The strategic value of RLEP lunar polar exploration

 

White paper DRAFT v. 2,  April 19, 2006

 

Ben Bussey, Jeff Plescia, and Paul Spudis, APL

 

Strategy

 

The Vision for Space Exploration calls for a return to the Moon with robots and humans.  A key objective is to learn how to use lunar resources to support the long-term presence of humans on the Moon and to enable further exploration.  This objective implies extended presence on the surface of the Moon, the development of a significant infrastructure, and the exploitation of local resources to the greatest extent possible.  A constraint on the Vision is the philosophy of “pay-as-you-go.”  To facilitate pay-as-you-go and reduce risk, you need to “know before you go.”

 

There are a number of decisions that need to be made regarding the lunar return architecture.  One key decision is what resources are most important and how they will be used.  For example, oxygen and water may only be required to make up for losses to a close life support system.  Alternatively, these resources may be harvested for the production of fuel for cislunar operations as well for journeys farther afield.  The latter option is a necessity to enable pay-as-you-go, permanent presence on the Moon and further Solar System exploration.

 

ISRU

 

Among the first questions to be answered are the extent to which ISRU is viable, which resources will be used, and how they will be extracted.  Hydrogen and oxygen, two important resources, can be found virtually anywhere on the Moon.  Over most of the Moon, hydrogen is found in very low abundance (less than 100 ppm) from solar wind implantation and oxygen is about 45% of the lunar soil by weight.

 

There are three issues with respect to exploitation of lunar resources: 1) the energy required to extract the substance of interest; 2) the efficiency and complexity of the process of extraction (e.g., batch vs. continuous processing); and 3) the infrastructure needed on the Moon to establish resource production (e.g., mass needed on the Moon).  A number of processes have been identified to extract O₂ from the both the mare and highlands regolith; they have varying energy requirements, production efficiencies, and infrastructure.  Most processes are inefficient (less than a few percent yield) and require significant energy (tens of kWh/kg).  Some are feedstock-sensitive, e.g., ilmenite reduction requires high-Ti mare regolith.  Demonstrations of these various techniques can be done within the constraints of a robotic mission, but most would need significant infrastructure for industrial-scale production (e.g., tens of metric tons/year, as in the production of propellant.)

 

The Known Moon

 

The Apollo and Luna missions provided an excellent database for understanding the chemical and physical properties of the equatorial lunar regolith. Thus, our understanding of the requirements, feedstock, and extraction methodologies for a variety of resource processing schemes is reasonably complete for equatorial and mid-latitude regions of the Moon.  Some issues remain with respect to handling significant volumes of granular material; this information could be addressed by RLEP or early in the series of human lunar return missions.

 

Energy costs for extracting both O₂ and H₂ from equatorial soils is given in Table 1.  Although taking advantage of abundant “free” solar thermal energy (available half the time at the lunar equator), extracting H₂ from typical lunar soils is a very energy intensive exercise.  Because no process is totally efficient, the higher the energy input, the more waste heat that must be rejected or transferred.  So while solar thermal energy is “free,”  the mass of radiators and other supporting equipment grows exponentially as energy and power use expands.  For this reason, all previous studies of ISRU have identified the availability of water ice and other volatiles to enable low energy resource extraction as a key question that must be answered before a cost/benefit analysis of ISRU can be completed.  Once an ISRU industry is started on the Moon, higher energy processes may be developed and gradually introduced using lunar materials as the source of its high mass components (retorts, pipes, etc.)

 

The Unknown Moon

 

The poles of the Moon are both different and largely unknown.  Permanently shadowed regions may hold significant quantities of water ice (and other volatiles) mixed with the regolith.  Water ice present in significant amounts (greater than 1-2 wt. %) would be a valuable resource.  Water derived from ice can be electrolyzed to produce both H₂ and O₂ in a relatively low energy process (Table 1).  The energy savings, in terms of ore reduction, would be significant over the production of both these substances at the equator.

 

Table 1.  Energies required for selected lunar resource processes
Operation	Specific Energy
Equatorial Moon
Excavation of regolith	0.01 kWh/kg regolith (electric)
Reduction of SiO2 to Si + O2	10.4 kWh/kg O2 (electric)
Extraction of hydrogen from dry regolith1	2250 kWh/kg H2 (thermal)
Polar regions
Excavation of regolith	0.01 kWh/kg regolith (electric)
Extraction of water from icy regolith2	2.8 kWh/kg H2O (thermal)
Electrolysis of water	4.7 kWh/kg O2 (electric)
Electrolysis of water	48 kWh/kg H2 (electric)
1. Assumes 100 ppm H2, heated 800° C above ambient  2. Assumes 1% ice, heated 100°  C above ambient

 

We do not now understand whether water ice is actually present at the poles and if so, in what quantities and manner it is distributed.  If present, ice would provide an important resource that might be extracted with relatively low amounts of energy.  Water electrolysis is a mature terrestrial technology used extensively at industrial scales (e.g., submarines.)  Extracting oxygen from silicates has rarely been done at large scales, so an entirely new industrial base would have to be created to do that on the Moon.  (It is done in the aluminum-smelting industry, which is one of the most energy intensive terrestrial mining processes.)  So while theoretically possible, oxygen production by breaking silicate and oxide bonds is much more difficult.

 

While hydrogen could be extracted from the solar wind in the regolith, such processing requires a large investment in initial surface infrastructure.  Previous studies of the economics of lunar ISRU suggest that finding the highest concentration of volatiles closest in distance to permanently lit areas is a high priority for robotic precursor missions.  Finding such deposits allow “bootstrapping” of capability from RLEP-scale infrastructure, giving us leveraging capability early in a program of lunar return.  In terrestrial mining, the easiest accessible ore allows the industry to get started, otherwise costs would be prohibitive.  Once started, it permits lower grade and less accessible ores to be extracted because the initial production produced enough wealth to finance the later, more difficult steps.  The industrial revolution in Britain started using wood for charcoal.  Once the easily harvested wood was gone, coal mining started, necessitating the development of the Watt steam engine to pump water out of the mines, which required more use of coal for more steam engines.  The same thing happened in the development of oil and gas; the first oil wells (e.g., Drake in PA in 1859) were shallow (depths of 50 feet or so) but that started an industry to produce kerosene to replace depleted whale oil, whose growth allowed deeper drilling and more exploration.  So while water is desired not to be on the “critical path” to human lunar return, in fact, it is – because of the economics of mining and processing.  Something too difficult and expensive to do (such as cracking oxygen out of rock) will not be as enabling.

 

RLEP

 

One of the objectives of a robotic program is to provide data necessary for the definition and decision on architectures for the human return to the Moon (know before you go).  In the case of lunar resources, RLEP should provide critical knowledge to understand whether water ice is present at the poles, its form and concentration, and provide a basis for understanding what would be required to process the regolith to extract it.  An RLEP mission to explore a permanently shadowed crater (e.g., Shackleton) would provide such data if suitably designed and instrumented.  Exploration of a permanently dark, cold  crater is challenging, but represents a step that must be taken because of the huge potential benefit polar water could provide.

 

The proximity of the dark areas of the poles to areas of permanent sunlight could make mining feasible by providing a local power source capable of being transmitted by several means (e.g., cable transmission, beaming, recharging by return to sunlight).  Space systems are designed routinely to operate in eclipse periods provided they have adequate power storage and thermal design.  In space, it’s often easier and requires less power to stay warm in the cold than to keep cool under high temperatures, so while challenging, working in the polar darkness is a problem well within the engineering state-of-the-art.

 

Once an RLEP mission has determined the presence, form, distribution and concentration of polar ice, a decision can be made as which ore (equatorial mare regolith or polar ice) is the most viable from a programmatic perspective.  The problem is more complicated than a simple comparison of the energy required for break the O-Si or H-O bonds.  Equipment and energy will be required to mine the regolith, move the regolith to a processing location, and store the products.  It may be possible to extract polar volatiles in situ (by mobilizing subsurface volatiles), akin to a gas or molten sulfur well on Earth.  Such a technique would require no significant soil movement and handling; no other identified lunar ISRU process allows this.  These factors are dependant on the nature of the deposit and distances involved (which is a partly function of ore grade).  It is the sum total of these considerations that permit a decision to be made in an informed manner.

 

After a mission to characterize the potential of polar resources and a decision regarding which ore to exploit, subsequent RLEP missions should demonstrate regolith mining (certify technologies to collect, move and process regolith), conduct extraction experiments (processes, efficiencies, possible problems), and experiment with conversion and storage techniques and processes.  These demonstrations would allow us to fully understand the advantages and problems of selected methods of resource extraction and make informed decisions on the which techniques to pursue.

 

Poles vs. Low Latitude

 

In addition to ore grade, environmental conditions drive surface operations.  Operations at the equator can be conducted only in two-week increments, unless a suitable power source is provided to permit night operations.  (Nuclear) There are also large temperature extremes between noon and midnight (250° C temperature differential).  In the lit areas at the poles, the mean surface temperature is lower but relatively constant (~ –50°± 10° C).  Sunlight is always at a grazing incidence and there are areas where sunlight is present for most of the time with only relatively brief periods of eclipse.

 

We can choose to go to the equator today; we have significant knowledge of the Moon and its materials in equatorial regions to design and outfit a human lunar outpost now.  We have enough samples and detailed surface information to design an ISRU operational plan; we know its level of difficulty and the likely costs of producing a given amount of product per unit time.  If such a path is chosen, there is no need for any RLEP mission whatsoever.  But, such a decision now, while avoiding near-term RLEP costs, would result in higher risk and possibly much higher overall cost to the implementation of ISRU in human lunar return over the program lifetime.

 

Summary and Conclusions

 

1. Harvesting polar resources may give huge leverage to lunar ISRU.  Both H₂ and O₂ are obtained for comparable amounts of energy input that yield O₂ only at the equator.  While H₂ is only a small percent of the mass of water, bringing hydrogen from Earth in liquid form requires much larger volume with associated costs and mass.  Previous studies have shown that this limits the use of ISRU largely for local life support replenishment.  Without H₂ from the Moon, lunar resources  cannot be used as a base to expand human activity in the Solar System, a key goal of the VSE.

 

2. Polar ice may be most suitable for continuous processing; although we don’t know this, we DO know that equatorial soils are NOT amenable to it.

 

3. Early propellant production can leverage cislunar transport significantly.  Potentially could pay for itself after a few years of operations (a small facility could produce ~ 50 mT/year, the typical payload mass of a cargo-based LSAM.)  Economic studies suggest this is only possible if there is an accessible source of lunar hydrogen.

 

4. Because both polar environment and deposits are potentially enabling, an effort should be made to answer the questions about their utility.

 

5. This is a good architectural objective for RLEP.  Use early landed missions to address whether such is worth pursuing.

 

6. There are significant consequences to NOT finding out about polar conditions; you are driven down a road of known difficulty (and it is considerable.)  Likely non-continuous processing, little or no H₂ production, ISRU to remain experimental (rather than productive) for a much longer time.

 

7. Processes and infrastructure developed for equatorial O₂ mining may be only marginally applicable to polar needs, if the poles are later found to be desirable.

 

8. Over the next six years (2006-2012), NASA will spend about $100B.  An RLEP program of 1-2% of that total is not unreasonable, considering that a goal of the VSE (returning to the Moon to stay) is a prime objective.  NASA should be able to spend a few percent of its total budget to address a central issue in its primary mission.  An RLEP program that produces a few tens of metric tons of pre-emplaced water and other products (e.g., radiation shielding) more than pays for itself if it reduces the total number of required LSAM missions over the life of the program by one or two.  Today the combined space lift capability of the United States and Russia is used to maintain a continuous human presence in low Earth orbit and much of the transported mass is water.  Finding accessible water on the Moon would be the breakthrough that enables permanent self sufficiency off  Earth, a key goal of the Vision.

 

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