Since President Kennedy, the Pesident by tradition sets long term objectives for human spaceflight for NASA. President Bush's vision was a return to the Moon. President Obama's vision treats the Moon as of so little interest, that the next step on the way to their long term goal of Mars is to pluck a boulder from an asteroid to create a new moonlet orbiting the Moon for astronauts to visit. With a new President, there is often a change of human space flight policy. The Moon is far more scientifically interesting than we realized at the time of Apollo, not only more interesting than a boulder from an asteroid, as we will see, you could go as far as to say that the level of science interest for the Moon is not dissimilar from that of Mars. So, might the new President pivot to the Moon as the next step for astronauts after LEO?
It's impossible to deduce anything by way of detailed space policy from the statements so far by either candidate. Both make feel good comments about the US leadership in human spaceflight and the many benefits of the program, but say nothing of substance. Hillary Clinton often mentions that she wrote to NASA saying she wanted to be an astronaut as a child and they wrote back saying they don't accept girls (something that changed soon after). She is also keen on mapping potentially hazardous asteroids. But she doesn't go any further to say what her human spaceflight policies would be.
As for Trump, after saying the usual things both candidates say about space, when asked for "any other comments" at the end of a series of questions for Space News Magazine, on the 10th October, Trump shows so little interest in this topic that he just answers "No". Seriously! Check here. On 26th October he gave a campaign speech in Florida saying he is going to boost jobs. The rest of what he said didn't make much sense to me, except that he said he would increase public / private co-operation in space, which is Obama's policy too so not sure why he thinks that is an innovation. So I'll refer you to: Trump: "I Will Free NASA" From Being Just a LEO Space Logistics Agency on SpacePolicyOnline.com
ASTEROID REDIRECT MISSION - SNAGGING A BOULDER TO CREATE A MOONLET FOR ASTRONAUTS TO VISIT WHICH IS NOT THE MOON
Under President Obama, US has made it its objective to send humans to the Mars surface as soon as possible, and to avoid the Moon which they see as a distraction that won't help to put human boots on Mars. President Obama in his famous speech to the White House in 2010 said we don't need to go back to the Moon because "We’ve been there before. Buzz has been there" aluding to Buzz Aldrin's "Been there and done that" remark - he was in the audience at the time. Buzz Aldrin later explained that he meant that remark facetiously, i.e. as a joke. In his "No Dream is too High" he says (pages 63-4)
"I had made a presentation in 2009 for the Augustine commission... in an effort to encourage further exploration and to avoid duplication of our efforts, I made the comment "Why go back to the Moon again? Been there and done that." I was saying it facetiously.
I learnt a lesson from that experience. Be careful what you say nowadays, because it might be repeated - maybe even by the president of the United States ... Just because I am PASSIONATE about motivating people to explore Mars doesn't mean that I think we should forget about the Moon. I know we can enjoy numerous benefits by exploring and building an outpost of some sort on the Moon."
The view that we should go back to the Moon, either as well as to Mars or first, before Mars, is shared by many astronauts, also it's the policy of ESA, Russia, and of course China etc. Indeed NASA is the only space administration that has a Mars first policy. SpaceX is the only private company that has this policy - Bigelow aerospace is firmly Moon first. There are many other Moon firsters, see Moon firsters - ESA, Russia, Many astronauts, former US Vision for Space Exploration etc - I'll come back to this briefly later.
At any rate the outcome is that by presidential directive, the US has to aim for Mars but can't send humans to the Moon on the way there. Yet for safety reasons, their first human mission beyond LEO since Apollo could hardly be a mission to Mars, so what do you do as a first step, something interesting, that hasn't been done before (so not just going into orbit around the Moon)? They could send the astronauts to a near Earth asteroid, but it turns out that although it doesn't take much fuel to get to some of the asteroids, it is still is a multi-month mission to get there and back, a big step up from a mission to LEO. The safest way to do space exploration is a step by step approach as we found out with Apollo, do something that stretches you pretty much to the limit at each stage, but not over the limit.
So they ended up with the idea of using a robotic spacecraft to snag a small asteroid, or snag a small boulder off a larger asteroid and return this to an orbit around the Moon. This creates a tiny moonlet of the Moon, which gives them a place to visit which is not the Moon.
Artist's impression of NASA's redirect mission plucking a large boulder from an asteroid to return to cislunar space to make a mini moonlet of the Moon for astronauts to visit, in preparation for later interplanetary missions. See gallery of images at Space.com.
It's an interesting mission in its own right. But you'd think it would be more interesting to go to the Moon - or perhaps, a mission to the L2 position above the far side of the Moon exploring the surface via telerobotics - a mission that with its isolation from Earth would give useful preparation for telerobotic exploration of Mars from orbit. The boulder moonlet, if we do that, could be an interesting stop over on the way, or perhaps a basis for making a base at L2, or L1 (a similar point of gravitational equilibrium above the near side of the Moon) or both, or for use in constructing some kind of tether transport system from the Moon to Earth etc.
However the vision outlined by President Obama is to go to Mars and not to visit the Moon. So the moonlet would be the only place they visit and explore in the Earth Moon system before they head off further afield.
So, you might think from that, that the Moon must be very dull. As dull as a block of concrete, for a tiny boulder from an asteroid to be a more exciting mission for the astronauts. But far from it. The Moon has turned out to be a very interesting place, far more so than they thought after the Apollo missions. Apollo astronauts only went to a few selected places on the near side of the Moon, near the equator and of course places that were reasonably flat for a safe landing. Also they were short duration, the longest only three days, and only the last one had a scientist on board, the geologist Harrison Schmidt.
To be clear, I'm not saying that the asteroid redirect mission is uninteresting. There's a lot of interest in these asteroids, and the mission itself has a lot of plus points in its favour.
- We may be able to mine them in future, so it is of interest to those who are keen on space mining.
- It will give us experience in working with very small asteroids including returning the material to this distant retrograde orbit around the Moon, and exploring them at close quarters with humans, which may be useful for later space mining.
- We may wish to redirect asteroids so that they miss Earth if any are on a collision course and so this mission is of interest for those who are keen to find out more about asteroids for the purposes of asteroid detection and deflection.
- Many of the asteroids are also ancient untransformed material, older than the Moon, and depending on which type of asteroid is targeted, such a large sample to study close to Earth may also tell us a lot about the early solar system. It's certainly a worthy mission to collect an asteroid or a boulder from one of them for study.
- It might also be a good start on a base near to the Moon, for instance a base at L2, or indeed, a counterweight for a lunar tether, or used for Hoyt's cislunar transport system.
For more about this, see Jonathon Goff's Op-ed | 10 Reasons Why an Asteroid Redirect Mission Is Worth Doing
But should this be our main objective in space prior to interplanetary missions, or is it just one of many things we could do? Perhaps they could pivot to the Moon. More on this later, but first let's look at why the Moon is so interesting to science.
OPTION OF PIVOTING TO THE MOON
It's an interesting time for US space policy. Will the new president follow the Mars first approach of Obama or will they follow some new direction, for instance return focus to the Moon?
The spacesuits, habitats etc being developed for Mars are being developed as dual purpose so that they would also work on the Moon. For instance both Mars and the Moon have a dust problem and the suitport spacesuit would work on either body to protect astronauts from the dust. The SLS / Orion launch system would work equally well to send astronauts to Mars or the Moon. It would be easy for the US to pivot to the Moon if that was the President's wish.
As an intermediate step, they could do the asteroid redirect mission, and then follow it up with a mission to the L2 position at the far side of the Moon, using the experience with handling the asteroid to help them with maintaining a station at L2. The astronauts then could take part in the surface exploration of the far side of the Moon by controlling assets on the surface via telepresence. This would be both useful experience for telepresence exploration of Mars and at the same time very useful for lunar exploration too. They could explore the far side, and amongst other things, could also build radio telescopes by controlling rovers that unroll the wires across the surface. More on that in a minute in the section on optical and radio telescopes for the Moon. With enough resources they could maintain space stations at both L1 and L2 and so do telepresence exploration of the entire lunar surface.
MOON COMPARED TO A CONTINENT LARGER THAN ANTARCTICA
We are at the same stage here as the very first Antarctica explorers, setting foot on a continent sized land mass that we know little about first hand. Indeed, a continent far larger than Antarctica; the Moon is as large as Russia, the USA and China put together.
This article consists mainly of extracts from my "MOON FIRST Why Humans on Mars Right Now Are Bad for Science", and Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart kindle books.
RECORD KEEPER OF INNER SOLAR SYSTEM (INCLUDING ASTROBIOLOGY AND EARLY LIFE ON EARTH)
The ice at the poles of the Moon could be the "Record keepers of the early solar system" as Greg Delory put it.Then there is more ice offset from the north and south pole. As mentioned already, this may be ancient deposits from over three billion years ago before the volcanic activity, which changed the polar axis slightly by shifting material.
Then there are ancient regolith layers covered with lava which preserve a record of surface lunar conditions and solar activity over billions of years.
There are other surfaces on the Moon recently formed with few craters - changed by some activity, perhaps emissions of gas, in the very recent geological past Some think it may still have ice, deep down, or even trapped water, from a layer of water rich material that might have accumulated on the early Moon at the same time our oceans formed soon after the original impact of the Mars sized protoplanet with early Earth - Water on the Moon, Yvonne Pendleton, page 3.
We don't yet know the age of the ancient South Pole - Aitken basin, a huge 2,500 km diameter crater which extends over the far side, the "oldest, deepest and largest basin recognized" on the Moon. Just a few samples of rocks returned from it would establish this.
The Moon might also be of interest for evolution and exobiology. That's because meteorites from Earth must hit the Moon in large numbers, after the largest impacts on Earth such as the dinosaur extinction Chicxulub event, (perhaps a million fragments around a cm in size, in that case). Though there is no atmosphere, the gravity is so low that meteorites from Earth can hit the surface at quite slow speeds (comparatively). This has lead to many ideas in the literature of ways that organics from life on Earth or other sources of interest to biology may be there for us to discover. We can only find out for sure by exploring the Moon on the surface:
- Possibility of an ancient regolith layer buried beneath later lava preserving actual organics from the first few hundred million years of Earth evolution.
- Suggestion that the lunar ice at the poles could preserve ancient biogenic organics.
- Suggestion that effects of cosmic radiation on the ices and other chemicals at the poles of the Moon may be creating complex organics - not quite life probably but the ingredients for life.
- Possibility of meteorites from Earth on the Moon - which again, if they landed in frozen areas of the Moon, may preserve organics from millions or even billions of years. They could also preserve fossils as fossil diatoms are still recognizable, and the smallest ones intact after a simulated impact on the Moon. There may be as much as 200 kilograms of material from Earth per square kilometer of the lunar surface. See also Paul Spudis's Ancient Life on the Moon — From Earth (In Airspace Magazine)
- It may also have early rocks from Mars, Venus and small bodies in the early solar system, including possibly biological evidence from those objects also . See section 3.1.1 of Back to the Moon: The Scientific Rationale for Resuming Lunar Surface Exploration.
- It may also record the composition of the solar wind, galactic environment, and volatiles from comets in the early solar system. See section 3.1.1 again
- Must have organics from meteorite and comet impacts at least and probably from the solar wind too which brings not just hydrogen but also heavier elements to the Moon.
When you consider the meteorites and the regolith, the Moon must be a treasure trove of proto Earth secrets, and of solar system secrets too. It would start with ejecta from the early Earth as it cooled down after the impact that formed the Moon, Venus too if its atmosphere back then was thin enough for an asteroid impact to eject material with escape velocity, and Mars for sure. Also material from comets, and asteroids, nearly every major thing of note that happened in the inner solar system probably left a record there. The record should also include ejecta from eevery major impact on Earth, right up to perhaps fragments of ammonites from the Chicxulub impact which must have showered the Moon with large numbers of meteorites.
The Apollo samples were recently re-analysed and the composition of amino acids suggests some extraterrestrial sources, The analysis was a tricky one due to contaminants from Earth in the form of rocket fuel, organics taken to the Moon by the astronauts, and organics introduced while handling them on Earth, which suggests we need to take care to avoid this sort of thing as we explore the Moon.
So, another thing we can do on the Moon is to find out how much organic contamination accompanies human explorations. E.g. 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. Especially if the aim is to study ice deposits in craters and such like. Organic Measurements on the Lunar Surface: Planned and Unplanned Experiments
The far side is the best place to build radio telescopes anywhere near to Earth.
- It is radio dark, shielded from Earth by the Moon. Even in the most remote places on Earth you don't get away from radio interference. Radio astronomers have particular wavelengths that are supposed to be allocated to them. The rest of the spectrum is crowded with signals
- Long wave telescopes could be built simply by rolling out lines of cable across the surface
- Let's you observe the unexplored frequencies below 100 MHz - these are blocked by the ionosphere on Earth. This would open up the last unexplored frequency regime in our radio observations of the universe. They would use a long baseline interferometry to simulate a larger telescope.
- Later on, we could build vast Arecibo dish type radio telescopes in natural craters on the Moon. Frank Drake once calculated that it should be possible to create Arecibo type telescopes with a diameter of 30 km or more on the Moon (see abstract on page 91) - Arecibo is 305 meters in diameter.
There are challenges as well of course. If observing during the lunar night (so least interference from the Sun) then it's both cold and without solar power. Either you use RTGs or power storage for the lunar night.
The Moon is also good for optical telescopes. See Lunar Telescope FacilityFinal Design Report (MIT, 2007)
- Far side is optically dark for much of the lunar month, with no stray light entering the telescope either from the Earth or the Sun
- Stable base for large base line interferometry. Easier to keep widely separated telescopes at the same orientation to each other than in free space because the ground is stable. It is possible to do this with free flying optical telescopes but easier on the Moon.
- No fuel for station keeping to keep the telescopes in position. The moon does that for you.
- Great for infrared passively cooled telescopes in the craters of eternal night near the poles, for instance using liquid mirrors
Artist's impression of a large liquid mirror infrared telescope on the Moon, passively cooled near the lunar poles. By a useful coincidence, a slowly spinning liquid in a gravitational field naturally forms an optically perfect parabolic shapd mirror. Because there is no atmosphere, the optics would be better than for a liquid mirror telescope on Earth, indeed it would be diffraction limited (i.e. the best that is theoretically possible).
These telescopes are so simple to build and lightweight that it should be feasible to build a 4-8 meter liquid mirror telescope soon after a lunar base is established, and larger mirrors later on, with 100 meter telescopes well within reach. Other advantages for the Moon include low shipping mass, ease of assembly, and low maintenance.
A lunar infrared liquid mirror telescope would have a mirror like this
which keeps its natural perfect parabolic shape through rotating the disk slowly. The photo shows the six meter diameter Large Zenith Telescope in Canada. On the Moon, this mirror would be made of ionic fluids (salts in a liquid state) rather than mercury, due to their very low vapour pressure which would let them work in the vacuum conditions of the Moon.
If the telescope is anywhere except at the poles, it will scan a circular strip around the sky. Also because the Moon's axis wobbles with a period of 18.6 years, it actually sees a fair bit more than that circular strip. Most of the lunar libration we see from Earth is due to changes in viewing geometry because of the Moons iregular orbit, it's slant relative to the Earth's equator, the varying distance from the Moon etc.
Shows how the Moon librates as seen from the Earth. Most of this is due to changes in the viewing geometry as seen from Earth.
However, there is also a small variation due to actual motion of the Moon's axis. The Moon's axis precesses, and traces out a circle in the sky of radius 1.5 degrees every 18.6 years. That's quite a lot in astronomy - the Moon as seen from Earth spans only half a degree, so if we can use this unit of the Moon's apparent diameter as seen from Earth- we can see three Moon's worth above of sky above and below the track of the telescope depending on the position in that 18.6 year cycle.
(Earth does have a subtle effect also - nutation - it's precession period is 52,000 years but the the position of its axis varies by an extra half a degree over that same 18.6 year time period as the Moon, enough to be useful for liquid telescopes here as well.)
That's also why the peaks of sunlight at the poles have periods of darkness, but only for a few days of the year. This is actually an advantage for polar infrared telescopes on the Moon. For more details, see Liquid Mirror Telescopes on the Moon (NASA news, 2008). A liquid mirror infrared telescope on the Moon 20 meters in diameter could detect objects a hundred times fainter than the ones visible to the planned James Web Space Telescope. All the materials for a 20 meter telescope would amount to only a few tons, a single load on a heavy boost rocket. A hundred meter diameter telescope could detect objects a thousand times fainter than for the James Webb. Though not steerable, this would be especially useful for detecting faint red shift galaxies from the early univerese, and supernovae from these early times, .
The dust can be dealt with, for instance by making a region around the telescope into glass, which is easy to do on the Moon as use of a microwave turns the regolith into glass as easily as you boil a kettle. Unlike Mars, there are no dust storms to deal with. Just electrostatic elevation of the dust. See Lunar Glass.
Another big surprise recently is that the Moon was recently geologically active, over the timescale of millions rather than billions of years. It might even be active today.
First there are the lobate scarps, fault lines that cut across small craters on the Moon. Small craters tend to be geologically young as they get obliterated by later craters. So these faults are thought to be young, possibly as young as a few hundred million years.
The Apollo seismometers recorded moon quakes, and though most are probably due to impacts, tides, and day / night temperature changes on the Moon, it leaves the possibility open that perhaps the Moon is still active today. These scarps may be a sign of it.
Then, they also found graben - trenches formed when the crust pulls apart. And these are as young as 50 million years old. That is so young that it suggests the Moon is still active.
This was a big surprise as the lobate scarps suggested that the Moon was shrinking. So how can it be expanding as well?
Then, there's this strange feature on the Moon, unlike most of the terrain there, the Ina depression.
Ina depression as imaged by LRO. It's 2.9×1.9 km and 64 m deep. Higher resolution available here. It is one of four similar features around the Imbrium basin.
First discovered by Apollo 15 in 1971, it's now known to be one of many such. They are small features, less than 500 meters across, and seem to be widespread on the near side of the Moon. Some may be as young as 100 million years old. Some of the younger features may be as young as 50 million years old. This means you can't rule out the possibility of future eruptions.
The LRO team found 70 "Irregular Mare Patches" (IMPs) from 100 meters to 5 km in diameter. To find out more, see Volcanoes erupted on the Moon within the past 100 million years which also has a link to the full Nature article which you can read through their article sharing initiative.
Whatever it is, it seems to be geologically recent, as there are few really small craters, and larger features have sharp edges and haven't been degraded. This suggests an age of only millions, rather than billions of years. It's spectra shows it to be bluer than the surrounding terrain (slightly) - a spectral signature consistent with freshly exposed Mare materials. Schultz et al interpret it as mare exposed by a sudden outgassing from the interior blowing away more than 12 meters thickness of overlying an regolith or pyrolastic material (rubble like in texture, result of fountaining lava in the past).
The Moon's surface layers are not just depleted in water compared to Earth, they are also depleted in the more volatile metals potassium and sodium too (by comparison with the less volatile zinc). One recent idea is that when two protoplanets collided to form the early Earth, and the debris condensed to form the Moon, volatile rich layers may have condensed first, then dry layers accumulated on top of them (links also to the original Nature paper) . That paper is mainly used to explain why the surface layers are so dry. They weren't able to determine if the interior would have volatiles in it, but it's possible.
So, it's not necessarily as dry all the way to the center as it is on the surface. Perhaps it has water and other volatiles deep down. If so, these outgassing ideas may be easier to accept.
Then you have the Transient Lunar Phenomena. Moon observers over the years have often noticed short term brightening of the Moon's surface, especially in the Aristarchus plateau. A small bright patch will appear, then disappear, just a bit too slowly to be just a flash from an impact on the Moon. Our eyes are very good at picking up such things, but also easily fooled. So it's a controversial observation. But they may be the result of outgassing, again, if they are real.
So anyway, Arlin Crotts of Columbia University noticed a correlation of the TLPs with sites where argon and radon gas is detected. This can't come from the solar wind and must be outgassing from below the surface. The usual explanation is that it's the result of slow leaks from radioactive decay from below the surface. He thinks that there may also be explosive outbursts of gas which may lead to the TLPs.
He developed these ideas in a series of papers called "Lunar outgassing, transient phenomena, and the return to the Moon", where he also suggests that the Moon may have ice some meters below the surface replenished from below
- I: Existing data
- II: Predictions and Tests for Outgassing/Regolith Interactions
- III: Observational and Experimental Techniques
In the third paper he proposes using ground penetrating radar in orbit around the Moon to search for subsurface ice. He also suggests various ways to monitor the Moon from orbit searching for outflowing gas as well as attempting to observe the TLPs directly.
Then there's this curious phenomenon, the Lunar Swirls. The most prominent of these is Reiner Gamma which is visible in a backyard telescope, but becomes much more impressive with high magnifications.
Lunar swirls in Reiner Gamma. The swirls are always associated with magnetic anomalies (a discovery made serendipitously in 1972 using two small satellites released into lunar orbit by Apollo 14 and Apollo 15 to study the Moon's magnetotail), and this is one of the strongest magnetic anomalies on the Moon. Though not all magnetic anomalies have swirls.
The light coloured swirls in these photographs are not associated with any geological feature but seem to be laid on top of the surface, and they don't seem to have any noticeable thickness or have any effect on the topography.
- Perhaps the magnetic field deflects the solar wind away so keeping these parts of the surface lighter coloured.
- Or perhaps they are a result of plasma from a comet impact removing part of the surface regolith, and the magnetic field may be a frozen remnant of the magnetic field of the comet itself, recorded into the iron it melted during the impact.
- Or perhaps it's made from the electrostatically levitated dust you get as the terminator line between the dark and light side of the Moon passes over its surface. Perhaps this dust accumulates preferentially in the lunar swirls. The finest lunar dust is bright, which makes this hypothesis plausible. Recent research on the electrostatic dust levitation.
Whatever they are, they are intriguing and who doesn't love a mystery? Some of the magnetic anomalies on the Moon might also be tracers of iron nickel / platinum asteroid impacts, if so do the swirls tell us anything about them? Or are they something else, nothing to do with any metal asteroid impacts (e.g. for the comet hypothesis)?
LUNAR GEM STONES?
Could there be lunar gems?
Surprising discovery in 2008 - the near side of the Moon has large deposits of relatively pure chromite spinel, which is a gemstone on Earth. This was discovered from orbit. The moon rocks have small amounts of spinel mixed up in them, but this was a much stronger signal. Could the Moon have spinel gemstones?
If they exist, they probably wouldn't be worth the cost of returning to Earth unless they have something distinctive about them due to formation in lunar conditions.
On top of that you also have the unexpected. What I've described so far are things we can expect, or know to search for and investigate, on basis of what we know so far. But usually we get surprises when we explore new places in the solar system. And though in some ways the Moon is well understood, in other ways it is barely explored at all on the surface, never mind below the surface. It wasn't that long ago that ice on the Moon was a big surprise to many astronomers. It may have many other surprises in store too. We have spent hardly any time exploring the Moon so far, with only one expedition with a geologist on it. Imagine if we had given up on Antarctica as "done" after the first few expeditions that succeeded in landing a human on the continent?
(I've shared this section before in my An astronaut gardener on the Moon - summits of sunlight and vast lunar caves in low gravity where I also go into the possibility of building a base in them and gardening within them).
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?
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 is the 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 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. It may be a shallow slope down into a lava tube type cave shown at bottom right. If it's a lava tube cave, it should widen out to a circular cross section further from the entrance.
Another interesting pit is the Marius Hills pit entrance, original destination for the astrobiotics lander now due to land on the Moon some time in 2017:
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. 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.
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.
As an example of one way the Moon could surprise us - Earth is often hit by iron meteorites, so the Moon should be also. The main question is, how Dennis Wingo has hypothesized in his Moonrush book, that the Moon may also have valuable platinum group metals which could be mined, the result of the impacts of these iron meteorites.
Taking this further, there's a hypothesis by Wieczorek et al that magnetic anomalies on the Moon around the south pole Aitken basin may be 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
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)
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 scepticism 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
VALUE OF THE ICE FOR SCIENCE AND AS A RESOURCE.
The ice is of scientific interest, whether it consists of a thick layer of ice, is fluffy, or it is just ice mixed in with regolith. Any of those forms of ice have the potential for preserving organics from the early solar system.
Astronauts will have to crack closed system recycling to stay on the Moon for long periods of time, and that's also necessary for long duration interplanetary spaceflights. So for a small base it really doesn't matter much if they source water on the Moon or take it with them. I don't see them as having to mine tons of water on the Moon each year to keep going. If they do have to do that, that would mean that interplanetary spaceflight is still quite tricky, I think they need water recycling and oxygen regeneration at least as good as near future plans for the ISS and probably a fair bit better, more like BIOS-3 if they are going to be there for years on end indefinitely like an Antarctic base.
So whether the ice is easy to extract and use s more relevant longer term if you want to export ice to LEO for commercial reasons or set up really large domed cities and colonies there. Potentially the Moon may well have vast reserves of ice, hundreds of millions or billions of tons there, but we won't know for sure until we get a chance to explore it properly.
MOON FIRSTERS - ESA, RUSSIA, BIGELOW AREOSPACE, MANY ASTRONAUTS, FORMER US VISION FOR SPACE EXPLORATION etc.
Amongst astronauts, Canadian astronaut and former ISS commander Chris Hadfield has also come out firmly in favour of Moon first, as reported by Wired Magazine This is also the message of former ISS commander Leroy Chiao who wrote an article about it for Space.com, also Jeffrey Hoffman is another astronaut who says "We basically just scratched the surface during Apollo, you know,".
NASA is the only space agency with this policy of Mars first. The ESA has come out firmly in favour of sending humans to the Moon first. ROSCOSMOS (Russian space agency) is of the same opinion. President Bush also favoured this approach, when the Vision for Space Exploration Program was announced.
SpaceX is the only commercial spaceflight company with this objective. Robert Bigelow, founder of Bigelow Aerospace, manufacturer of the BEAM inflatable habitat currently being tested on the ISS says the next logical step is the Moon and all their planning is based around a future Moon base. The US non profit organization Space Foundation also say the same.
There are many books also advocating Moon first, including the ones by Paul Spudis , most recently, The Value of the Moon, Dennis Wingo, CEO of Skycorp, and author of Moonrush., Madhu Thangavelu, David Schrunk, and other contributing scientists to the voluminous appendices of The Moon: Resources, Future Development and Settlement.
EFFECT OF NASA POLICY ON PRIVATE SPACE FLIGHT
It used to be that US's human spaceflight policy and NASA's policy were the same thing. With private human spaceflight perhaps on the horizon for the near future, private companies may be able to set their own objectives for human spaceflight to some degree, but the NASA direction will influence them too. For instance if NASA had an objective to join in with ESA in their lunar village, that would be a boost for Bigelow aerospace's moon base habitat long range plan.
This first exploratory robotic phase looks likely to be an international effort, with missions from different countries and from private enterprises adding together to give us a complete picture. And the exiting thing is, that it seems it's really going to happen this time. Starting next year, we really are going to get lots of new robotic missions to the Moon. Not just China, though China plan to send missions to the lunar north and south poles next year, 2017 and to get a sample return from the far side in 2018.
We'll also get several attempts at the Lunar X prize landing on different points on the Moon in 2017, with several confirmed launch reservations, and others expected to follow soon.
Then, astrobiotic, one of the X-prize contenders, has now offered to carry payloads from other Lunar x contenders on their first mission , offering a price of 1.2 million dollars per kilogram to the lunar surface on their Griffin spacecraft. They have partnered with DHL for the logistics on Earth. They plan to send their first mission to the Moon’s Lacus Mortis region which seems to be the location for a skylight entrance to a lunar cave. They will start by a Formula 1-like race on the Moon between their own Andy rover and the other rovers they carry with them to the Moon to see who can travel 500 meters fastest to win the lunar X prize (they will share the prize money, whoever wins). I assume that is, unless one of the other contenders gets an earlier mission to the Moon and beats them to it. The other rovers in their manifest include Team Hakuto from Japan and Team AngelicvM from Chile.
Team Hakuto from Japan plans to explore a possible lunar cave skylight with their Moonraker lander (named after legend of English smugglers in Wiltshire, not the James Bond film). This mission is of especial scientific interest and of interest to human exploration of the Moon so let's look at it in a bit of detail:
Moonraker, pulling the two wheeled Tetris. After competing for the 500 meters travel time on the Moon for the Lunar X prize with the other rovers brought there by astrobiotics (assuming it's not won already by another lunar X mission), its mission goal is to go on to lower Tetris into the skylight of a lunar cave. The rover has four independently driven wheels (it can also use them to "turn on the spot"). It has four instead of six wheels to give space for larger wheels as this helps reduce slip on steep slope. (Six wheels have advantages for travel over rugged terrain, but for their particular challenge of approaching a lunar cave over possibly steep terrain, preventing slip was more important to them). When it gets to the cave entrance it will use itself as an anchor as it lowers Tetris down into the cave, to explore it.
They plan to target the partially collapsed skylight in the Lacus Mortis region (originally their target was the Marius pit). For details of those features, see the section of this booklet: Example lunar cave skylights - Lacus Mortis, Marius pit and the King-y natural bridge. For details about why the caves are interesting, see Lunar caves and Lunar caves as a site for a lunar base. The team is led by Professor Kazuya Yoshida, designer of the Minerva II asteroid hopping robots for the Hayabusa 2 mission to an asteroid (currently on its way to an encounter in 2018).
The Andy rover (for Team astrobiotics itself) may also include the VESA gravimeter which could explore variation in gravity around the pit and use that to map out any underground voids, which should show clear gravitational signatures if a large lava tube cave exists underground.
They may also include their innovative tetramorph cube rover - a fast and lightweight semi-autonomous folding rover small enough to pack inside the 30 by 30 by 30 cms of a cubesat - and can then unfold and do pioneer exploration to scout out difficult territory that you can't risk exploring for the first time with the main rover. Early prototype of tetramorph unfolding:
Visualization of tetramorph in folded and unfolded position. See also Tetramorph Avionics: My Experience of Building a Lunar Rover
The same techniques of tether bots, gravimeters, and high speed high risk exploration pioneer rovers could be used to explore the similar Mars cave "skylights" robotically, after testing first on the Moon.
The lunar flashlight cubesat will launch piggyback on the first SLS "Exploration Mission 1" in late 2018. It will use laser light to look at the permanently dark shadows at the lunar poles to search for volatiles.
Lunar flashlight - artist's impression
Other confirmed cubesats for the SLS launch include the Lunar Polar Hydrogen Mapper shown above - cubesat from Arizona university to map water resources on the moon - see also the section Cubesat explorers and rep rap printing
NASA is also partnering with the commercial lunar companies like astrobiotic in its lunar CATALYST program. India also plan a second lunar probe, Chandrayaan 2 at the end of 2017 or early 2018 , following on from its successful lunar orbiter Chandrayaan 1. This time it will be a combined orbiter and lander-rover. The rover will be 29 - 95 kg and operate for at least 14 days under solar power. Japan has a plan, not yet approved, to send a lunar rover landing by 2018. Then there's the ESA and Russia's Luna 27 to the south pole Aitken basin, some time in the early 2020s.
So, there's a fair bit of this robotic scouting going to happen anyway. The main idea is to increase the emphasis on this initial scouting phase, and slightly de-emphasize the later plans, for the time being, until we find out a little more.
There is ice at the poles, for instance, the LCROSS impact shows that. But how easy is it to extract it? We might get many surprises. Perhaps with a thorough understanding as a result of these studies, we position a human base a few tens, or hundreds of kilometers away from what seems the most obvious place right no, based on our knowledge to date. Perhaps the best place for a base is indeed on one of the peaks of (almost) eternal light, as Dennis Wingo and Paul Spudis both suggest, and the ESA do also. However, there are quite a few of those peaks at both poles, and the best one in terms of light might not be the best for humans in other ways.
For an example, we don't know yet, but there might be deposits with 100% pure ice or other volatiles, who knows, if we explore enough. Perhaps these are on the surface in some unexpected place, or perhaps below the surface. These might be much better for in situ resource utilization than the obvious first choices based on a few prospecting trips and optimizing for the amount of light available to the base. Perhaps we would need a better scientific understanding of the Moon before we can find them. This is just one possibility.
We may also find sites of exceptional scientific interest that need to be protected. Dennis Wingo spoke about developing the North pole for industrial work, so separated by a lunar diameter from the South pole which is for scientific research. That makes sense on present day understanding, but what if there is some uniquely interesting scientific site at the North pole? Perhaps it might even be the other way around, after we've done the preliminary studies?
So we need to know not just where to locate the base, but where not to put it also. And generally explore anything that's unusual and interesting, as you don't know what it will turn up.
Or we may get surprising discoveries about the lunar caves which make them go to the top of the list. The caves have an even temperature just as for the poles. It's true that the lunar poles are dramatically better sites for solar panels, when you work out the average power generated, as Dennis Wingo shows in his recent paper. But should the amount of solar power available be the main factor for choosing a landing site? It is a factor of course, but solar panels are quite lightweight. When you have a stable surface to spread them out on, as for the Moon, and no weather to disturb them, they can even be thin film, spread over a large area. They can also perhaps be made on the Moon itself.
You can compensate for less power by having more panels - and adding facilities to store the power for the 14 day lunar night. That is definitely possible, using hydrogen fuel cells, or good batteries, or in many other ways. Suppose for instance that some of the lava tube based lunar caves are large enough to fit an entire city inside, with smooth, close to airtight walls, easy to turn into large habitats with natural cosmic radiation protection? That by itself might outweigh the solar power advantages of the peaks of (almost) eternal light. Or there may be resources in the caves that we don't know about yet, or in some of them only, available nowhere else. Also, if there is ice near to a cave, a cave close to the poles say, that would combine the best of both worlds. Or they may have other advantages including ones that nobody has thought of yet.
Also, if we have biologically closed systems, nearly all the water can be recovered and used again just as water. So it might not be so important to have the main human base close to the ice. It might of course be that the best place for the humans is close to the ice, especially if the ice is the basis for an industrial operation that they have to oversee. But again, maybe that's not the best place for them. Maybe the ice has to be kept free of organics for scientific reasons, or even industrial reasons, and that it's best if the humans are some way away from it. Just as a "for instance". Or perhaps, the main base is somewhere else and there is an outpost, only visited occasionally, near the ice deposits.
Or it might be that Dennis Wingo and Paul Spudis are right and that the peak of (almost) "eternal light" with the most solar power is the best place to send humans, and that this also is a place so rich in ice that it will be a source of fuel not just for the Moon but for cislunar space as well.
If so, we might find this out early on. We might find a place that is so good as a base, that we feel there is no need to look any further. But if we base our entire strategy on this, before we know for sure, and then it turns out to be wrong, that could be a huge setback. Also, if it is more or less right but less than optimal, we might miss some opportunity that could have reduced the cost of the human mission considerably.
You could probably go on almost for ever looking for better and better sites for a base. I'm not suggesting that. But I think perhaps a few extra years searching in a more open ended way early on could bring dividends later.
As an example, Russia hope to send humans to the surface of the Moon some time around 2029. If that was the timescale, we'd have about 13 years for robotic exploration of the Moon and more research in LEO before we go there, plenty of time for quite a decent preliminary study of the lunar surface at the rate of several robotic missions a year, which seems quite likely based on the ones already planned for 2017 to 2018. But a dramatic discovery such as thick layers of pure ice at the poles might speed that all up.
This is not to suggest we stop human spaceflight as we explore the Moon with robotics. Rather, I think we need to be more adventurous with human space flight as well, at the same time. But before we start adventurous human exploration further afield, I think that just as with the early Gemini missions, we need to start being adventurous closer to Earth first.
I think that we need to start working on human factors research. The ISS is not really a human factors research facility. First - why is there so much emphasis on growing food for astronauts in zero g? It's interesting pure research to discover that some plants manage fine in zero g. But as for practical applications, we don't really know whether or not astronauts will typically need to grow food in zero g in the future.
Also the ISS has given us lots of information about the harmful effects of zero gravity on health. Also lots of research into using exercise in zero g to help counteract some of the worst effects. But there's been no research at all into possibilities of using artificial gravity to counteract those effects. Before we do deep space missions, we need to look into this. See my Can Astronauts Spend Years In Space - And How Quickly Can They Recover On Return To Earth? for more details about the motivation for doing this.
We can start research into both of these in LEO.
ON THE WAY TO NOT JUST MARS BUT THE ENTIRE SOLAR SYSTEM
I won't go into this in detail here because the focus is on the science interest of the Moon. However, my articles suggest a similar approach to Bill Nye and the Planetary Society see: Bill Nye to Trump, Clinton: Stay the Course in Space
As with the Planetary Society, I argue that our near future objective for humans to Mars should be to explore Mars with humans in orbit rather than from the surface for planetary protection reasons, why send humans as rapidly as possible to the one place in the solar system where our microbes can most confuse the search for life? Though I also argue there should also be no set date for landing humans on the surface, rather that the decision whether to do that should depend on what we find out about the planet from orbit.
Further, I suggest that there are many other places we can explore of great interest in the solar system, Mars is not the only one. But the Moon is the logical place to start, of great interest in itself, and also a gateway to the solar system. If we can crack the Moon for human spaceflight, then we have a good chance of sending humans throughout the solar system, while if we can't maintain a base on the Moon I don't see how interplanetary flight will ever be safe or feasible. For details see my Op-ed President Obama, Why Humans On Mars Right Now Are Bad For Science
I go into all this in more detail in my Moon First books, and this article consists largely of extracts from them:
"MOON FIRST Why Humans on Mars Right Now Are Bad for Science", available on kindle, and also to read for free online.
I'm also the author of Case For Moon First, available online and on kindle, which expands on this theme of the Moon as the gateway to our solar system and the place to go first, to explore for its own sake, and leading to expanding exploration of the solar system all the way from Mercury through to Jupiter and beyond.
WHERE TO FIND "CASE FOR MOON FIRST"
- Buy it from Amazon as a booklet for kindle
Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart - kindle edition
- Read it online on my website (free).
- Some of you might also find Pocket useful - I've been suggested this by a keen reader of my posts. It lets you read articles offline without any internet connection. It's free (with a premium version which few people need).
In President Obama, Why Humans On Mars Right Now Are Bad For Science I've talked about planetary protection issues for humans on the surface of Mars and suggested we make orbital missions and missions to its two moons and telerobotic exploration of Mars our long term goal for humans to Mars, and leave any decisions about whether to land humans on the surface to a later date. This is also an important theme of the two books.
I've made a new facebook group which you can join to discuss this and other visions for human exploration with planetary protection and biological reversibility as core principles. Case for Moon for Humans - Open Ended with Planetary Protection at its Core
And on Science20
KINDLE BOOKSHELF ON MY AUTHOR'S PAGE
And I have many other booklets on my kindle bookshelf