. CASE FOR MOON FIRST - 06 Moon science surprises

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MOON SCIENCE SURPRISES

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, far larger than Antarctica; the Moon is as large as Russia, the USA and China put together.

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 3.1.1again

• 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 every 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

IDEAL LOCATION FOR MANY TYPES OF RADIO AND INFRARED TELESCOPES AND OTHER OBSERVATORIES

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 30 MHz - these are blocked by the ionosphere on Earth. This would open up the last virtually unexplored frequency regime in our radio observations of the universe. Frequencies below 10 MHz are blocked out completely on Earth by the ionosphere (see page 8 of this paper). They would use a long baseline interferometry to simulate a larger telescope. There have been attempts to observe these frequencies before,

• Radio Astronomy Explorer RAE 2. RAE 1 made the surprising discovery that Earth itself is an emitter of very low frequency radio radiation. in the form ofAuroral kilometric radiation.

• (Click to watch on YouTube)

• This interference makes it hard to observe in this area of the radio spectrum and they set it to orbit the Moon instead of its planned orbit around Earth. However the RAE did not have the resolution to be able to pick out any individual sources. So this area of the radio spectrum is virtually unexplored. Even the Moon is not entirely free from distortions so the sites would need to be evaluated first, but probably in one of the eternally dark craters or in the shadow of a mountain such as Mount Malapert for shielding from man made radio interference. Their 2016 paper however proposes other locations such as the Earth Moon L2, the Sun Earth L2, and independent orbit around the sun (a sun based distant retrograde orbit around the Earth I think). Still the far side of the Moon would still be a good place for these observations as well.

• 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.

• Artist's impression for NASA by Pat Rawlngs of a radio telescope on the Moon constructed similarly to the Arecibo telescope in a large lunar crater.

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 Facility Final 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 shaped 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 irregular orbit, it's slant relative to the Earth's equator, the varying distance from the Moon etc.

(Click to watch on YouTube)

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 26,000 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 universe, 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.

There are many things we can do from space observatories can also be done from the Moon, in the future, if there is some advantage in doing them on the Moon e.g. because there is a base already there, ease of maintenance, using in situ materials etc.

However there are a few other forms of astronomy that the Moon is uniquely suited for, not yet mentioned. From this list of Lunar Exploration Objectives - picking the ones not yet mentioned and ones for which the Moon has special advantages:

• Detect the seismic signature of nuggets of strange quark matter passing through the Moon (if they exist) using a network of seismometers

• Lunar laser transponders on the near side to increase the accuracy of measurement of the distance to the moon - these could also be used to test general relativity predictions to higher precision.

• Search for NEOs - an ideal site for extra telescopes to monitor NEOs that pass close to the Earth Moon system, due to the moon's long lunar night and lack of atmosphere

• Study of the dynamics of the magnetotail of Earth - the Moon is at just the right distance to do these studies, either satellites around the Moon or instruments on the surface.

• Study of the solar wind including its role in formation of water at the lunar poles (Sweden is going to put an experiment on Chang'e 4 mission to the lunar far side, to do this in the near future).

GEOLOGICALLY ACTIVE MOON

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.

See NASA's LRO Reveals 'Incredible Shrinking Moon'

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.

Graben on far side of the Moon found by Lunar Reconnaissance Orbiter

This was a big surprise as the lobate scarps suggested that the Moon was shrinking. So how can it be expanding as well?

Seems in some places it is shrinking and in other places it is pulling apart. If the entire Moon was molten and then cooled this shouldn’t happen - It may be because the outside of the Moon was remelted at one point due to bombardment by asteroids, and that can lead to this situation where as it cools down part expands, part contracts.

(click to watch on Youtube)

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.

(click to watch on Youtube)

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).

However, there's a new interpretation from geologists from Brown university. They think that Ina depression may consist of magmatic foam - a mixture of gas and lava to make a highly porous texture. If so, then newly formed regolith gets jostled into gaps in the rock by seismic activity, making it seem younger. And impacts into porous material like this makes smaller craters. An impact that would make a 100 foot diameter crater in regolith would make only a 30 foot diameter crater in a more porous material. Taking this into account they get a revised age of 3.5 billion years which puts it into the same timeframe as other volcanism on the Moon. Jim Head, professor at Brown university and co-author of the paper with the leading author, graduate student Le Qiao puts it like this.

“We think the young-looking features in Ina are the natural consequence of magmatic foam eruptions on the Moon. These landforms created by these foams simply look a lot younger than they are.”

Ina depression relief map; red is higher, blue is lower. Geologists from Brown university suggest that these mounds rising from an ancient caldera may consist of magmatic foam, which leads to smaller craters and so makes it seem younger than it really is

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.

LUNAR SWIRLS

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.

More swirls at Mare Marginis on the lunar far side.

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)?

EXPECT THE UNEXPECTED

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?

USING THE MOON TO BUILD HABITATS IN FREE SPACE

The Moon may be the key not just to lunar habitats, but also habitats in free space. The original plan for space habitats in the 1970s was to use a rail gun on the Moon to fire material away from the surface for the radiation shielding of the Torus. This is the most heavy part of the structure. The idea was to land a small bulldozer on the Moon, and then bulldoze the regolith, load into a railgun - or a mass driver as it is usually called in this context, and fire it to the construction site for the habitat. This then would be used for the 4.5 tons per square meter cosmic radiation shielding; the heaviest part of the structure.

As O'Neil put it in his testimony to the US house of representatives (subcommittee on space science and applications) in 1975, talking about his smaller Island One:

"The removal of half a million tons of material from the surface of the moon sounds like a large-scale mining operation, but it is not. The excavation left on the moon would be only 5 yards deep, and 200 yards long and wide: not even enough to keep one small bulldozer occupied for a five-year period."

The Stanford Torus would need 20 bulldozers for 5 years, more of a job, but it's still not a huge impossible undertaking to source the shielding on the Moon. With the low lunar gravity, and escape velocity of only 2.3 km / sec, then this could be done with solar powered or nuclear powered rail guns on the Moon capable of sending material to orbit. This is something we could test on the Moon.

The most important metric here I think is the amount of mass per colonist. For a Stanford torus you have 1000 tons per colonist including the regolith shielding or equivalent. Put like that it does seem a lot more than for any surface habitat. But you need roughly similar amounts of regolith shielding for surface habitats. And you still need a bulldozer or similar to use the shielding to cover your habitat. If the infrastructure to get this material from the Moon to the space colony is just a small item in the total budget, as O'Neil suggests, then this difference is not as significant as you might think.

When you look at structural mass, you only need 15 tons per colonist for the Stanford Torus, and you need 4.4 tons per colonist of atmosphere for a total of 20 tons per colonist, or five colonists for every hundred tons. That is the only mass you need to launch from Earth for early stage habitats - and later on you can hope to mine this too from asteroids, using the metal rich asteroids for metals, or get it from the Moon. This is very similar to any other space station / space colony.

We can compare this with the ISS which has 419 tons for up to 6 astronauts so about 70 tons per astronaut. As a Stanford Torus habitat, that much mass could support 20 colonists. You'd expect the Stanford Torus to be more efficient through economies of scale, and apart from the radiation shielding, there isn't much difference between the Stanford Torus and anything else.

Also, the Stanford Torus is not necessarily the most efficient. The "Kalpana one" for instance has multiple levels inside the torus and the shielding rotates with the habitat which simplifies the engineering and reduces the total mass per inhabitant. The Vademecium (grand prize winner for the NASA Ames Space Settlement contest in 2006) consists of a torus with an elliptical cross section which reduces the amount of atmosphere needed per colonist.

The only colonies that need significantly less mass than this are the lunar caves and the Venus cloud colonies. The cloud colonies require less mass per colonist than any other habitat, because they don't need to hold in the atmosphere, meteorites are much less of a problem, and with pressure the same inside and out, tears in the fabric, if ripstop, don't matter much either. They can be made of thin materials similar to airships. Just about all the mass consists of the biosphere itself in that case. See Venus cloud colonies - a surprising low maintenance solution.

The lunar caves of course eliminate the need for additional shielding completely and reduce the amount of mass needed to hold in the atmosphere and probably also reduce the maintenance costs for the external envelope of the habitat (you could make a low maintenance airtight enclosure by covering the interior of the caves with a layer of glass formed from the regolith dust). See Lunar caves and Lunar caves as a site for a lunar base.

However free space habitats have other advantages such as easy generation of artificial gravity, direct sunlight available 24/7 year round, and ability to position them in LEO, GEO, near to NEOs for mining and so forth.

So, you can position them wherever you want, set the artificial gravity as high as you like, and you can set your own day / night levels as you like not tied to any planet just by tilting mirrors. You could simulate any planet or moon or exoplanet in your orbital habitat easily.

EXPORTING MATERIALS FROM THE MOON

We've just mentioned the idea of using a lunar mass driver. This is a 1970s artist's concept drawing of the idea, the large area of solar cells to the left powers it.

However, unless it can be constructed largely using in situ materials, then there is a lot of material to send up to the Moon to construct it. This is a 2010 study that (making various assumptions) suggested that a lunar mass driver would have a long payback time. The more frequently you can send the payloads, the shorter payback time. You might want to do regenerative breaking to recover as much of the energy as possible from the launch vehicle. Some ideas for mass drivers on lunarpedia.

Then, there's the possibility of generating fuel using the ice at the lunar poles. With 600 million metric tons at least, if those figures are correct, then it seems that we could make fuel without significantly impacting on the amount of ice there.

There are other ideas also for exporting materials from the Moon, such as the lunar space elevator. which was promoted in a successful kickstarter by the Liftportgroup. Though we don't yet have the technology to build a space elevator on Earth, we could in principle build one on the Moon. It would extend from the surface through the Earth Lunar L1 point,

There are different design ideas, but one proposal in 2011 was for an 11,000 kg tether made of Zylon HM fiber which would be 264,000 km long, transport microrovers to the lunar surface, and return 10 kg at a time from the lunar surface, so that is a mass to payload ratio of 1,100 to 1. For techy details see LADDER.

For more background information about lunar tethers, see. "Lunar Space Elevators for Cislunar Space Development Phase I Final Technical Report"

Hoyt's cislunar tether transport system

Another ingenious idea, and one with a much lower structure to payload ratio than a lunar elevator is Robert Hoyt's Cislunar tether transport system. More details:CISLUNAR TETHER TRANSPORT SYSTEM

It uses momentum exchange tethers. This shows the idea for Earth - a rotating tether that could pick a payload from a suborbital spacecraft up from the upper atmosphere, and boost it into LEO. If the tether rotates at exactly the right spin rate, then it can be stationary relative to Earth when in the upper atmosphere, closest to Earth in the tether spin - and spinning at just the right speed to boost the payload to LEO at the upper part of the spin. This is too hard to do at present. But it could capture a payload from a suborbital spacecraft moving at Mach 12, and boost it to orbit.

Well this gets especially interesting if you do it on the Moon. Robert Hoyt designed a cislunar transport system which looks like this:

"The Cislunar Tether Transport System. (1) A payload is launched into a LEO holding orbit; (2) A Tether Boost Facility in elliptical, equatorial Earth orbit picks up the payload (3) and tosses it (4) into a lunar transfer trajectory. When it nears the Moon, (5), a Lunavator Tether (6) captures it and delivers it to the lunar surface."

At the Moon end of the transport system, the tether can be stationary relative to the Moon's surface when closest to the Moon. There are many details to the idea - that's just the outline of how it works. See his Cislunar tether transport system.

His summary reads:

"We have developed a preliminary design for a 80 km long Earth-orbit tether boost facility capable of picking payloads up from LEO and injecting them into a minimal-energy lunar transfer orbit. Using currently available tether materials, this facility would require a mass 10.5 times the mass of the payloads it can handle. After boosting a payload, the facility can use electrodynamic propulsion to reboost its orbit, enabling the system to repeatedly send payloads to the Moon without requiring propellant or return traffic. When the payload reaches the Moon, it will be caught and transferred to the surface by a 200 km long lunar tether. This tether facility will have the capability to reposition a significant portion of its “ballast” mass along the length of the tether, enabling it to catch the payload from a low-energy transfer trajectory and then “spin-up” so that it can deliver the payload to the Moon with zero velocity relative to the surface. This lunar tether facility would require a total mass of less than 17 times the payload mass. Both equatorial and polar lunar orbits are feasible for the Lunavator™"

The interesting thing about this is that it needs almost no fuel!

"By balancing the flow of mass to and from the Moon, the orbital momentum and energy of the system can be conserved, eliminating the need to expend large quantities of propellant to move the payloads back and forth"

That works because the LEO is lower in the gravitational well. So whenever you send material from the Moon to Earth, the system actually gains energy, a bit like a ball rolling down a hill. This is the whole beauty of the Hoyt system, what makes it so very clever.

When it picks up a space plane from a suborbital flight to send it to the Moon that lowers its orbit of course. But each time it then receives a payload from the Moon and that raises its orbit again. There is no need for any rockets, or ion propulsion or anything to keep boosting its orbit. That is why it has essentially zero delta v. Both tethers are powered by the flow of materials downwards in the Earths gravitational potential well from the Moon. There is no other propulsion involved apart from minor adjustments.

That's once there's a lunar base. Before then then it uses electrodynamic propulsion:

In the period before a lunar base is established, however, the tether facility will use electrodynamic propulsion to reboost its apogee by driving current through the tether when the tether is near perigee. "

However, later on once the base is established it can start catching payloads to reboost itself

"Once a lunar base and a lunar tether facility have been established and begin to send return traffic down to LEO, the tether facility can restore its orbit by catching and de-boosting these return payloads."

Explained in more detail:

"The tether will then pick up a payload from a circular, 450 km orbit and toss it to the Moon so that it will reach the Moon as the Moon crosses its ascending node. The facility will then drop to a lower energy orbit. At approximately the same time, the return payload will be released by the lunar tether and begin its trajectory down to LEO. When the return payload reaches LEO, the Earth-orbit tether facility will catch it at perigee, carry it for one orbit, and then place it into the 450 km initial payload orbit. Upon dropping the return payload, the facility will place itself back into the high-energy orbit."

This is also why it has very little overhead. It doesn't need propulsion. Just capture and release. Fine tuning of the orbit is done by reeling the tether in and out at various stages in the orbit.

It can achieve a launch only every 4.5 months or once a sidereal month with some use of tether reeling. All these quotes are from Cislunar tether transport system. :

"Once a lunar base is established and begins to send payloads back down to LEO, the orbit of the tether system can be modified slightly to enable frequent opportunities for round-trip travel. First, the facility’s orbit will be raised so that its high-energy orbit has a semimajor axis of 12577.572 km, and an eccentricity of 0.41515. The tether will then pick up a payload from a circular, 450 km orbit and toss it to the Moon so that it will reach the Moon as the Moon crosses its ascending node. The facility will then drop to a lower energy orbit. At approximately the same time, the return payload will be released by the lunar tether and begin its trajectory down to LEO. When the return payload reaches LEO, the Earth-orbit tether facility will catch it at perigee, carry it for one orbit, and then place it into the 450 km initial payload orbit. Upon dropping the return payload, the facility will place itself back into the high-energy orbit. The perigee of this orbit will precess at a rate such that after 4.5 lunar months (123 days) it will have rotated 180°, and the system will be ready to perform another payload exchange, this time as the Moon crosses its descending node. If more frequent round-trip traffic is desired, tether reeling could again be used to hold the orientation of the tether’s orbit fixed, providing transfer opportunities once per sidereal month."

One of the strong points in this design is the low mass to payload ratio of 10.5 to 1 for the tether in LEO and 17 to 1 for the tether in lunar orbit. It becomes especially favourable when you compare it to the propellant mass needed to send a rocket from LEO to the Moon and back, of 16 times the mass of the payload. A tether requires only 28 times the payload mass in LEO, so less than double the mass for the rocket, but can transfer many payloads, not just one.

"Comparison to Rocket Transport

"Traveling from LEO to the surface of the Moon and back requires a total !V of more than 10km/s. To perform this mission using storable chemical rockets, which have an exhaust velocity of roughly 3.5 km/s, the standard rocket equation requires that a rocket system consume a propellant mass equal to 16 times the mass of the payload for each mission. The Cislunar Tether Transport System would require an on-orbit mass of less than 28 times the payload mass, but it would be able to transport many payloads. In practice, the tether system will require some propellant for trajectory corrections and rendezvous maneuvers, but the total delta V for these maneuvers will likely be less than 100 m/s. Thus a simple comparison of rocket propellant mass to tether system mass indicates that the fully reusable tether transport system could provide significant launch mass savings after only a few round trips. Although the development and deployment costs associated with a tether system would present a larger up-front expense than a rocket based system, for frequent, high-volume round trip traffic to the Moon, a tether system could achieve large reductions in transportation costs by eliminating the need to launch large quantities of propellant into Earth orbit."

Agreed, if you want to go to Mars and it's your only destination you are interested in for humans outside of Earth, and for a short visit only, then perhaps it wouldn't make sense to set up all that infrastructure to mine lunar water, or lunavators, or lunar elevators or to build mass drivers, just for a single mission to Mars with humans, or even several missions perhaps.

But if you are visiting the Moon because it is interesting in its own right, then the fuel on the Moon could then easily become a key towards opening the rest of the solar system to easier spaceflight, as well as supplying fuel and water to LEO as well. And the other ideas may be useful also, especially if it turns out that the Moon is a useful source for water for space. Ideas like the lunavator might even make it economical to return materials from the Moon such as nickel, cobalt, etc., quite low value, but in large quantities.

BALLUTES - RETURN OF HIGH VALUE RESOURCES SUCH AS PLATINUM TO EARTH

Hoyt's cislunar tether or the propellants made on the Moon can return materials to LEO. That's great for materials that need to be supplied to LEO such as propellant and water, but what about exports to Earth itself?

To return materials to Earth, you have to target the Earth's atmosphere and to hit it slowly enough so that the materials you mine don't burn up in the atmosphere. You might do aerobraking first to reduce speed, and then over a period of time, skimming the atmosphere, lower the orbit to make the landing gentler.

When you are ready, then actually landing the materials on Earth is relatively easy. To make sure there is no risk of damage to Earth, if the parachute or aeroshell fails to deploy, make sure you send the materials in small amounts, small enough to burn up in the atmosphere in its entirety without the aeroshell - and equip each one with an aeroshell and parachute.

Conventional aeroshells are likely to be too complex to create on-site at least at the early stages of development, and too heavy and so too costly to export from Earth. But there's an alternative here, the ballute, an inflatable balloon that works like an aeroshell.

See the New Scientist article Inflatable cushions to act as spacecraft heat shields, and this article Profitably Exploiting Near-Earth Object Resources

It's lightweight, so you could make them on Earth and then send them up to the facility for returning to Earth.

Space mining would probably start with mining of water for use in LEO and other spacecraft missions - because of the high cost of supply of materials to orbit, making it far easier for the mining to turn a profit and pay for itself. But later, funded by the sale of water to space agencies, it could then move on to mining metals and other resources useful for Earth itself.

COMPARISON OF EXPORTS FROM THE MOON WITH ASTEROID MINING AND SUPPLY FROM EARTH

For those who worry that lunar exports might deplete the Moon of its volatiles, then mining the lunar volatiles for fuel is only needed for perhaps a few decades, or until we find an easy way to access other sources in the asteroid belt or find easy ways to lift water from Earth. If we get easy transport to orbit at dollars per ton (e.g. orbital airships, space elevators, or Earth based mass drivers), then the Earth would take over from the Moon and asteroids at that point for any materials abundant on Earth such as water, or air.

I think there's a good chance that eventually it will be as easy to get into space as to fly to another continent. If it costs a similar amount to transport of materials to space, or across the Atlantic, then it would surely be economical to send Earth water and air into orbit and would compete with sources in space. I think there's a good chance this will happen long term, perhaps on timescales of decades, and very likely on the centuries timescale. And of course there's no risk of Earth running out of water or air as a result of exporting the comparatively tiny quantities that would be needed in space. See my Projects To Get To Space As Easily As We Cross Oceans - A Billion Flights A Year Perhaps - Will We Be Ready?

Meanwhile, if there are hundreds of millions of tons of lunar volatiles, they should last until then, and will be a useful source of income for lunar development. That's enough water so that even if you had a million people on the Moon, you could have a full lane of an Olympic swimming pool's worth of water for each one from the local resources, and only a tiny amount of that would be exported for fuel.

Less delta v is needed to get water and other materials from Near Earth Asteroids to Earth, some have a delta v of less than 1 km / second. Though the ones that have the least delta v are the ones with orbital period closest to Earth which means that the times when they are most easily accessible from Earth come around less often, and close flybys might be a decade or more apart.

The Moon scores here in the early days because it is always easy to get to. Then, if the lunavator idea worked for the Moon, then you could get materials back to Earth with almost no extra fuel needed, so that could make the Moon even more attractive than an asteroid.

There are similar ideas for asteroids also, this time the idea to harness the spin of an asteroid to send materials to Earth using a tether that is attached to it and spins at the same rate as the asteroid itself. For the basic idea, see Asteroid Slingshot Express - Tether-based Sample Return.

In the long term future, perhaps the Moon would be most useful for materials that are rare in most asteroids but common on the Moon, such as Aluminium and Titanium. Both lunar and asteroid mining has the potential to move some of our industry into space, and help reduce the impact of mining on the Earth.

BUZZ ALDRIN'S "BEEN THERE AND DONE THAT" - MEANT FACETIOUSLY - AS A JOKE

Buzz Aldrin, a keen advocate for Mars colonization, and inventor of the idea of the Adler cyclers for transport to Mars, is often quoted as saying "Been there, done that". But actually, he says we should go to the Moon first, with a quote from his own website:

“Aldrin and other experts believe Nasa is overlooking an important part of space exploration: a permanent, manned base on the moon that would prepare us for the mission to Mars.”

and in more detail here. (He has been reported as saying to leave the Moon alone - that's back in 2009, perhaps his views were changed by the more recent discoveries of ice on the poles, or was he just joking?)

In his "No Dream is too High" (page 81), he talks about a presentation he gave in 2009 to the Augustine commission when he said "Why go back to the Moon again? Been there and done that.". But he explains that he meant it facetiously, i.e. not to be taken seriously. A year later President Obama echoed his words, but seriously, in his speech to the White House in 2010: "Now, I understand that some believe that we should attempt a return to the surface of the Moon first, as previously planned. But I just have to say pretty bluntly here: We’ve been there before. Buzz has been there"

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."

He goes on to say that after explaining his Mars ideas to Stephen Hawking, then - after a long time to compose his sentence as usual, "We waited a long time for Stephen's response. Finally he managed to say "Why not the Moon first?" " . So I think we can count Stephen Hawking as a Moon firster too :).

He then goes on to say "Although I don't want to ignore the Moon, I do want the next generation to go where no humans have previously traveled. There are entire vistas yet to be explored.". Then later in the book, page 203, he says "Returning to the Moon with NASA astronauts is not the best use of our resources. Instead we need to direct our efforts to go beyond the Moon to establish habitation and laboratories on the surface of Mars".

In his Mission to Mars, he says

"The moon is a different place since I traveled there in 1969.

"... Thanks to a fleet of robotic probes recently sent to the moon by several countries, there's verification that the moon is a mother lode of useful materials. Furthermore, the moon appears to be chemically active and has a full-fledged water cycle. Simply put, it's a wet moon.

"New data on our old, time-weathered moon points to water there in the form of mostly pure ice crystals in some places. For example, sunlight-starved craters at the poles of the moon — called 'cold traps' — have a unique environment that can harbor water ice deposits. Gaining access to this resource of water is a step toward using it for life support to sustain human explorers. Similarly, the moon is rife with hydrogen gas, ammonia, and methane, all of which can be converted to rocket propellant."

"Fresh findings about the moon from spacecraft have revealed the lunar poles to be lively, exciting places filled with complex volatiles, unique physics, and odd chemistry, all available at supercold temperatures ...

"In short, our celestial neighbour in gravitational lock, the moon, can be tapped to help create a sustainable, economic, industrial, and science-generating expansion into space. The question is, What should America's role be in replanting footprints on the moon?"

So, he's perhaps somewhat in two minds about it, but even with his strong focus on humans to Mars, he's not at all saying that we should ignore the Moon.

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