The Value of the Moon

by Paul D. Spudis

  When enough prospecting data have been obtained, the next most pressing need is to demonstrate the process of resource extraction and storage on the Moon. Although water extraction is probably the simplest processing of extraterrestrial materials imaginable, in order to be taken seriously by some in the space engineering community, an actual end-to-end system demonstration is needed. Such a demo mission could be quite small; a fixed lander in the sunlight, fed with feedstock from the shadowed area, could heat the soil, collect the water vapor, liquefy it, and store it. Once this demonstration has been accomplished, the production of large amounts of water becomes merely a matter of scale.

  Some lingering mysteries about the lunar surface environment also need to be addressed. It has been postulated that the passage of the day/night line (the terminator) across the surface induces an electrical charge, one of possibly dangerous magnitude. This effect could be measured and that possible risk retired through a series of measurements from a fixed lander over the course of a lunar day. Observing the postulated levitation of fine-scale dust by electrical fields should also be studied on the surface, although evidence obtained recently from the orbital LADEE mission suggests that this phenomenon, if it occurs at all, is minor and of local extent.4

  Consolidating Our Lunar Presence

  As previously described, I believe that the most efficient and least expensive way to return to the Moon will require performing much of the preliminary, early work with robotic assets, followed later by people.5 In the early stages of lunar return, robotic machines operated from Earth can begin the harvesting and processing of lunar water. We should initially plan to build up enough capability to fuel a return trip back to Earth before humans arrive. Such a capability requires the production of about 100 tons of water per year. This isn’t as great a quantity as one might imagine: 100 tons of water is roughly the amount contained in a tank the shape of a cube 15 feet (4.5 m) on a side, or roughly the volume of water in a single backyard swimming pool. Because water is the most enabling resource with the widest possible range of use, it is the first priority for utilization.

  Significant mining activity on the Moon will require power and lots of it. Fortunately, there is enough surface area in the polar quasi-permanent sunlit zones (see figure 3.1) to establish networks of multiple solar array power stations. A single station could consist of a tall (~10–20 m), narrow (~2–3 m wide) array of solar cells that can be articulated around its vertical axis (see figure 7.1) to track the Sun as it slowly moves around the horizon over the course of a lunar day. Such an individual station would be low mass (~1 ton). As a modular system, these pieces could be connected together to provide whatever level of power is needed. Initial robotic mining capability would require roughly 150 kilowatts, power that could be provided by eight to ten individual power stations. As outpost capability and size grows over time, additional power stations delivered from Earth can satisfy generating needs. This potential for growth in a surface power system is possible up to about the ten-megawatt level, after which we would probably need to consider the deployment of a nuclear reactor. A thorium molten salt reactor could be sized to provide virtually unlimited power (hundreds of megawatts) for a wide variety of uses and while initially supplied entirely from Earth, could ultimately be operated from locally mined sources of thorium on the Moon.

  In civil engineering, one of the most important material resources on Earth is “construction aggregate”—the sand, gravel, and cement building materials that make up the infrastructure of modern industrial life. Aggregate is easily one of the most important and valuable economic resources of all mined terrestrial materials, more so than gold, diamonds, or platinum. We depend on aggregate for many different types of objects; they are the fundamental building materials of roads and structures. The use of aggregate in building goes back to ancient civilizations, such as the concrete used for construction in ancient Egypt. The Romans devised a recipe for a concrete so durable that the molded arches, walls, and self-supporting dome of the Pantheon, built more than two thousand years ago, still stand today. Aggregates in terrestrial use typically depend on a lime-based cement that bonds the particulate material together. Both lime (CaO) and abundant water are needed to make concrete on Earth.

  By necessity, a permanent presence on the Moon will require an infrastructure that uses as much local material as possible. Aggregate materials probably will become the primary building blocks of industrial society off planet, just as it has on the Earth. The composition and conditions of local materials will require some adjustments as to how we use lunar aggregate. A quick assessment reveals some interesting parallels, as well as differences, with terrestrial use.

  On Earth, gravel pits are carefully located to take advantage of the sorting and layering produced by natural fluvial activity. We harvest gravels from alluvial plains and old riverbeds, where running water has concentrated rocks, sand, and silt into deposits that can be easily excavated, loaded, and transported to sites of construction. The highly variable currents, as well as the velocities of flow of our terrestrial streams and rivers, sort the aggregate by size. This natural sorting creates layers of gravel- to cobble-sized stones for the fastest flowing waters. Finer-grained material is likewise concentrated where water speeds are low, and sand and silt settles out from the suspended sediment (the “bed load”).

  Unlike the aggregate processing by water on Earth, lunar surface rock has already been disaggregated into a chaotic upper surface layer (regolith) by impact. Regolith is ground-up bedrock; impacting objects of all sizes constantly pummel the surface, breaking, fracturing, and grinding up the Moon’s bedrock, a process of impact that has greatly slowed from the much higher level experienced earlier in lunar history. The regolith is a readily available building material for construction on the lunar surface. It is an aggregate in the same sense as on Earth, but with some significant differences. We could make lime and water from the Moon’s surface materials but that would require too much time and energy. Thus, we should adapt and modify terrestrial practice to take advantage of the unique nature of lunar materials. The fractal grain size in the regolith means that we can obtain any specific size fraction we want through simple mechanical sorting (raking and sieving). Instead of water-set, lime-based cement, we can use the glass in the regolith to cement particulate material together, that is, sinter the aggregate into bricks and blocks, as well as roads and landing pads, using thermal energy (figure 9.2). Both passive solar thermal power (concentrated by focusing mirrors) or electrically generated microwaves can provide the energy to melt grain edges into a hard, durable ceramic.

  Figure 9.2. Robotic rover carrying microwave sintering equipment to fuse local regolith into ceramic pavement for use as a landing pad for spacecraft. Power for melting the soil is provided by solar array at center. (Credit 9.2)

  The use of aggregate on the Moon will likely be gradual and incremental. Our initial presence on the Moon will be supported almost entirely by materials and supplies brought from Earth. As we gain experience in using lunar resources, we can incorporate local materials into the structures. Simple, unmodified bulk soil is an early useful product. It can be used in building berms to protect an outpost from the rocket blast of arriving or departing spacecraft, and to cover surface assets for thermal and radiation protection. The next phase will be to pave roads and launch/landing pads to limit the amount of randomly thrown dust and to provide good traction for a multitude of wheeled vehicles supporting the outpost. The fabrication of bricks from regolith will allow us to construct large buildings, initially consisting of open, unpressurized workspaces and garages, but ultimately habitats and laboratories. The new technology of three-dimensional (3D) printing will allow nearly autonomous machines to construct the lunar outpost through the use of regolith aggregate assembled into structures by 3-D printers working in conjunction with Earth-controlled construction robots.6 Making glass by melting regolith can produce building materials of extreme strength and durability; anhydrous gl
ass made from lunar soil is stronger than alloy steel, with a fraction of its mass.

  Metals are abundant in the Moon and can be extracted from the local materials. The basic process is one of simple chemical reduction, accomplished through a variety of low-tech processes, all of which were known to eighteenth-century industry. Carbothermal reduction of ilmenite, an iron and titanium oxide, has been demonstrated in the laboratory to produce oxygen; it also produces native metal as a by-product. The use of fluorine gas as a reducing agent has also been well studied. Metal production techniques require large amounts of electrical power, as it takes significant energy to break the tight metal-oxygen bonds in common rock-forming mineral structures. For this reason, it is likely that metal production will come late in lunar industrialization; initial surface structures and base infrastructure pieces are likely to be made from lower-energy products, like composites and aggregate.

  Although most products made on the Moon will be used locally, eventually we can export lunar products into space. The gravity well of the Moon is a drawback for large mass delivery—its escape velocity is about 2.38 kilometers per second, much smaller than that of Earth (11.2 km/s), but still substantial. In order to use large quantities of lunar materials for space construction, we need to develop an inexpensive means to get material off its surface. Fortunately, the Moon’s small size and lack of atmosphere make this possible by building a system that literally throws material off the Moon into space. A “mass driver” can launch objects off the lunar surface by accelerating them along a rail track using electromagnetic coils that hurl encapsulated material into space at specific velocities and directions.7 We can collect such thrown material at a convenient location, such as one of the libration points. From there, it is a relatively simple matter to send the material to wherever it is needed in cislunar space. A mass driver is not a science-fiction concept; such systems are used to launch planes from the flight decks of aircraft carriers.8

  Surface Activities and Exploration

  An early goal for lunar return is to become self-sufficient in the shortest amount of time possible. This does not mean that a significant amount of surface exploration and science is not also attainable. By virtue of being on the Moon for extended periods, we will have many opportunities to study lunar processes and history in unprecedented detail. Our scientific tasks include understanding the nature and details of the regolith and its interaction with the space environment (a topic addressable any place on the Moon). Such study has both practical relevance, to better conduct resource processing and to improve product yield, and academic interest, since details of regolith dynamics remain elusive. An example of a simple, easy-to-complete experiment is to dig a trench in the regolith several meters deep.9 In the exposed wall of this trench would be several billion years of solar and impact history available for our inspection, sampling, and detailed study.

  The Moon has experienced the processes of impact, differentiation, volcanism, and tectonism. These processes occur on all rocky planets in the solar system. The antiquity of the lunar surface ensures that near-complete examples of these processes are on display for our enlightenment. By using the Moon as a window into early planetary history, we improve our understanding not only of its history and evolution but also the history and evolution of all the planets. As one example, the Earth and Moon have occupied the same volume of space for the last 4.5 billion years, a space where the impact flux affects both objects. As a result of Earth’s highly dynamic surface environment, these ancient events have not been preserved. However, the lunar surface preserves the impact record of the Earth-Moon system dating back to at least 3.8 billion years ago. The study of a multitude of lunar craters can tell us about changes in the impact rates over time, a topic relevant to the extinction and evolution of life in the geologic past.

  A common article of faith in many academic and space circles is that robotic spaceflight is the preferred method of scientific exploration. Many famous space scientists, including James Van Allen and Carl Sagan, argued for the superiority of unmanned missions over human ones. Indeed, many phenomena in space, such as plasmas and magnetic fields, cannot be sensed directly by humans, and in some cases, such as detecting the tenuous lunar “atmosphere,” the presence of people interferes with the property being measured. I agree that while some scientific activities cannot or should not be done by people, in other areas, a human presence is not just beneficial—it is critical.

  The Moon is a natural laboratory, a place where important scientific questions can be answered. The conceptual visualization of the four-dimensional—three spatial dimensions plus time—makeup of planetary crusts is achieved through fieldwork. Fieldwork is not merely a matter of picking up rocks or taking pictures. The “field” is the world in its natural state, where the phenomena we study are on display and where we observe facts and clues that permit us to reconstruct past processes and histories.

  A good example of the difference in capabilities between humans and robots is illustrated by the experience with the Mars Exploration Rovers (2003–present). Over the course of their first five years on Mars, these machines traversed many kilometers of terrain, examined and analyzed rock and soil samples, and mapped the local surface. These robotic rovers, giving us an unprecedented view of the martian surface and its geology, have returned many gigabytes of data. They are truly marvels of modern engineering. Yet, after all this extended, robotic exploration, we are unable to draw a simple geological cross section through either of the two MER landing sites. We do not know the origin of the bedded sediments, strikingly shown in the surface panoramas; we do not know whether they are of water-lain sedimentary, impact, or igneous origins. We do not know the mineral composition of rocks for which we have chemical analyses. Without this information, the planet’s processes and origins cannot be determined.

  Even after more than a decade of Mars surface exploration, we still do not know things about the field site that, given an afternoon’s reconnaissance, a human geologist could have deduced. In contrast, we have an incredibly detailed conceptual model, albeit incomplete, of the geology and structure of each of the human visited Apollo landing sites. The longest stay on the Moon for these missions was three days, most of which was spent inside the Lunar Module.

  A robotic rover can be designed to collect samples, but it cannot be designed to collect the correct, relevant samples. Fieldwork involves the posing and answering of conceptual questions in real time, where emerging models and ideas can be tested in the field. It is a complex and iterative process; geologists can spend years at certain field sites on the Earth, asking and answering different and ever more detailed scientific questions. Our objective in the geological exploration of the Moon is knowledge and understanding. A rock is just a rock—a piece of data. It is not knowledge. Robots collect data, not knowledge.

  Because people control planetary exploration robots remotely, it has been argued that human intelligence already guides the robot explorer. Having done both types of field exploration on Earth, I contend that remote, teleoperated robotic exploration is no substitute for being there. All robotic systems have critical limitations—important sensory aspects, such as resolution, depth of field, and peripheral vision. Robots have even greater limitations in physical manipulation. Picking a sample, removing some secondary overcoating, and examining a fresh surface is an important aspect of work in the field. The physical limitations of teleoperated robots are acceptable in repetitive, largely mechanical work, such as road construction or mining, but in creative, intellectual exploration they are woefully inadequate. The makers of the MER rovers recognized this need by including an abrasion tool to create fresh surfaces; regrettably, it became worn down and unusable after a short period of operation.

  Ultimately, we need both people and machines, each with their own appropriate skill bases and limits, to explore the Moon and other planets. Machines can gather early reconnaissance data, make preliminary measurements, and do repetitive or exhaustive manual
work. Only people can think. And thinking, and then taking action and working with those informed results in real time, is what fieldwork is all about.

  On the Moon, we will learn more about the universe, and by doing so, we will also learn how to study the universe. Recognizing that people and robots bring unique and only partly overlapping capabilities to the task of exploration, we may find that a combined, specialized approach that builds upon the strengths of both, and that mutually supports the weaknesses in each, is the most efficient and beneficial way to explore.10 It is easy to conduct thought experiments in how telepresent robots could replace people on planetary surfaces, but we have no real experience in using them. By experimenting with these techniques on the Moon, we can learn the optimum approach for specific exploration tasks. Simple reconnaissance may be conducted with minimal human interaction, but detailed field study might require continuous, real-time human presence. Knowledge of the problems appropriate for each technique is something that can be acquired and understood on the Moon. Such understanding is vital to future exploration and for comprehending other planetary objects.

  Building a Transportation Infrastructure

  In contrast to the “build, launch, use, throw away, then repeat” paradigm of the past, we seek to create a permanent spacefaring infrastructure that incorporates reusability for as many assets as possible. Although much of the current focus in space development is on reusable launch vehicles, reusability is actually much easier to achieve for vehicles that are permanently based in space. These spacecraft do not have to undergo the thermal and mechanical stresses of launch and reentry. Cislunar transportation consists of multiple steps, including the marshaling of assets at certain points, such as rendezvous and preparation in LEO and L-points (see figure 9.3) followed by transport to the next marshaling area (involving a rocket burn to increase or decrease orbital energy). Since these activities put little stress on vehicle systems, there is no technical reason not to design as much reusability into them as possible.