. CASE FOR MOON FIRST - 04 The Moon is resource rich

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THE MOON IS RESOURCE RICH

Mars colonization advocates often contrast Mars with the Moon. The Moon may be described as being as uninteresting for human colonization as a lump of concrete. But actually it turns out that the Moon is very rich in resources. We need a "Case for the Moon" here like Robert Zubrin's "Case for Mars".

Moon advocates perhaps don't hit the news as much as the Mars advocates but there are many of them, and they are just as enthusiastic about their vision as the Mars advocates. Paul Spudis is one, with his most recent book, The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources. Another is Dennis Wingo, CEO of Skycorp, and author of Moonrush, see his recent paper, and appearance on the Space Show. Others include Madhu Thangavelu,David Schrunk, and other authors and contributors to The Moon: Resources, Future Development and Settlement. See also David Schrunk's paper Planet Moon Philosophy , and their appearance on The Space Show.

It was also the policy of the US too during the Bush administration, with his Vision for Space Exploration program. And the ESA and Russia are strongly behind the idea of sending humans to the Moon first.

So, let's look at some of the suggested lunar resources for this Moon first approach.

CO₂ ON THE MOON

The Moon doesn't have a CO₂ atmosphere, but it has dry ice at the poles. It has an estimated at least 600 million metric tons of ice (based on the mini SAR and lunar prospector data, indirectly detected through radar), and possibly much more. If the proportions are the same as for the LCROSS impact measurements, 2.12% of that may be CO₂, so if those figures are correct (which needs more confirmation), that could be more than twelve million metric tons of CO₂ in the form of dry ice.

That's plenty, but we might not even need it. It is only a trace gas in our atmosphere, less than 0.04%. And, this may surprise you, but actually, feces is nearly all water. About 1 kg of carbon dioxide is exhaled every day, compared to only 30 grams (0.03 kg) of dry weight in feces.

Plants don't need a constant supply of carbon dioxide from outside of the habitat. Indeed, if you grow enough plants for food, then they get all they need to make the plant matter and the oxygen from the carbon dioxide you breathe out, and a small amount from the feces which can be burnt or composted.

With each crop, to close the cycle, the plants also produce almost exactly the amount of oxygen that humans need to breathe, indeed that's the main advantage of growing your food, that it saves on the 0.84 kg of oxygen you need to supply per crew member every day (from table 2 of this paper). Oxygen supply is the main issue here, and after that, food.

If you have to import some of your food, as has always been the case so far, then an excess of carbon dioxide builds up, and you have to remove because it is dangerous to humans long term at concentrations of above one or two percent in the atmosphere.

The first crop cycle, for, say, 40 days (it took 39 days for dwarf wheat to reach maturity in an experiment on the ISS in zero g) does need a net input of CO₂ to the plants for them to grow. But this will be provided by the astronauts just breathing it out, so if you supply them with food for the first month, then they will then provide enough carbon dioxide for the next crop cycle onwards.

Or if you use robots to set up the greenhouse before the astronauts arrive and bring the first crop to maturity ready for them to eat, then you need to supply that 1 kg a day of CO₂ per astronaut for one month. So that's 40 kg per astronaut.

You could take carbon out of the atmosphere in the habitat if you store and accumulate the plant wastes. Typically half the crop is plant wastes and if you stored all the plant wastes, yes, you'd need a constant input of CO₂. However that would lock up valuable oxygen into the plant wastes and remove carbon from the system as well, so it's not too likely that you would do that.

If you burn plant wastes too as is the most likely thing to do in a space colony, or compost them, then you have a closed system, and any imported food will mean an excess of carbon in the system which will build up in the atmosphere rather rapidly. This was the main thing that threatened the lives of the Apollo 13 crew, they had plenty of oxygen but had to rig up a way to scrub the carbon dioxide to survive.

On the ISS it used to be vented into space. Nowadays they react it with the hydrogen got from splitting water to generate oxygen, in the Sabatier reaction This converts the carbon of the carbon dioxide to methane which is then vented into space. That's still not a closed system as it depends on constant input of water and food to provide the carbon and hydrogen that's lost to space in the methane.

The ISS's half kilogram for 0.04% (see next section) would then correspond to just half a day's worth of wastes for one person. If you suppose similar amount of atmosphere to the ISS, and six people, importing all their own food, the carbon dioxide would build up at a rate of about half a percent a day, so would over 1% within two days, and reach 10% within 20 days, a level which leads to convulsions, coma and death.

If they import half their food, and don't scrub the carbon dioxide they die within 40 days, but probably much sooner. If they import a quarter of their food, with no carbon dioxide scrubbing, they die within 80 days.

That's based on this paper which says concentrations >10% may cause convulsions, coma and death. That's for short term exposure, so with continuous exposure with gradually increasing concentrations, they would probably die well before then. Levels of up to 3% however can be tolerated for more than a month without any adverse effects (see table 2 page 66b of this paper).

No space habitat to date has had to import CO₂, and until you have near perfect recycling it won't be needed. Once you are able to grow all your own food, then you may need a tiny amount, but the more perfect the recycling, the less you need. If you had perfect recycling, you'd only need as much as is necessary to get the plants started. Usually half of the plants grown for crops consist of plant wastes, but that also doesn't really change anything. The CO₂ you get from burning the wastes or composting it, added to the CO₂ breathed out by humans, is almost exactly the amount the plants need to grow the next generation of crops.

So, it's not too likely that you will have a shortage of CO₂ in space.

One way or another, if you import food, you will have an excess of carbon in the system which has to be scrubbed and got rid of somehow, usually as carbon dioxide or methane. While if you manage a nearly biologically closed system, you need hardly any materials supplied from outside to keep it going, just need to deal with leaks. The water in urine, sweat and grey water can be recycled, something that is already done in the ISS. For techy details: upgrades to the ISS water recovery systems. The feces can be dealt with also, without need to build a sewage plant in space, for instance oxidized at high temperatures 400 C and high pressures using supercritical water.

It's true that in the carbon cycle on Earth, volcanoes recycle carbon dioxide, which was originally taken out of the atmosphere millions of years ago, however not by us eating plants, or plants just growing and decaying as that returns roughly the same amount to the atmosphere as was taken from it. It is taken out through steady build up of plant residues, for instance peat, coal and oil, and through build up of limestone and chalk in the oceans and through organics falling into the ocean from algae growing on the surface. Most of our CO₂ is stored in carbonates such as limestone, or as organics in the oily shales, and that's what gets subducted and then returned to the atmosphere in volcanoes. It's a much slower process.

If we ever attempted to terraform Mars or the Moon or anywhere else long term, we would need to have some other way to return the limestone and chalk and other carbonates to the atmosphere, as it doesn't have continental drift to subduct them and cycle them around through volcanoes. However this is not a concern for space habitats in the near future - they aren't going to be troubled by the effects of a build up of oil rich shales, limestone and chalk.

For more about all this see my Could Astronauts Get All Their Oxygen From Algae Or Plants? And Their Food Also?

NITROGEN

Moon has nitrogen too, in the form of ammonia at the poles. If it is correct that there are 600 million tons or more in the form of ice up to 2 meters thick, with 6% ammonia, then there may be 25 million tons of nitrogen there .

We need nitrogen as a buffer gas in the atmosphere to protect us from oxygen toxicity.

There is another way to avoid oxygen toxicity, and that is, to use oxygen at low pressures, as they do for spacesuits. Spacesuit gloves are stiff and difficult to use and full Earth pressure would make them much harder to use. Apollo also used a pure oxygen atmosphere even after the fire of Apollo 1, as a simpler system, but you have to take care to use materials that won't burn easily. Since Apollo all space flights have used mixed oxygen / nitrogen for habitats and pure oxygen only for spacesuits. So, lunar habitats will surely do the same.

We don't have to keep resupplying nitrogen to a space habitat because it is a one off amount of mass (plants use nitrogen but it's part of a nitrogen cycle so could be returned to the atmosphere using denitrifying bacteria, and anyway compared to carbon, it is a small amount of the total mass of the plant). So how much mass do we need to provide?

Well, ignoring reserves of nitrogen, we can work out how much is needed for the habitat air itself. The pressurized volume of the ISS is 32,333 cubic feet or around 915.5686 cubic meters. At 1.225 kg / m³, at Earth sea level pressure which is what they use, it's 1.122 tons of air. So that's less than a ton of nitrogen, and of the total mass of the ISS, 419.725 tons, only 0.27% is atmosphere. As for CO₂, at 400 ppm, that much air would contain a negligible half a kilogram (0.04% of 1.122 tons)

So, it's useful to be able to get your nitrogen and oxygen in situ, but it's not a deal breaker if you have to get it from Earth if you have decent closed system recycling.

It's the same for any size of habitat, for city domes too, the atmosphere is small fraction of the total mass, not including regolith shielding, just the unshielded habitat mass.

For a large habitat such as the Stanford Torus then the atmosphere is a higher percentage of the total mass, but that's because it has less structural mass needed, because the surface area (heaviest part) goes up only as the square when the volume goes up as the cube.

For the Stanford Torus, the structural mass is 150,000 tons, atmosphere 44,000 tons, for 10,000 people.Per person that's 15 tons of structural mass and 4.4 tons of atmosphere, not counting the regolith shielding (which they planned to send from the Moon using bulldozers and a mass driver)..

For the ISS, with 6 people, 419.725 tons, that's about 70 tons per person of which 0.187 tons is atmosphere That's a fair bit of atmosphere. There are more efficient ideas for the Stanford Torus, the Vademecium design which has a flatter torus so reducing the amount of mass for atmosphere. However, the reduction in structural mass compared to the ISS more than compensates.

The situation is the same for most large space habitats. The larger it is, the less structural mass per person but the more atmosphere per person if it is similar in shape to a smaller habitat. Venus cloud colonies are different, they have much lower mass requirements per person similar to an airship, and in principle, they can get all of their atmosphere from the Venus atmosphere too.

Nitrogen doesn't seem likely to be a major issue in the early stages at least. It may be more of an issue if we wish to fill an entire lunar cave with nitrogen, more on that below: Can we fill lunar caves with air.

LUNAR CAVES

We can only see a few meters into the lunar caves from the surface, so we don’t know how far they extend, especially since the regions near the pits are probably partly filled in with debris as well. But they could be huge; potentially they can be large enough to fit in a large city, the size of Philadelphia, with space to spare

• • Lava tube caves on the Moon could be stable up to five kilometers wide in the lower gravity. The black silhouette here shows the city of Philadelphia superimposed in one of these suggested tubes. We know there are rills on Mars this wide and can see cave entrances into them on the surface, in photographs taken from orbit, but we can't see far into them so don't know how large the caves are yet. These huge lava tube caves may have been detected indirectly though, through gravitational anomalies in the Grail spacecraft measurements: Scientists May Have Spotted Buried Lava Tubes on the Moon - see also Grail data points to possible lava tubes on the moon.

Such huge caves are only possible because of the low lunar gravity, as they would collapse on Earth. Similar caves on Earth are far smaller as would be any similar caves on Mars. We don't know for sure if such large caves do exist, but it does have many cave entrances photographed from orbit, which proves that at the least, it has caves with entrances similar in size to Earth cave entrances. Then the extensive systems of rills and the Grail data are suggestive of larger caves to be discovered.

Some of the possible lava tube gravitational signatures are over 100 kilometers long and several kilometers wide. If the Moon does indeed have caves 100 km long and kilometers wide, that's similar in size to the O'Neil cylinder space habitat with a land area of several hundred square miles (the O'Neil cylinder consists of a pair of cylinders, each 20 miles long and 4 miles in diameter, with total land area 500 square miles).

Each such cave could house several million people. This may be a long shot, but isn't it amazing, to think that the Moon could have caves as vast as this, similar in size to an O'Neil cylinder, and we simply wouldn't know yet?

EXAMPLE LUNAR CAVE SKYLIGHTS - LACUS MORTIS, MARIUS PIT AND THE KING-Y NATURAL BRIDGE

The Lacus Mortis area has possible volcanic cinder cones, as well as the more common shield volcano features, rilles, and a partially collapsed cave entrance with a gentle slope leading into it. This was the proposed destination for the Astrobiotics mission in 2014.

Partially collapsed "skylight" in the Lacus Mortis region of the Moon.

Photos of the Lacus Mortis pit from various angles, which were used to build a 3D model of the pit, assuming that it is a cave entrance.

3D model shown from various angles. The cave was assumed to be oval shaped as a result of fill by debris form the collapse - further from the entrance, if it's a lava tube cave, it should widen out to a circular cross section.

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One of two possible volcanic cinder cones in the Lacus Mortis area. Though the Moon has many rilles and shield volcanoes, volcanic cinder cones are very rare indeed, if that is indeed what this is.

Another interesting pit is the Marius Hills pit entrance, original destination for astrobiotics:

The "skylight" on Marius hills (see page 7) was the original objective for the astrobiotics Skylight mission as envisioned in 2013 - it may be an entrance to a much larger lunar cave as it is located on a lunar rille

This shows the topography - it's about 40 meters deep The crispness of the landform suggests the collapse happened less than a billion years ago, and the lack of any raised rim or eject suggests it formed through collapse, not through a meteorite impact.

This image shows an oblique view. It's viewed from an angle of 45 degrees, and the light from the sun is at an angle of 34 degrees from the vertical. As a result they were able to confirm that the area of the floor illuminated in this image continues at least twelve meters under the overhang. Papers here, and here .

This shows the location of the Marius pit along a lunar rille. Image from page 5 of Exploration of Planetary Skylights and Tunnels

Another "honorable mention" goes to the region of King crater, which is of special interest for its remarkable natural bridge.

Lunar natural bridge feature King Y, probably caused by a double collapse. It's about 7 meters in width and a 20 meters walk to cross it.

The lunar caves may also have unusual minerals that formed as the lava that created the cave slowly cooled and differentiated.

The NASA PERISCOPE project, currently a phase II concept study, could potentially give us a way to see into lunar caves from orbit using femtosecond laser photography which lets you "see around corners" to parts of the cave that were never within the line of sight of the orbiter.

We may may get our first views into the interior of a lunar cave from ground level some time in 2017, with the Japanese Hayuto Lunar X prize contender Moonraker, which will explore the Lacus Mortis pit "skylight" and then lower its two wheeled rover Tetris into the pit . For details of this mission, see Robotic missions to the Moon, already planned, or near future, from 2017 onwards, below.

See also Lunar caves as a site for a lunar base

VOLATILE RESOURCES - INCLUDING ICE

We have pretty good evidence now of ice at the poles, in permanently shadowed craters, thought to be relatively pure and at least a couple of meters thick according to radar data from a NASA instrument flying on India's Chandrayaan-1 lunar orbiter.

It's not a direct detection however, so there is still room for skepticism about it, as rough material would have the same radar signature as radar transparent ice. But craters that are rough when new, are rough both inside and outside the crater rim. While these signatures are found only inside the craters and not outside the rims, which they interpret as meaning that they are caused by ice. The temperatures are also right for ice.

If it is ice, it could be "fluffy ice".

"We do not know the physical characteristics of this ice—solid, dense ice, or “fairy castle”—snow-like ice would have similar radar properties. In possible support of the latter, the low radar albedo and lower than typical CPR values for nonanomalous terrain near the polar craters are 0.2–0.3, somewhat lower than normal for the nonpolar highlands terrain of the Moon and are suggesting the presence of a low density, “fluffy” surface."

(page 13 of Evidence for water ice on the moon: Results for anomalous polar)

In either case, it is not just a little ice; if this is what they detected, there's estimated to be at least 600 million metric tons of this, and possibly much more.

It also contains other volatiles. We know for sure that there is some ice on the Moon, by the LCROSS impact experiment. Relative to H₂O at 100% they found H₂S at 16.75%, NH₃ at 6.03% SO₂ at 3.19%, C₂H₄ at 3.12%, CO₂ at 2.17%.

So, if the rest of the ice at the poles has a similar constitution to the impact site that's a lot of nitrogen (in the ammonia) and CO₂ on the Moon at the poles.

• • • The green circles here surround craters at the lunar north pole thought to have layers of ice, with an estimated total of at least 600 million metric tons of water.

    • (0 degrees longitude at bottom)

On the other hand, caution is needed as this is not direct detection. The LEND results (searching for hydrogen through reduced emissions of neutrons of a particular type) are particularly puzzling, as there is almost no resemblance between their map and the miniSAR map.

LEND map - in this picture blue is reduced neutron emission and shows likely locations of hydrogen. 0 degrees longitude is at the top.

They did detect hydrogen, but puzzlingly, it was not correlated with the permanently shadowed regions - there was some hydrogen in permanently shadowed regions, and some also in illuminated regions. A recent paper suggests that ice mixed in the regolith in illuminated regions may be ancient ice that survived a minor shift of the lunar axis. According to one hypothesis, this may be ancient deposits from over three billion years ago before volcanic activity, which changed the polar axis slightly by shifting material.

A new LEND mission has been proposed involving low passes over the poles at altitudes as low as a few kilometers, for higher resolution results.

The Moon may also have ice at lower latitudes too, as there are permanently shaded regions up to 58 degrees from the poles (only 32 degrees from the equator). Though these regions are too warm to have ice on the surface, there may be ice there underground. See Ice may lurk in shadows beyond Moon's poles (Nature, 2012).

At any rate, the Moon does seem to have resources of ice at the poles (though memorably, Patrick Moore in one of the last Sky at Night programs that he did said that he'd believe there is ice at the poles when someone brought him a glass of water from the Moon). More research is needed to find out how much there is and where it is.

Some scientists - particularly Arlin Crotts, think it may have ice several meters below the surface over the entire planet, and that it may have volatile resources deep down. There are signs that suggest it is still geologically active, and one possibility is that the activity may be due to volatiles deep down escaping to the surface. For more on this, see Geologically active moon

METALS

Critics often say that the Moon is undifferentiated and doesn't have any processes to concentrate ores. Although the Moon doesn't have any liquid water so all the processes involving concentration of resources through water erosion won't work, it still has many processes that can concentrate ores. Including:

• Fractional crystallization - as a melt cools down, some minerals crystallize out at a higher temperature than others so form first. They then settle or float, so remove the chemical components that make them up from the mix, so changing its formula, leading to new crystals to form in a sequence.

• Gravitational settling, lower mass material floats to the top.

• Volcanic outgassing can concentrate materials such as iron, sulfur, chlorine, zinc, cadmium, gold, silver and lead.

• The processes that lead to volatiles condensing at the poles - which it seems can also concentrate silver too

• Processes unique to the Moon (perhaps electrostatic dust levitation may concentrate materials)?

• Volatiles brought in as part of the solar wind

• Asteroid and micrometeorite impacts bring materials from asteroids to the lunar surface such as iron and possibly platinum group metals etc.

The Moon has many valuable ores for metals. For instance, the highland regions (probably the original crust of the Moon) consists mainly of Anorthite (a form of feldspar, formula CaAl₂Si₂O₈) which is 20% Aluminium, compared with 25% Aluminium for Bauxite on Earth. So aluminium ores are abundant on the Moon, indeed orders of magnitude more abundant than they are in typical asteroids, but it does require a lot of energy to extract the aluminium from the ore. Either a nuclear power plant or large areas of solar panels. Crawford, in his "Lunar Resources: a Review"* , says this about aluminium on the Moon:

"Aluminium (Al) is another potentially useful metal, with a concentration in lunar highland regoliths (typically10-18 wt%) that is orders of magnitude higher than occurs in likely asteroidal sources (i.e. ~1 wt% in carbonaceous and ordinary chondtites, and <0.01 wt% in iron meteorites; . It follows that, as for Ti, the Moon may become the preferred source for Al in cis-lunar space. Extraction of Al will require breaking down anorthitic plagioclase (CaAl2Si2O8), which is ubiquitous in the lunar highlands, but this will be energy intensive (e.g. via magma electrolysis or carbothermal reduction; Alternative, possibly less energy intensive, processes include the fluoridation process proposed by Landis , acid digestion of regolith to produce pure oxides followed by reduction of Al2O3 (Duke et al.), or a variant of the molten salt electrochemical process described by Schwandt et al."

Mining this for the aluminium would create calcium as a byproduct, which is useful as a conductor in vacuum conditions, a better conductor than copper weight for weight - you need half the mass for the same amount of electricity. (Copper does better than calcium on a per volume basis because it is 5.8 times denser, it is also of course much more practical in an atmosphere because calcium reacts vigorously with air, but that's not a problem for conductors that operate in a lunar vacuum, and in space applications the reduced mass may be an advantage).

"Calcium metal is not used as a conductor on Earth simply because calcium burns spontaneously when it comes in contact with oxygen (much like the pure magnesium metal in camera flashbulbs). But in vacuum environments in space, calcium becomes attractive.

"Calcium is a better electrical conductor than both aluminum and copper. Calcium's conductivity also holds up better against heating. A couple of figures mining engineer David Kuck pulled out of the scientific literature: "At [20C, 68F], calcium will conduct 16.7% more electricity than aluminum, and at [100C, 212F] it will conduct 21.6% more electricity through one centimeter length and one gram mass of the respective metal." Compared to copper, calcium will conduct two and a half times as much electricity at 20C, 68F, and 297% as much at 100C, 212F.

"Like copper, calcium metal is easy to work with. It is easily shaped and molded, machined, extruded into wire, pressed, and hammered.

"As would be expected of a highland element, calcium is lightweight, roughly half the density of aluminum. However, calcium is not a good construction material because it is not strong. Calcium also sublimes (evaporates) slowly in vacuum, so it may be necessary to coat calcium parts to prevent the calcium from slowly coating other important surfaces like mirrors. In fact, calcium is sometimes used to deoxidize some metal surfaces. Calcium doesn't melt until 845C (1553F).

"Utilization of lunar materials will see the introduction of industrial applications of calcium metal in space."

From the section on Mining the Moon in Permanent - by Mark Evan Prado, a physicist in the Washington, D.C., region working for the Pentagon in advanced planning in the space program.

The Moon is deficient in copper, at least on the basis of what is known so far, but as well as calcium, aluminium is a good conductor.

The LCROSS experiment found silver (a superb conductor) and mercury at the impact site, but the concentration is not known, except that it is far higher than the levels in the Apollo samples, and is probably in a layer below the surface, as the signal was delayed. See LCROSS mission may have struck silver on the moon.

It has abundant iron. In addition to ores (which would need a lot of power to extract), it actually has free (unoxidized) iron metal (see section 5, Metals from Crawford)"* . Some of this is nanophase iron mixed in with glass, and hard to extract, but some of it consists of small particles of iron (less than a micron in diameter) mixed in with the regolith, and it may be possible to extract it with magnets. I will go into this in more detail in the next section.

Mars doesn't have any free iron except for the occasional rare iron meteorites.

The iron is valuable for steel, and is also a conductor, though not nearly as good as Aluminium or Calcium. It would be useful for some applications such as electric railroads on Mars, and is a conductor easy to access in the early stages.

However it also contains a fair amount of nickel. Nickel and iron are useful for making nickel / iron batteries. These could be useful for making batteries on the Moon with in situ resources, for instance to help last through the lunar night.

"Iron-nickel batteries are very rugged. Their lifetimes which can exceed 20 years are not affected by heat, cold or deep cycling. They are not easily damaged by rapid discharging or over-charging. On the downside, they have poor performance at low temperatures but they can be kept warm with insulation (e.g. simple regolith) and thermal wadis. Also, they only have a charge to discharge efficiency of 65% and will self discharge at the rate of 20% to 40% per month. Despite these shortcomings, they might be the Moon-made power storage systems of choice due to their simplicity and the availability of their component materials on the Moon. Moreover, these materials are among the easiest of materials to produce on the Moon."

See Electrical Energy Storage Using Only Lunar Materials.

Then, you also have titanium. This is especially interesting as it is rare in asteroids. Apollo 17 samples are 20% high purity Ilmenite, a Titanium ore which is found in the lunar mare. And better than that, the Lunar Reconnaissance Orbiter, with its spectral mapping of the Moon, discovered deposits that are up to 10% titanium, more than ten times higher than titanium ores on Earth. (Phys.org report, NASA image). Titanium is an industrially desirable metal, stronger per unit weight than Aluminium (though it is a poor conductor).

Titanium is also widely used in medicine for hip replacements, dental implants, etc., as "one of the few metals human bone can grow around firmly", see also this new titanium / gold alloy four times tougher than titanium

Titanium is especially useful for medical applications because it

• Forms an inert and stable titanium oxide layer spontaneously

• Has a high strength to weight ratio

• Doesn't leach into blood and other aqueous environments because of its low rate of ion formation

• Is one of the few materials that can integrate directly into living bone tissues (osseointegration) without any soft tissue layers in between

Crawford writes (page 17)"* :

"Therefore, in the context of a future space economy, the Moon may have a significant advantage over asteroids as a source of Ti. The fact that oxygen is also produced as a result of Ti production from ilmenite could make combined Ti/O2 production one of the more economically attractive future industries on the Moon.

For more on this, see major lunar minerals. And for an in depth study, read Crawford's review"* .

So, yes, there are plenty of metals on the Moon, but it might take a lot of power to extract them, apart from the iron, if that can be separated out using magnets.And that's mainly based on the Apollo results which explored a small region of the lunar surface which has been found to be in some ways unrepresentative. The Moon may have many other surprises in store.

Many ores on Earth would not be detected from orbit, and it seems the Moon has a fairly complex geology as well from the Apollo results and from the lunar mapping showing variation in concentrations of various metals.

Global map of iron oxide concentrations on the lunar surface, with the colours showing 2% increments from black (0%) to white (16%). (Source: NASA/Clementine) - The method used is described here (and briefly here).- and paper covering it in detail with an earlier version of the map here.

What they did is to compare the light reflected at just two wavelengths 750 and 950 nm and also look at the effects of space weathering which darkens the soil so there are two things there - the ratio of the intensities and the intensity at 950 nm -and then by doing that they are able to work out the iron concentration at a particular point. Then to interpret this they used ground truth from the returned Apollo samples. It's rather indirect and only possible because of the Apollo ground truth.

They also found similarly large variations in the thorium and potassium content.

The big splodge on the second, far side image, is probably due to the large impactor that created the Aitken basin digging up materials from the lower crust and mantle that are likely to be richer in iron oxide.

As one example of one way the Moon could surprise us - Earth is often hit by iron meteorites, so the Moon should be also.

Dennis Wingo has hypothesized in his Moonrush book, that the Moon may also have valuable platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum) which could be mined, the result of the impacts of these iron meteorites. Iron meteorites often have high concentrations of these metals, and gold also (which doesn't count as PGM).

Taking this further, there's a hypothesis by Wieczorek et al that magnetic anomalies on the Moon around the south pole Aitken basin may result from the remains of the metal core of a large 110 km diameter differentiated asteroid that hit the Moon to form the basin. If so, they could be useful sources for platinum, gold, etc.

From Wieczorek et al, the North and South poles are marked N and S. Notice the magnetic anomalies clustered around part of the rim of the South Pole Aitken Basin. This is thought to be the result of an impact by a 110 km diameter asteroid. Wieczorek et al hypothesize that the magnetic anomalies trace out the remains of the metal core of this asteroid. If so these could be rich ores, including iron, nickel, also platinum and other platinum group metals (gold, rhodium etc). See page 16 of Crawford's Lunar Resources: A Review*

Platinum is a particularly useful metal (the other PGM's have similar properties to platinum and are also useful). It is heavy, soft, malleable as gold and silver, easy to draw into wires, very unreactive, and has a high melting point. Out of gold, silver, platinum and copper, platinum is the densest and the hardest and the least reactive (the others are somewhat better in terms of electrical and thermal conductivity, and malleability, but it's not too bad at those either).

So, it's not just useful for catalytic converters, fuel cells, dental fillings and jewelry. We'd probably use it a fair bit in other ways too if it didn't cost so much.

The platinum group metals might be valuable enough to return to Earth from the Moon, just as suggested for the asteroids, depending on how easy they are to return. Of course, you can't just take the current market value of platinum, multiply by the amount of platinum available in a large meteorite - or on the Moon if Wingo and Wieczorek et al are right - and conclude that you'd get trillions of dollars by returning all that platinum to Earth and selling it here.

You need to fulfill a need or eventually nobody will buy it, and whatever you use it for it has to be worth the expense of returning to Earth. If it's just to replace copper, for instance, in wires, it wouldn't be worth returning unless you could reduce the transport cost back to Earth right down.

Dennis Wingo suggested in Moonrush that it could be worth exporting it to Earth for use for fuel cells, as an application that could be high value and yet need a lot of platinum.

The gold could be useful too, on the Moon at least. You don't normally think of gold as more decorative than useful but it is used a fair bit in electronics.

Also when gold is combined with the abundant titanium on the Moon you get Ti₃Au, an alloy with 70% less wear, four times the hardness and increased biocompatibility compared with pure titanium (and twice as hard as titanium / silver and titanium copper alloys). It's also 70% less wear than titanium, lower friction and four times harder with a hardness of 800 HV in the Vickers hardness test. Density about the same as steel.

(density of titanium: 4.43 g/cc. using the atomic masses of gold and titanium, multiplying by (196.96657+3*47.867)/(4*47.867)*4.43 = 7.88 approx. By comparison, density of steel is 7.75 g / cc).

Crawford's paper focuses on its medical applications, you can alloy titanium with copper or silver, which are twice as hard as pure titanium, but this is four times as hard. It's also 70% more resistant to wear which will make it last longer and lead to less debris. And has excellent biocompatibility properties. But I wonder if it might also have lunar applications, with the hardness especially and resistance to wear.

Probably only the platinum group metals would be worth returning to Earth, unless the costs of transport back to Earth go down considerably (maybe through the use of Hoyt's cislunar tether transport system ). However, whether or not they are useful for Earth, they are well worth using on the lunar surface once you have industry there.

The Moon has some advantages over Mars for metals, such as the pure nanophase iron mixed in with the regolith, which can only exist in oxidized form on Mars except for rare metal meteorites.

Also, it's unlikely it will be commercially worthwhile to return metals from Mars while there are definite possibilities of returning metals from the Moon. See Exporting materials from the Moon for future suggested low cost methods for export from the Moon. For discussion of whether anything physical could be worth the expense of export from Mars, see Commercial value for Mars

POSSIBILITY OF EXTRACTING METALS FROM LUNAR REGOLITH

Crawford touches on the idea of extracting metals from the lunar regolith using magnetic sieving. There are practical difficulties to overcome, but if they could be, it might be the easiest way of all to extract metals from the Moon, especially if you choose sites with high concentrations of meteoritic iron

As we saw in the last section, Clementine found areas with more than 16% iron oxides with a low resolution survey from orbit around the Moon - but that's not meteoritic iron but rather, most likely, subsurface iron oxides brought to the surface. As for meteoritic iron they can't seem to detect it directly from orbit but the magnetic anomalies may mark out the effects of meteorite impacts on the Moon, so those might be associated with regions where the regolith is enriched in meteoritic metal.

So - let's look at this a bit closer, and see what research so far suggests.

HOW MUCH IS THERE IN THE REGOLITH?

The free iron on the Moon has three main sources:

• Directly from iron meteorites that impact on the Moon.

• Nanometer sized "blebs" released from the rock by the hydrogen in the solar wind reacting with iron oxides

• Particles of iron released from the bedrock as a result of the impacts.

It's in powder form already, and naturally alloyed with nickel and cobalt. The blebs, or "nanophase iron" are found inside impact glass particles, so would be hard to extract. Some of it is found also as thin patinas less than a micron thick on the surface of soil grains (see page 189 of this paper). *

The rest is made up of tiny particles of pure iron, so the obvious thing to try to do is to separate them out using powerful magnets. They are rather small though, most are less than a micron in diameter which could be a challenge.

Crawford in his paper from 2014 tries an estimate at possible concentration (section 5, Metals) *

"A more readily available source might be native Fe in the regolith, although the concentrations are quite low (~0.5 wt%; Morris, 1980). Native Fe in the regolith has at least three sources meteoritic iron, iron released from disaggregated bedrock sources, and iron produced by the reduction of iron-oxides in the regolith by solar wind hydrogen. The latter component occurs as nanometre sized blebs (often referred to as ‘nano-phase Fe’) within impact glass particles (‘agglutinates’) and is likely to be difficult to extract.

Morris (1980) found the average concentration of the other two components to contribute 0.34±0.11 wt%. At least in principle this might be extractable, perhaps through some form of magnetic sieving, but given the very small sizes of the individual Fe particles (generally < 1µm), the practicality of such a process is uncertain.

...If the practical difficulties could be overcome, a native Fe concentration of ~0.3 wt% is not an entirely negligible amount, corresponding to about 5 kg for 1 m3 of regolith. Moreover, meteoritic iron will be associated with siderophile elements, some of which (e.g. nickel, the PGM's and gold) are valuable owing to their catalytic and/or electrical properties"

He goes on to say that ,if we can separate them out, then from a cubic meter of regolith, then that 0.3 % by weight gives five kilograms of iron, 300 grams of nickel and about 0.5 grams of platinum, rhodium, palladium etc. (platinum group metals), and as pure metal, not the metal oxides. He doesn't say the quantity of gold but perhaps it would be around 0.01 grams of gold (at typical 1 ppm).

He bases the iron figure there from on a paper from 1980 by Morris and particularly its conclusion that the lunar soils have 0.5% by weight of iron, and their suggestion that 40% of that may consist of particles 40 - 330 Angstrom (0.004 to 0.033 microns). This uses a model to interpret the data.

He bases the PGM's on a 1994 Kargel paper, which gives a figure of 99.5 ppm of Platinum Group Metals (PGM's) for the top 10% of Fe bearing asteroids. It's 100.1 ppm when you add gold to the PGM's

This figure may need to be reduced if you use the average quantities of PGM's

Though, I wonder if that figure is a suitable one to use for the richest deposits on the Moon?

Crawford writes:

"Much higher localised concentrations of native Fe, and associated siderophile elements, could potentially be found in the vicinity of any Fe-rich meteorites which partially survived collision with the lunar surface"

So - could there be deposits on the Moon with varying chemical composition depending on the accident of which type of M type meteorite hit that particular location in the distant past?

Here I'm just speculating myself.

On that reasoning, his top 90% composition seems likely to apply to at least some ares of the Moon with debris of the larger meteorite impacts. And the value of the deposits for the Aitken basin impactor might then depend on the PGM concentrations of its iron core, and there could potentially be other deposits that are more valuable if they have higher concentrations of PGM's

MANY QUESTIONS ABOUT PLATINUM IN LUNAR REGOLITH

So - how much of the lunar regolith is pure metal? And how much consists of those hard to extract "blebs"? And of the metals that might be extractable,, how much consists of the platinum group metals?

You'd think it would be easy to answer these questions - at least for the samples of lunar regolith returned from the Moon and from the work on asteroid composition and evidence from iron meteorites on Earth. However I haven't been able to find much detailed work on this topic in my Google Scholar searches.

Most of the asteroid mining articles I found seem to refer back to those papers by Kargel in 1996 and 1994 and an earlier paper from 1973. This is a 1994 Kargel paper, it has 21.5+4.0+16.5+14.5+14.0+29.0 = 99.5 ppm of PGM for the top 90th percentile in richness of Fe bearing asteroids.

This 1973 paper is behind a paywall for me, but it's summarized in table I of this paper.

Based on the Iridium concentrations, multiplied by 7, it gives an estimate of the total concentration of PGM's of 75 ppm for the top 10% of meteorites for type IIAB iron meteorites (see chemical classifications of iron meteorites). They give an average of 7 ppm. That compares to 2-6 ppm for good terrestrial mines. See section 3. Accessibility in this paper.

Also, what about metal particles eroded from the bedrock?

This paper from 2012 has evidence of a pure platinum / rhodium / iron alloy particle in the regolith.They give evidence that it originated from the lunar geology, not from meteorites, saying

"This provides mineral embodiment of the geochemical rhodium anomaly occurring in basaltoids from this area of the Moon"

Again this doesn't tell us anything about quantities. But it it does show that the regolith has some particles of platinum alloys eroded out of the basalt.

If any of you reading this know more do say. And I'm going to try to find out more by contacting researchers in the field.

HOW MANY CRATERS ON THE MOON ARE FORMED BY IMPACTS OF PLATINUM RICH IRON METEORITES?

(WORK IN PROGRESS ROUGH CALCULATION)

This is a preliminary rough idea, to get us started, thinking about the possibilities. From figure 1 of this paper about 4% of NEOs are M-type. So we are talking about 0.4% of all lunar craters likely to be formed by iron rich meteorites. There I'm using the present day percentage of M type NEOs from the present day as an estimate for the percentages that have hit the Moon in the past - a simplification but I don't know of a better estimate.

So, then, how many craters are there? As a very rough estimate here, there may be half a million craters > 1 km on the Moon, lowest estimate he comes up with is 181,000 just rough estimates:

Let's use 100,000 as the figure. Then it looks like there may be at least 4,000 craters of 1 km or larger by M type asteroids.

I'm not sure how the impactor size to crater size works on the Moon, but again a rough guess maybe 100 meters or larger. Then the ones in the top 10% would leave us with 400 craters of interest.

Each one would consist of around 4 million tons of iron (7.874*(4/3)*PI*50^3) and so, at 100 ppm, that's around 400 tons of PGM's for each one. However only some of that would be on the surface.

The world platinum production is 161 tons.

If you go to craters above 20 km in diameter we have an exact figure of 5,185 craters. Out of those our 0.4% would give a probable 20 large craters in the top 10% of meteorite PGM abundances - and another 180 with lower abundances. And now you are talking about large quantities, 8,000 times the mass for those smaller craters, using rough estimate that crater size scales by impactor size, so 20 times the diameter means 8000 times the volume.

Some of that metal may be in the form of metal meteorites instead of mixed up in the regolith, or buried pure ore deposits.

COULD WE EXTRACT IT?

So could we extract the metals if they are there? Well there is one interesting experiment here from 2013. Jayashree Sridhar et al of the NASA Johnson Space Center have done the experiment using actual samples of lunar regolith. See Extraction of meteoritic metals from lunar regolith.

They say that the problem is daunting because much of the metal is in the form of nanophase iron deposited on the rims of soil grains, which would not be easy to extract. The main issue is to find a way to separate the pure meteoritic iron.

Anyway their experiment is not meant as a model to show how to extract the metals on the Moon. It's just an initial study.

They ground up the regolith and immersed it in isopropanol. Then they tried various arrangements of magnets. Their most successful run had an arrangement of four magnets, two strong and two weak, with the regolith ground to a medium size of 1.5 microns. They were able to extract 61 particles of meteoritic metal and 9 of nanophase iron l (so 87% by number were meteoritic metal) with the amount of nanophase iron going up and meteoritic metal going down when they repeated the process. With bulk regolith (not ground up) the pattern was similar but with fewer particles, only 9 and 2 respectively for the first run.

They concluded:

Experimental results indicate promise for the extraction of meteoritic metals from lunar regolith. However, more work is needed to refine the technique and understand more about the variables that affected our results.

Sadly, there is nothing in the paper to give the masses of the original samples or the extracted particles or their PGM abundances.

If any of you reading this know more, do say!

HOW TO SEPARATE OUT THE PLATINUM GROUP METALS - AND THE IRON AND NICKEL - MOND PROCESS

Of the various suggestions for ways of extracting the PGM's, the Mond process seems most promising to me - just the second half, as you already have the pure metals.

Perhaps lunar miners could use something analogous to Bruce Damer's idea for asteroids of using CO in an enclosure warmed by the sunlight and iron and nickel extracted in attached 3D printer - with the PGM's as residue.

So then the other metals would be used on the Moon - and then you return the residue to Earth for more processing as a very concentrated source of PGM's and gold. Paul Spudis suggests this in this article- the residue would be 0.5% PGM by weight so that's 5000 ppm at that point - and if you can get that process to work on the Moon it may be economically worth returning the residue to Earth for reprocessing.

I don't know if it is going to be commercially viable to return the PGM's - but if it is going to be viable at all, to set up a space settlement supported by commercial industry - it seems most likely to be viable on the Moon perhaps even before asteroids - given the easy accessibility (the lowest delta v asteroids have orbits most similar to Earth's and so phase in and out slowly so are most accessible only once every decade or so) and also the low costs of transport to Earth.

Those advantages of ease of access to the Moon and easy export may offset the extra step you need on the Moon to extract the metal from the regolith. Also perhaps there may n be fragments of the iron meteorites that impacted on the Moon much as there were on Earth, which would help kickstart the industry by providing metals that could be exported very easily.

All that's especially true if we can build Hoyt's cislunar tether transport system, powered by the difference in gravitational potential between the lunar surface and the Earth (so moving materials from the Moon to Earth is actually moving it "downhill" and you can exploit that through clever use of momentum exchange tethers).

I think ti's a reasonable argument that the Hoyt tether system could reduce the amount of fuel needed to return materials to Earth to almost zero with the movement of materials from Moon to Earth actually generating power.

If we have robotic flights to the Moon as often as once a week, e.g. maybe when Skylon is operational - or some other reduction of costs - then it would save mass to build Hoyt's system within just one year of operation, and it can be done with existing materials. At that point Kargel's scenario of large quantities of PGM available to return to Earth might well come into play.

I'm a little surprised that Hoyt's system does not get more attention than it does, given how much of a game changer it could be. With easy access then platinum exports from the Moon may be amongst the most viable - that's the key idea behind Dennis Wingo's "Moon Rush". It's the main export that's most often mentioned. The others are ice, covered above under Volatile resources, which I think has an excellent case - and Helium 3 which is not so clear cut as we will see below.

Then another export is of power in the form of solar power, which I cover below in Possibility of using lunar solar power for Earth

LUNAR GLASS

Larry Taylor was lucky enough to get hold of enough of the genuine lunar dust from Apollo to experiment with and he found it was as easy to make the dust into glass as it is to boil a cup of tea with a microwave. This doesn't work with the regolith simulants. The genuine lunar dust is mingled with iron intimately as a result of space weathering by micrometeorites and that's why it is so easy to microwave.

This is a beneficial side effect of all the micrometeorite impacts on the Moon (which you don't get so much on Mars with its thin atmosphere, just enough to filter out micrometeorites). The Moon's "soil" or regolith contains large quantities of glass, created during the impacts. It also has free iron, as we saw, at half of one percent of the soil, in tiny micro beads of iron (nanophase iron) which concentrate the microwave energy. Again, you don't have this on Mars.

As a result, it is really fast to melt the regolith using microwaves. It took only 30 seconds to melt small lunar sample at 250 watts (typical of a domestic microwave). You can melt the soil to glass as easily as you can boil water using the microwave in your kitchen. See lunar lawnmower. This only works with genuine lunar soil and not the simulants. We have nothing analogous to lunar soil on Earth, as Larry Taylor, principle author of this paper found: Microwave Sintering of Lunar Soil: Properties, Theory, and Practice. He says the microstructure of the genuine lunar regolith, with nanophase iron beads scattered throughout, would be almost impossible to simulate.

His idea (see Products from Microwave Processing of Lunar Soil on page 194 of the paper) is to run a "lunar lawnmower" over the soil with two rows of magnetrons (such as generate microwaves in a microwave cooker). The first row would sinter it to a depth of half a meter using microwaves. Then the second row completely melts the top 3-5 cm of the soil, which then crystallizes to glass. As it does this, it will heat up and release most of the solar wind particles notably hydrogen, helium, carbon and nitrogen. So it could also capture these assets as it goes along, including the Helium 3, if this turns out to be of economic value.

He has various ideas of how this could be useful, for instance for tracks on the lunar surface, or a landing pad. He put it like this, interviewed by NASA:

"Picture a buggy pulled behind a rover that is outfitted with a set of magnetrons," that is, the same gizmo at the guts of a microwave oven. "With the right power and microwave frequency, an astronaut could drive along, sintering the soil as he goes, making continuous brick down half a meter deep--and then change the power settings to melt the top inch or two to make a gtlass road," he suggested."

"Technical challenges remain. Sintering moondust in a microwave oven on Earth isn't the same as doing it on the airless moon. Researchers still need to work out details of a process to produce strong, uniformly sintered material in the harsh lunar environment.

But the idea has promise: Sintered rocket landing pads, roads, bricks for habitats, radiation shielding--useful products and dust abatement, all at once.

"The only limit," says Taylor, "is imagination."

Their techy paper is here:The Lunar Dust Problem: From Liability to Asset. Others have suggested this could be used, for a solar panel paving robot to make solar panels, and other applications.

Then, there's Behrokh Khoshnevis' idea for making a landing pad on the Moon using tiles made of lunar glass in situ. The idea is to make the surface into lots of tiles by injecting a material that can't be sintered easily using microwaves into the soil first to outline the edges of the tiles, then use microwaves to melt the soil in between.

(click to watch on Youtube)

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Subscribe to Inverse! https://goo.gl/AGT2uT USC Professor Behrokh Khoshnevis has designed a machine that can build roads and landing pads on the moon and on Mars by fusing interlocking panels of space rock together under high heat. ——— Inverse sparks curiosity about the future. We explore the science of anything, innovations that shape tomorrow, and ideas that stretch our minds. Our goal is to motivate the next generation to build a better world. http://inverse.com

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==

This would make a tiled flat surface for supply vessels to land on. It would also help with the problem of lunar dust by removing dust from the landing area. You canread the details here. He used lunar regolith simulant, so presumably by Larry Taylor's results, it would work even better with genuine lunar samples.

SOLAR CELLS FROM LUNAR MATERIALS - SOLAR PANEL PAVING ROBOT

Once you have glass, so easy to make because of the nanophase iron (see previous section), it might not be such a big step to make photovoltaic cells on the Moon. After all the glass substrate is most of the mass of solar panels.

Here the Moon has another big advantage, the high grade vacuum so you could use vacuum deposition to make the cells in situ. To start with you'd make the cells themselves from materials sent from Earth. Later on you would mine those materials also on the Moon.

This was first tested in the Wake Shield Facility exploiting the increased levels of vacuum in the wake of the Space Shuttle. They were able to grow very pure thin film materials, so demonstrating a path to easy production of solar cells in high vacuum conditions in space, for instance on the Moon.

This is a report from the Center for Advanced Materials at the University of Houston, suggesting the possibility of an autonomous solar powered lunar photovoltaic cell production rover

It would use silicon extracted from lunar materials to make the cells themselves. The panels then are based on low efficiency silicon cells vacuum deposited on glass. This is not easy to do on Earth but would be possible in the ultra high vacuum conditions on the Moon. Techy details of this suggestion are here. The layer by layer details are on a later page here, which describes a thin layer of solar panels with the following composition, thickness shown in microns:

All the elements are sourced locally, and the total thickness is between 4.3 and 28.5 microns. There are various ways you can do the extraction, and, magma electrolysis may be best.

It would require transporting a small mass to the Moon in the form of the rover which then over several years of driving could build a 1 MW facility on the Moon.

Idea for a robot to drive over the surface of the Moon leaving solar panels in its wake wherever it goes, using only indigenous lunar materials to make the panels. The panels would be only 1% efficient, but given that there is no shortage of real estate on the Moon, that might not matter. It might be more important to make the panels in situ without any imports from Earth than to make them highly efficient

Structure of the panels

For making glass on the Moon see the section above: Lunar glass

BASALT (LIKE GLASS FIBER)

The basalt itself is a natural resource. If reasonably pure and consistent in composition, it's ideal for making basalt fibre, which is like glass wool, but much better in some ways. The regolith consists mainly of powdered basalt. So might well be ideal for making basalt fibre. See:

• Basalt Fiber Properties

LUNAR DUST INHALATION COMPARED TO MARS DUST

Both the Moon and Mars have dust, and in both cases also, it's a fine dust much finer than you normally encounter on Earth, so fine that it can't be expelled from our lungs by the cilia, the hairs on the cells that line the airways. The lunar dust is sharper edged, on the other hand the Mars dust is oxidizing, and may also burn your skin.

The Mars dust also has perchlorates in it, at levels 10,000 times higher than on Earth. These are food for some microbes, but harmful to humans as they impair uptake of iodine which is needed for functioning of our thyroid glands, which regulate our metabolism. They may also have other effects. See This chemical might make Mars more dangerous.

Mars perchlorates are also likely to be decomposed by ionizing radiation into the reactive chlorates (ClO₃) and chlorites (ClO₂) which have more serious and immediate effects "such as respiratory difficulties, headaches, skin burns, loss of consciousness and vomiting" (quote from page 3 of this paper). Mars also has dust storms, which the Moon doesn't have - and it's likely to be harder to keep dust out of a habitat on Mars if you have to go out during a dust storm.

Though we have the reports of the astronauts and samples of dust to study, still there's a lot we don't know about lunar dust. The main problem is that the lunar dust would have been "activated" by UV and proton radiation with more reactive chemicals, dangling bonds, etc. See Toxicity of lunar dust which goes over what we know as well as the many knowledge gaps.

Mars and the Moon are rather similar here. In both cases the dust is both a hazard and a resource. For Mars, see Perchlorate on Mars: A chemical hazard and a resource for humans. For more on the Moon, see "Risk of Adverse Health Effects from Lunar Dust Exposure" (Chapter 13, page 317 of Human Health and Performance Risks of Space Exploration Missions from the NASA Human Research Program, 2009). In the case of the Moon we also have actual experience of humans inhaling the lunar dust, and can also examine and test the dust returned to Earth. Some of the astronauts found it a sensory irritant, but they did tolerate it fine short term. That's just for a few days however.

In both cases, steps would need to be taken to keep the dust out of habitats and spacesuits.

COPING WITH THE DUST

For the Moon, one solution is to make your landing pads into lunar glass, by microwaving the regolith. Turning it into glass gets rid of the dust, and is easy to do, because of the nanophase iron, see the section Lunar glass.

Another option is to exploit the nanophase iron which is mixed in with the dust as part of the impact generated glass. A system of coil magnets attracts the dust and propels it to the next magnet in a kind of "magnetic vacuum cleaner" that could be used to remove the fine dust from the surface which would also be used as a resource.

Figure 2 from this paper

Neither of these solutions would work on Mars of course, or at least not for long, with the dust storms spreading the dust.

The other thing you can do is to use the SuitPort

How it works is that there's no airlock, instead you crawl into it from the back. Like this:

It has separate seals for the spacesuit and for the habitat or rover, with a gap in between. After you get into the spacesuit, you seal them both and then separate from the habitat. You lose about a cubic foot of air, so much less than for a normal airlock. Techy details of how it works here. It is fully automated and, from the suit donning instructions in the patent you can see that everything is initiated by the astronaut with no need for help from inside the habitat.

The big advantage there is that the suit remains always attached to your habitat on the outside, so you never bring it inside. Most of the dust during the Apollo missions came into the habitat on the outside of the suit, so that will get rid of nearly all of it. And you can don it quickly also, as it supports cabin pressure so you don't need to do prebreathing and to use pure oxygen, but just use the same air pressure as the habitat.

There's another more far future idea, which someone came up with in the late 1980s, of a sump type liquid airlock. That would remove just about all dust,with no loss of air, but would mean the spacesuit has to be capable of being submerged in liquid.

The dust can be tolerated short term as the astronauts on the Moon did fine, but some found it a sensory irritant. It might be unhealthy long term though, and the dust is too fine for the cilia in your lungs to get rid of it. So it's a significant issue, though Mars dust would probably be a fair bit more hazardous, so it's still plus points for the Moon here in the comparison.

I think actually that probably in the future human astronauts on the Moon will do a lot more via telepresence and telerobotics than they do at the present in the ISS. On the ISS then the astronauts do EVAs quite often. But it might be that this gets quite rare in the future, with them usually doing the EVAS from inside via telepresence.

Telepresence might seem distancing,but in some ways you might actually experience it more directly, because you wouldn't have a spacesuit in the way and would be much more mobile, with haptic feedback etc. So I think as robotics advance, it's quite possible that doing telerobotic exploration like that may come to be thought of as more immediate an experience than being out there with a spacesuit between you and the lunar surface. That's just my own thought there :). Probably still do some EVAs though and then the dust would be a hazard.

HELIUM 3

I should mention this, since the topic is brought up so often in discussions of lunar settlement. However I don't see this as a major plus point for the Moon at present.

The Moon is a source for helium 3, deposited in the regolith by the solar wind, and some say that helium 3 will be of value for fusion power in the future because it is not radioactive and doesn't produce radioactive waste products. If so, small amounts of helium 3 from the Moon could be worth a lot on Earth and be a useful commodity to export. Apollo 17's Harrison Schmidt is a keen advocate of helium 3 mining on at a reasonable rate at a reasonable rate the Moon.

However, we don't yet have fusion power plants at all, and one able to use helium 3 is a tougher challenge. Frank Close wrote an article in 2007 describing this idea as "moonshine" saying it wouldn't work anyway. Frank Close says that in a deuterium - helium 3 tokamak, at normal temperatures for a tokamak, the deuterium helium 3 reaction proceeds so slowly that the deuterium would instead fuse with itself producing tritium and then fuse with the tritium (the original article is here, but it's behind a paywall). For a critical discussion see also the Space Review article The helium-3 incantation

See also Mining the Moon by Mark Williams Pontin. If you can use much higher temperatures, six times the temperature at the centre of the sun by some calculations, the helium 3 will fuse at a reasonable rate, but these are temperatures way beyond what is practical in a tokamak at present. The reason such high temperatures are needed for a tokamak is because the plasma is in thermal equilibrium and has a maxwellian distribution which means that to achieve a few particles at very high temperatures you have to heat up a lot of particles to lower temperatures to fill up the maxwellian distribution so that just a few will react. This is potentially feasible for the lower temperatures of DT but not feasible for the higher temperatures of ³He³He.

However if you use electrostatic confinement, a bit like a spherical cathode ray tube with the fusion happening at the center where the negatively charged "virtual cathode" is, then the particles are all at the same high energy and the result is much more feasible with lower power requirements. This is the approach of Gerald Kulcinsky who achieves helium 3 fusion in a reactor 10 cm in diameter. However though it does produce power, it produces only one milliwatt of power for each kW of power input so is a long way from break even at present.

Gerald Kulcinski who has developed a small demonstration electrostatic ³He³He reactor 10 cm in diameter. It is far from break-even at present, producing 1 milliwatt of power output for each kilowatt of input. See A fascinating hour with Gerald Kulcinski

Perhaps this line of development will come to something. Perhaps one way or another we will achieve helium 3 fusion as the enthusiasts for helium 3 mining on the Moon hope. However it is early days yet, and we can't yet depend on this based on a future technology that doesn't exist yet.

However even if we do achieve helium 3 fusion, it might not be such a game changer for the lunar economy as you might think. Crawford says (page 25)"* that to supply all of our energy from Helium 3 would mean mining 5000 square kilometers a year on the Moon, which seems ambitious (and would mean the whole Moon would only last 200 years). So, even if we develop Helium 3 based fusion, and it turns out to be a valuable export, it's probably not going to be a major part of the energy mix.

Even more telling, he also calculates that covering a given area of the Moon with solar panels would generate as much energy in 7 years as you'd get from extracting all the Helium 3 from that region to a depth of three meters.

Also - there are many other ideas being developed for nuclear fusion, such as laser fusion, and the Polywell which has the same advantage that no significant radiation is produced when it uses fusion of boron and hydrogen. I think it is far too soon to know whether or not the helium 3 on the Moon will be an asset in the future when we achieve nuclear fusion power. For a summary, see ESA: Helium-3 mining on the lunar surface.

This doesn't mean that there is no point in helium 3 mining however. As Crawford suggests (page 26)"* , Helium 3 is useful for other things, not just for fusion power. It's used for cryogenics, neutron detection, and MRI scanners, amongst other applications, so some Helium 3 from the Moon could be a valuable export right away, even if it doesn't scale up to the huge quantities you'd need for Helium 3 based power generation on Earth.

You'd get it automatically as a byproduct while extracting the more abundant volatiles from the solar wind in the regolith, so it might well be a useful side-line to help support lunar manufacturing economically as part of the mix along with everything else.

THORIUM AND KREEP (POTASSIUM, PHOSPHORUS AND RARE EARTH ELEMENTS) ,AND SOME URANIUM

The Moon has some uranium, which is a bit of a surprise for such a heavy element, but when bound with oxygen it is rather lighter and can occur in the lunar crust as on Earth. It is especially rich in Thorium, in the lunar Mare. This is useful as a fuel for nuclear fission reactors, which have to be designed to burn thorium instead of uranium to use it. It's not likely to be worth returning to Earth as thorium is abundant here. But it could be very useful in space, at some point in the future.

Nuclear power stations built on the Moon wouldn't have the same pollution hazards and hazardous waste issues as stations on the Earth. Perhaps this may be a way to power space colonies, and interplanetary ships fueled from the Moon, so avoiding the need to launch nuclear power plants from Earth to orbit.

Thorium is a tracer for KREEP - potassium, phosphorus and rare earth elements. Also associated with chlorine, fluorine, sodium, uranium, thorium, and zirconium, so KREEP ores could be sources for all those elements on the Moon.

When the Moon cooled down from the original molten state, then olivine and pyroxene crystals form first, and sink to the bottom of the magma ocean (both made of iron and/or magnesium plus silicon and oxygen). Meanwhile anorthite also forms (made of calcium, aluminum, silicon, and oxygen), which is less dense and floats to the top (forming the lunar highlands). Some of the other elements like nickel are able to squeeze into the crystal lattice and get removed at the same time. But the larger elements can't, and are left in liquid state. They are last to solidify and form the KREEP deposits. It forms in between the olivine and pyroxene deep down, and the floating anorthite on top and may have been liquid for a long time.

For some reason, not fully understood, then KREEP deposits on the surface of the Moon are concentrated on the near side of the Moon near the Imbrium basin, with a small amount also in a separate concentration on the far side. The Imbrium impactor probably excavated the KREEP deposits on the near side. But it's puzzling that the much larger Aitken basin didn't lead to large deposits on the far side. Perhaps for some reason KREEP is concentrated on the near side of the Moon. For more about this see The Moon is a KREEPy place by the planetary geologist Emily Lakdwalla which I summarized here.

The abundances of rare earth elements on the Moon are much less than rare earth ores on Earth, and despite the name, they aren't very rare here on Earth. So it's not likely that they'll be worth returning. However the most concentrated spots - the ones marked white in this figure - haven't been sampled on the surface and the spatial resolution is low, tens of kilometers. So it's possible we'll find more concentrated ores on the Moon.

It's a similar situation for uranium and thorium. The abundances on the Moon from this map are too low to count even as a low grade ore on Earth. But with such low resolution, there could be richer ore deposits when we look at it closely. (Here I'm summarizing what Crawford says about lunar KREEP ores in his survey, see section 7, Rare earth elements and following)"*

EARTH LENGTH DAY ON MARS VERSUS ADVANTAGES OF CLOSE TO 24/7 SOLAR POWER AT THE LUNAR POLES

Yes the Mars near to 24 hours day is a remarkable coincidence. But - it's not so much of an advantage over the Moon as you might perhaps think. The 24 hour day of Mars actually leads to huge differences of temperature between day and night in Mars' very thin atmosphere. At night in the Martian "tropics" the air gets so cold that carbon dioxide freezes out as dry ice, carrying water with it to form the Martian frosts photographed by Viking. While in the day time the temperatures can get well above zero at times.

You might think the Moon is even worse with its 14 Earth day long night. But there's one other big difference between the Moon and Earth, that turns out to be very important. Unlike Earth and Mars which have an axial tilt of over twenty degrees, the Moon has a tilt of only a bit over 1 degree. In a very "messy" system, it's remarkable that the Moon's axis is so vertical.

Diagram from NASA. With the Moon's orbit tilted by 5.14°, Earth tilted by 23.4°, yet the Moon's axis is pretty much vertical, only tilted by 1.5424° to the ecliptic (the apparent path of the sun through the sky). This remarkable coincidence is the reason the Moon's poles are so habitable.

This means it has no seasons, and as a result, you get points at the poles that are in almost constant sunlight, the peaks of eternal night, and just next to them, permanently shaded craters.

So, yes, the lunar night is a major challenge. But it does have the advantage of these peaks of (almost) eternal light at its poles where the temperatures are much more even, neither too hot nor too cold. (The average temperature may seem rather chilly at -30° C, but it varies by only 10° C in either direction and that's warm enough that a habitat there can be kept at a comfortable temperature of 20° C with the aid of a solar collector.

Some of the lunar caves probably have an internal steady temperature of around -20 °C (see page 5 of this paper)

Hamiwari sun tracking solar collector - the light is collected, focused and sent through fiber optics to the interior of the spacecraft or habitat, where it can be used as a light source for algae or growing crops, or to help keep it warm. Details of how this would work for spacecraft, see page 319 of Peter Eckart's book: Spacecraft Life Support and Biospherics.

See peaks of (almost) eternal light in the online NASA astrobiology magazine). Compare that with Mars, where for for 200 days of the year, even at the equator, it gets so cold at night that dry ice condenses out as frost (mixed with water ice), while in the day time the temperatures can get well above 0 C and you can see that thermal regulation would be far easier at the lunar poles.

SOLAR POWER AT THE LUNAR POLES

The best place for long term settlement on the Moon as they explain in the ESA video also, may be these peaks of (almost) eternal light. They are best for power too, as they get solar power almost 24/7. There are a few hours and occasionally periods of several days in the year (Earth days there) when they don't get solar power but for most of the time there is just sun 24/7. So they do much better than Mars in that respect. See also the section lunar poles (below).

So, when you are at the lunar poles, it's like a perpetual Arctic or Antarctic equinox with the sun skimming the horizon, seeming to circle around you once every 28 days. If you have solar panels mounted upright, which rotate once every 28 days,they can then follow the sun and get maximum solar power. The reason that can happen is because the Moon has an axial tilt of only 1.5424°. Basically, the Moon doesn't have seasons like Earth. Earth has seasons because of its axial tilt of around 23.4° at present (varies slightly by around one degree or so between 22° 2′ 33″ and 24° 30′ 16″ with a period of 41.040 years).

You can also reflect sunlight to wherever it is needed, to power rovers exploring the permanently shadowed craters, or to supply power for mining the ice, or for solar power. To do this they could use TransFormers or shape transforming two dimensional robots. They have actuators to transform their shape, antennas to communicate, and reflectors and solar cells amongst many other possible components. For details of the concept see this article from 2014. For the idea of using them to reflect solar power into the lunar craters see this article.

Using TransFormers as programmable flexible mirrors to power a rover, or to help extract ice for ISRU on the Moon. From this paper.

The Moon also has the lunar caves. These would shield from radiation, lead to a more even temperature again, may be possible to be made air tight, and as we saw, are thought to be possibly as much as several kilometers in diameter (only the Moon with its low gravity can have caves as large as that).

LOW POWER LEDS FOR PLANT GROWTH THROUGH THE LUNAR NIGHT

So, what happens to plants, won't the caves be too dark, for them? Well, yes, lights would be needed in the caves or anywhere on the Moon except the poles, during the long 14 day lunar nights. But they would be needed for plants on Mars too, during the dust storms, which block out 99% of the sunlight, often for weeks on end.

This used to be a major issue, because the lights for growing plants would consume a lot of the power of a space habitat with a greenhouse. But it's not a big issue any more. This is something that has changed a lot recently with the invention of LED lights. Even if you have to produce all the lighting for your plants through electric power, that's feasible. You need about 100 watts of supplied power for lighting for one square meter.

I am going there by this High Efficiency Full Spectrum LED Grow Light - uses 20 watts of power to illuminate 0.2 square meters. So that's 100 watts of supplied power needed per square meter. It is recommended for crops that require bright sunlight such as lettuces in this roundup in 2015: Top 10 Best LED Grow Lights

That's 3 kW per person for 30 square meters. However you only need it for 12 hours a day for 14 days. That makes it a total of 504 kWh of storage needed per person to provide lighting for the lunar night. You could generate power from fuel, or use batteries which are topped up when you have an excess of solar power. Power for the LEDs for your greenhouses doesn't seem likely to be a major issue for the lunar caves. You need power anyway during the night. For ideas of how this could be achieved see Power during the night.

PLANTS CAPABLE OF GOOD YIELDS WHEN KEPT IN DARKNESS FOR THE LUNAR NIGHT

It's also possible to deal with the lunar night by reducing the temperature of the plants from 24 °C in the lunar day to 2.5-3 °C in the lunar night (which helps maintain plant vitality during darkness). This was tested in an experiment by the Russians for BIOS-3. Of the ten crops tested, most were able to cope with this regime. The ones that couldn't cope were tomatoes, sedge nuts, and cucumbers. Wheat, barley, peas, turnips, dill, carrots, beet and radish were all able to survive a simulated repeated 14 day lunar night. The edible crops were reduced 30-50%. The most promising ones were carrots (73.5% yield), Beet (yield actually increased to 122%), turnip (57%), dill (72%), and radish (61.5% yield). See table 3 here (I've converted the figures to percentages).

So if you use that approach you'd need up to double the growing area, or around 60 square meters (probably a fair bit less depending on the mix of crops), but would be able to use natural sunlight so would not need to supply light to the crops during the lunar night at all. You could also have a mixed regime, supplying light during the lunar night only to the plants that require it, such as tomatoes, cucumbers, etc, and the ones that most benefit from light during the lunar night.

DEALING WITH PERIODS OF DARKNESS AT THE LUNAR POLES

Even though the "peaks of (almost) eternal light" experience nearly continuous sunlight, they do go dark during eclipses. Also depending on the local topography, they have periods of darkness of several days at a time.

One way to deal with this is to raise the fibre optics solar collectors and solar photovoltaic panels on towers.

This idea goes back at least to 1990. If the Moon was a perfect sphere, then you would be able to achieve sunlight 24/7 (except during eclipses) with a tower 622 meters high.

Figure 1 from Arnold Reinhold's paper

See the section Description of idea in this paper A Solar Powered Station at a Lunar Pole by Arnold Reinhold. That's a major construction project but it would be easier to build on the Moon.

However the Moon isn't perfectly spherical of course, and the peaks of (almost) eternal light are quite high already. So your tower wouldn't need to be as tall as that. As it turns out, you can make quite a difference with a tower only 10 or 20 meters high.

This figure shows the difference it makes to put the solar collectors or solar panels on a pole 10 meters high and then 20 meters high, to raise them above nearby obstacles, from this paper.

With the best location in this paper, near the South pole (see page 1077), a mast 10 meters high increased the average solar visibility from 92.66 to 95.83, that reduces the number of days of darkness in the year from 26 down to 15. So it can make quite a difference.

It also made a difference to the night events:

This table shows how the periods of total darkness are shorter at 10 meters,. The longest period of total darkness at the surface is 5.88 days, but it's only 2.75 days at 10 m. For partial darkness, the periods are 10.5 and 7.4 days respectively. From this paper.

Another approach is to find points close to each other such that one is in daylight when the other is in darkness. You can then route power from one to the other via a power line, or you can transmit the power using microwaves or lasers. One example here, page 9 two points 12 km apart, together have sunlight 97% of the year.Another example here, points A and B 10 km apart on page 562, figure 6b - B is illuminated for 82% of the year but when you combine it with A, then the result is illuminated 94% of the year.

This is another example

The white lines here show the local horizon as seen from this site in the South pole area on the Shackleton rim, and the yellow regions show the parts of the horizon that block the sun (less than 50% illuminated) on the worst day of the year. (Figure 11, page 16, explanation on page 6 of this paper). As you can see, many of the features that block the light are close to the site itself. Based on the analysis in the paper, this site needs a fast recharge power supply for a maximum of 62 hours or about two and a half days. With a coarser plot, the result was 156 hours, which shows how much a difference the local terrain makes. The results need to be validated against more independent images.

This is work in progress, as results are sensitive to details of the local topography, and the field is changing a lot with new high resolution data. We could do with even better elevation mapping of the poles.

So, the actual figures here might change as we learn more, but the basic idea is clear, that solar collectors on quite a short mast of 10 meters can often significantly reduce the length of the longest period of darkness and that it can also be reduced by using several collectors a few kilometers apart and transmitting power from one to the other.

Another idea which has been looked at is to use Earthlight to power a base on the Earth facing side of the Moon during the lunar night, when it is always between half and full phase. However, as Landis calculated, it's 10,000 to 20,000 times less bright than the sun. Even with one micron thick thin film mirrors, this doesn't seem to be very practical. See page 11 of this paper.

LONG SHADOWS ON THE MOON, ESPECIALLY AT THE POLES

I haven't found any papers about this yet, so will just describe the issues. If you know of good citations do say. So, the thing is that shadows on the Moon are so very dark, that it's hard to see any features in them. Apollo 11 had to land during a narrow time window of sixteen hours every 29.5 days, so that they could see the lunar features during the descent flight path to the Moon. They could only land when the sun was at an elevation of between 5° to 14°. Too high and the sun would be directly behind them so that they couldn't make out any shadows at all, the landscape would be "washed out". Too low and most of the landscape would be shadowed.

The sun angle at the poles would be far lower, from 0° to around 1.5°, which would probably make it hard to impossible to do an Apollo style visual landing. Most of the landscape would be hidden by the long shadows of boulders, at ground level. We don't need to worry so much about the landing with modern technology and detailed 3D maps of the Moon, but the long shadows have other problems.

It will be hard to just walk around outside the base without artificial light. Although you would be walking in full sunlight, most of the ground at your feet would be in pitch darkness. There'd be some scattered light from the brighter pats of the landscape, but the bright landscape will also stop your eyes from dark adapting, so you won't see much in the darkness. So I think you will need bright lights to shine into the shadows to walk anywhere around the base. I don't see this as much of an issue, but it is something to think about.

Another problem is that if you park a rover next to the habitat, then at certain times of the lunar month, the rover will block the sunlight. It's the same for anything else you park or any structure you put up next to the village. The habitats will block each other as well.

So - this is just a suggestion, but I wonder if it would work best to have a multi-level village? Some of the habitats could be raised on legs above the landscape rather like Belgium's first ever "zero emissions" Antarctic base, the Princess Elisabeth Antarctic Station,

You could park your rovers beneath it. You could also have ramps or lifts to get down to the ground level from above. Of course it would have to be more rounded than this one.

The habitats could also be at slightly different layers and if you had a greenhouse, it could be at the top, center, a few meters above everyone else. The solar panels and solar collectors would be much higher on five or ten meter posts to avoid local relief, so they would not be affected.

HEAT REJECTION

Often it's as much of a problem to keep a space habitat cool as to keep it warm. The ISS has large heat rejecting panels, six of them, each able to reject at least 11.8 kilowatts of excess heat from the ISS. (See also "Explore the Space Station" with the radiators labeled)

This is another advantage of the lunar poles is that it's really easy to design heat radiators. You just need to set the radiators flat on the ground, and then the sun will only ever catch them edge on. They are already in the optimum position for heat rejection, with no need for the radiators to move. (See page 584 of this paper).

It's harder to do this at lower latitudes. One approach is to use heat pumps to increase the temperature to the point where the heat can radiate easily even in the lunar day. Or the heat rejection radiators can be shaded so that they remain cool. Or you can use a combination of both approaches.

POWER DURING THE NIGHT

If the base is sited at the lunar poles, you still have occasional periods of darkness with the peaks of eternal light in the local "winter" of up to several days long. If the base is sited in a lunar cave at lower latitudes you need power for the 14 Earth day long lunar night. One way or another you need a way to deal with periods of darkness.

The caves and bases anywhere except at the poles would also need extra power for growing plants in the lunar night. Until the invention of LED lighting, this was a major issue. However, with modern LEDs requiring only 100 watts per square meter, and efficient growing methods for plants with only 30 square meters per colonist, then you need only around 500 kWh per colonist per lunar night (see Lower power LEDs for plant growth through the lunar night).

Those 500 kWh may sound a lot, but it corresponds to a manageable 750 kg of regenerative fuel cells per colonist to last the night, as far as agriculture is concerned. If you can use hydrogen electrolyzed from water during the day, it's more like 17 kg of hydrogen storage per colonist to last them through the lunar night, though that doesn't include the mass needed to store the hydrogen, separate it and recombine it. Hubble uses nickel hydride batteries. The ISS used to use them as well.

A new type of battery of this type under development stores 0.14 kWh per kilogram, so if those were available, you would need about 3.57 tons of batteries per colonist, which yields when kept in darkness for the lunar night.

There are many other ways to store the power overnight including a lunar wadi idea of covering part of the regolith with an insulator at night to keep the heat in and using it like geothermal power as a heat storage - or molten salt which is what is typically used on Earth if you want to store large amounts of power for long periods.

So agriculture is not an issue for power. The ISS uses more than 3 kW per person, it's 75-90 kW so getting on for 15 kW per person though that is also including extra power to power up batteries to use during the night passes. Add an extra averaged 1.5 kW for agriculture it would be 16.5*24*7 = 2772 kWh or around 4 tons of batteries per colonist.

Its 4 kWh per kg for molten salt which would reduce that to 693 kg per colonist and even less if you use hydrogen storage which goes up to 120 kWh per kg, so 23 kg per colonist but with overhead of course. Keeping liquid hydrogen cool would be much easier at the lunar poles with the cold traps close by, and if you are creating it anyway for fuel to export to LEO, that's the most natural way of all to store power during the night.

For a detailed working out of power available and requirements for early stages of a lunar polar base (with Shackleton crater at the south pole as an example site for the base), including methods of transmission of the power, and storage on the Moon, see Power System Concepts for the Lunar Outpost.

We've also seen the possibility of making nickel iron batteries using lunar materials Electrical Energy Storage Using Only Lunar Materials.

This level of power is also just within the range you can supply continuously using RtGs, though it does require a fair number of them.

RTGs (Radio Thermal Generators) are used for some deep space missions and typically produce of the order of 100 to 200 watts to the spacecraft, by way of example, New Horizons had 202 watts available on arrival at Pluto. Cassini's mission to Saturn used 33 kg of Plutonium 238 (not the isotope used for nuclear bombs) with a total power output at the end of its mission of 600 - 700 watts.

RTGs are currently in short supply but both the US and ESA are working to deal with this issue. The ESA's approach is of especial interest for a lunar base, as they are working on americium RTGs with a lifetime of centuries instead of decades, for a power output of 1-2 watts per kilogram. You can get high quality americium as a byproduct from plutonium storage (the UK has a hundred tons of Plutonium in long term storage). The alternative of plutonium 238 (not the type of plutonium useful for nuclear weapons) has an energy density of 3 watts per kilogram. See Can nuclear waste help humanity reach for the stars?

This is a useful level of power, but the energy density is rather low. The 3 kWh peak power for plants during the day could be be supplied using 100 kg of plutonium 238 per colonist (at 3 watts per kg) or about 150 kg of Americium 241. That's about the equivalent of five Cassinis per colonist for crops during the night. Note, plutonium 238 is different from the plutonium 239 used for nuclear bombs. It is not capable of a runaway chain reaction.

Top of their list for storage of power are cryogenic regenerative fuel cells, batteries and flywheels. Flywheels have a much lower energy density (0.03 kWh / kg compared to 1.5 kWh/ kg for regenerative fuel cells) but have some advantages including a long lifetime. To supply power to rovers in the dark craters they suggest cables, or beamed power (via laser or microwaves) or using fiber optics to transmit light which they could use for solar power.

Other ideas suggested include hydrogen produced by electrolysis of water, stored and then recombined.

Hydrogen storage has amongst the highest energy densities, at 120 kWh per kilogram (the 502 kWh would then need only 4.2 kilograms of hydrogen generated per colonist during the lunar day), though of course that's just the mass of the hydrogen not including the mass of the system used to separate and store the hydrogen and to use it as fuel.

Table from Hydrogen Storage (UK Office of Energy Efficiency and Renewable Technology article about fuel cell technologies for electric cars).

One of the most common ways of storing energy on Earth is gravitational storage. However, that would be difficult on the Moon, because of its low gravity, a sixth of Earth's. You'd need to raise a 150 ton boulder by 30 meters to store 1 kilowatt hour. See page 4 of Landis's paper.

Thermal storage using molten salt is a good way to retain power for long periods of time. This is a frequently used technology on Earth used with solar power plants, to supply power during the night (here is a recent example which will have a 1100 MWh storage capacity in the form of molten salt). Molten salt can retain its heat for up to a week in facilities on Earth. It can achieve up to 1 kWh per kilogram, and some technologies go well over to over 4 kWh / kg (figure 8 here, review paper that goes into the various technologies currently being explored).

Superconducting magnetic storage is also another way to store energy, efficient, can stand many recharge cycles. However it has a low energy density of watt hours per kilogram. Once the superconducting circuit is charged, it won't lose its charge, can keep it indefinitely and the energy can be recovered at any time with only 5% energy loss. The main issue here is to keep the circuit cold enough, so it is mainly used for short term storage at present. High temperature superconductors may help with that in the future, but it still seems an outlier for the Moon because of its low energy density. This article is a useful overview of some of the main methods used for storing power on Earth with their advantages and disadvantages.

One other method that's been suggested for storing energy for the lunar night is to use the reversible reaction of CaO with H₂O to produce Ca(OH)₂ which liberates large amounts of heat. The CaO can be obtained from the lunar soil as a byproduct of aluminium extraction from anorthite. During the day, the CaO and H₂O are dissociated again at high temperatures (550 °C) using sunlight for heat. This reaction is tricky to use for power storage on the Earth because of the presence of CO₂which leads to formation of calcium carbonate, steadily removing calcium oxide from the system. But on the Moon there is no CO₂ naturally present so it would be easy to keep carbon dioxide excluded from the power generation and storage system, and the system could be cycled over and over.

Another interesting idea is a heat engine, powered by the difference between the lunar day and night. At the end of the day, you cover part of the sunlit surface with a reflective layer to hold in the heat, and then use the temperature difference between this and the lunar night to generate electrical power. See page 37 of the L5 society report.

There are several other ideas also in the Sacramento L5 society report. Including a long discussion of ideas for beaming energy from large solar panels in orbit and related ideas.

If we start with a lunar colony, as for the ESA village, then in the early days, for the first temporary visits to a polar colony on the Moon, astronauts could avoid the local lunar "winter" and could still be there most of an Earth year. Later you could put panels on several of the lunar peaks of almost eternal sunlight. You could use cables to transmit the power over a short range, and then microwaves or lasers for longer distances. Then the only periods without any solar power would be during the rare total lunar eclipses, requiring only hours of storage rather than days. In that way you would sort out the details of long term storage through the lunar night later on after you already have a fair bit of experience of living in the Moon.

POWERING THE WHOLE MOON IN THE FAR FUTURE

Eventually, especially as metal refining takes off on the Moon, you could use HVDC for long distance transmission as is often done on Earth. Eventually perhaps have solar panels at various latitudes on the Moon, right down to the equator, and positioned so that there are always some panels in sunlight, and then you could have a power grid on the Moon which supplies solar power 24/7.

We can do similar things on Earth, long range transmission of power from places where power is readily available. We don't have to go as far as the other side of the planet - actually you can get abundant power even at night in the Sahara if you use the sun to store the heat as molten salt in the daytime to use to supply power at night.

Desertec was a project to supply pretty much all of Europe with solar electric power from the Sahara desert. The reason they never did that is mainly political, and the cost of setting it up in the first place. Desertec would have supplied power from Sahara to the whole of Europe including Germany, even up to Norway and Sweden. They would have used HVDC cables, including existing cables that connect the countries to each other already plus new ones for the project.

In this map, the existing HVDC cables are shown in red, and proposed ones, which would be used to connect Europe together to the power generating facilities in the Sahara desert are in blue.

The longest existing HVDC link in the world is 2000 km in China.from Xiangjiaba–Shanghai transmits 6400 MW of power over that distance according to the details here (see page 8). (would need to check but probably accurate).

A quarter of the circumference of the Moon is 2728.5 km which is the furthest you'd have to send power to get it to the middle of the night side from the day side of the Moon. It's not much more than the Chinese grid system - and you'd have less transmission loss in a vacuum.

With any of these approaches, even if you have cables spanning the Moon, you can't do anything about the times when the moon is eclipsed by the Earth. However, that would only be for a few hours per year for the partial eclipse stages, and totality for the longest duration total lunar eclipses lasts a little over 100 minutes.

POSSIBILITY OF USING LUNAR SOLAR POWER FOR EARTH

This is a bit further ahead, but it is worth thinking about, whether solar power for the Moon could actually be useful for Earth also. Some scientists think it could be.

The advantage of doing this on the Moon is that you can use indigenous materials to make the solar panels. For a small amount of launch mass to the Moon you could have a rover that travels over the surface leaving solar panels in its wake. See Lunar glass and Solar cells from lunar materials - solar panel paving robot (above)

It's easy to see this working to supply power to the Moon, but some have suggested it could also be used to generate power on Earth. So, taking this even further, with a large scale operation of this type, using only 1% of the surface area of the Moon, you could supply 2 kilowatts of continuous power per person to a population of 10 billion on the Earth. See Solar Power via the Moon. More details here.

Or, further ahead, maybe this is more interesting as a talking point than a likely near future concept, the Japanese Shingzu corporation has suggested we could build solar panels in a band around the Moon - at the equator

See Shimizu dream - Lunar Solar Power Generation - Luna Ring.

(click to watch on Youtube)

Earth would get solar power only half the day, so they send the power to satellites in orbit around Earth, which then beam it down to the other side of Earth. Of course they need large receivers to collect the power from the Moon, but only 1% of what they'd need to collect it directly from the sun - that could be worth doing if it is significantly easier to make solar panels on the Moon.

On the other hand there are ideas to use large thin film solar panels in space or large thin film mirrors to concentrate the light onto solar panels or furnaces, launched from Earth to LEO. So would the lunar solar plants be a major saving compared to those?

Another way that the Moon could help the Earth though, with solar power, is to make the solar cells from lunar materials, and then ship them to GEO or lower orbit. The idea of using lunar materials to make solar power satellites goes back at least to the 1970s, see Construction of Satellite Solar Power Stations from Nonterrestrial Materials

LUNAR RAILWAYS

This section is mainly based on Peter Kokh et al's Railroading on Moon and Mars and "Railroading on the Moon"

If you have railways on the Moon, you can move large habs around. You might as well standardize to wide gauge lines too, as there's no difficulty finding space for the railway lines. Kokh et al suggests a good width might be double the width apart of a normal railway on Earth. Also there would be no height restrictions, at least to start with, so you could permit double story "compartments" as well. Your train "compartments" could be as large as a 747. Also, in a vacuum, they could travel at similar speeds to our airplanes as well, probably on maglev lines.

A maglev train like this could be double the width of any trains on Earth or wider, as we can set any gauge standard we like on the Moon. Its carriages could be as large as a 747. Artist's impression, illustration by Madhu Thangavelu and Paul DiMare © from The Moon: Resources, Future Development and Settlement

This way you could have transport between habitats, transport for tourists to see scenic sites, and you could also move the habitats themselves, for instance to build new lunar "villages". Also as on Earth you could use the railways to transport materials for building and repairing the railways themselves. They would probably be built with integrated solar panels along the length of the tracks to power them in the lunar day.

There is no air resistance of course, so the shape of the train doesn't matter. You could even transport complete habitats, far larger than the one in the illustration. Artist's impression, illustration by Madhu Thangavelu and Paul DiMare © from The Moon: Resources, Future Development and Settlement

They could also be used as launch assist, Kokh suggests that: "The LunaRail track could also serve as a launch-assist system, using 70 Mw to accelerate a payload at 3 g for 81 seconds to an escape velocity of 5,355 mph in a distance of about 60 miles.""

That also reminds me of an idea I've read before (I can't find it right now) of having a railroad all the way around the equator - or any other line of latitude, and to keep habitats moving on it so they are constantly in sunlight.

They could travel so that they are constantly at late afternoon on the Moon, giving nearly a fortnight to fix and repair any issue and restart before it gets dark if there is any problem. Habitats moving at 16 km/ hour would circumnavigate the lunar equator once a month so could stay in sunlight 24/7 apart from eclipses.

The habitats could even travel continuously, for instance along a line of latitude, to stay always in sunlight with full solar power - they wouldn't have to travel fast and if they kept towards the lunar evening, then if there were any faults, they'd have 14 Earth days to repair them before darkness approached. Artist's impression, illustration by Madhu Thangavelu and Paul DiMare © from The Moon: Resources, Future Development and Settlement

• This is the technical article: "Railroading on the Moon"

• Shorter non technical article Railroading on Moon and Mars

• Lunarpedia section: Railroads

He doesn't mention it but perhaps HVDC lines could help solve the power issue for railroads once you have railways all the way around the Moon. See Powering the whole Moon in the far future.

One thing you'd have to deal with on the Moon for both railroads and habitats are the moonquakes. On Earth, railways start to get significant damage withearthquakes of magnitude of getting on for 6 see figure 1 in this article. Lunar quakes recorded with Apollo go up to magnitude 5.5.

MOON QUAKES - CAN THEY DAMAGE LUNAR HABITATS, RAILWAYS ETC

This section is based on the NASA page on Moonquakes

Buzz Aldrin deploying a seismometer in the sea of tranquility on the Moon. Apollo 12, 14, 15 and 16 also took seismometers to the Moon and their seismometers operated as a four station seismic network until they were switched off in 1977.

They found that there were four types of moonquake. The deepest, about 700 km below the surface, are probably caused by tides. Then you get vibrations from meteorite impacts, and thermal quakes from the expansion of the surface in the heat of the day and contraction at night.

Then finally you get the shallow quakes, only 20 or 30 kilometers below the surface. These are the strongest, magnitude 5.5 in the Richter scale. Nobody knows quite what causes them. They may be due to slumps of the rims of large young quakers. Since the seismometers were all near the equator, lunar seismologists are especially ignorant of what happens at the poles. They want to deploy a network of ten or twelve seismometers on the Moon before we send humans back there, to help determine where are the safest places to go.

Even the shallow moonquakes are not that strong compared with earthquakes. Magnitude 5.5 on Earth is enough to crack plaster and move heavy furniture. However they last a lot longer on the Moon. Weathering on Earth makes the ground much more compressible than it is on the Moon, where moonquakes can make the Moon ring like a bell. So these shallow moonquakes can last for well over ten minutes, compared with only a couple of minutes for Earth.

This table shows deep and shallow moonquakes recorded from the Apollo 16 landing site. It shows the x, y and z components as well as a short period z component. A meteorite impact (not shown here) has large x, y and z components but a very low short period z component. A thermal earthquake would show as a short duration signal in the SPZ graph only. The shallow earthquakes would be most damaging for habitats on the surface.

Lunar habitats would need to be designed with the moonquakes in mind. They would need to be built from materials that are somewhat flexible, and they'd also need to be able to withstand repeated bending and shaking, so it would be important to know the fatigue threshold. The same also would be true for the lunar railways, lunar telescopes, mass drivers and so on (for mass drivers, see Exporting materials from the Moon).

MICROMETEORITE DAMAGE

Since the Moon has no atmosphere, then astronauts there have to cope with micrometeorite damage.

Micro-meteoroid impact damage - close up of handrail on the ISS. (Credit: NASA/JSC Image & Science Analysis Group).

These can damage astronaut's gloves. Larger ones can create holes in solar panels. This is an example from the ISS.

The hole in the solar panels towards middle left of this image was caused by a micrometeorite, or space debris. We have plenty of experience with protecting the ISS and other space stations from micrometeorites now, and the same precautions would be needed on the Moon. This photograph was tweeted by Chris Hadfield in April 2013. Solar panels have to be designed so they still work if you have a hole in them.

They are also a risk for astronauts on EVA. You can't protect against them completely. A report for the ISS in 2007 found that the chance of a micrometeorite or orbital debris penetrating the spacesuit of an astronaut doing an EVA is 6% over 2700 hours for a two-person EVA (i.e. 6% chance for two astronauts working for 337.5 "eight hour days"). So the risk isn't high, and it hasn't yet happened. But if we have many astronauts working out of doors on the Moon, for long periods of time, sooner or later one of them surely will be hit by a micrometeorite. The safest thing is to stay inside your habitat and use telerobots.

Habitats can be protected more thoroughly. The first defence is a whipple shield.

Whipple shield for the Japanese Kibo Laboratory on the ISS - before hypervelocity testing. A micrometeorite hitting the top surface gets disintegrated and some of its energy dissipated so that when it hits the habitat inner wall, it's no longer able to penetrate it.

Often extra layers or stuffing is added as well like this.

Whipple shield for Kibo with the stuffing added.

With these defences in place, the ISS is still not totally safe. The same report estimated an 8% chance of a micrometeorite or orbital debris leading to the ISS being abandoned within a 10 year period.

The habitats on the Moon would have a thick layer of regolith on top of the habitat so that provides extra shielding. But rovers and other vehicles traveling across the surface of the Moon would be more vulnerable as well as, obviously, astronauts doing EVAs. The study would need to be done afresh for the Moon anyway, as with a different gravity field, the impact velocity distribution of the micrometeorites would be somewhat different and there's no orbital debris of course. I haven't found a similarly detailed risk assessment for a lunar base - if you know of anything do let me know!

MICROMETEORITES - COMPARISON OF THE MOON AND MARS

This is one of the few comparisons where Mars comes out top. It's thin atmosphere is still thick enough to screen out micrometeorites. On the other hand Mars does get ten times as many of the larger meteorites because it is closer to the asteroid belt.

Mars doesn't need any mitigation strategy for micrometeorites therefore. But the Moon does.

That 6% chance of a micrometeorite penetrating a spacesuit if you have two people working 8 hours a day EVAs for a year was higher than I expected. It's not so likely you get it happening on the ISS though it could happen. But if you have people working out of doors every day on the Moon, it is something that is pretty much certain to happen.

If they are test pilots or astronauts they take on huge risks at times, the Apollo astronauts knew what they were doing is risky, way above that 6% risk for just a single mission. But if you think of it as ordinary work / construction work, then it could be unacceptably high. And in either case you want to know what the risk is that you are taking.

Depends on whether those are risks of injury or death also. In the construction industry, there's a 3% chance of injury every year. So it's similar to that except of course this is only one of several risks from EVAs.

But if it was a 3% risk of dying, well most industries don't have a high risk of death as a result of doing your job. The risk of fatal injury from construction is 0.46 per 100,000 workers per year. Most people run more risk from the journey by car into work than from their job itself (unless the job involves driving of course).

So, if you want to compare it with everyday risks, it's better to compare with motor vehicle accidents,which in the US runs at 1 in 8,938 per year. If you want to reduce the risk from an EVA from the ISS to the risk of a motor vehicle accident in the same year, that means you need to do a maximum of 2700/(8,938/33.333) or 10 hours of EVA per year.

So, as it is now for an astronaut doing a single EVA, or a couple of EVAs, it's not like a huge personal risk at all. It's similar to their risk of a motor car accident in a year. It would be the same also for tourists. The risk from a single EVA would not be excessive. Indeed the biggest risk at present is getting into space in the first place.

But if you have people working every day in construction work on the Moon it would be a major risk, way beyond what you'd expect from any normal activities on Earth, if it is a risk of death. If it only causes injury it's similar to the construction industry.

So, what can we do about it? Well, first, astronauts do already have micrometeorite "armour" in their suits. The Thermal Micrometeorite Garment. They may be able to use other future materials such as this metal foam that can stop bullets. This deals with some of the risk, but that 3% per year figure was based on astronauts with micrometeorite armour already.

Then, I expect most construction work in the future to be done by telerobotics. After all the ESA plan to do all the regolith shielding using 3D printers controlled from Earth. The astronauts on the Moon might well stay inside their habitat or in roaming rovers for most of the year, and only do a few EVAs per year. They might be like the current astronauts on the ISS - do EVAs but only occasionally. Then the risk would not be excessive.

The other way to do it is to have the astronauts in mobile habitats. This is the NASA Multi Mission Space Exploration Vehicle exploring both an asteroid and the Moon

And this is how it works:

Anyway, an idea to suggest here for the Moon - these mobile habitats could have, not just whipple shields, but no reason really why they can't also have thick extra layers of shielding added to them by the same 3D printers that protect the habitats themselves as in the EVA plan. Those layers of shielding in their plans are hollow, cellular, filled with vacuum so light. I don't see why they couldn't also be applied to mobile habitats. They would then be very safe, also protected from solar storms too.

In the further future with lunar railways, you could use the same approach to protect the carriages from solar storms and even the largest of the micrometeorites.

Or there's this early 1967 Soviet idea of a self propelled self burying rover.

After traveling across the surface, it finds a soft spot to dig into the regolith for protection from solar storms. This would also protect the astronauts from micrometeorites.

Then, longer term, the Moon may develop an atmosphere as a result of industrial processes there. Indeed, it might be hard to avoid it. Hoekzema in An Atmosphere for the Moon concluded that

"Creation of a thin lunar atmosphere is perfectly feasible, even with present day technology, and in some scenarios even hard to avoid."

Once you have heavy industry on the Moon, it would be relatively easy to generate an atmosphere thick enough to stop micrometeorites and the worst of solar storms, yet it would be so thin it would be invisible to the eye. A hundred million tons of atmosphere would stop all micrometeorites and many of the larger meteorites as well. He estimated that a single large modern steel works on Earth, if located on the Moon, would liberate between a million and ten million tons of oxygen a year. The solar wind can only remove around 100,000 tons of the lunar atmosphere per year. So as soon as you get to the point where you add a million tons per year, the atmosphere will rapidly thicken.

It might well be something you want to prevent, see Lunar vacuum as an asset. But if moon colonists wanted it, it would be relatively easy to thicken the atmosphere to the point where micrometeorites are no longer an issue. It's also possible to go further and terraform or paraterraform the Moon but I'll leave that to later. SeeTerraforming the Moon

SOLAR STORMS AND RADIATION SHIELDING - MOON AND MARS

Solar storms can be deadly, and can lead to a lethal dose within hours. If astronauts were on the surface of Mars, the protection of the Mars atmosphere provides significant protection (the pressure varies a lot, seasonally and diurnally, but Curiosity measured on average 21 grams per square centimeter shielding from the atmosphere). The Moon has no protection however, and the same also applies to astronauts in transit to Mars. The effects are the same as for radiation sickness,

"Following large doses received in minutes to hours, acute radiation syndrome (ARS) can result. Depending on dose and dose rate, nausea, vomiting, skin damage, and blood cell depletion resulting in infections and bleeding. Following acute doses substantially larger than those expected during space travel, serious gastrointestinal and central nervous system damage may result leading to death within days or even hours depending on the dose and dose rate received"

Radiation Hazards and the Colonization of Mars

Solar storm particles are low energy compared to the galactic cosmic radiation, so you can protect against them by using a "storm shelter" with about 4 grams per square centimeter of aluminium (figure from this paper, page 11), and less than that for low mass materials like polyethylene. The idea is that you go straight into the solar storm shelter as soon as you get warnings of a solar storm. If the astronauts also spend significant amounts of time inside the solar storm shelter during their mission it also reduces their overall dose (e.g. they could sleep in it, eat in it etc. during transits to Mars and in Mars orbit).

It's possible to get early prediction of solar storms, hours or even days before the main event, but the main problem with this is that 90% of them are pretty much harmless. You can't tell if it's going to be one of the most hazardous storms until at most an hour before the worst part of the storm hits. So astronauts doing EVAs are at most risk. They can avoid most of the risks by canceling their EVAs whenever they get warnings of storms - but that would mean many false alarms. If they can get to a storm shelter quickly, then it's no problem. For instance, you could provide a storm shelter inside a rover on the Moon, so it might be a case of making sure they can get back to the rover within an hour when a solar storm is predicted of any size.

Solar storms can happen at any time though they are most common during solar maximum. By contrast, Galactic cosmic radiation levels are highest at solar minimum (the increased solar activity deflects some of the cosmic radiation from the region around the sun).

If astronauts were caught out in a storm on the Moon they could be rushed back to the best medical facilities on Earth. But astronauts in space on their way to Mars or in orbit around Mars would not be able to be treated like that, though that risk is easily mitigated by having a solar storm shelter inside the spacecraft.

There's another technical difficulty, that we can't observe the far side of the sun from Earth, so when the astronauts are at the other side of the sun, the methods we use to predict solar flares for Earth and the Moon wouldn't work. So other methods would be needed to predict solar flares for Mars astronauts.

In detail

The Apollo astronauts were in significant danger from solar storms. There actually was a solar storm during the Apollo missions, in August 1972, but luckily there were no astronauts on the Moon at the time. It was after the Apollo 16 mission ended in April and before Apollo 17, the last mission to the Moon in December. They would have been exposed to 300 REM which is a dose that carries significant risk of death. See Sickening Solar Flares

If they had been on the Moon at the time, from the observations of a giant sunspot, even without not knowing in advance if it would send a solar storm to Earth, maybe they would have just cancelled the EVA and stayed in or close to the lunar module. In that case they could have used it as a storm shelter which would have reduced their exposure to a very survivable 35 rem.

However a detailed study here, estimates that with the protection they had from their spacesuits, there was only a 1% chance of death without adequate medical treatment.

For a storm two times as strong as the 1972 event, they estimate a 12% chance of mortality without treatment, and for a storm four times as strong, they estimate an 88% chance of mortality without treatment in the LEM, and an 87% chance of mortality if they are in a spacesuit for the worst 8 hours (supposing that they get back to a solar storm shelter afterwards).

So for these extremely strong, but rare solar storms, worse even than the 1972 one, then the risk of mortality is high on the Moon, if you are out doing an EVA in a spacesuit or thinly protected pressurized chamber like the LEM, hours of travel away from shelter.

All those risks go down to 0% mortality however, if they can get into a shelter with ten grams per square centimeter of aluminium, or a thinner layer of polyethylene (low mass atoms, especially, hydrogen are a much more effective protection from the low mass solar protons than higher mass atoms).

These estimates need to be treated with some caution because it's based on data from radiation exposure during nuclear explosions, and nuclear power plant incidents, which is a very different type of radiation. Also the details of the spectrum of energy levels for the solar protons varies a lot from one storm to another. (SeeRadiation Hazards and the Colonization of Mars)

The Apollo 16 or 17 astronauts of course would have been rushed straight into radiation treatment when they got back to Earth, if exposed to the storm. Based on this evidence, it seems that they would have survived even if caught out in the middle of the 1972 event in their spacesuits, or even a somewhat stronger one with immediate medical treatment.

The most dangerous solar storms can start rapidly, within tens of minutes or an hour.

Figure from page 13 of this Risk of Acute Radiation Syndromes Due to Solar Particle Events. I think the curves are for grams per square centimeter of aluminium shielding. Anyway I include it here mainly to show the rapid onset of the solar storm, of the order of ten hours or so - and you don't know if it is going to be one of the dangerous storms until about the last hour before the peak.

So to be safe, you need to be within a few tens of minutes travel of a storm shelter at any time. Though most likely during solar maximum, they are unpredictable and can happen at any time.

So, these are things that we will have to take account of if we send astronauts back to the Moon. But it's not too dangerous so long as you have a storm shelter close at hand and can get back to it quickly. Astronauts in a lunar rover with a storm shelter inside the rover would be fine. If you went on a long EVA on foot far from shelter, or in an open rover like the one the Apollo astronauts used, you'd be at much more risk.

It’s the same on the way to Mars or in orbit around Mars also, unless you are in a cave of course. In Mars orbit, Phobos’s crater Stickney facing towards Mars all the time may give a fair bit of protection from solar storms, because a base there has the sun either hidden behind Mars or below the crater rim for all except a few hours of each Deimos orbit - it is tidally locked with Stickney facing towards Mars.

See also Overview of energetic particle hazards during prospective manned missions to Mars

We can get early warnings of solar flares, but this is challenging. See Risk of Acute Radiation Syndromes Due to Solar Particle Events

One of the main issues is that more than 90% of the solar storms are minor with only small radiation doses to critical organs. But astronauts might have to cancel their EVAs anyway because there's no way to know in advance what's going to happen until hours after the storm starts. It might be possible to have a one hour warning, but not much more. Attempts at a longer term warning would lead to many false alarms because of that 90% figure. The astronauts might have to put off EVAs ten times to avoid a single event like the 1972 storm.

The main increased risk here for Mars is that it's impossible to get back to Earth for emergency treatment after a solar storm if an astronaut does get caught out in it, while on the Moon, you'd have a better chance of surviving with emergency treatment within three days in the best hospitals on Earth. But either way you'd want to avoid exposure in the first place so I'm not sure this is a big issue.

There's one other - relatively minor - issue for Mars. We can't always see the side of the sun facing Mars from Earth, so would have to find some way to deal with that before we can predict solar particle events for Mars in the same way that we can for the Moon and Earth.

So the main risk here is for EVAs on the surface of the Moon (assuming that we have solar storm shelters in human occupied Mars transit spacecraft and orbiters). If you want to reduce the risk to a minimum, without numerous false alarms, you just have to keep within an hour or less of a solar storm shelter whenever there's a chance of a solar storm. The shelter could be in a rover, so that's not as restricting as you might think.

There are ideas for new materials that could make solar storm shelters much lighter. For instance, one material being researched is hydrogenated boron nitride nanotubes. The material is low atomic mass anyway, and then the tubes are packed with hydrogen atoms for more shielding. Boron is an efficient neutron shielder so will help deal with neutrons knocked out of the material by the high energy proton impacts. They are still in development, but could be useful for spacecraft, habitats and spacesuits.

As for the high energy galactic cosmic radiation, there's not a huge difference between the ISS, the Mars surface, and interplanetary space. The Earth's magnetic field can only protect against the lower energy particles. The fastest ones just go straight through without noticing it.

The flux of galactic cosmic radiation at the ISS is about half that in interplanetary space for GCR, and the same is true for the surface of Mars. While for solar storms it is very much reduced. But down on the Earth's surface, those low energy particles wouldn't get through the atmosphere anyway. Our atmosphere is like a protection layer of ten meters thickness of water above us, by mass. Even a Stanford Torus doesn't have that much shielding - they only have 4.5 tons per square meter cosmic radiation shielding.

Because those more energetic particles are so penetrating, the usual way to protect against them is through heavy shielding. So even with those advances in solar storm protection, if they work out, we will still need to cover permanent residences with layers of regolith shielding as well. Even for the ISS, if astronauts want to stay in low earth orbit for periods of many years at a time, the would need more protection from the cosmic radiation. The main risk for these, as a low radiation dose sustained over a long period of time, is neurological damage and an increased risk of cancer which could strike at an early stage.

Another approach that could work in the future, though not available yet, is a magnetic "force field" to protect the region around a habitat or spacecraft from solar storms and the lower energy cosmic radiation. CERN, in collaboration with the European Space Radiation Super Conducting Shield project are using advances in super conductor technology to develop a super conducting magnetic field to protect spacecraft and their occupants.

The aim is to create a magnetic field 3,000 times stronger than the Earth's to protect astronauts in a region of diameter ten meters enclosing the spacecraft or habitat.Only the very most energetic particles will get through, and the flux from these would be negligible. If this works out, the mass is not that huge, only 53.8 tons to protect a region of 10 meters in diameter.

More conventional earlier version of the SR2S - this also works but would be getting on for double the mass, a hundred tons upwards to protect ten meters diameter. See also SR2S technical accomplishments.

GREENHOUSE CONSTRUCTION - COMPARISON OF THE MOON AND MARS

Mars does have an atmosphere, but it is a near vacuum. Less than 1% of Earth's atmosphere. This is not enough pressure to be a significant advantage over the Moon for greenhouse construction, as the lowest pressure greenhouses typically would be pressurized to around 10% of Earth's atmospheric pressure.

Example: if you have a greenhouse on the Moon at 10% of Earth' pressures, the atmosphere exerts an outwards pressure of one ton per square meter of the greenhouse. On Mars, on average, the outwards pressure on your greenhouse would be 0.93 tons per square meter. (assuming 0.7% of Earth's atmospheric pressure). It depends where you site it - at the bottom of the Hellas basin it is a little more, but remember you have to engineer for the lowest rather than the highest pressure there and it varies seasonally. The maximum pressure is 12.4 mbars for Hellas Planitia which means it requires at least 0.876 tons per square meter for the greenhouse though it is likely to be more than that to allow for the lower pressures in winter. At any rate it's a lot of outward pressure, not that different from the Moon.

The 10% figure here is nearly the lowest pressure greenhouse you can have and still have humans able to visit it using only an oxygen mask and not a full body pressurized spacesuit. If you go all the way down to 6.18%, right at the limit of tolerance with just an oxygen mask, then the figures are a pressure of 0.618 tons per square meter for the Moon and 0.548 tons per square meter for Mars average pressures. At lower pressures than that, the moisture lining your lungs, on your skin, in your mouth etc. will boil at blood temperature, so you can't survive without a pressurizes spacesuit.

Detail of lunar colony showing a greenhouse inside a base. Detail from image from NASA, 1989. This was for the Lunar Oasis proposal for a ten year program to establish a self sufficient science outpost on the Moon to act as a test bed for space settlements.

If you build a greenhouse on the Moon or on Mars it has to withstand getting on for a ton per square meter of outwards pressure. It might be easiest to just put it inside your habitat as shown here. With a modern habitat it could be illuminated with efficient LED lights which require little by way of electricity and don't have problems of excess heat to get rid of. You could also pipe light into the habitat using optical fibres connected to solar collectors

Hamiwari sun tracking solar collector . The light is collected using fresnel lenses - when used for plants, then harmful frequencies in the UV and IR range are not collected, so it is optimized for photosynthesis.

Details of how this would work for spacecraft, see page 319 of Peter Eckart's book: Spacecraft Life Support and Biospherics.

Alternatively you could use a spherical or tube shaped greenhouse.

Although you often see images of flimsy looking greenhouses similar to those on Earth in space colony art, an Earth type greenhouse wouldn't be able to withstand the vacuum or near vacuum atmosphere outside it, either on the Moon or on Mars. For instance a greenhouse rectangular in shape with a triangular roof would be an unlikely design for a greenhouse in space. It's more likely to be dome shaped or tube shaped with a reasonably equipotential surface unless it is very small or the coverings are immensely thick and strong.

FERTILITY OF LUNAR AND MARS SOIL

The easiest way to grow plants for food in space is to use soilless gardening with hydroponic solutions or with aeroponics where plants are grown with roots suspended in a fine mist (uses much less water).

This leads to huge savings in the precious area you need to grow crops. Instead of one acre of farmland needed per person for conventional agriculture (4000 square meters approximately), you can grow 95% of the food, water and oxygen for an astronaut from just 30 square meters, with a conveyor belt system, of rapidly growing crops such as wheat, sedge-nut, beet, carrots, etc. For details see Sending humans to Mars for flyby or orbital missions - comparison of biologically closed systems with ISS type mechanical recycling (also relevant to long duration lunar missions). So early stages of agriculture on the Moon are sure to use more intensive methods such as this.

Soil based gardening can also be used with the methods of biointensive mini gardening. By using good gardening practices and by careful choice of crops you can grow all the food for one person in 4,000 square feet, about 372 square meters, or less than a tenth of an acre. That's intermediate between conventional agriculture and the conveyor belt type system of BIOS-3.

Grow biointensive - sustainable mini farming - this method needs only 372 square meters of growing area per person.

We can get an idea of how efficient these methods are by working out the total land area needed to feed the world on a vegetarian diet by all the methods. With a million square meters to a square kilometer, then we just need to multiply the numbers by 7,500 to get the area in square kilometers needed to feed a population of 7.5 billion. We get

• BIOS-3, 30 m², so 225,000 km²

• (smaller than the UK, see list of sovereign states and dependencies by area)

• Biointensive mini gardening. 325 m² per person, so 2.44 million km²

• (smaller than India at 3.288 million km²)

• Conventional agriculture, 4000 m² per person, so 30 million km²

• (a little under the area of the US, China and Russia combined)

By comparison, the Sahara desert is 9.2 million km². With the BIOS-3 system, we would need only 2.5% of the Sahara desert to feed the world. The total land area of the Earth is 148 million km². But of course much of that is desert, mountains, ice etc, some is uncultivated and animals require more land area than plants.

The surface area of the Moon is 38 million square kilometers. Indeed, a medium sized lunar crater 535 km across has sufficient land area to feed the whole world using the BIOS-3 system if you had the materials to cover it with greenhouses and enough air and water to produce growing conditions inside all those greenhouses.Mare Imbrium with a diameter of 1146 km has enough surface area to feed the world four times over.

Early experiments involving adding lunar material to hydroponic solutions suggested that the lunar soil was very fertile indeed as the plant growth was enhanced.

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NASA scientists grow plants in lunar soil. Ferns, lettuce, and corn all seemed to grow well in space minerals

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However later research suggested that the reason for this was a deficiency of trace elements in the hydroponics solutions used and that the plants were equally stimulated by some terrestrial soils.

A recent experiment using lunar soil simulants instead of the lunar regolith itself, and growing the plants in them directly found that the soil was not particularly good for plants, and that Mars simulants were better. However the Mars simulant contained trace amounts of organics and also held water better than the lunar simulant. Also heavy metals are an issue for Mars soil (they could not eat the results of their experiments because of the possibility of heavy metal concentration) as also is the case for the Moon.

None of this simulates the situation of gardening on the Moon or Mars using the soil itself as that would involve composting the residues of the first generation plants with micro-organisms to make a good top soil. You can't really expect plants to grow well directly in ground up rock without topsoil. Even after a single cycle of composting the residues, the result could be much more fertile.

Experiments with material from Apollo 16 showed that cabbage seedlings accumulated high concentrations of aluminium. Other experiments by Kozyrovska et al show that marigolds grown in terrestrial anorthosite accumulated heavy metals such as zinc, iron, nickel and chromium (lunar anorthosite forms the light areas on the Moon).

However when the marigolds were grown in a community of microorganisms in a model plant microcosm, this protected the marigolds against toxic doses of the heavy metals and also helped deliver essential nutrients to the plants.

Apollo 15 "Genesis Rock" made of the lunar anorthosite which forms the light areas of the Moon. Marigolds grown in terrestrial anorthosite accumulate zinc, iron, nickel and chromium, however addition of bacteria helped correct these imbalances.

The Genesis Rock on the Moon before it was collected.

Marigold flowers (tagetes patula) photo by Dori - the experimenters tested marigolds in terrestrial anorthosite (a mineral common on the Moon), grown with a community of micro-organisms then marigolds are able to deal with the heavy metals in terrestrial anorthosite (a common lunar rock). This suggests that terrestrial plants can be used to convert lunar rocks into good top soil and that microbes can help protect plants from the high levels of aluminium found in the Apollo cabbage plant experiments.

Kozyrovska et al write:

"Our idea was to use the lunar soil for the pioneer plant growth and to convert the resulting plant biomass into so called protosoil, utilizing an optimized consortium of microorganisms."

Their experiments were promising. See Bioaugmentation in growing plants for lunar bases. The toxic heavy metals seem to be a surmountable issue. Another approach suggested by Haisong Liu et al. is to supply soil, and even seedlings, from Earth to get the process off to a quick start.

I'd like to suggest another idea here. Since hydroponics work well in the BIOS-3 experiments, one could also start by growing the first generation of plants hydroponically. Then you could use the plant residues to inoculate the lunar soil with organic material along with beneficial bacteria.

Also after whatever method is used for the first areas of good top soil on the Moon, the soil itself can then be used to inoculate larger areas of lunar soil with some of the top soil created earlier. The same methods could of course also be used on Mars, and Mars also has heavy metals in its soil. So in this comparison the two come out roughly equal.

See also Earth length day on Mars versus advantages of close to 24/7 solar power at the lunar poles

LANDING AND TAKING OFF - COMPARISON OF THE MOON AND MARS

The atmosphere does help with aerobraking on Mars, but at a price of a much trickier landing. Typically you have to reduce speed from Mach 5 to Mach 1 and then convert to a lander, and land it on the surface all within 90 seconds. Then typically the parachutes only reduce the impact velocity to around 200 mph, so for the final few meters of the landing you need retropropulsion.

The lunar gravity is only a little under a half of the Mars gravity, so you might think there wouldn't be much difference, but there's a huge difference because of the rocket equation. You can see this difference with the Apollo lunar landers. Their small rocket motors were not only able to land two humans and their supplies on the Moon, but take off as well, and all with no use of atmospheric braking.

OTHER MOON MARS COMPARISIONS

Much is made of the value of CO₂ for fuel generation on Mars from a hydrogen feed stock. However, Mars needs much more fuel for take-off than Earth. The volatiles at the poles of the Moon might be a source for rocket fuel to LEO, and the solar power, which is more than double on the Moon and even more so at the poles where it is not only double the intensity of Mars sunlight, but also present nearly 24/7.

As for the extreme cold - yes the Moon has the coldest temperatures of the entire inner solar system in the polar craters of eternal night. But that's actually an asset, it can be used for passive cooling of infrared telescopes for instance. Especially since it is right next to areas where the temperatures are much warmer, for habitats, and again, pretty much the same year round.

Much is made of the coincidence of the Mars 24 hour day. But it is easy to create night, for plants to grow in darkness, you don't need a lot of shielding. Just a thin sheet of any opaque material is all you need to shield out the midday sun and create a shadow on the Moon. And you need light for the plants during the lunar night - well you need light for plants on Mars during the Martian storms so you would need the capability to supply plants with LED lighting for several weeks on end anyway.

And the 24 hour day of Mars actually leads to huge differences of temperature between day and night in Mars' very thin atmosphere. At night in the Martian "tropics" the air gets so cold that carbon dioxide freezes out as dry ice, carrying water with it to form the Martian frosts photographed by Viking. While in the day time the temperatures can get well above zero at times. The steady temperatures of the lunar poles of the Moon would be much easier to engineer for.

So none of those seem particularly to be advantages for Mars over the Moon. Indeed, I think you could say that, despite appearances, the Moon is actually more habitable than Mars in many ways, for the first settlers to get there at least. Especially so if it has volatiles at its poles.

WHAT ABOUT GRAVITY - ISN'T THAT A BIG ADVANTAGE FOR MARS OVER THE MOON?

One advantage Mars might have over the Moon is its somewhat higher gravity level, slightly more than double lunar gravity and about a third of Earth gravity. But we know so little about human tolerance of gravity, that we can't really assess this yet, or even say for sure that it is an advantage. It might be. But you can't extrapolate a straight line based on only two data points, Earth gravity and zero gravity.

Examples of possible curves to join the two known dots of full g and zero g. Discussion on page 59 of this report

I'd go a bit further than this though and ask the question: do we even know for sure that the graph is monotonic (steadily increasing)? Perhaps that's quite likely, but for one example, might not the optimal gravity level for human health be less than full g, for instance? We know that hypergravity and zero g are bad for our health, when compared with full g. But we don't yet know for sure that all levels of partial gravity (hypogravity) are bad for our health.

Also, what if lunar gravity (say) is really good for some health conditions or particular individuals? For instance, does reduced gravity help with heart conditions because there is less blood to pump around because of the reduced plasma, and less work for the heart to do? Or does it make things worse, because of the reduced plasma and faster heart rate? Does it benefit particular age groups (e.g. elderly or young) or is it particularly bad for them? Is there a difference in effect for men and women, also what about women in pregnancy? Are there genetic differences in responses to partial gravity?

So, from a mathematical pont of view, potentially there could be a lot more variation than that. Here is a graph I did to illustrate this idea

The numbers here are arbitrary. I just used 100 for zero g health, 500 for Earth gravity, and various numbers in between to make a nice graph, with theonline Line graph maker. (If you want to duplicate it, I used 100 48 520 500, 100 650 400 500, 100 350 200 500, 100 150 600 500, and 100 40 50 500).

Also the graph may depend on your age, sex, health conditions, genetics, etc. Indeed each person might have a slightly different graph here and different optimal partial gravity levels for health.

Lines 1 and 5 show the possibility that some levels of partial gravity could be even worse for health than zero g, and lines 1, 2 and 4 show the possibility that some levels of partial gravity could be better for health than full g. Lines 2, 3 and 5 shows the possibility that lunar gravity could be better than Mars g for health, perhaps for some people or health conditions.

Can we rule out any of these possibilities yet?

We don't know yet:

• If lunar gravity is bad for health. It might even be beneficial, or beneficial for some people or health conditions or ages.

• What the optimum gravity health prescription is for humans, and whether this varies depending on age, for pregnant women, for young babies etc.

• Whether a steady gravity level is best or one that varies (e.g. higher gravity when asleep or while eating or exercising, might varying gravity levels during the day be beneficial for health?).

• How easy it is to augment lunar gravity, and what human spin tolerances might be for artificial gravity on the Moon.

If you just want to sleep, eat, exercise, not use it for doing things that need fine control, then the coriolis effects don't matter either. People can adapt to coriolis effects also to the extent that when they stop spinning they feel that there is force acting in the opposite direction.

Also, we just don't know the gravity prescription for health. Maybe if you spin just a few minutes a day while eating or during exercise, it makes all the difference. Maybe lunar g is okay. We don't know until we find out more.

Also humans are not a single system, but a complex interaction of many things. It might be that your circulation (heart rate, blood pressure etc) works best at one gravity level, the cells of your body are healthiest at another, your muscles keep their best muscle tone at another, your immune system works best at another, and it is easiest to lose excess heat while exercising at another gravity level.

It may be that it is best to sleep at one level of gravity, to eat meals at another, to exercise at another, and to work at another, and to read a book at another, and another is optimal for using a toilet, another is best for ice skating, another is ideal if you want to be able to run as fast as you can unaided. It might be that in future space Olympics, all the sports use different gravity levels :). Maybe a space sprinter wouldn't think of running in anything except Mars gravity, but for hurdling, you wouldn't dream of any other level except lunar g. Again just making up numbers there for fun :). While human flight, I think that is supposed to be possible in lunar g, and surely would be harder in Mars gravity, so might be a new lunar sport. But who knows, maybe it is optimal in 1/100 g, or maybe that is only used for newbies and kids trying to fly for the first time.

Flying - one thing that the Moon surely is better for than Mars. Illustration by Madhu Thangavelu and Paul DiMare © from The Moon: Resources, Future Development and Settlement

So, you can't say if lunar or Mars gravity is better. The answer is just, that we don't know yet. And it might also be a case of "best for which people and which activities?".

What though, if we do need full Earth gravity for health, or higher than lunar gravity or Mars gravity? Can we augment it with artificial gravity? You'd think it would be possible as we can generate hypergravity on Earth, and people have even lived for months at a time in rotating rooms here during research into effects of spin motions on humans. So one way and another it should be possible on the Moon too, but how easy would it be to do? And could you build really large structures like this?

ARTIFICIAL GRAVITY ON THE MOON TO AUGMENT LUNAR GRAVITY

One day perhaps we will have centrifuges to generate artificial gravity on the Moon. We might build small rotating rooms or short arm centrifuges, or we may build large structures for lunar gravity. For instance, in the low lunar gravity, the lunar caves may be up to several kilometers across, similar in width to an O'Neil cylinder. If these caves exist, you have large spaces to use to build rotating structures to augment lunar g.

But how could these work? What kind of a structure could hold up a rotating habitat kilometers in diameter on the Moon? One big difference from spinning space habitats is that we will have lunar gravity acting along the rotation axis. In particular, how could you keep it balanced and avoid huge torques on the central pivot when people move around in the habitat? If you can use short arm centrifuges, then the problems are less acute, but it still seems a tricky engineering challenge.

Solution for large diameter centrifuges on the Moon

There's a solution though. For the larger diameter centrifuges, then simplest idea is to just build a train going round and round a circular track. Then there's no limit, you can have circular tracks kilometers in diameter if you so wish, either in the lunar caves or on the surface. Also you don't have to think of narrow carriages as on trains on Earth. As we saw in the section on Lunar railways, lunar gauges could be wide gauge, perhaps twice as wide as most track gauges, perhaps three meters between the tracks, or more. Your "carriages" could also be multistory, with no height restrictions, so in principle they could be as big as a 747 or larger.

If you use circular railways, you don't have any engineering problems of torque on a central axle or pivot. Whatever the diameter of the track, the force outwards is only your that exerted by the train itself and its passengers under 1 g. This is no more of a problem for wheels to support than a conventional train on Earth. This idea is often discussed online, but not so much in the academic literature. But there is a 1996 paper: Artificial Gravity Augmentation on the Moon and Mars

"One method of augmenting gravity is a extraterrestrial railroad. A vehicle on a circular track banked with respect to the horizon creates centripetal accelerations related to the speed of the vehicle and the diameter of the track. Incremental accentuation of gravity may be accomplished by switching the vehicle to a track of larger diameter and steeper bank. Rotation creates accelerations on the vestibular canals of the inner ear that will limit the angular velocity of the vehicle. Colonists would have the opportunity to work part of each day in simulated Earth gravity and easily access the planet's surface. The magnitude of gravity that will protect us is unknown, as is the frequency and duration of exposure. This must be investigated. An extraterrestrial railroad, as one solution to this problem, does not involve exotic technology and is readily expanded."

So the suggestion is that you have banked tracks for the train to run on. As you transition from lunar gravity to full gravity the train would move to steeper and steeper banked tracks so that the floor always feels level to the passengers. The transition would go in the opposite direction when the passengers want to leave the train, the train would move to the less steeply banked tracks first.

Another idea suggested in the forums is a tilting train:

A JR Hokkaido KiHa 283 series tilting DMU on a Super Hokuto limited express service on the Hakodate Main Line, photo by Japanese wikipedia user: 出々 吾壱

For full g it would need to be tilted by 80 degrees instead of the 8 degrees shown in in this photo. For large amounts of tilt, perhaps this would work best if the carriage is in an inside compartment only indirectly coupled to the outside while the train is moving. This idea could also be used with a smaller amount of tilt to keep the floor of a train level while transitioning between tracks with different degrees of bank, and then finally to a stop.

To deal with issues of friction between the trains and the tracks we can

• Use maglev trains so that there is no physical contact with the tracks (though with safety mechanisms to make it impossible for them to leave the tracks)

• Run them in high vacuum - would be the default situation on the surface. In lava tube habitats, then you don't fill the entire tube with air, just the habitats.

• Convert the regolith to glass below the track and for a few meters to either side to reduce dust problems and regularly clear the tracks of dust (not needed very often probably as the dust does get levitated but probably not in huge quantities.

Also bear in mind that in a future where we can build large circular tracks like this, it's also gong to be easy to lay out large areas of solar panels on the surface and we can also design power storage during the lunar night, and by then we may have small nuclear power stations too. So, though it's not going to be as efficient as a spinning habitat in space which spins pretty much endlessly once you set it spinning, it's probably not going to be a huge power drain on a working habitat. SeePower during the night.

However perhaps we don't need such large scale systems for artificial gravity on the Moon. First, for all we know to date, lunar gravity might already be healthy for humans. In that case we don't need it at all.

Artificial gravity only intermittently during the day.

Or we might need gravity only intermittently during the day for health. Here is a study that found that full gravity in a centrifuge for just one hour a day made a big difference to muscle loss in bed rest volunteers lying with head slightly lower than their feet to simulate zero g. Other studies come to similar conclusions. Other experiments use "dry immersion" to simulate zero g, where a volunteer is immersed in warm water with an elastic suit to keep them dry, which has similar physiological effects to head down bed rest, but the changes take effect more quickly.

Dry immersion used to simulate zero gravity in some experiments, figure from Artificial Gravity by Clements et al. Drawing by Philipe Tauzin (SCOM, Toulouse)

The volunteer is submerged in warm water, protected by an elastic sheet to keep their skin dry, to protect them from skin softening due to immersion in water (skin maceration).

A 2016 survey of the literature to date, "Centrifugation as a countermeasure during bed rest and dry immersion: What has been learned?" concludes

"Centrifugation for as little as 30 min per day was found to be effective in mitigating orthostatic intolerance and strength in postural muscle after 5 days of bed rest, but it was not effective in mitigating plasma volume loss."

In other words it helps with muscle wasting and difficulty in standing up in full g after prolonged exposure to the simulated zero g (orthostatic intolerance) but 30 minutes exposure per day is not enough to help with the reduced amount of blood supply to the body which is another effect of zero g on astronauts. They suggest more experiments are needed to find out effects of longer periods of artificial gravity per day. All the studies they survey involved male subjects, and women respond differently, they appear to be able to regulate blood pressure in ways that men cannot. They also suggest that the spin rate be adjusted to achieve full g at the heart rather than the feet, and that it could be useful to let the subjects choose what level of artificial gravity they want to use. Many other suggestions in the detailed survey.

We have actually tested artificial gravity in space, the Neurolab experiment, flown in 1998 able to produce artificial gravity of between 0.5 and 1 g in an off axis rotator. Although the astronauts were only tested for a few minutes a day, the four astronauts tested didn't have the same problems with standing upright on return to Earth that most (64%) astronauts have. It's a small sample but the probability that this happened just by chance is 1 in 60. See page 24 of this report.

Tolerance of high spins in zero g, and adaptation to higher rotation rates

Also, from some early experiments in zero g, and anecdotally (Tim Peake's demo of tumbling at 60 rpm for a couple of minutes in the ISS), astronauts in zero g can tolerate spins that would make them nauseous on Earth.

The same may apply in reduced gravity too. From some parabolic flight experiments by Lackner and DiZio in 2000, summarized on page 21 of this report

“the severity of side effects from Coriolis forces during head movements is gravitational force-dependent, raising the possibility that an artificial gravity level less than 1 g would reduce the motion sickness associated with a given rotation rate”

Also it's possible by acclimatization to gradually adapt to higher rotation rates without ever experiencing motion sickness.

So one way or another, we might be able to tolerate higher spin rates on the Moon. We would need to do experiments to find out if this is the case. But if we can use higher spin rates there, the centrifuges can be smaller for the same amount of artificial gravity, and still be tolerable. See my section below on: Small centrifuge based artificial gravity experiments in LEO

Design ideas for smaller centrifuges on the Moon

A small centrifuge could be based on the same principle of an external track around the centrifuge rather than a central pivot. It's just like a model railway going round and round :). Same idea as the larger one but on a smaller scale.

It could consist of a drum inside a container and motors that drive the drum around and just wheels (or maglev) between the drum and the outermost walls of your habitat. This would seem an especially good approach if the outermost walls are very strong, for instance, if it is in a cave or you can pile a ramp of regolith around the outside of your habitat.

The same approach could also be used for a rotating room such as a dining area, say. Or a gym or sleeping compartments

I'd like to suggest another idea which I haven't seen yet in papers or discussions online - a central pivot with living quarters suspended from it like a swing carousel, or indeed maybe a smaller scale swing carousel inside a habitat for sleeping or for eating or exercise. On the Moon, if spun fast enough for full Earth gravity, they would be angled about ten degrees from horizontal.

Another related idea is to build a velodrome style track on the Moon for cyclists to use to keep healthy.

Dunc Gray Velodrome in the City of Bankstown, Australia, cycling venue for 2000 Olympic games. Photo by Adam. J.W.C. It bends at a maximum angle of 42°. A more steeply banked velodrome on the Moon, banking to a maximum angle of 80° would let cyclists generate between half and full g by cycling round and round. For normal cycling speeds of 10 to 15 meters per second, or 22 to 33.5 miles per hour, the lunar velodrome could be 50 meters in diameter, see Human Powered Centrifuges on the Moon or Mars.

That's rather fast though. Fit astronauts could do it for short periods of time. Here are some example speeds from a sporting cyclist. He gives an example of reliability rides involved riding 50 miles in 3 hours or 16.6 miles per hour.

The AG we want to generate by rotation to combine with lunar gravity to create a sensation of full gravity for our astronauts is sqrt(9.807²-1.622²) or 9.6719 m/s/s.

It would be rather like the "wall of death" motorcycle stunts (typical radius 3 to 5.5 meters) :

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Photos of the event can be found here: http://rhobbs.co.uk/8lp Ken Fox Wall of Death (Complete Show) at Haddenham Steam Rally 2009.

07:34

https://www.youtube.com/watch?v=fNM9GFV9adg&feature=youtu.be

==

- but at the much lower lunar gravity, you only travel at jogging speed, and you haven't got far to fall and the gravity is much lower also. So I see no reason why you can't have a "jogging track" inside the habitat similar to the jogging track inside Skylab.

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Jogging on the walls on Skylab

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Jogging in weightlessness? :) Get real! It's fake. Jogging in weightlessness is impossible, period.

07:18

https://www.youtube.com/watch?v=0_3NqeRJizg&feature=youtu.be

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Arial","sans-serif";mso-fareast-font-family:"Times New Roman"; color:#065FD4;text-decoration:none;text-underline:none'>http://rhobbs.co.uk/8lp Ken Fox Wall of Death (Complete Show) at Haddenham Steam Rally 2009.

07:34

https://www.youtube.com/watch?v=fNM9GFV9adg&feature=youtu.be

==

Jogging starts at 3.30 and they jog at around 10 rpm, so probably experienced around 1/3 g at their feet, taking the radius as 3 meters. Jogging at around three meters per second, or about six miles per hour. The longest jog is for one minute 50 seconds (including various gymnastic tumbles in the middle).

The speed you'd need to cycle to reach full g for artificial gravity depends on the radius. So for a radius of 5.5 meters, like the larger "wall of death", that's a speed of 7.3 meters per second or 16.3 miles per hour, so that's the reliability rides type speed. For a radius of 3 meters, it's a speed of 5.39 meters per second, or 12 miles per hour.

But this is only going to provide artificial gravity while exercising, and indeed, only while cycling. That leads to another idea, what about a moving walkway? Similar to the idea of a train, but you can do it inside a habitat. The Trottoir Roulant Rapide in the Montparnasse—Bienvenüe Métro station in Paris. however moved at 9 km / hr or 5.6 miles per hour, 2.5 meters per second.

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Experimental 185 metre long high speed moving walkway at Montparnasse - Bienvenüe métro station in Paris, France. At first it operated at 12km/h but too many people were falling over so the speed was reduced to 9km/h. It has been estimated that commuters using a walkway such as this twice a day would save 15 minutes per week and 10 hours a year. Using this walkway is like using any other moving walkway, except that for safety there are special procedures to follow when joining or leaving. Staff (seen here in yellow jackets) vet who can use it as you must have at least one hand free to hold the handrail. So, if you are carrying bags, shopping, etc or are infirm you must use the regular walkway to the right. On entering there is a 10 metre acceleration zone where the 'ground' is a series of metal rollers - you MUST stand still with both feet on these rollers and use one hand to hold the handrail and let it pull you so that you glide over the rollers; the idea being to accelerate you so that you will be travelling fast enough to step onto the moving walkway belt. Once on the walkway you can stand or walk; there is no special sensation of travelling at speed. At the exit there is a deceleration zone where again you MUST stand still and let the handrail pull you as you slow down, again whilst gliding over metal rollers. Then you just walk off. ------------------------ This clip is hand held, and at times it is a little unsteady. Towards the end of the clip note how the other people leaving the moving walkway stand with both feet firmly on the ground and one hand on the handrail whilst passing through the deceleration zone. For more information visit these webpages: http://citytransport.info/Niche.htm http://en.wikipedia.org/wiki/Moving_w... http://fr.wikipedia.org/wiki/Trottoir... -------------------------------------------------- In May 2009 it was announced that because of its unreliability and the number of users having accidents this high-speed moving walkway will be replaced with a standard moving walkway. It closed in September 2009.

01:12

https://www.youtube.com/watch?v=pBJN1X3LeJw&feature=youtu.be

==

They ran into problems during the acceleration phase with the elderly and frail tending to fall over plus the need to hold onto the rail, leading to injuries, but that's perhaps not an issue for early days on the Moon with fit astronauts. Also you'd have the much lower lunar gravity so the risk of injury would be less. Still you'd also need it to move faster than this, a little more than double this 2.5 meters per second for this to generate full gravity even for a small 3 meters diameter track. A walkway like this on a 3 meters radius track would generate about 2.67 m/s² or about 27% of Earth gravity at your feet, less at your head of course.

Variation in amount of Artificial Gravity across the width of the track

Variation in amount of Artificial Gravity across the width of the track

Another issue here is that unlike artificial gravity in space, the amount of AG will vary across the width of the track. The amount of this effect depends on the radius of a centrifuge. Suppose for instance it's a 4 meter radius centrifuge (larger than the Skylab jogging track, head two meters from the axis when standing up), and the track is 2 meters wide (or you move 2 meters sideways on a wider track).

And - let's use an exact figure for the angle, may as well, asin(1.622/9.807) in degrees = 9.52 degrees for the angle from the vertical of our centrifuge wall. So now we have 2×1.622/9.807 = 0.33 meters difference in radius between top and bottom of our centrifuge. And the AG we want to generate by rotation to combine with lunar gravity to create a sensation of full gravity for our astronauts is sqrt(9.807²-1.622²) or 9.6719

So then with a 4 meters radius centrifuge, 14.85 rpm, then it is 10.47 m/sec² AG at the widest radius at the top of the slope from the lunar perspective - (and of course, 9.807 m / sec² at the bottom) so you would get heavier by 6.7% by walking two meters sideways. I'd have thought even that would be reasonably tolerable.

Depends what you are doing, fine control or just eating a meal or sleeping, or using the toilet - or exercise. But we are pretty good at learning to adjust to things, and I wouldn't be surprised if we could learn to adjust to a variation in weight of 6.7% even for tasks that involve fine control. The only way to find out for sure would be to test it, and we couldn't simulate it on Earth.We couldn't really simulate it that easily in space either, so perhaps after preliminary experiments in LEO to find out if we even need more than lunar gravity, and if so, what the gravity prescription is for health and if it is needed 24/7 or just for a short while each day, we would just have to test it on the Moon. If it is a big problem we need larger centrifuges but I'd have thought it has a decent chance of being okay.

Summary

It would need careful study I think. So the options seem to be, so far:

• Train, would work well for larger diameter tracks but you do need to make the tracks and habitats like large train carriages, capable of moving on wheels. You need to work out how to get on and off the train and how to deal with the tilt with track or train banked to 80 degrees.

• 50 meter diameter velodrome banked to 80 degrees - super fit cyclists would be able to generate full g for short periods of time

• A smaller, perhaps three meter radius velodrome could double up as a jogging track like skylab - would generate more than lunar gravity, around a third of full g. An experienced reasonably fit cyclist could reach full g and keep this up for hours on end (twelve miles per hour), and a fast runner could achieve full g for short periods of time. However can only achieve full gravity while exercising

• Inclined moving walkway - would need technological development to be safe and easy to use at the high speeds needed (similar to cycling speed or faster). Could generate up to a third of Earth gravity however reasonably easily for fit astronauts, using technology similar to the Trottoir Roulant Rapide. This idea could be combined with the velodrome idea - you just switch off the moving walkway to turn it into a velodrome or jogging track.

• Spinning rooms for smaller centrifuges or sleeping compartments. These seem the most feasible for an early stage.

• Hammock or swing carousel type - suspended from cables attached to a central pivot - work on small scale but less mobility as you have to remain within your swing or hammock. Still you could eat food there, read, sleep, do some exercises. Easiest of all for early experiments.

I found very little on this topic in my literature search. Most is in forum posts apart from the two papers I mentioned. Here are some of the discussions and posts I consulted, though of course they are not peer reviewed, used as a source of ideas:

• Forum discussion of the idea here: Surface-based centrifuge for lunar colonization

• More ideas here in a blog post: Banked track Magnetic levitation based artificial gravity

• Discussion on Case for Moon first facebook blog

• Lunarpedia entry here Artificial Gravity on Luna

And the papers I mentioned already:

• Human Powered Centrifuges on the Moon or Mars.

• Artificial Gravity Augmentation on the Moon and Mars

If you know of anything else on this particular topic of artificial gravity on the Moon, do say!

I'm using spincalc here for AG calculations

Effect on plants

The levels of artificial gravity needed for phototropism is between 0.1 and 0.3 g. So some higher plants would grow normally on the Moon without need for centrifuges, while others might need up to Mars levels of gravity. Here Mars may have an advantage for growing plants.

I think it's possible that the Moon may also have advantages though, because you'd have much less by way of gravity along the spin axis. As a result our otoliths which sense linear acceleration along the spin axis on Earth would be less stimulated so potentially, there could be less conflict between the otoliths and our vestibular system. We would have to do research to find out why it is that astronauts are able to tolerate spin motions in zero g, which they can't tolerate in full g. Once we understand this better then it would be a major question to see if this also applies to some extent to lunar gravity. If it does we could use much smaller centrifuges for artificial gravity on the Moon for longer periods of time.

See the section below: Small centrifuge based artificial gravity experiments in LEO

LUNAR VACUUM AS AN ASSET

The vacuum of the Moon is also actually an asset, so much so that we might need to take special precautions to preserve it.

"It seems absurd to expect that the lunar vacuum could be lost by small-scale operations on the moon. However, high-vacuum and ultra-high vacuum is needed for many industrial processes, some of which may be accomplished on the moon. Some processes which require vacuum and thus would be simpler to manufacture or use on the moon include vacuum tubes, semiconductor manufacture, solar cell manufacture, and particle accelerators."

Degradation of the Lunar Vacuum by a Moon Base - Geoffrey A. Landis

The carbon dioxide atmosphere of Mars is so thin it counts as a laboratory vacuum, you would need a pressurized spacesuit or you'd die quickly because the water lining your lungs would boil. But it's not thin enough to be a useful vacuum. And as we've seen, carbon dioxide is not needed as an input for greenhouses - in a closed system it just circulates around to food and back again. The Moon would seem to have the advantage here, too.

MARS OR MOON SPECTACLES AND THE OLD WOMAN YOUNG WOMAN ILLUSION

One of the things I've discovered as a result of writing this book, is that the Moon is not only much closer, and a safer place to send humans - it's probably also in many ways more suitable for human habitats than Mars! That will probably surprise you if you come to it after reading Robert Zubrin's "Case for Mars". Probably you will say something like this:

"It's got no 24 hour day, no CO₂ atmosphere - it seems dull as concrete - and some of the ideas for colonizing Mars will work on the Moon, but others will not. You can't make methane from a hydrogen feedstock on the Moon without a CO₂ atmosphere for instance. So how can the Moon possibly be an easier place for humans to live than Mars, when only some of Robert Zubrin's ideas will work there?"

But if you look at it on its own merits, then things suddenly turn around, like those images of a young girl and an old lady

One of the early versions of the Young Girl Old Lady Illusion.

The 24 hour day of Mars actually turns into a disadvantage of Mars because of the huge day to night temperature changes. Similarly we've seen that on the Moon you won't miss the low grade laboratory vacuum levels of carbon dioxide, which is not needed in a closed system greenhouse. And instead the hard vacuum becomes a major advantage of the Moon over Mars. Instead of methane fuel, you use the abundant solar power to split the water ice into hydrogen and oxygen, a much better rocket fuel, and you can also use the hydrogen to make fuel cells. We've seen several examples like this.

I think the general point here is that if you try to analyse the Moon as if it was Mars, then it seems like it has little by way of resources. But when you analyse it on its own terms, it is rich in resources. It also has several unique resources not available on Mars. You need to look at the Moon with "moon spectacles" to see its advantages.

TRASH ON THE MOON - TESTING GROUND FOR PLANETARY PROTECTION MEASURES FOR A HUMAN BASE

How do you deal with the trash on the Moon? This is not an exciting or glamorous subject I know, but it is part of the reality of space travel. This is something that's easy to forget about, how much trash astronauts generate every year. We don't notice it so much on the ISS because it is easy for the astronauts to dump it, and it all burns up cleanly in the atmosphere. The Apollo crew didn't spend enough time on the Moon for it to be that noticeable.

Well, the Moon is also a good place to study the effects of humans and their wastes and trash on their surroundings in space. In such challenging conditions we are bound to have wastes of all sorts pile up around the base. Think how much refuse the Progress rockets return to Earth, to burn up in the atmosphere:

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Astronaut Mike Fossum demonstrates how to pack trash before it’s removed from the space station.

02:25

https://www.youtube.com/watch?v=JP5D0QK88Lw&feature=youtu.be

==

See all that trash which you can see filling up the Progress in those white bags in this video. The ISS discards that much trash into our atmosphere every few months. Imagine all of that piling up around a base on tiny Phobos or Deimos? The Moon would be a great place to see what happens to all that trash, and find out if there are ways we can deal with it. How much can we recycle? Or do we just bury it all? Some of it is "smelly stuff". For instance on the ISS the astronauts don't have facilities to wash their clothes. As soon as their socks, smalls, shirts etc begin to get a bit smelly or dirty they just put them into a bag and eventually it ends up burning up in the Earth's atmosphere at a very high temperature, along with the Progress vehicle itself.

On the Moon, all those bags will pile up around their base unless they find another way to deal with the trash. They would recycle what they can of course, but that might not be easy in such challenging conditions. How do you recycle a spacesuit that doesn't work any more for instance? They will have quite an incentive to find a way to do some recycling. For instance, to wash their clothes, but that's not the easiest of things to do in a spaceship. Incinerate most of it in an oven??

A base on the Moon surely isn't going to send all its trash back to Earth. It's not like the Progress attached to the ISS. You need 1.6 km / sec delta v to launch it as a payload from the Moon into orbit, and if you were to send it back to Earth to burn up in our atmosphere, you'd need another kilometer per second for a Trans Earth Injection. You would use up a lot of good fuel to burn your trash in Earth's atmosphere. They surely won't do that. So your base would have piles of trash build up around the base.

Then the thing is that, like the Apollo 11 footprints, any trash you leave on the surface of the Moon will still be there thousands, and probably millions of years into the future. It won't degrade , rust away, and mix into the landscape.

Buzz Aldrin standing near a leg of the lunar module - notice how many footprints they left on the Moon? These will still be there a million years from now.

It will be the same for the area around any lunar base. It will soon be totally covered in footprints - unless they either turn the dust around the base to glass or they rake the soil. In the same way, all the trash they leave on the surface of the Moon will also still be there a million years from now too.

It would be the same for a base on tiny Phobos or Deimos. Also any other small region such as the most favoured "peaks of almost eternal sunlight" at the lunar poles. It's the same for any geographically small region of the Moon, for instance the region around a lunar lava tube cave entrance (unless they put all the trash into the cave?). It will soon be covered in footprints and trash after humans have been there for a few months / years. The Moon would be a great place to get a first idea of what the scale of impact of this will be, as we explore the solar system, and to make our first experiments in dealing with this issue and learning what we can do about it.

I can't find much on the problem of trash on the Moon. It's often mentioned in passing but not discussed in detail. But here is an abstract from 1988 which gives a succinct summary of the main issues. After describing the main sources of trash including the descent platforms (which remain on the surface when the modules return to orbit, and they estimated as 4.9 tons each), paper, cloth, wood, plastics, ceramics and glass, aluminium and steel, they then go on to describe possible disposal methods

The primary disposal process on the lunar surface of the by-products of shredding, wet oxidation or solar furnaces, will be burial. The initial temptation of filling nearby craters will be unavoidable for the initial settlement, but cannot continue for any extended time. As lunar mining becomes a reality, ample landfill opportunities should arise. An alternative to crater filling or direct burial would make use of areas of perpetual shade if available near the colony. These areas, if large enough, exhibit absolute darkness and temperatures near 150 K that would make for excellent limited disposal sites.

Most of that surely still applies ,though I'm not sure if piling trash into areas of perpetual shade would be favoured so much nowadays given the interest in any ice or volatiles that might have accumulated there. Perhaps in caves though? Would some of the lava tube caves, or sections of them, be earmarked for trash disposal?

Also trash on the Moon will have many materials in it that would be almost impossible to make locally and that would cost a lot to send up from Earth. Recycling would be hard to start with but get easier as they develop more technology and industry on the Moon. Perhaps in the not so distant future our descendants or even our older selves may return to those trash pits from the early twenty first century and mine them for valuable resources. If so it might be worthwhile segregating the trash before burying it to make things a bit easier for the future astronauts on the Moon who wish to re-use it.

Also, though it may seem the best thing to do is to compact it to take up as little space as possible, crushing, and burning it, is that really the best solution? Maybe in the not so distant future, some of the materials may be useful if not incinerated or compacted? For instance tanks, or a spacesuit, perhaps equipment that has some small thing wrong with it that the astronauts couldn't fix on the spot with the materials they had - either in the future the astronauts may find a way to recondition them, or to re-use the materials or components for some less demanding task.

These are just a few thoughts on the subject. If anyone reading this knows of good material on trash on the Moon do say!

Rocket exhausts, microbial spores and organics mixing with levitating lunar dust

The Moon is also an ideal place for planetary protection scientists to study survival of microbial spores in space conditions. There are many spacecraft on the Moon that have been there for decades, crashed or landed there, and we can study them to see if there is any viable life on them. This could give us good ground truth on planetary protection. See also Organic Measurements on the Lunar Surface: Planned and Unplanned Experiments (powerpoint presentation).

The scientists actually used Apollo 11 for planetary protection studies. First, they estimated that the lunar surface where the Apollo 11 astronauts collected their samples had only 10,000 to 100,000 viable microbial spores from Earth per square meter (between one and ten spores per square centimeter). They did do a few analyses of selected samples up to Apollo 14 for colony forming spores, inoculating them into with different media, and no colonies formed. Here is a detailed description of the process of examining a sample from Apollo 11, no colony forming life was found after inoculating 3,000 petri dishes each with 4 mg of the sample (total of 12 grams). In another paper, one of the four samples they tested was actively biocidal (perhaps heavy metal toxicity) but other soil samples were able to support colony forming spores, but didn't. However, those are only a few analyses of selected samples and with early 1970s technology.

The returned Apollo samples actually highlight the need to take care about contamination of the Moon if we are interested to find out about native organics. A NASA study in 2015 re-analysed some Apollo samples, using modern methods searching for amino acids.The analysis was a tricky one because there were several ways the amino acids could form (native to the Moon, produced by reactions with rocket fuel contamination in the lab, or from Earth organics).

In the end they decided that most of the organics came from Earth microbes (because of preference of the "left hand" form and the carbon 12 / 13 isotope ratios) but some probably came from meteorites. The samples are now known to have up to 70 parts per billion of organics from life. However much of that could be contamination of the samples after return to Earth (see this paper). Sadly this is a bit inconclusive, but it does suggest that, even with the Moon we may need to take care to avoid contamination of the samples by Earth organics, either in situ on the Moon, or after return of the samples to Earth, when exploring areas where the amounts of organics on the surface are of scientific interest.

The microbes, rocket fuel, and trash brought left on the Moon by the Apollo astronauts after their up to three days stay is nothing to the trash that would accumulate around a human base. So, another thing we can do on the Moon is to find out how much organic contamination and other types of contamination accompanies human explorations, and how far it spreads.

You might think that organics on the Moon would only spread as far as the humans themselves travel, away from the base? Most of it just localized around the base?

So, yes, of course the Moon doesn't have any weather as we know it, and it doesn't have global dust storms like Mars either. So, the organics won't spread as easily and as far as they do on Mars. However, it does have electrostatically levitated dust, which surprisingly can levitate even particles as large as 140 microns in diameter (line 215 of this paper). The dust is levitated through UV radiation and plasmas.

A microbe in a 140 nm particle would be protected from UV (though of course affected by the electrostatic discharges that levitate the particles in the first place). So, if you introduce foreign material to the Moon, it could spread some distance in this levitated dust. Perhaps even viable microbes. The rocket exhausts for Apollo 12 for disrupted about two tons of dust around the landing site that lead to localized dust storms observed during the next few sunrises as measured from the Apollo 12 landing site levitated to a height of one meter above the ground.

So how far can the dust spread? The finest dust might have gone right up to orbital altitudes. At least, the Apollo astronauts sketched what seem to be rays of sunlight hitting the dust at sunrise and sunset from orbit. However recent observations by LADEE show that there is no dust at altitudes between three kilometres and 250 kilometres. At least not any more, if there was back then.

Lunar horizon glow as photographed from orbit by Clementine spacecraft. The bright dot at the top is Venus and the Sun is behind the Moon. The Moon has an exosphere - an atmosphere so thin that the molecules rarely encounter each other. However LADDEE proved that there is no dust above 3 kilometers.

With the Moon's surface area larger than Africa, a few spores spreading out through dust levitation aren't going to do much to confuse science results kilometers away, unless there are habitats for life there. Every indication is that there are no habitats that Earth life could colonize on the Moon. There are ideas to turn the dust into glass for a landing pad, so perhaps that would help with the dust too. So, it may not be a major issue, but this is certainly something to study and monitor. It could be an issue for regions that we want to keep free of organics and other contaminants close to a human base.

Some areas of the Moon could be particularly sensitive to organics from Earth. In a review paper from 2007, the authors suggest that perhaps this categorization may need to be revisited depending on what we find out about the lunar ice. If this ice and the other volatiles there are of especial interest for study of prebiotic chemistry for instance, perhaps we might need to set up "organic special regions" on the Moon that need to be kept free of organics.

"Other locations, like the permanently shadowed craters at the Moon’s south pole, may contain water ice or hydrated minerals and other valuable scientific and physical resources. If, for instance, these sites contain ice with signs of prebiotic chemistry, one can envision the establishment of organic special regions to protect these native lunar organics for careful scientific study."

Although the human bases at the poles would be right next to permanently shadowed craters, perhaps the organics wouldn't spread far in the cold desiccated conditions there. After all, there is no UV light to help levitate the dust there, and there would be some shielding from solar storms. Also, there are hundreds of square kilometers of permanently shadowed ice at the poles, an estimated 1,850 square km of ice at each pole . However if the organics do spread beyond the base - well the Moon is an ideal place to study such things. It has minimal planetary protection issues compared to anywhere else. If it does still have some contamination of the polar ice by organics from the human base, well this is a chance to study the situation.

The historical lunar landing sites are another area that may need protection from contamination by Earth microbes. They are valuable for planetary protection scientists,as places to study the effects of a brief human presence on the Moon several decades later. They are a:

"valuable and limited resource for conducting studies on the effects of humankind’s initial contact with the Moon" (quote from page 774 of this paper).

These sites, with microbes exposed to cosmic radiation, UV etc are also decades long unplanned experiments in the interplanetary cruise stage of panspermia - the ability of microbes to remain viable in the conditions of interplanetary space for transfer from one body to another. (see page 771)

Another planetary protection question for the Moon (in its broadest sense) is whether our landers would change the Moon's very tenuous "atmosphere" or exosphere with rocket exhausts. Each Apollo lunar landing added 10 tons of exhausts to the atmosphere with a persistence half life of approximately one month. That might not seem a lot but it is a noticeable amount for the lunar atmosphere which has a total mass of approximately 25 tons.

We have a golden opportunity right now to observe its atmosphere "as is". The rocket exhausts from Apollo should have dissipated long ago. Amongst other things we can study the movement of water vapour in the lunar atmosphere and see where it comes from. Well it will be a while probably before we get humans landing on the Moon again, but we do have man robots due to land there in the near future, including (if they keep to schedule) the remaining Google Lunar X Prize contenders in 2017, and Astrobiotic, in 2018 (they dropped out of the Lunar X-Prize saying the 2017 timetable was unachievable). So what effect will they have on the lunar atmosphere? Well when the Chinese Chang'e 3 landed on the Moon on 14th December 2013, we had an excellent chance to find out as NASA's LADEE is still in orbit analysing the lunar atmosphere.

Artist's concept of NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) Image Credit: NASA Ames / Dana Berry

They came to the surprising conclusion that the Chinese lander hadn't modified the lunar atmosphere at all, or at any rate, if there were any changes, they were beyond their detection limits They studied it from a distance of 1,300 km so they couldn't observe the dust it kicked up (which only lasted for about 15 seconds in the Chinese descent video). No exhaust products were detected and the lunar atmosphere wasn't changed in any way.

This is good news for robotic missions to the Moon at least. This was a particularly large lunar lander for a robotic mission, so it seems that any effects from rocket exhausts are only local and don't have any global effects on the atmosphere, and the exhaust products don't travel large distances either.

Here is what they said in detail:

"Surprisingly, the LADEE science teams' preliminary evaluation of the data has not revealed any effects that can be attributed to Chang'e 3. No increase in dust was observed by LDEX, no change was seen by UVS, no propulsion products were measured by NMS. Evidently, the normal native lunar atmospheric species seen by UVS and NMS were unaffected as well. It is actually an important and useful result for LADEE not to have detected the descent and landing. It indicates that exhaust products from a large robotic lander do not overwhelm the native lunar exosphere. As the descent video shows, the interval of time that dust was launched by the lander is very short, perhaps less than 15 seconds. LADEE would probably have had to be in just the right place at the right time to intercept it. Also, significant amounts exhaust products apparently cannot migrate to large distances (hundreds and thousands of miles) and linger with sufficient density to be measured. "

What, though, about more local effects on the lunar surface, especially the lunar ice? We don't have any ground data on that yet, but we have some theoretical modeling. This shows the modeled effect of the Apollo 17 landing exhausts on the lunar surface near their landing site:

Figure 28 from this paper showing their modeled rocket exhaust contamination of the lunar surface from Apollo 17 superimposed over Google Moon. The contaminated area spans 522 kilometers of the lunar surface. The red range rings contain 50% and 67% of the total contamination respectively.

There would surely be scientific interest in the organics on the surface of the ice at the lunar poles. This modeling suggests we may need to take care about the effects of rocket exhausts from spacecraft landing in the vicinity of the lunar village, especially once the larger rockets start landing with astronauts on board. So what can we do about this?

The authors of the paper looked at future missions "where contamination by exhaust gases is not desired" and recommended that:

• Most of the braking is done with the engine pointed over the horizon and with the exhaust gas velocity much greater than the lunar escape velocity.

• Once the lander has lost enough momentum, its braking engine and excess fuel could be dumped to reduce the landing mass

• With the lunar lander now lower in mass, it can make the final descent to the surface using less fuel.

Perhaps I can make another suggestion of a way to reduce the exhaust problem even more, once we have frequent travel to the Moon. Hoyt's cislunar tether transport system . uses counter rotating momentum exchange tethers. At the Moon end of the transport system, the tether can be stationary relative to the Moon's surface whenever it is closest to the Moon, so that you can pick up payloads from the surface and deliver them to the surface with no need to use any rocket fuel to do this.

His lunar tether masses only seventeen times the payload mass, so you don't need that much traffic per year for this to be worth doing, just for economic rather than for planetary protection reasons. Once you get that much mass in orbit around the Moon, from then on, landing on the Moon and taking off again is essentially fuel free, and what's more, you get an automatic boost from the tether to take your spaceship down to LEO. Or indeed you could use the same method to go all the way down to Earth's atmosphere on an Earth return trajectory. This would not only reduce the amount of fuel needed to land on the Moon and take off to essentially zero, and make it more economical to travel to and from the Moon. It would also protect the lunar surface and exosphere from the effects of rocket exhausts. For details see the Exporting materials from the Moon section of my Case for Moon First.

Then, there's another potential benefit for Hoyt's cislunar tether. Perhaps you could dispose of lunar trash in the Earth's atmosphere as for the ISS, after all as there is essentially no cost of fuel to do that. That is, if you wanted to. Perhaps the trash may come to be very valuable on the Moon with much to recycle in it, once they had the capability. They might value the elements and components even, things like screws, sheets of metal etc, even paper, cloth and so on, that would take a lot of work or import costs from Earth to make on the Moon. But if they so wished, with Hoyt's tether, they could dispose of trash from the Moon in the Earth's atmosphere easily. There's a lot to be thought over and worked out here.

In conclusion, the Moon is an ideal place to look into such issues. Before we send humans to Phobos or Deimos or wherever they go next, we'd better know what humans will do to a celestial body when they set up a settlement there. This is especially important if the aim is to study ice deposits in craters, or other sensitive locations. In the case of Phobos, its regolith should have records of organics and even spores of life possibly, from pretty much the entire history of Mars mixed up with it, brought there by asteroid impacts into early Mars. So, it might well be important to keep the Phobos regolith, or parts of it, clean from organics and other contaminants from Earth. Our experiences on the Moon may help to give us the experience we need to design suitable precautions for missions like this.

PROTECTION OF THE HISTORICAL APOLLO LANDING SITES

Also - the lunar X Prize will have many smaller missions go to the Moon, commercial ones. We may see the first of them towards the end of this year. There are five teams in the competition, SpaceIL plans to use the SpaceX Falcon 9, but it’s had trouble fitting its mission into their faring. Hakuto has made a pact to land with Team Indus and both will use the Indian rockets - it’s proven technology but they are having trouble raising the funding. Moon Express will launch on Rocket Lab’s Electron, and Synergy Moon on Interorbital System’s Neptune. All are sharing the nose cone with other payloads on their rockets.

Will Anyone Win the Google Lunar XPRIZE?

Astrobiotics, the top favourite for a long time, have pulled out saying they can’t be ready to launch until 2018. But they are planning a “FedEx service to the Moon” - their Griffin lander will carry other lunar missions to the lunar surface.

In the future we may see tiny rovers image the landing sites.

Artist’s impression of a Google Lunar X-Prize rover at an Apollo landing site. Image Credit: Google Lunar X Prize

(I actually got the image from: Team Indus joins Google Lunar X-Prize finalists, Astrobotic drops out)

That was actually one of the challenges for a bonus prize for the Lunar X prize rather controversially, some think the area immediately around the historic landing sites should be kept pristine and the rovers not permitted to drive over them.

That’s particularly because of the prospect of them landing by accident on top of the flag or some such:

““I’d like to see them demonstrate their ability to do a precision landing someplace else before they try it next to the Apollo 11 site,” Logsdon says. “You wouldn’t have to be very far off to come down on top of the flag or something dramatic like that.” “

Preserving Tranquility

The Lunar Legacy Project say in their Introduction

“Our goal is to preserve the archaeological information and the historic record of Apollo 11. We also hope one day to preserve Tranquility Base for our planet as a World Heritage Site. We need to prepare for the future because in 50 years many travelers may go to the moon. If the site is not protected, what will be left?”

Another group with a similar mission aim is "For All Moonkind".

However none of the finalists plan to take up this part of the challenge as far as I know. So this is not an issue for a while.

Perhaps in the future then we will get Lunar Parks set up, recognized world wide, and rovers and humans will only be able to approach within some set distance of the landing site. The historical lunar landing sites occupy only a tiny part of the moon’s surface. It’s sometimes called our The Eighth Continent, the second largest after Asia, at 37.9 million square kilometers, it’s larger than Africa, and five times the size of Australia.

So, it's only protecting a very small area, a bit like protecting Stonehenge on Earth. It seems reasonable enough to me.

Though there would need to be some way for scientists to access them if we are going to study the effects of that brief human presence on the Moon, as we discussed in the last section: Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base . I wonder if that is best done using remote controlled robots for minimal impact? Especially for the biological surveys?

There are no internationally agreed treaties or guidelines yet. But NASA has published a set of guidelines. From: NASA's Recommendations to Space-Faring Entities: How to Protect and Preserve the Historic and Scientific Value of U.S. Government Lunar Artifacts, the requirements include, amongst other things:

• No overflight. A landed spacecraft mustn't fly over a region of 2 km radius centered on the landing site, or half a kilometer for impact sites like the Ranger probes. That helps avoid risk of accidentally crashing on the sites or of contaminating them with exhaust or debris.

• The landing ellipse also must be outside of that region. The distance of 2 km places them over the lunar horizon of 1.8 km from the landing site. This 2 km distance is ideal also for dealing with the sandblasting effects of rocks and dust thrown up by the landing. These can have velocities from 300 to 2000 meters per second. The larger ones wouldn't reach the landing site. The smaller ones would fly right over it and miss. There would be some that hit it but they would be small in number.

• Use natural barriers where possible like hills, crater rims, ridges, terrain slopes to block the spray of the landing spacecraft from the heritage sites.

• Exclusion zone for rovers etc after landing - they can approach the sites from their landing site 2 km away, but must come no closer than 75 meters for Apollo 11, and 225 meters for Apollo 17. For Apollo 12, 14 - 16 then they recognize that they may need to come closer for scientific investigation, so footprints are not protected, but there's a 3 meter exclusion distance for the descent stage, and 1 meter for the experiments, sampling sites and other artifacts.

For details see the NASA Recommendations.

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https://robertinventor.online/booklets/Online-Case-for-Moon.htm

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