The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources (2016)
6
Why? Three Reasons the Moon Is Important
Throughout all of the various attempts to give our national space program a long-term, strategic direction, the Moon has waxed and waned in significance. Despite many attempts over the last thirty years to ignore it or focus exclusively on robotic space science or human missions to Mars or the asteroids, the logic of lunar return has not been refuted. Undeniably, the Moon will figure prominently in any plans for human spaceflight beyond low Earth orbit, if not by the United States, then by some other nation with the foresight and the will to take the lead.
Previous attempts to define the “mission” on the Moon—the quest for various rationales for lunar presence—has produced multiple themes, goals, and objectives, the most infamous being the six themes and 186 objectives adumbrated at the 2006 NASA Exploration Workshop.1 It really isn’t that complicated. I will attempt to cut through this programmatic fog in order to examine the fundamental reasons why the Moon is not merely important but also critical for the development of permanent spaceflight capability. Whatever long-term space goal we adopt, the Moon will play a key role in enabling us to achieve those objectives. The value of the Moon lies in three principal attributes: It’s close, it’s interesting, and it’s useful. I will examine each attribute in turn, evaluating its significance to the development and exploration of space.
It’s Close: The Value of the Moon’s Proximity
Unlike most other space destinations, the Moon is Earth’s companion in space. The Earth-Moon system orbits the Sun as a single planet. Thus, the Moon is always accessible from the Earth. This is in marked contrast to other deep space targets such as planets and asteroids, all of which have independent solar orbits and thus, are optimally accessible only during certain short, periods called “launch windows.” In the case of Mars, good launch windows, those requiring the minimal amount of energy for transfer, expressed as “delta-v” or change in velocity, occur about every twenty-six months. Other targets, such as near Earth asteroids, may have more frequent windows separated by months but lasting only a few hours to a few days; some have even fewer launch windows.
The Moon is always available. Fifty years ago, the Apollo launches were scheduled within very tight launch windows because the Lunar Module had to land on the Moon in the early morning hours, when cast shadows make the surface relief stand out clearly. In the case of a future lunar outpost, one of the first items to emplace on the surface will be a beacon, a radio device that allows future landers to land completely “blind” at any time of the lunar day or night. Departures and arrivals will be conducted for convenience, with timing imposed not by celestial mechanics but by the operational schedules of the flight systems manager. A series of radio beacons would enable the development of a completely automated flight system, one that could transport goods and people between Earth and a lunar outpost.
The Moon is accessible via many different orbital approaches (figure 6.1). Direct paths, requiring the maximum amount of velocity change (delta-v), are possible from Earth, resulting in transfer times on the order of three days; minimal modification permits lower total energy requirements and adds another day or so to transit time. Staged approaches can be conducted using the L-points or low lunar orbit as a staging location. The advantage of such an approach is that assets and pieces of a complex system can be assembled at a staging node, with the surface mission conducted from that point. The Apollo system used low lunar orbit (100 km circular) as a staging area. Staging from one of the L-points—usually L-1, about 60,000 kilometers above the center of the near side of the Moon—has many benefits, including its utility as a marshaling area for lunar exports when water production meets that level and for constant line-of-sight communications with both Earth and Moon. Finally, it is possible to send large payloads of cargo via “slow boat” transfer routes using efficient, low-thrust, high-energy techniques, such as solar electric propulsion. These transfers spiral out to lunar distances over periods of weeks to months. But while they pass through the Van Allen radiation belts multiple times, they impose no hazards because they carry only cargo and not people.
Figure 6.1. Zones of cislunar space. Low Earth orbit (LEO) is the location of the International Space Station and the limit of most human missions. Geosynchronous orbit (GEO) is the location of communications and weather satellites. The Earth-Moon L-1 and L-2 points are possible staging locales for trips to and from the Moon. Low lunar orbit and the surface are within the gravity well of the Moon. (Credit 6.1)
If problems arise during lunar journeys, the return to Earth takes only a few days. On planetary missions, a return to Earth may take many weeks to months, if possible at all. An abort capability is critical for the planning of human missions. This was demonstrated most dramatically during the flight of Apollo 13 in April 1970.2 An explosion of an oxygen tank in the Service Module crippled the spacecraft’s electrical system, making the Command Module inoperative. Because the Lunar Module was still attached to the vehicle (they were on their way to the Moon), the crew was able to use it as a “lifeboat” to survive for the three days it took to swing around the Moon and return to Earth. The ability to abort a flight in progress, for safety or other operational reasons, distinguishes the Moon from other planetary destinations. This benefit is an enormous advantage during the early stages of space development, as the reliability of new systems has yet to be demonstrated. Catastrophic loss of crew can bring a nascent program to a halt, and in some cases, result in its early termination.
Ease of communication with the Earth is another advantage of the Moon’s proximity. The leverage provided by this short time-delay ranges from the merely convenient to the operationally essential. For typical human operations on the Moon or at lunar distances, round-trip radio time is a bit under three seconds, a noticeable but easily handled delay, as listening to any of the Apollo audio files will attest. The critical value of the short time-delay of lunar distance comes with robotic teleoperation. As will be discussed in more detail later, a strategy for acquiring early operational capability on the Moon will come from the emplacement and use of robotic assets. These robots will prepare and construct the infrastructure of the lunar outpost, as well as begin work on resource harvesting, water extraction, storage, and processing, work that can be operated, or at least supervised, remotely from Earth. Because of the tens-of-minutes time delay for radio propagation, the remote operation of machines on Mars makes it difficult to accomplish even the simplest of tasks. In contrast, the proximity of the Moon permits us to operate assets on the lunar surface in near real time.
The many advantages of the Moon’s closeness make it a logical and useful destination in any trans-LEO human spaceflight architecture. The creation of capability in space will be accomplished more easily and safely by first learning how to operate in space at lunar distances. With experience and competence acquired on the Moon, we will be more confident and skilled when we move outward to more distant destinations. By using the Moon to learn these skills and techniques, we learn how to crawl before we attempt to walk.
It’s Interesting: The Scientific Value of the Moon
The Moon offers scientific value that is unique within the family of objects in the solar system.3 It is a recorder of history and process, an ancient world containing materials unprocessed since their formation more than four billion years ago. The Moon records its own history and the history of the universe around it. Its environment permits unique experiments in the physical and biological sciences. Additionally, it is a natural laboratory for understanding the processes that created our solar system and that currently drive the geological evolution of the planets.
The Moon has undergone a complex and protracted geological history that we can study to understand early planetary evolution. From Apollo data, we found that the Moon is a differentiated object, with a metallic core, mantle, and crust. Its segregation into this tripartite condition was the result of global melting early in solar system history. If a body as small as the Moon could undergo global differentiation, it is likely that all the terrestrial planets did likewise. The study of early lunar geologic history is a guide to the interpretation of the history of all the rocky planets, and the Moon records events of an epoch for which evidence has been erased from the eroded, dynamic surface of the Earth. After this differentiation, the Moon underwent a protracted impact bombardment, hit by objects from the microscopic to the asteroidal; these collisions formed craters that span similar size ranges. While we understand the impact process in broad outline, details of the physical and compositional processes remain obscure, especially questions about how they scale with size. The Moon’s abundant craters (figure 6.2), on display for our study and enlightenment, offer innumerable examples of this process.
Figure 6.2. Examples of fresh lunar craters. Rümker E (38.6°N, 302.9°E; 7 km diameter) is a simple crater, with a bowl shape and small, flat floor. Large blocks are visible near its rim crest. The complex crater Aristarchus (23.7°N, 312.5°E; 40 km diameter) shows wall terraces (from slumping after crater excavation), an extensive flat floor (impact melt sheet) and a central peak (brought up from the deep crust). (Credit 6.2)
Billions of years ago, internal melting of the mantle of the Moon produced copious iron-rich magmas that rose upward to the surface and erupted as vast sheets of basaltic lava. These lavas make up the lunar maria, the dark smooth lowlands of the Moon. They are concentrated on the near side (for reasons that still elude us) and are made up of hundreds of individual flows with differing compositions, volumes and ages. By understanding the sequence of lavas over time, their source regions, and changes in composition, we can reconstruct the thermal and compositional evolution of the lunar deep interior. Again, because volcanism is ubiquitous on the terrestrial planets, knowledge of the lunar experience helps us to better understand this process across the solar system.
The principal geological process on the Moon for the last three billion years is bombardment by a constant micrometeorite “rain” of tiny particles. The flux of debris acts as a giant “sandblaster,” grinding surface rocks into a fine powder. This layer of disaggregated rocky debris, the regolith, is exposed to space and thus, implanted with particles from sources external to the Moon. Because the Moon has no atmosphere or global magnetic field, plasmas and streams of energetic particles from the Sun, and the universe around us, impinge directly on its surface, becoming embedded onto these lunar dust grains. Thus, the Moon contains a unique, detailed record of the output of the Sun and galaxy through geological time.
The solar wind is the most common source of particles, a stream consisting mostly of protons that collide with and stick to the lunar dust grains. As this process is constant, particles from the Sun emitted at varying times in history may be recovered from the ancient regolith and used to reconstruct the output of the Sun and galaxy as it was in the distant geological past. A special case occurs when an ancient regolith is buried by a lava flow. In this instance, the covered regolith becomes a closed-system, shut off from further particle implantation. The solar wind gases, preserved in such a closed-system, record a “snapshot” of the ancient Sun, dated by the ages of the bounding rock units above and below the ancient paleoregolith.
Accessible regoliths on the Moon cover a time range of at least the last four billion years. The Sun is the principal driver of Earth’s climate, and by recovering solar output over time, a record unavailable anywhere on Earth, we can understand its cycles and singular events for the duration of the history of the solar system. Some initial results, from our study of the Apollo samples, suggest that the ancient Sun had a different composition of its nitrogen isotopes than it does now, a puzzling result not predicted by existing theories of stellar evolution. What other new and unexpected secrets of the Sun and stars lie embedded on the Moon, awaiting discovery?
Because of the antiquity of the Moon, and its proximity to the Earth, the lunar surface retains a record of the impact bombardment history of both bodies. We know that the collision of large bodies has had drastic effects on the geological and biological evolution of the Earth and occur at quasi-regular intervals.4 Because our very survival depends on our understanding the nature and history of these collisions as a basis for the prediction of future events, the impact record on the lunar surface is critical to our understanding of this hazard. By dating a large population of individual craters on a surface of known age, we can establish whether the periodicity of the impact flux is real. Such periodic impacts may have driven the process of evolution on Earth. These studies could uncover fundamental, unknown aspects of the history of life on Earth and in the solar system.
With no ionosphere, and a far side that is the only known area in the solar system permanently blocking the radio noise and static of Earth, a radio telescope on the far side of the Moon can examine low frequency wavelengths that are impossible to detect from Earth’s surface or in LEO. The seismically quiet lunar surface permits the construction of extremely sensitive and delicate instruments, such as interferometers at optical wavelengths. An array of such telescopes could achieve resolutions at the micro-arc second level, allowing the direct observation of phenomena such as star spots and the hemispheres of terrestrial planets in nearby systems. Such capabilities would revolutionize our understanding of the evolutionary paths of stellar and planetary systems.
Finally, the environment of the Moon is itself a scientific asset of great value. The hard vacuum and extreme thermal regime permit unique material science experiments. The low gravity of the Moon allows us to quantify the effects of fractional gravity on physical and biological phenomena. The Moon is an isolated and sterilizing environment, permitting experimentation with hazardous materials and processes. Facilities on the lunar surface allow us to conduct dangerous or hazardous experiments that would be unwise to pursue on the Earth. These unique properties make the Moon an unparalleled asset for scientific experimentation and laboratory work.
It’s Useful: The Utility of the Moon
While the previous two attributes of the Moon are extremely important, its greatest value is its capacity to create new spacefaring capability through the exploitation of its material and energy resources. The idea of using the materials of other worlds to provision ourselves, and to supply and support spaceflight, is a very old one, but to date, it has not been attempted. Yet, development of this single activity could completely change the paradigm of spaceflight. Currently, anything that we need in space must be transported to Earth orbit at enormous cost, usually on the order of at least $1,000-10,000 per kilogram. This high cost applies to everything: It costs the same amount of money to launch a kilogram of high-technology electronics as it does a kilogram of water. If we could provide low-information density materials (like water, air, and rocket propellant) from local sources already present in space, we could accomplish much more for less money. In a nutshell, this is the driving motivation for the use of off-planet resources, or, in the term used in the business, in situ resource utilization (ISRU).5 This is a skill that we must master in order to become a truly spacefaring species.
Although the physics and chemistry of extracting and using the resources of the Moon are simple and straightforward, there has been great resistance to incorporating ISRU into any spaceflight architecture. There are many reasons for this attitude, ranging from unfamiliarity with the processes involved to a natural and at least partly understandable conservatism in engineering design. For initial ISRU efforts, we would only undertake the simplest processes, such as bulldozing regolith to make blast berms around landing pads and to cover habitats for radiation shielding, along with heating polar regolith to extract water ice. These are minimal, low-risk activities that provide useful products and pieces of outpost infrastructure. The techniques needed to begin ISRU are no more complex than everyday eighteenth-century industrial processes.
The resources of the Moon are simple and require minimal processing. First, bulk regolith (soil) has many uses as thermal and radiation shielding and for construction. Although loose soil can be used as is, regolith can also be fused by microwave sintering or passive solar thermal heating (such as a concentrating mirror) into ceramics or aggregate for building material. Roads and landing pads can be manufactured by sintering the regolith in place using a microwave-heating element mounted on a rover.6 Microwaves fuse loose regolith into brick and ceramic because of the fine-scale, vapor-deposited free iron that coats the surfaces of dust grains. This coating permits RF energy to be efficiently coupled and transferred into heat, so that the grain boundaries fuse together to make glass. A microwave with a power level comparable to a kitchen oven can fuse the upper surface into a paved road or landing pad several centimeters deep. Fused regolith structures can be made as large or as long as needed. Structures and pieces can be produced with 3-D printer technology using fine regolith as feedstock.
The Moon’s poles possess critical resources needed for long-term human presence on the Moon and in space. They have two key attributes that the rest of the Moon does not possess: water ice (and other volatile substances) and areas of near-permanent sunlight. We have verified the presence of water ice using several techniques of remote sensing, including hydrogen detection, near-infrared and ultraviolet reflectance, laser albedo, radar, and a physical impactor. In addition to water—the most cosmically abundant volatile substance in the solar system—other volatile species are present in the polar ice, including methane (CH4), carbon monoxide (CO), ammonia (NH3), hydrogen sulfide (H2S), and some simple organic molecules. All of these volatile substances can be chemically processed to help support a human presence on the Moon.
Questions remain over how much water and other volatiles are present in total, on their distribution laterally and vertically, and over what physical form the different chemical ices take. These volatiles probably come from sources external to the Moon—the impact of water-bearing objects, such as cometary nuclei and volatile-rich meteorites. As such, they are deposited in extremely small amounts, in a vacuum and over a very long period. The likely nature of such a deposit would be a very porous mixture of dust grains and amorphous (noncrystalline) ice. In astrophysics, such a compositional fabric is called a fairy-castle structure and is a common state of materials in space.
The dark areas where ice is stable are extremely cold, always less than —169°C (104 K), but in some cases as cold as —248°C (25 K) and widespread at both poles. These dark areas are typically found in crater interiors but in some cases as extended regions of shadow. The “cold traps” are all equally likely to contain ice, but current evidence suggests, for reasons we do not fully understand, that the ice is distributed heterogeneously (see figure 5.1). In addition, because lunar soil is an excellent thermal insulator, it is possible that extensive deposits of ice might be present in the shallow subsurface, in areas that receive partial solar illumination.
We need to survey the potential mining areas to determine their content and grade. This is best accomplished by using a small robotic rover that traverses the polar areas and measures ice content and composition over many locations. The dark areas are close to the lit regions, as the grazing sunlight at the poles, both illuminates and shades. Although there are no areas of “permanent” sunlight, certain regions near both poles have been found to be in sunlight for more than 90 percent of the lunar year.7 Solar arrays mounted on a high mast could be in sunlight for longer periods; this possibility is a subject for current research. An outpost located in these areas would be able to generate electrical power on a nearly constant basis, with periods of darkness bridged by power storage, such as the use of a rechargeable fuel cell.
Another advantage of these “quasi-permanent” sunlit areas is that they are thermally benign. At the equator of the Moon, the surface is heated during the daytime, which is fourteen Earth days long, reaching temperatures of up to 100°C. During the coldest part of the nighttime, also fourteen Earth days long, the surface may assume temperatures as low as —150°C, a 250° swing from the hottest part of the day. The high temperatures of lunar noon put stress on systems designed to keep machinery cool, while the cold night temperatures require moving parts to be heated. Within the sunlit areas near the poles, illumination is always at grazing incidence—that is, the Sun circles around near the horizon—and maintains the surface temperature at a near-constant —50°C. In such an environment, minimal power is required to maintain thermal equilibrium for complex machinery. Along with the pervasive presence of highly abrasive dust that can wear down parts and make machinery inoperative, the extreme thermal environment is one of our biggest technical challenges in developing the resources of the lunar poles. Mitigating strategies for each of these difficulties are currently the subject of intensive research.
In addition to the constant solar power available at the poles, the Moon contains substances that, in the future, may be used to generate energy for use on the lunar surface and in space. Several regions of the western near side contain elevated amounts of the radioactive element thorium, which can be used to fuel nuclear reactors to generate electrical power. Via several nuclear reactions, thorium breeder reactors can produce their own fuel, making it possible for us to construct space reactors on the Moon. The use of nuclear power would allow us to survive the long lunar night and permit habitation of equatorial and mid-latitude regions of the Moon. The availability of abundant power also enables large-scale industrialization of the Moon.
In the more distant future, some have proposed that the rare isotope helium-3, implanted in the lunar regolith by the solar wind, could be harvested to generate electrical power in a relatively “clean” nuclear reaction, one that does not generate excess neutrons and “dirty” reaction products.8 The fusion of deuterium (hydrogen-2) with helium-3 produces fewer neutrons and positively charged He ions, permitting the efficient conversion to electrical power over the standard deuterium-tritium (2H-3H) fusion. In fact, a variant of this process, whereby helium-3 fuses with itself (3He-3He), produces no harmful by-products at all. Potentially, helium-3 fusion could solve the world’s energy problems if a suitably large source of the isotope could be found—it is present on Earth as a component of natural gas, but in extremely small amounts.
It has been proposed that we mine the lunar regolith for helium-3 and import the product back to Earth for commercial electrical power generation. The difficulty with this idea is twofold. First, we do not yet have reactors that can burn helium-3 nuclear fusion fuel. It takes a great deal of energy to start this reaction and then to contain and control it; no fusion reaction to date has achieved “breakeven,” the point at which the fusion reaction liberates more energy than it takes to start it. Research on this problem has been going on for decades; it is unlikely that we will see commercial applications of fusion power generation for many years. Second, although there is helium-3 in the lunar regolith, it is present in concentrations of less than about twenty parts per billion. This low concentration is for sampled sites in the lunar equatorial maria; we do not yet know the concentration of helium-3 in the polar volatiles. Extracting helium-3 from the mare regolith will require the mining and processing of hundreds of millions of tons of regolith, a scale of resource processing that may eventually occur, but certainly not in the early stages of lunar habitation. The mining of helium-3, often alluded to as the ultimate “pay dirt” on the Moon, is not likely near-term (~20 years) but may turn out to be significant in the multidecadal time scales of future lunar development.
Water is the most useful material in space. In its native form, we can drink it and use it to reconstitute food, cool equipment, and jacket habitats for radiation protection, as well as for hygiene and sanitation needs. An electrical current can disassociate water into its component hydrogen and oxygen. These gases can be stored and used; oxygen can be used for breathing, and both gases can be recombined in a fuel cell to generate electricity. Used this way, water is a medium of energy storage. Finally, the hydrogen and oxygen can be cooled into cryogenic liquids and used as rocket fuel, the most powerful chemical propellant known. Because of its utilitarian value, water is truly the “currency” of spaceflight.
The real lunar “El Dorado” consists of the water ice and the permanent sunlight near the poles. It is a location known to contain resources of material and energy that we can access and use. It is a place where we can learn the skills and technologies needed to become permanent residents of space.
Why Not Mars?
Virtually the entire space community, from those inside the agency to others working on spacecraft, missions or data analysis, presume that Mars is the “ultimate goal” for human spaceflight.9 In 1965, the imaginative pull that decades of science fiction and speculation about Mars as an Earthlike planet had dealt us were dashed when we found by direct investigation that the real Mars is a distant, cold, dry desert, with virtually no atmosphere. Subsequent missions over the years have shown that it may have been warmer and wetter in the past, which led to the idea that microbial life might have originated there. This single idea is largely responsible for the subsequent fixation on Mars as the “next destination” for humans in space. The obsession with “searching for life elsewhere” has hijacked our thinking about the future of people in space. It is virtually impossible to advance an idea or concept involving people at some space destination other than Mars, without proving that it “feeds forward” to our “ultimate destination.”
We do not know how to send people to Mars at this time. The difficulties with a human mission to Mars fall into several categories: technical, programmatic, and fiscal. The manned space program has conducted long-duration spaceflight, built a heavy-lift launch vehicle, and conducted landings on the Moon. But for a variety of reasons, getting humans to Mars is much more difficult. Mars is much farther away from Earth, varying between 140 to 1,000 times (55 to 400 million km) the distance of Earth to the Moon (400,000 km). No known trajectory can shorten the months of transit; most robotic missions take nine months. Although issues of crew deconditioning caused by microgravity appear to be mostly resolved from flight experience on the ISS, months of exposure to hard cosmic radiation and the occasional possible solar particle event, requires some type of shielding. People need to breathe, eat, and drink, so those consumables must be carried with them. Mars is bigger than the Moon (its gravity is about 3/8 that of the Earth, compared to the 1/6 g of the Moon); thus, it requires more energy to descend and land on the surface of Mars. This applies to the return trip as well. The larger gravity well of Mars means that bigger landers and more fuel are needed. Although there is an atmosphere on Mars, it is more than one hundred times thinner than Earth’s, so we cannot rely solely on aerothermal entry to slow down the spacecraft; a significant propulsive maneuver is required. This issue, the EDL (entry, descent, landing) problem,10 is one for which we have no solution at present.
The composition of the martian atmosphere is virtually pure carbon dioxide (CO2) and thus, not breathable; the thinness of the atmosphere requires people to wear pressure suits. The surface is not completely shielded from cosmic rays and solar UV radiation; Mars does not have a magnetosphere like the Earth, which means that it is a hard radiation environment, limiting the permissible time for surface exploration. The soil on Mars is very fine, probably consisting of clay minerals, and owing to the presence of perchlorates and other highly oxidizing substances, it appears to be highly reactive chemically. If inhaled—and some dust inevitably will be brought into the crew cabin—dust could result in caustic chemical reactions in the bronchia of the crew’s lungs. In addition, no one knows how well humans will cope with the reduced gravity of the martian surface after spending multiple months in microgravity.
The biggest problem with a human Mars mission comes right at the beginning. In order to carry the fuel, consumables, equipment, and vehicles that the crew will need for this trip, it will require the launch of between 500 and 1,000 metric tons into low Earth orbit.11 Getting this mass into orbit will require eight to twelve launches of a heavy lift rocket, each carrying about 130 metric tons to space. The vast bulk of this mass is the fuel required for the trip, most of which is burned at the beginning of the voyage to insert the vehicle into its planetary trajectory. But that’s not the end of the issue. The fuel will probably be cryogenic hydrogen and oxygen; if we use storable propellant, multiply the mass needed by factors of two to three. After it is delivered to orbit to await the eventual mission to Mars, the super cold cryogenic fuel will quickly boil off from solar heating. It would be a race against the clock to get enough fuel in one place in orbit at the right time. For now, we have no solution to the problem of fuel boiling off in the landers, both for descent and ascent, during the cruise phase of a manned mission to Mars.
Other problems arise in terms of the scheduling of the multiple HLV launches and coordinating their payload manifests. Only two HLV launch pads (Launch Complex 39) exist at Cape Canaveral. One is currently unavailable, leased to a private company. Thus, we would need to launch all of these vehicles from a single pad. To get the pieces of the Mars mission in one place and ready to go, we must deal with an enormous scheduling and manifest problem, as well as the logistics of multiple HLV deliveries. After that, the next hurdle would be assembling and fueling the Mars vehicle in space.
The cost of a Mars mission conducted in this manner is estimated at several tens of billions of dollars per trip. Is such a cost politically viable? Regardless of the propaganda spun by a hopeful New Space community, there are no magic bullets to lower this enormous cost. We still need the same mass in LEO, and the “lowering” of launch costs, which in any event is only on the order of factors of two or three at best, might turn a $500 billion mission into a $450 billion mission. For context, we currently spend about $18 billion per year on our civil space program, of which about $8 billion is designated for human spaceflight.
Faced with these realities, it should be evident that Mars is very far from Earth, technically and fiscally. But the hardwired dreams of living on Mars have left space advocates of all persuasions chasing their tails, locked in a 50-year exercise by the promises of politicians or administrators who tell us, “Yes, we will be embarking on a new program to send humans to Mars.” What follows, as night follows day, is that people get spun up and start conducting feasibility studies; new vehicles are designed, and lovely color artwork showing people rappelling down the walls of one of the canyons of Valles Marineris is produced. And then, yet again, the dismal mathematics of a Mars mission becomes evident. But, we are told, not to worry: The mission is at least a couple of decades into the future. Somehow, the money and the political support for more money still will magically appear at the right time. Certainly, if we can assemble a Mars advocacy group, one that shows we have clout and that strikes fear in the hearts of our elected officials, we will get more money. To date, these methods and declarations have accomplished nothing. But, our leaders tell us, this will change as soon as we find a way to get the public excited about space—that “excitement” causes money to flow into the space program. After 50 years, is it not time to admit that this approach isn’t working?
An article of faith among the true believers is that interest in the Moon and planning for lunar bases has kept them from achieving their lifelong dream of strolling across the red plains of Mars. The reality is exactly the reverse: It is the fixation with sending people to Mars that has kept us from doing any human missions beyond LEO. Looking over the history of post-Apollo planning, from Nixon’s Space Task Group in 1969 to the Vision for Space Exploration in 2004, all efforts to get people into trans-LEO space have run aground on the realities of the enormous technical and cost difficulties of human Mars missions.12 During the VSE, NASA was more concerned with devising a lunar “exit strategy” than it was with getting people back to the Moon in the first place.13 The dirty little secret is that most politicians love human Mars missions not because they have any desire or interest in doing them but because it is an excellent and proven way to keep the space community pacified by selecting a goal that is so far into the future that no one will be held accountable for its continuing non-achievement. What a remarkable accomplishment for America’s efforts in space: once we had a real space program that some thought was faked, and now we have a fake space program that many believe is real.
The only way we will ever get people to Mars is through the construction of a transportation system that enables the routine movement of cargo and people throughout space. An Apollo-style crash program to send humans to Mars is highly unlikely to ever materialize. We need to acquire and learn certain spacefaring skills and technologies, including reusable space-based vehicles, staging nodes in deep space, in situ resource utilization, and the manufacture of propellant from water. If we possessed these capabilities, a human mission to Mars, while still challenging, would become more feasible. We can learn those skills and acquire those technologies on the Moon.
Why not Mars? Because it’s too far, too difficult, and too expensive.
Why Not Asteroids?
At first glance, it might seem that asteroids, specifically the near-Earth objects (NEO), answer the requirements for future human destinations. NEOs are beyond low Earth orbit, they require long transit times and so simulate the duration of future Mars missions, and we have never visited one with people. However, detailed consideration indicates that NEOs are not the best choice as our next destination in space.
Most asteroids reside not near the Earth but in the asteroid belt, a zone between the orbits of Mars and Jupiter. The very strong gravity field of Jupiter will sometimes perturb the orbits of these rocky bodies and hurl them into the inner solar system, where they usually hit the Sun or one of the inner planets. Between those two events, they orbit the Sun, sometimes coming close to the Earth. NEOs can be any of a variety of different types of asteroids, but are usually small, on the order of tens of meters to a few kilometers in size. As such, they do not have significant gravity fields of their own, so missions to them do not “land” on an alien world, but rather rendezvous and station-keep with them in deep space.
The moniker “near Earth” is a relative descriptor. These objects orbit the Sun just as the Earth does, and depending upon the time of year, vary in distance to the Earth from a few million kilometers to hundreds of millions of kilometers. Getting to one NEO has nothing to do with getting to another, so visiting multiple NEOs during one trip is both difficult and unlikely. Because the distance to a NEO varies widely, we cannot just go to one whenever we choose: Launch windows open at certain times of the year, and because the NEO is in its own orbit, these windows occur infrequently and are of very short duration, usually a few days. Moreover, due to the distances between Earth and the NEO, radio communications will not be instantaneous, with varying time lags of tens of seconds to several minutes between transmission and reception.
Although there are several thousand NEOs, few of them are potential destinations for human missions. This is a consequence of two factors. Because space is very big, even several thousand rocks spread out over several billion cubic kilometers of empty space results in a very low density of objects. Second, many of these objects are unreachable, requiring too much velocity change from an Earth departure stage; this can be the result of either too high of an orbital inclination (out of the plane of the Earth’s orbit) or an orbit that is too eccentric (to varying degrees, all orbits are elliptical). These factors result in reducing the field of possible destinations from thousands to a dozen or so, at best.
There are few asteroid targets and it takes months to reach one. Long transit time is sold as a benefit by advocates of asteroid missions: Because a trip to Mars will take months, a NEO mission will allow us to test out the systems for Mars missions. But such systems do not yet exist. On a human mission to a NEO, the crew is beyond help from Earth, except for radioed instructions and sympathy. A human NEO mission will have to be self-sufficient to a degree not present on existing spacecraft. Crew exposure to the radiation environment of interplanetary space is another consequence of long flight times. This hazard comes in two varieties: solar flares and galactic cosmic rays. Solar flares are massive eruptions of high-energy particles from the Sun, occurring at irregular, unpredictable intervals. We must carry some type of high-mass shielding to protect the crew from this deadly radiation, and this “storm shelter” must be carried wherever we go. Because Apollo missions were only a few days long, the crew simply accepted the risk of possible death from a solar flare. Cosmic rays are much less intense, but constant. The normal ones are relatively harmless, but high-energy versions (heavy nuclei expelled from ancient supernovae) can cause serious tissue damage. Although the crew can be partly shielded from this hazard, they are never totally protected from it.
When the crew finally arrives at their destination, more difficulties await. Many NEOs spin very rapidly, with rotation periods on the order of a few hours at most. This means that the object is approachable only near its polar area. Because these rocks are irregularly shaped, rotation is not the smooth, regular spin of a planet, but is more like that of a wobbling toy top. If material is disturbed on the surface, the rapid spin of the asteroid will launch this debris into space, creating a possible collision hazard to the human vehicle and crew. The lack of gravity means that “walking” on the surface of the asteroid is not possible; crew will “float” above the surface of the object, and just as occurs in Earth orbit, each touch of the asteroid surface (action) will result in a propulsive maneuver away from the surface (reaction).
We would need to work quickly at the asteroid because we would not have much time there; loiter times near the asteroid for most opportunities are a few days. Why so short? Because the crew wants to come home. The NEO and Earth continue to orbit the Sun, and we need to make sure that the Earth is in the right place when we arrive back at its orbital position. In effect, we will spend months traveling there in a vehicle with the habitable volume of a large walk-in closet, have a short time at the destination, and then spend months on the trip home.
In general terms, we already know what asteroids are made of, how they are put together, and what processes operate upon their surfaces. Most NEOs will be ordinary chondrites. We know this because ordinary chondrites make up about 85 percent of all observed meteorite falls. This class of meteorite is remarkable not for its diversity, but for its uniformity. Chondrites are used as a chemical standard in the analysis of planetary rocks and soils to measure the amounts of differentiation or chemical change during geological processing. One chondrite is pretty much like all the others.
Questions that could be addressed by human visitors to asteroids concern their internal makeup and structure. Some appear to be rubble piles, while others are nearly solid. Why such different fates in different asteroids? By using active seismometry (acoustic sounding), a human crew could lay out instruments and sensors to decipher the density profile of an asteroid. Understanding the internal structure of an asteroid is important for learning the internal strength of such objects; this is an important factor in devising strategies to divert a NEO away from a collision course with Earth.
An alleged benefit of travel to an asteroid is that they have resource potential. I agree, putting the accent on the word “potential.” Our best guide to the nature of these resources comes from the study of meteorites—NEOs that have already collided with the Earth. The resource potential of asteroids lies not in the chondrites, but in the minority of asteroids that have more exotic compositions. Metal asteroids make up about 7 percent of the population and are composed of nearly pure iron-nickel metal, with some inclusions of rocklike material as a minor component. Other siderophile (iron-loving) elements, including platinum and gold, make up trace portions of these bodies. A metal asteroid is an extremely high-grade ore deposit, potentially worth billions of dollars, if we were able to get these metals back to Earth.
However, from the spaceflight perspective, water has the most value. A relatively rare asteroid type contains carbon and organic compounds, as well as clays and other hydrated minerals. These bodies contain significant amounts of water (up to 20 weight percent). Finding a water-rich NEO would create a logistics depot of immense potential value.
A key advantage of asteroids as a resource is a drawback as an operational environment: They have extremely low surface gravity. Getting into and out of the Moon’s gravity well requires a change in velocity of about 2,380 meters per second each way; to do the same for a typical asteroid requires only a few meters per second. This means that a payload launched from an asteroid rather than the Moon saves almost 5 kilometers per second in delta-v, a substantial amount of energy. From the perspective of energy accessibility, the asteroids beat the Moon as a source of materials.
Yet there remains the challenge of working in very low gravity, as well as other difficulties that exist in mining and using asteroidal, as opposed to lunar, resources. First is the nature of the feedstock or “ore.” Water at the poles of the Moon is not only present in enormous quantity, tens of billions of tons, but is also in a form that can be easily used: ice. Ice can be converted into a liquid for further processing at minimal energy cost; if the icy regolith from the poles is heated to above 0°C, the ice will melt and water can be collected and stored. The water in carbonaceous asteroids is chemically bound in mineral structures. Significant amounts of energy are required to break these chemical bonds to free the water, at least two or three orders of magnitude more energy than to melt ice, depending on the specific mineral phase being processed. So extracting water from an asteroid (present in quantities of a few percent to maybe a couple of tens of percent) requires significant energy; water ice at the poles of the Moon is present in greater abundance (up to 100 percent in certain polar craters) and is already in a form that is easy to process and use.
The processing of natural materials to extract water has many steps, from the acquisition of the feedstock, to moving the material through the processing stream, to the collection and storage of the derived product. At each stage, we typically separate one component from another; gravity serves this purpose in most industrial processing. A challenge to asteroid resource processing is to devise techniques that do not require gravity, including related phenomena, such as thermal convection, or to create an artificial gravity field to ensure that things move in the right directions. Either approach significantly complicates the resource extraction process.
The great distance from the Earth and poor accessibility of asteroids compared to the Moon works against resource extraction and processing. Human visits to NEOs will be of short duration, and because radio time lags to asteroids are on the order of minutes, direct remote control of processing will not be possible. Robotic systems for asteroid mining must be designed to have a large degree of autonomy. This may become possible but presently we do not have enough information on the nature of asteroidal feedstock to design, or even envision, the use of such robotic equipment. Moreover, even if we did fully understand the nature of the deposit, mining and processing are highly interactive activities on Earth and will be so in space. The slightest anomaly or miscalculation can cause the entire processing stream to break down, and in remote operations, it will be difficult to diagnose and correct the problem and restart it.
The accessibility issue also cuts against asteroidal resources. We cannot go to a given asteroid at will; launch windows open for very short periods and are closed most of the time. This affects not only our access to the asteroid but also shortens the periods when we may depart from the object to return our products to near-Earth space. In contrast, we can go to and from the Moon at any time, and its proximity means that nearly instantaneous remote control and response are possible. The difficulties of remote control for asteroid activities have led some to suggest that we devise a way to “tow” the body into Earth orbit, where it may be disaggregated and processed at our leisure. I shudder to think about being assigned to write the environmental impact (if you’ll pardon the expression) statement for that activity.
So where does that leave us in relation to space resource access and utilization? Asteroid resource utilization has potential, but given today’s technology levels, it has uncertain prospects for success. Asteroids are hard to get to, have short visit times for round-trips, difficult work environments, and uncertain product yields. Asteroids do have low gravity going for them, which is both a blessing and a curse. In contrast, the Moon has the materials we want and in the form that we need. The Moon is close and easily accessible at any time and is amenable to remote operations controlled from Earth, in near-real time. We should go to the Moon first to learn the techniques, difficulties, and technology to conduct planetary resource utilization by manufacturing propellant from lunar water. Nearly every step of this activity, from prospecting and processing to harvesting, will teach us how to mine and process materials from future destinations, on both minor and planetary-sized bodies. Learning how to access and process resources on the Moon is a skill that transfers to any future space destination.
The Moon: Our Next Destination in Space
The Moon is the first extraterrestrial object after leaving Earth orbit and it is a highly desirable place to visit and utilize. Why would we not want to explore and use it? Yet, as we have seen, two presidential attempts to return to the Moon in the past twenty-five years have both ended in failure, stifled by bureaucratic process and the continuing siren call of Mars. Other nations clearly see the value of the Moon. Why can’t we?
In part, America is the victim of its own early success on the Moon. The Apollo missions and the associated robotic missions that preceded them, were great technical and emotional triumphs. They produced sights and experiences that have yet to be surpassed, even by the technically more challenging (but also more prosaic) flights of the space shuttle and the construction of the ISS. It wet our appetites for more. Because of Apollo, there is a sense that we’ve been there, and overly anxious explorers don’t see a reason to return. This ignorance and quick dismissal about what the Moon has to offer is exploited by space advocates who have other agendas: to quest for life, to step onto new worlds, to build colonies and transform other planets. None of those motivations by themselves have had any better success in generating more—or even adequate—funding for the civil space program. In particular, the constant and recurring obsession with human missions to Mars has kept us from pursuing the more valuable and emphatically achievable near-term goal: a permanent return to the Moon.
Simply put, most people are indifferent to space. This has been true since the beginning of spaceflight, even during the Apollo program.14 They are neither overly enthusiastic nor hostile to it; they are at best, mildly interested in space, occasionally becoming enthusiastic and patriotic in times of significant accomplishment. For years, space advocates have had the obsessive certainty that if they can impart to the public the same zeal that they feel for Mars or space colonies, or whatever their cause, that they will be showered with more money, forever. That hasn’t happened and it won’t. At best, there will be a modest level of ongoing federal funding—more or less what NASA has received since the end of the Apollo program.
We must craft a program that will endure for decades, a program that makes steady, constant progress and returns tangible benefits with the levels of funding likely to be made available. Our challenge is to work with what we have. Yet, how can we craft a program that aims for big goals, like space settlement or planetary missions, under existing constrained budgets? I have spent the last few years exploring that question, and I believe there is a clear path forward.