We became convinced that a space program large enough to need, and to benefit significantly from, nonterrestrial resources would require a great expansion of energy, power, and transport beyond the capabilities of today. Sunlight is already in use as a primary energy source in space, and nuclear energy has been used on a small scale. Photovoltaic panels, together with chemical or nuclear energy sources brought from Earth, have been sufficient up to now. In the future, more advanced and much larger solar and nuclear energy systems may be built; but, even then, energy supply may limit our rate of progress. For example, a program is under way to develop the SP-1 00, a space nuclear power plant intended to produce 100 kilowatts of electricity with possible extension to a megawatt. But even a small lunar base would consume several megawatts.
Harnessing sunlight on a large scale and at low cost thus remains a priority research and development goal, as does the creation of high-capacity systems for converting and storing solar and nuclear energy in space. Many studies have described the candidate techniques, including solar furnaces, solar-powered steam engines, solar-pumped lasers, and nuclear thermal power plants.
Although solar energy is ubiquitous and abundant, and compact nuclear energy sources can be brought up from Earth, it is still necessary to have machinery in space for capturing, storing, converting, and using the energy. Perhaps nonterrestrial resources can be used in the creation of some of this machinery. For example, as has often been proposed, lunar silicon could be used for photovoltaics; lunar glass, for mirrors.
A more important energy initiative might be the development of new storage and management concepts, such as the establishment of water, oxygen, and hydrogen caches cryogenically stored in the lunar polar cold traps. Fluidized-bed heat storage, molten metal cooling fountains, and storage by hoisting weights are other examples of energy storage and management benefiting from attributes of the lunar environment; namely, a large supply of raw materials, vacuum, and gravity. Consideration should also be given to the siting of solar and nuclear power plants on the Moon. For example, a solar plant located at one of the lunar poles would be capable of nearly continuous operation, in contrast to a plant at an equatorial location which would be in darkness 14 days out of 28 (figs. 5 and 6).
We found that transport costs would be dominant in any program large enough to make significant use of nonterrestrial resources. We recommend continued pursuit of technologies offering the prospect of large reductions in Earth-to-LEO transport cost. A preliminary economic model of the effect of lunar resource utilization on the cost of transportation in space was developed by the study. This model, developed in more detail, shows that delivery of lunar-derived oxygen to LEO for use as propellant in space operations would be significantly cheaper than delivery of the same payload by the Space Shuttle, assuming a demand for about 300 metric tons of oxygen delivered to LEO. If Earth-to-LEO costs could be reduced using unmanned cargo rockets - Shuttlederived launch vehicles or heavy lift launch vehicles, the cost of lunar-derived oxygen would also be reduced. At this demand level, if Earth-to-LEO costs were lowered to about 2/3 their present value, it would be cheaper to bring all oxygen up from Earth. But, if demand for liquid oxygen as propellant in LEO were to grow by a factor of 2 or more, then lunar-derived oxygen would be competitive with Earth-derived oxygen using any currently contemplated launch vehicle. This model points out that considerable reduction in unit cost for lunarderived oxygen delivered to LEO can be achieved as the volume and scale of operations increase. The model assumes that all hydrogen is transported from Earth. If lunarderived hydrogen were available, the cost of providing lunar-derived oxygen would be considerably reduced at all production rates.
While enhancing Earth-to-orbit capacity, we should be preparing to expand our range. For example, an orbital transfer vehicle (OTV) is needed for traffic to and from GEO. Extending the space-based OTV concept to meet the needs of a lunar base transport system should be considered from the outset of OTV development. The development of an efficient OTV capable of LEO-to-Moon transportation was identified by the economic model just summarized as the single most important factor in the cost of supplying lunar-derived oxygen to space operations.
Also, we support the findings of other studies, such as NASA's 1979 report Space Resources and Space Settlements, to the effect that it may be practical and desirable to transport lunar material using means other than the OTV-derived vehicles that will be carrying humans to and from the Moon. The lunar environment encourages consideration and development of electromagnetic launchers and other unconventional transport devices.
We recognize a need for transport of both equipment and personnel from place to place on the lunar surface and probably also a need for at least short-range transport of raw and processed lunar materials. Much of this transport would logically be provided by teleoperated vehicles. Teleoperated systems, robotics, and automation developed for the space station may have direct application in lunar operations. Such systems would be absolutely required by any program to mine and utilize material from near-Earth asteroids.
Finally, we recommend that alternative advanced propulsion technologies be developed to permit comparison and selection of systems for transport beyond Earth orbit. Examples include solar thermal propulsion, solar electric ion thrusters, nuclear electric propulsion, laser-powered systems, and light-pressure sailing.
Locations, Environments, and Orbits
Another natural resource is afforded by orbits and places in the solar system. The geosynchronous orbit, used for communications and observation, is a resource that has led to the largest commercial development in space and offers an even greater payoff in the future. The combined gravity fields of the Sun and planets offer a resource that has already been used for modifying and controlling spacecraft trajectories through swingby maneuvers. Aeromaneuvering in planetary atmospheres and momentum exchanges using tethers offer additional means of trajectory control.
In future space activities, unique space environments may become important resources. Examples include the Moon's far side, which is shielded from the radio noise of Earth and would thus be an excellent location for a deep-space radio telescope. Lunar orbit or the gravitationally stable Lagrangian points in the Earth-Moon-Sun system may be good locations for space platforms. As has already been pointed out, the lunar poles have the potential of providing constant sunlight to power a lunar base. And aerobraking in the Earth's upper atmosphere may make it possible to bring both lunar and asteroidal material into low Earth orbit for use in space activities.
We found that any future program intending to make major use of nonterrestrial resources, especially the materials of the Moon, must include a substantial human presence beyond Earth orbit. This finding leads to the conclusion that some form of extended human living in deep space, such as a lunar base, is a necessity (fig. 7).
The space station is the obvious place to conduct the proving experiments that will enable confident progress toward productive lunar living, including use of local resources. While this summer study group did not attempt to lay out an entire plan of events leading up to establishment of a lunar base, we recognized some of the steps that are logical and likely to be considered essential. One of these is a suite of experiments, in the space station, demonstrating the soundness of methods and processes to be used at the lunar base.
Since a number of these methods and processes are gravitydependent, it is necessary to demonstrate them at simulated lunar gravity, 1/6 g, and this cannot be done on Earth. We therefore recommend that space station facilities include a 1/6 g centrifuge in which lunar base experiments and confirmation tests can be carried out.
Lunar Orbit Space Station Artist: Michael Carroll
Proximity to lunar-derived propellant and materials would make a space station in orbit around the Moon an important transportation node. It could serve as a turnaround station for lunar landing vehicles which could ferry up liquid oxygen and other materials from the lunar surface. An orbital transfer vehicle could then take the containers of liquid oxygen (and possibly lunar hydrogen) to geosynchronous or low Earth orbit for use in many kinds of space activities. A lunar orbit space station might also serve as a staging point for major expeditions to other parts of the solar system, including Mars.
Polar Solar Power System Artist: Maralyn Vicary
At a base near a lunar pole, solar reflector (the large tower in the background) directs sunlight to a heat collector, where it heats a working fluid which is used to run a turbine generator buried beneath the surface. At such a location the solar power tower can track the Sun simply by rotating around its vertical axis. Power is thus provided continuously without the 2-week nighttime period which is characteristic of nonpoloar locations. The triangle in the background is the mining pit. In the foreground, two scientists collect rock samples for analysis at the base.
Any material that is already in space has enormous potential value relative to the same material that needs to be brought up from Earth, simply because of the high cost of lifting anything out of the Earth's deep gravity well. On an energy basis, it is more than 10 times as easy to bring an object into low Earth orbit from the surface of the Moon as from the surface of the Earth (see figure 1). Residual propellants and tanks or other hardware left in orbit can constitute a resource simply because of the energy previously invested in them.
These facts of nature underlie many proposals for the use of nonterrestrial materials. For example, as discussed in the transportation section, there could be a payoff if lunar oxygen, abundant in the silicates and oxides of the Moon and extractable by processes conceptually known, were to be used in large quantities for propulsion and life support in space operations. A sketch of a concept for extracting oxygen from lunar materials is shown in figure 8. Byproducts of this process might include useful metals.
The materials of near-Earth asteroids may complement the materials of the Moon. On the basis of evidence gained to date, the Moon is lacking in water and carbon compounds -important substances that are abundant in certain classes of meteorites and thus may be found among the small asteroids that orbit the Sun near us. Water from asteroids could provide hydrogen for use as rocket fuel in space operations. On an energy basis, many of the near-Earth asteroids are even easier to reach than the Moon. And there are more energy advantages in a payload return from an asteroid because of their very low gravity. This same low gravity may require novel techniques for mining asteroids (figs. 9 and 10). Low-energy transit times to asteroids are relatively long (months or years, in comparison to days for the Moon), so the voyages to obtain these asteroid materials will probably be automated rather than manned.
Our first finding with regard to materials and processing is obvious but still needs to be stated explicitly because it is so important.
The United States will have no access to nonterrestrial materials unless there is a substantial change in the national space program. Because of recent budget limits and a concentration on applications in LEO and GEO, we have no capability to send humans to the Moon. The option of an entirely automated lunar materials recovery operation, while it might be technically feasible, appears to us unlikely to gain approval. With regard to asteroidal resources, automated return to Earth orbit is mandated by trip times, but the processing would still require human supervision. These findings have two consequences: first, that the utilization of nonterrestrial materials awaits the creation of some new system for highcapacity transport beyond LEO; and, second, that large-scale utilization awaits the creation of a lunar base or an asteroid mining and recovery scenario.
Our other findings regarding lunar, asteroidal, and martian materials presume that the nation has found some way to get over the hurdles just described. With the required transport and habitat infrastructure in place, the question reduces to one of considering possible ways to process and use the materials.
Lunar oxygen, raw lunar soil, lunar "concretes," lunar and asteroidal metals, and asteroidal carbonaceous and volatile substances may all play a part in the space economy of the future.
Because oxygen typically constitutes more than threequarters of the total mass launched from Earth, an economical lunar oxygen source would greatly reduce Earth-based lift demands. Since transport to LEO accounts for a major portion of the total program cost, use of nonterrestrial propellants may permit faster growth of any program at a given budget level.
Another potential use of nonterrestrial resources is in construction, ranging from the simple use of raw lunar soil as shielding to the creation of refined industrial products for building large structures in space.
At the outset, we believe that lunar material will be used rather crudely; for example, by piling it on top of habitat structures brought from Earth. Even that conceptually simple use implies a significant dirt-moving capacity on the Moon. In any event, the use of local material for radiation and thermal shielding is probably essential because of the prohibitive cost of bringing up an equivalent mass from Earth.
Going beyond just raw soil, it is reasonable to ask whether or not a structural material equivalent to concrete could be created on the Moon. Studies by experts in the cement industry suggest that lunar concrete is a possibility, especially if large amounts of energy and some water are available (figs. 11 and 12). Even without water, it may be possible to process lunar soil into forms having compressive and shear strength, hence usable in structures. Examples include sintered soil bricks, cast glass products, and fiberglass.
Metals are also available on the Moon and asteroids. Metallic iron-nickel is a major component of most meteorites and probably most asteroids. Meteoritic iron, extracted magnetically from lunar soils, can be melted and used directly. Ultrapure iron, which could be produced in the Moon's vacuum and which would not rust even in the moist oxygenated air of a lunar habitat, may prove to be a valuable structural material. Other metals, including titanium and aluminum, are abundant on the Moon but are bound in oxides and silicates so that their extraction is more difficult.
In an early lunar base, the air, water, and food to support human life will have to be supplied from Earth. As experience is gained, both in a LEO space station and on the Moon, recycling will become more practical, allowing partial closure of the life support system and greatly reducing resupply needs. At some point, local raw materials can be introduced into the cycle. This may be one of the first uses of lunar oxygen and of hydrogen implanted in lunar soil by the solar wind. Then, on a larger scale, lunar materials may be used as a substrate and nutrient source for agriculture. Asteroids can supply substances, such as carbon compounds and water, in which the Moon is deficient. Asteroidal water may be particularly valuable, if no ice is discovered on the Moon and if the hydrogen trapped in lunar soil proves to be impractical to utilize.
A more complete understanding of lunar and asteroidal resources will require additional exploration. Such exploration can be done without making any decision to commit to utilization of nonterrestrial resources and will provide important new data which will help in making such decisions. We therefore recommend that NASA's Office of Space Science and Applications (OSSA) and Office of Space Flight (OSF) jointly sponsor and conduct the study, analysis, and advocacy of two automated flight missions: a lunar polar geochemical orbiter and a near-Earth asteroid rendezvous, each having a combination of scientific and resource-exploration objectives. Both missions could use spacecraft similar to the Mars Observer now planned for launch in the early 1990's. Also, to evaluate the resource potential of Mars and its moons, Phobos and Deimos, we recommend that the Mars Observer data analysis be planned to include resource aspects, such as the potential for in situ propellant production.
Lunar resource exploration might proceed in one of three ways:
Once serious planning for the use of a particular body of lunar material begins, it will be necessary to determine the extent of the potential mine in three dimensions. New instruments for probing to modest depths beneath the lunar surface may be required. We therefore recommend that limited depth mapping be included among the objectives of any lunar surface exploration mission.
Asteroidal exploration might proceed by sending an automated lander and sample return mission to the most favorable near-Earth asteroid. The asteroid rendezvous would have to be preceded by an Earth-based search for the right asteroid. The search for near-Earth asteroids, and their characterization by remote sensing using groundbased telescopes, is a good example of a scientific activity with strong implications for the use of nonterrestrial resources. This work is now going on with a mixture of private and public support and could readily be accelerated at low cost.
Laboratory research, on a relatively small scale, using lunar simulants could yield fundamental knowledge important in choosing which technology to develop for the extraction of lunar oxygen, hydrogen, and metals. At similar levels, useful research could be done using meteorites to assess the technology needed to process asteroidal materials for water, carbon, nitrogen, and other volatiles. We recommend that NASA encourage such materials research.
It is a finding of the present study that the processing of nonterrestrial materials, though conceptually understood, has yet to be reduced to practice despite numerous past studies, recommendations, and even some laboratory work. In view of the long lead times characteristic of projects bringing new raw materials sources into production, we believe that more active preparations will soon be needed.
Though laboratory research in this area, as outlined above, is necessary, there are some processes that are ready for technology development and competitive evaluation at pilot-plant scale both on Earth and in space. A logical next step would be processing demonstrations at reduced gravity in the space station and ultimately on the Moon. An example of the needed technology would be a solar furnace designed to extract oxygen and structural materials from lunar soil on the surface of the Moon (see figure 8).
So much remains unknown about the behavior of the living systems (humans, microorganisms, plants, and animals) that will occupy the space habitats of the future that this is a research field with a very likely payoff. As in the case of inorganic materials, some aspects of this problem have already come past the research stage and are ready for technology development and evaluation. We recommend that NASA's Office of Aeronautics and Space Technology (OAST) support biotechnology work in two areas: (1) plant life support and intensive agriculture under simulated lunar conditions, leading to experimental demonstrations on a 1/6 g centrifuge in the space station, and (2) biological processing of natural raw materials, lunar and meteoritic, to concentrate useful substances (fig. 13). Some such techniques are already in use on a large scale in the mining industry on Earth.
Products derived from the processing of space resources will be used mainly or entirely in the space program itself, at least up to our reference date of 2010. Plans and methods should be developed with this in mind. We do not find any early application of nonterrestrial materials or products made from them on the surface of the Earth. Rather, these materials can accelerate progress at any given annual budget level and thus increase the space program's output of new information, which continues to be its main product.
We found that, while Mars and its moons (fig. 14) almost surely provide a large resource and thus offer the best prospects for sustained human habitation, the most likely use of martian resources would be local; that is, in support of martian exploration and settlement rather than for purposes elsewhere.
