
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It has “whegs” instead of wheels - a kind of cross between a leg and a wheel that lets a rover traverse much rougher terrain than you can with a wheel.
They can travel quite long distances too as in this sped up video of a 250 meter “hike”
YOUTUBE VIDEO
PUFFER: Fast-motion video of a 250-m journey
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•Mar 12, 2017
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Pop-Up Flat Folding Explorer Robots, or PUFFER, is a small, origami-inspired robotic technology under development to provide a low-volume, low-cost mission enhancement for accessing new science from extreme terrains that are of high interest to future NASA missions. A “pop-up” robot that folds into a small, smartphone-sized weight and volume, PUFFER’s compact design means numerous robots can be packed into a larger “parent” craft at a low payload cost, then deployed on a planet’s surface individually to increase surface mobility. For more information, visit: https://gameon.nasa.gov/projects/puffer/
00:23
https://www.youtube.com/watch?v=wPtYBBbyTiw&feature=youtu.be
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It can travel about 2,050 feet (625 meters) on one battery charge on a flat dirt path like that.
This shows it climbing a steep slope - it can handle up to 45 degrees:
And slip into cracks
So far they have tested it in Rainbow Basin California, at a ski resort in Grand Junction Colorado, Big Bear California and on Mt Erebus in Antarctica.
They plan to field test them in the Mojave desert in the near future. PUFFER’s body inspired by origami is due to Jaakko Karras, at JPL who is also the project manager.
Its body is wrapped in Nomex, the same textile used for the air bags that cushioned the NASA Spirit and Opportunity rovers when they landed on Mars. The Nomex is integrated into the folding circuit boards and it helps protect it from high temperatures as it repels heat. It’s able to function in the Mars temperatures of –135 to 30 ˚C without a heater. The next stage is to find a way to integrate instruments into it. For instance to sample water for organics or add a spectrometer. It is a “Game Changing Development” program managed by GCD, and is part of NASA’s Space Technology Mission Directive. For more about it: Origami-inspired Robot Can Hitch a Ride with a Rover, PUFFER Prepares for Field Testing, JPL Robotics: Research Tasks, NASA's Adorable Pop-Up Rovers Are Designed to Explore Harsh Alien Terrains - and I did a page on quora here with the youtube videos embedded into the page.
TWO DIMENSIONAL PLANETARY SURFACE LANDERS
Another idea is to use flat two dimensional landers, using the same technology used for flexible thin film electronics in ultra thin laptops etc. They have the advantage that you can stack many of them into a small space. This is an artist's concept for Mars but they'd also be useful on the Moon.
Artist's concept of flat landers deployed on Mars
They can actually move around on the surface by including actuators in the sheet. They could be made of Kevlar woven fabric, as it has strength and resilience down to cryogenic temperatures. They could be covered with Mylar to protect the kevlar from UV. They could be powered by printed circuit board fuel cells or ultra thin flexible solar cells, or through power beaming from another orbiter or rover. They can communicate using either a flexible RF transmitter, or else, though with a lower data rate, via modulating laser retroreflector array (which changes its reflectance a bit like a bar code for a bar code reader, except one that's continually changing) to an orbiter or rover that beams a laser light at it. It can keep warm through a radioisotope heater unit if necessary.
Many instruments can be included too. 3mm diameter imaging cameras, or an imaging array for 3D video. Spectrometers and gas sensing films. A really tiny mass spectrometer, ground penetrating radar with a range of 50 meters with 15 meter resolution, and miniature laser based dust and particle analyser. They also had plans to develop more miniature instruments such as a miniature gas chromatograph and drills and penetrators that can be deployed from flat landers. For details see this article from 2014.
3D PRINTING ON THE MOON
In the ESA plan, the printer needs to be large to do the regolith shielding. For techy details see: Lunar Outpost Design, 3D printing regolith as a construction technique for environmental shielding on the moon.
One of the main differences from printing on Earth is that they use a large gantry system on Earth:
"The current D-shape printing process, like most 3D printers, uses a gantry system that is always of an order larger than the printed object. This is not, of course, a feasible set up for any large scale structure. We assume that to be able to print on the moon, a much more ‘bottom up’ approach must be taken. Smaller robots could deposit small amounts of regolith and selectively solidify them with a printing device ("
CUBE SAT EXPLORERS AND REP RAP PRINTING
This is something that's in use already on Earth, printers that print out their own components. The aim of the Rep-Rap project is to produce more and more of the printer from raw ingredients.
This shows a Rep-Rap printer printing one of its own components in 2015 (author says it has improved since then)
YOUTUBE VIDEO
Snappy-RepRap printing printer parts
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•Jul 11, 2015
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This is the first non-calibration print on my newly assembled Snappy-RepRap printed printer. It is printing a rail segment of itself. The print quality of this first prototype is crappy, and has been improved a lot since this video was taken.
01:52
https://www.youtube.com/watch?v=3KJbrb0P8jQ&feature=youtu.be
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This seems perfect for the Moon. Why not get the printer to print out a copy of itself? But even more so - why not get a small rep-rap printer to make a larger printer? Even a large beam could easily be made of many smaller components that snap together or indeed, "melt them together" since the lunar regolith is easy to change into glasses.
So, maybe you could send a small printer up to the Moon to make most of the components of a larger printer, which could then make an even larger printer, and at this point you can print out really large components in one go? This idea came out of a conversation with Sean Clifford on facebook. I haven't come across it anywhere else. Anyone reading this come across this anywhere?
The "Made in Space" project produced a 3D printer which has already been on the ISS:
Made in Space's first 3D printer, launched to the ISS September 21st, 2014.
Eventually these 3D printers could produce components printed in space and assembled by robots into large space structures like this:
So anyway the idea suggested here - expect surely someone has thought about it before - but why not use smaller 3D printers to print components for larger 3D printers in the same way?
The smaller 3d printers could be small enough to send to the Moon as cubesats. They might be too small to print the regolith shielding for a habitat, or anything large, in less than years of work, but if they make a larger 3D printer first, this could speed it up a lot.
Artist's concept for Lunar Polar Hydrogen Mapper - cubesat from Arizona university to map water resources on the moon. It's by Dr. Craig Hardgrove. One of two missions selected, as part of NASA's Small, Innovative Missions for Planetary Exploration (SIMPLEx) program, capped at a budget of $5.6 million, to ride along on the first flight of the Space Launch System (SLS) in 2018 - see CubeSats to the Moon
Then, there's the Vermont Lunar Cubesat project lead by Carl Brandon
Vermont Lunar Cubesat [VTC]. This prototype successfully orbited Earth, taking pictures, from September 2013, to 21st November 2015. A bit more about it. And Lunar Lander / Orbiter CubeSats(slides), for Carl Brandon's complete lander / orbiter system to land cubesats on the Moon. For more, see the website for the Vermont Technical College Cubesat Laboratory.
This could be a first step towards cubesats to the rest of the solar system. Cubesats to the Moon are a natural starting point.
So, could these be used for 3D printing on the Moon? Could you fit a Rep-rap printer into something as small as this? If not, one thing you can certainly do with cubesats is a lot of robotic explorer with low cost missions of costs of order a few million dollars each, with robots exploring on the surface and in orbit.
HUMAN HABITATS
We would probably use the Bigelow inflatable modules to reduce weight.
Robert Bigelow with a model of a Bigelow Aerospace lunar outpost
We could get started quite soon if the Falcon Heavy is a success, with its 53 tons payload, perhaps using the Dragon V2 for humans.
LIQUID AIRLOCK
This is a rather fun idea originally suggested in the Moon Miner's Manifest Classics - 1987-1988 (see page 31). It's not so likely in the early stages, because of the large amounts of water needed to construct it, but it may perhaps be of great value at a later stage, especially for bases that have a lot of traffic in and out. If the liquid is water, it has to be over sixty meters deep (62.3 meters), to equalize the pressure inside and outside the habitat. The depth can be much less, if it is a denser fluid. You then don't need any doors but can just dive through it and come out on the surface of the Moon.
Liquid airlock for the Moon. You need to look carefully at the picture. How it works is that you have a sump, like the sumps that cave divers dive into. On the inside, it is kept in position by the pressure of the air inside the habitat. On the outside is a vacuum.
It's like the way that In a barometer, the weight of the mercury counterbalances the pressure of the atmosphere outside, with a vacuum above the mercury. It's like an "inside out" barometer with the vacuum on the outside. The weight of the water in the column counteracts the pressure of the air in the habitat.
You might think that as for a barometer, mercury is the best fluid of all, because of its high density, at least, if it weren't for its toxicity. Especially for a surface base, the lower column height the better. Mercury would give a column height of only 76 centimeters on Earth, so on the Moon, (9.807/1.622)×0.76 = 4.6 meters. Should we look for the densest liquid we could send to the Moon?
Well, if supplied from Earth, the actual mass of the mercury would be the same as the mass of water. So in that sense, a denser fluid is no saving at all. If you can source the liquid in situ it's also a considerable saving, so if there is ice on the Moon that might well swing it in favour of water rather than some denser liquid from Earth. As for the column height, that's less of a consideration if the base is constructed inside a lunar cave, as many lava tube caves are likely to be at least 60 meters below the surface of the Moon. It might be useful to be able to float up 60 meters to the surface. A denser fluid would help with a surface base.
In the vacuum conditions of the luanr surface, exposed water would evaporate quickly, all gone in just one day, or less. But there are solutions to this, including, to cover it in a thin film of vacuum stable oil, or to replace water with the liauid metal Gallium (which is non toxic, and liquid at 30°C).
Sections:
Diving through the waterlock - is that possible in a spacesuit??
Funicular type railways driving though a sump and out onto the surface
High density liquids for a lower column - less than 10 meters for gallium
Diving through the waterlock - is that possible in a spacesuit??
The interesting thing about it is that you could dive through it in a spacesuit, if your suit is capable of being submersed, and come out onto the surface with no loss of air at all from the habitat. There is no need to evacuate the airlock, or anything. This means you can have it there available as an airlock all the time, "always open". Any time to get out of the base you just dive through the sump. This could be quite a saving in air if the airlock is in regular use and could also make it faster to go in and out - no need to wait for the air inside the airlock to cycle before the next group of people or equipment to exit.
In the other direction it would also keep dust out of the habitat.
Now, spacesuits aren't normally waterproof. The ones used by astronauts to prepare for space missions in swimming pools are just mock-ups of real spacesuits. They are designed to operate in an atmosphere or a vacuum. For one thing, to make the joints mobile, then the bearings lose air constantly at litres an hour, so water would leak in in the other direction. There may also be electronics that would be exposed to water if it was submerged. And the outer layers would not necessarily be waterproof either. Imagine drying off a spacesuit inside a habitat, to stop it getting mouldy?
So - this would need waterproof spacesuits. Perhaps an outer covering that's waterproof and also helps to keep the dust out? Or perhaps you just drive in / out with rovers - in that case the rovers have to be waterproof.
Or, astronauts could have a kind of overall outer suit outside of their spacesuit that they don before getting into the pool, watertight. Even maybe just a kind of big zip-lock bag, zip up get into a lift / hoist, unzip when they get to the top.
For cargo, then deliveries to the ISS are normally packaged anyway - just need to make sure the packaging is waterproof.
Even if not much used for individual astronauts, it could be very useful for large cargo deliveries, if you want to deliver 100 tons at a time, say, then it might take hours to cycle all that material through a small airlock able to take a ton at a time. Either you have large airlocks, or you have a liquid lock. If you do it that way you can just drop the cargo in from the top and remove it from the bottom continuously.
Need for very low vapour pressure liquid - water would lose around 60 meters depth of per day at 0 °C
The liquid needs to have a very low vapour pressure. If it was water you'd lose it through evaporation into space in vacuum conditions at a rate of tens of meters thickness of water per day. Indeed you'd lose nearly all of the 62.3 meters depth of your water lock in a single day of evaporation.
Calculation indented
With surface temperature of 273.15 °K (0 °C) and using the equation for mass loss of liquid water in a vacuum of
(pe/7.2) * sqrt (M/T) kg / m² / sec (equation 3.26 from Modern Vacuum Physics)
where M is the molar mass in kilograms, 0.018 kg for water, T is the temperature in kelvin, pe is the vapour pressure, which for water at 0 °C (273.15 °K) is 611.3 Pa, (Vapour pressure of waterat 0 °C), so putting all those into the formula we get:
(611.3/7.2) * sqrt(0.018/273.15) = 0.689 kg / m² / sec.
So you lose 24*60*60*0.689 or about 59.529 tons a day.
So you lose about 60 meters a day thickness of liquid water exposed to a vacuum, or about 21.9 kilometers thickness of water per year at 0 °C.
The rate of loss goes up if the temperature increases. So, what about room temperature? Well they do the calculations here: Modern Vacuum Physics where they use the vapour pressure for water at room temperature 295 K to calculate (2300/7.2) * sqrt(0.018/295) = 2.495 kg / m² / sec.
So at room temperature of 22 °C,you lose 24*60*60*2.495 or about 215.6 tons a day
So now you lose around 215.6 meters per day and 78.6 km per year.
It doesn't help much to have a layer of ice on the surface of the water as ice also sublimes rapidly into water vapour in vacuum conditions, unless it is extremely cold. Spacesuits often use sublimation of ice to cool down astronauts.
So water is no good, so what can one use instead?
Idea to use low viscosity room temperature ionic fluids
The original author suggests NaK. which is highly reactive with water and could burst spontaneously into flame if exposed to air. Not the safest of materials for a liquid airlock
However, this is my suggestion, you could use room temperature or lower ionic fluids. An ionic fluid is a salt, like sodium chloride, common table salt. That's liquid only at very high temperatures. If it is liquid at room temperature or below, you call it an ionic fluid. There, a salt in chemistry is a general term for the result of combining any acid with any base. When in solution, it has positive and negative ions. When you melt the solid salt, if you can do that without it decomposing or vaporizing, it also usually consist of positive and negative ions (cations and anions).
Anyway, so it turns out that if salts have a high molecular weight, they are often liquid at room temperature, and what's more, they can be liquid even at very low temperatures such as you'd get on the lunar surface. That makes them ionic fluids. Typically they have very low vapour pressures, so wouldn't boil away in a vacuum. They have been suggested for liquid mirror telescopes on the Moon.
The only thing is, that most are high viscosity which would make it hard to get in or out, but there is research into low viscosity ionic fluids. Also if the ionic fluid is immiscible and also low density, so that it floats on the surface of the water, you might only need a thin film of it to protect the rest of the water from evaporation. In that case, as a thin surface layer, it wouldn't matter much if it has high viscosity. But many salts will dissolve easily in water.
Vacuum stable light oils
Another idea is to use vacuum stable light oils designed for use in conditions of high vacuum. In this paper, one of the tetraalkasylane oils mentioned has a very low saturated vapour pressure. It's also less dense than water at 857.4 kg / m³ so would float on water. As usual calculation indented so it is easy to skip:
It's structure is:
Figure 1. The structure shown as SiCH-3 has a very low saturated vapor pressure at 25 C of only 2 * 10-10 Torr or about 2.666*10-8 pascals.
So it has
5 * Si: 5*28.0855 amu
4*(3+10) C: 52*12.0107amu
4*(6+21) H : 108*1.00794 amu
For a total of 873.8 amu
That makes the calculation .
(pe/7.2) * sqrt (M/T) kg / m² / sec (equation 3.26 from Modern Vacuum Physics)
where M is the molar mass in kilograms, 0.018 kg for water, T is the temperature in kelvin, pe is the vapour pressure
(2.666*10-8/7.2) * sqrt( 0.8738/(273.15+25)) = 2*10-10 kg / m² / sec. Or about 6.33 g / m² / year .
This time the calculation is for 25 C because that's the temperature at which the author measured the saturated vapour pressure.
An even better example is Pennzane X2000 which is used as a lubricant in space applications
Pennzane X2000 density 0.85, vapor pressure 10-12 torr or 1.333 * 10-10 pascals. This time measured at 40 °C.
Formula C65H13 0 (2 octyldodecyl cyclopentane)
So it's molecular mass is 65*12.0107 + 130*1.00794 amu = 911.7277 amu.
So it would lose (1.333*10-10/7.2) * sqrt( 0.9117277/(273.15+40)) = 10-12 kg / m² / sec. Or about 0.032 g / m² / year .
So, if you cover the liquid water column with a thin layer of this tetraalkasylane oil you'd lose only around 0.26 grams per square meter of the oil to the vacuum of space even if the water was kept at or above normal room temperature at 25 °C. And as for Pentane x2000 then you lose only 0.0013 grams per square meter per year.
You might not need much. Even a micron thick layer, even a monolayer would be enough to stop the water from evaporating - just as only the thinnest of layers of aluminium oxide is enough to prevent aluminium from combusting when exposed to air. A thin layer would also help with the issue of equipment getting covered with oil as it goes in or out of the lock. If it is just a monolayer then this won't have noticeable effects.
The oil wouldn't need to be photostable - there'd be some sort of a reception area probably covered above the upper entrance to the sump which would protect it from exposure to direct sunlight. Probably also it would be a shelter covered in regolith for protection from solar storms.
Funicular type railways driving though a sump and out onto the surface
Perhaps a liquid airlock might also be useful for moving cargo in and out. Instead of those big hanger like airlocks you get in science fiction movies, maybe you'd just have a truck that drives through a sump filled with ionic fluid - or even - a funicular railway type carriage gets pulled through it? You could have trains that run out of the base onto the lunar surface directly with no need for an airlock so long as they can withstand being submerged. This doesn't seem to be beyond future high tech. The more I think of it, the more possibility it seems to have for future tech.
High density liquids for a lower column - less than 10 meters for gallium
One thing that makes it harder on the Moon is that with a sixth of the gravity, the column is six times higher. A water column would have a height of (9.807/1.622)×10.3 or 62.3 meters which is quite a lot. The author talks about higher density fluids to keep the height difference down. Some ionic fluids are high density so that would help. If we can find a high density low viscosity ionic fluid, that's ideal.
However Wikipedia has a useful section on heavy liquids. Sodium polytungstate is the densest non toxic one I can see there, density 3.01, reducing the column height to 20.7 meters. Clerici solution is denser but toxic.
Gallium is even better at density 6.5 reducing height to 9.6 meters, and if you are using it as a heat sink for the habitat seems like a good choice too. Has to be kept above 30 C, That's a little on the warm side for a swimming pool. However even heavy things would float to the surface easily so that would be a plus. That would reduce the height to less than 10 meters. And - the high temperature wouldn't be much of an issue for cargo or people driving in / out in rovers or buggies. But would have to be very sure it's not going to freeze on you as you travel in / out
As with water, you could cover any of these high density liquids with a thin layer of an immiscible low vapour pressure liquid such as vacuum stable light oil. But now it doesn't matter even if it is rather dense, so long as it is less dense than the liquid it covers. However if we use Gallium it has an extremely low vapour pressure
Back to our forumula:
(pe/7.2) * sqrt (M/T) kg / m² / sec (equation 3.26 from Modern Vacuum Physics)
where M is the molar mass in kilograms, 0.018 kg for water, T is the temperature in kelvin, pe is the vapour pressure
vapor pressure 6.08e-36 pa (from this online calculator) molecular weight 69.723
So it would lose (6.083*10-36/7.2) * sqrt( 0.069723/(273.15+30)) = 1.3*10-38 kg / m² / sec. Or about 4*1O^-28 g / m² / year .
Even after a quadrillion years you still would only lose trillionths of grams per square meter.
You'd need to be heavily weighted to submerge in liquid Gallium, lead weights obligatory. But if you ever get into trouble just drop the weights and up you'd float to the surface.
The high column and low density of water as an asset
There's a plus there, however, for water and for low density ionic fluids, as then the height difference is comparable to the depths below the surface of the floors of the lunar cave entrances. When you dive into the sump at the bottom of the cave, your buoyancy will take you to the surface. That could be especially useful for cargo - a sump that goes all the way from the cave floor to the surface would make it easier to move goods up and down using weights, or floats.
The mass isn't that much for a moderate sized settlement. If you have 60 residents, then that's only one ton per resident, and they would have tons of supplies delivered per resident every year probably.
What if the water freezes?
You might wonder if the area exposed the the lunar surface would freeze. However usually the main issue with a habitat is to keep it cool. The ISS has huge heat rejecting panels. So long as enough heat is supplied from the bottom, the hot water would convect upwards and keep the surface warm. Indeed, it might well be an idea to have some kind of radiator system at the top connected to the sump to use it to convey as much heat as one can upwards from the habitat. After all the surface exposed to a vacuum might as well be in a vacuum flask, the vacuum is a good insulator and it would only lose heat through radiation. It's just a small radiator and wouldn't lose much heat into space.
What about leaks?
First, there is no real risk of the water leaking inwards. It's like a cup upturned and pushed down underwater. The pressure of the air in the habitat keeps the water out. It can only get in if the pressure inside the habitat goes down, at which point the water in the sump would gradually rise. Of course one should have a cover that can go over automatically in case of habitat depressurization to stop the water lock filling the habitat with water.
In the other direction though, it's a more serious failure mode. Suppose something goes wrong with the atmospheric pressure regulation and it increases to 1.1 atmospheres. The waterlock would then require a water height of 68.53 meters. There wouldn't be enough height tin the water column to accommodate this, even if there is water available in a reservoir to automatically top it up. So what do you do?
The failure would be spectacular, if the habitat pressure was self regulating. The sump would immediately start to sink, pushed out by the higher pressure inside the habitat. The habitat air system would respond by pumping in more air to compensate for the increase in volume to be filled. The air would eventually bubble around through the sump and up to the surface. The habitat environment control system would continue compensating for the loss of pressure pumping the air out onto the surface of the Moon.
You could design it though with some play in the system to deal with small pressure variations. First you go down a meter or so to go into the sump, so that a slight increase in pressure won't lead to the water level going so low air bubbles out through the siphon. Then to have a reservoir that automatically fills the sump to raise the water levels if the interior sump level starts to fall,, and enough head room at the top of the sump in the reception area above for the water to rise some meters before it overflows onto the surface of the Moon. And finally then those doors that automatically close if the internal pressure is too high to deal with by just adjusting the column height, or in case of some failure of the regulation system itself.
It would need safety doors to deal with this too. Perhaps just a compartment above the sump, an antechamber, with airproof and waterproof doors that slam shut in this situation.
Protection from lunar dust
A liquid airlock would also help keep out dust. A lot of the dust in the Apollo lunar module came in on the outside of the suits. So it would surely also come in on the outside of vehicles and their wheels, and there'd be a chance of it getting scattered in through an open airlock too from the outside. None of this would happen with a liquid airlock.
WATER LOCK
This is an idea from Bryce Johnson to use water inside the airlock on Mars as a way to protect from the dust and contaminants in the dust such as perchlorates. It could be adapted to the Moon as well. He explained the idea to me in a conversation in the Case for Moon facebook group:
"Crews come in from outside. Hatch is sealed. Air is introduced followed by water that not only neutralizes (or at least sequesters) contaminants but also begins the process of cleaning the suits. As a plus, the air is efficiently forced out of the airlock as the water rises. At no point is the water ever exposed to Martian vacuum.
"Evacuating the water only requires opening a valve to the pressurized inside of the shelter to let the air in. Opening a second valve drains the water out like in a bathtub. The contaminated water goes into a holding tank where solids are filtered out. Depending on the exact nature of contaminants, the water could be processed further to be reused. This system would NOT be part of the environmental system and the water is not intended for human/animal/plant consumption."
It has many of the advantages of a liquid airlock, but as the water is never exposed to the vacuum it can just use ordinary water.
Another suggestion (my own, on reading his idea) - in a larger habitat, with enough space for it, you could also have the hybrid idea of an ordinary airlock leading to a sump that astronauts go through to go into the inner habitat. Again the advantage is that you can use ordinary water as it is never exposed to vacuum. It might permit faster throughput of traffic as a hybrid between the water lock and the liquid airlock. It also provides an extra layer of protection if the outer airlock fails, especially if there is enough water to fill the airlock and seal it after a breach, with ice forming as the water cools down exposed to vacuum conditions.
CAN WE FILL LUNAR CAVES WITH AIR?
We will surely just bring air from Earth to start with, since it's only a small amount of the launch mass, only 0.26% in the case of the atmosphere in the ISS (not including stored nitrogen), as we saw in the Nitrogen section. But later on, if we have thousands or even millions living on the Moon, perhaps we might want to fill the entirety of a large cave with air. A cubic kilometer of air weighs 1.225 million tons, which is a lot of nitrogen to find somehow. Could we source it on the Moon? This is for a fair way into the future of course, but let's try to get a rough estimate of whether it is possible.
Let's try a cave 250 meters in diameter, and 100 km long, a medium sized lunar cave (if the Moon does have caves as large as the Grail data and modeling suggests), but huge for Earth. That's 4.9 cubic kilometers of air, so 6 million tons of atmosphere using our 1.225 million tons per cubic kilometer figure (1.225*PI*(0.25/2)^2*100).
That much nitrogen could be sourced on the Moon, using lunar railways to truck it from the poles, if they do have the 600 million tons of volatiles, and 6% nitrogen for a total of 25 million tons of nitrogen. Typical heavy trains on Earth can carry 20,000 tons upwards. So if we had their like on the Moon that would be 300 train trips to fill the cave using volatiles from the poles.
You could also fill the cave pressurized to a tenth of Earth normal, which is enough to grow crops and for humans to walk around using only an oxygen mask, and then have pressurized habitats and greenhouses and parks within it at Earth normal pressure. If you do it that way, you need 600,000 tons of atmosphere for our cave 250 meters in diameter and 100 kilometers long, requiring about 30 trips of a heavy goods vehicle to truck the volatiles from the lunar poles to the cave.
Also, we can do another calculation based on the 225,000 km² needed to feed the population of the Earth on a basically vegetarian diet with the BIOS-3 system. If that area was covered with greenhouses, typical height say 2 meters, that's 550 cubic kilometers which at a mass of 1.225 million tons per cubic kilometer is around 674 million tons. If we pressurized the greenhouses at a tenth of Earth normal, that would be 67.4 million tons. There'd be enough water too, if we use aeroponics. So, if there are 25 million tons of nitrogen at the poles, the Moon may have enough by way of volatiles to fill greenhouses sufficient to feed 2.78 billion people or 37% of the Earth's population using the BIOS-3 system.
That doesn't include the carbon dioxide for growing food. As we saw above (CO2 on the Moon), if you supply enough carbon dioxide to bring the first crop to maturity without astronauts there (so you don't have to feed them), you need about 40 kg of CO2 per astronaut for the first crop, assuming it takes 40 days for the crop to reach maturity. After that, carbon dioxide becomes a nuisance gas to be removed if you need to import food.
If there are twelve million tons of carbon dioxide at the poles, that's enough for 300 million people on the Moon.
However this might not even be needed at all. If you are sending astronauts to the Moon anyway, and have to send habitats for them, you can surely manage to find the extra 40 kilograms per person for the carbon dioxide and what's more the extra 9 kg of nitrogen per person, or 90 kg if you pressurize the greenhouses to Earth normal. Each person would need at most twice their own weight in volatiles even with all the greenhouses pressurized to Earth normal.
Of course I'm not suggesting we colonize the Moon with nearly three billion people, but it does show that the Moon has a fair bit of potential for gardening just using indigenous resources.
So, yes, we could fill some of the medium sized caves in their entirety in this way (depending on what we find by way of volatiles there).
However, eventually if we had millions living on the Moon and wished to fill all the lunar caves with nitrogen and oxygen, and if some are as large as 5 km diameter and 100 km long, then you have the same problem as you have with an O'Neil cylinder, that in such a large space, the mass of the atmosphere dominates, and nitrogen is in short supply in the inner solar system, apart from on Earth and Venus.
For a cave as large as that, 5 km across and 100 km long, you would need 1.225*PI*2^2*100 million tons = 1.54 billion tons of volatiles, most of that nitrogen. Perhaps you could get it from comets, but they have onlyaround 0.5% NH3 or so (hard to estimate). Perhaps it can be exported from Earth if transport costs go down hugely? Or perhaps you would build domed cities and air filled tunnels on the floors of the caves much as you would do it on the surface, and keep them in vacuum conditions. The reason for building in caves then would be for the protection from cosmic radiation, and micrometeorites and more stable thermal environment.
Anyway - that's for the distant future. It's clear we are not going to run out of volatiles on the Moon any time soon, so long as the volatiles are indeed abundant and easy to mine and transport.
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