Space Transport and Engineering Methods/Lunar Development

The Moon is obvious to anyone who has looked up at the sky. The same reasons for expanding civilization apply to it as to other locations. They include access to new energy and raw material resources for upgrading civilization, and to meet the other objectives listed at the start of Section 4.1.

&emsp;However, we do not start developing the Moon right away for several reasons. First, we must travel through the Low and High Orbit regions to get there, then navigate the Moon's gravity field to reach closer orbits or the surface. This requires more energy and new transport equipment than the orbital regions. Second, the surface has different conditions than the orbital region around it, or the orbital regions around the Earth. So surface equipment also needs new designs. Third, developing the Moon is made easier with support from the previous orbital locations. So we delay the start of Lunar development until after Phases 4A and 4B have been started, and needed (R&D) for this phase is completed in Phase 0M. The Moon's surface is roughly the size of Africa and Australia combined, so it is much too large to be developed all at once. Once started, Phase 5A will continue in parallel with previous phases, with progressive upgrades and expansions over time, both in orbit and on the surface.

&emsp;Our concept exploration for developing the Moon begins with describing the characteristics of the region and an industry survey to identify possible future activities. Motivations, economics, technology, and prior developments elsewhere will drive which of these activities can get started and when. Combining this information we can identify a development approach and specific Lunar projects, place them in approximate time order, and link them to other phases and projects. In the course of doing this, we identify needed R&D for Phase 0M, which is recorded in that section.

Lunar Region Features
The Lunar region includes the Moon itself, and orbits with average distances (semi-major axis) within 35,000 km of the Moon's center. At this distance the Moon's gravity exceeds the Earth's by 50% and is the dominant local force. Lunar orbits in general are unstable due to the influence of the Earth and Sun. The Moon also has mass concentrations from past impacts that create an uneven gravity field. Particular orbits, where these effects are minimized, can be stable for long periods. Otherwise propulsion must be used to correct them, or the orbit allowed to evolve over time. Since the Moon orbits the Earth, the Lunar region is embedded in the larger High Orbit region around our planet.



&emsp;As noted above, horizontal surface area of the Moon is quite large, 37.93 million km2 or about one quarter of the Earth's land area. This does not include the sloped exposed area from large-scale topography (Figure 4.12-1) and numerous craters and other small-scale features. See USGS Topographic Map 3316 of the Moon, 2015 for a more detailed version.

&emsp;The Moon orbits the center of the Earth-Moon system every 27.3 days with respect to the stars (inertial frame), and 29.5 days with respect to the Sun, which sets the length of the Lunar day. The Moon makes one rotation per orbit period, and therefore keeps approximately the same side facing Earth. It is not exact because the Moon's orbit is not circular, it has a slight residual pendulum motion, and the Earth's diameter is 1/30 of the Moon's distance. That size allows varying views of the Moon, depending on where you are. Orbit durations around the Moon vary from 108 minutes close to the surface, to 6.8 days at the upper edge of the region. The main environment and resource features of the region are as follows:

Environment Parameters
Temperature - The Moon has the same average distance as the Earth from the Sun, so the incoming Solar Flux of 1361 W/m2 is the same. That sunlight is partly blocked by the Moon's shadow in lower lunar orbits, and blocked 50% or more on the surface on a monthly cycle. There is no significant atmosphere to moderate temperature changes on the surface. Objects exposed to space and direct sunlight can vary from 100-400K at the equator, and as low as 30K in shadowed polar craters. The surface is covered by particles separated by vacuum, and is an excellent insulator. Soil temperatures more than 30 cm deep are not expected to vary more than a few degrees from average, despite two week intervals of daylight and night. The average soil temperature is about 240K at the equator (Vasavada et al. 2012), and colder with increasing latitude and lower sun angles. Heat flow from the hotter interior has been measured at 0.008-0.03 W/m2, depending on location. The concentration of radioactive elements in the crust and good insulation properties leads to a somewhat uncertain thermal gradient of 10-50 K/km with depth.

&emsp;Temperatures in orbit will mainly depend on the percentage of time in sunlight vs in the Moon's shadow, and how much light is reflected or emitted from the Moon at a given distance. The Earth appears about 2 angular degrees in size from the Lunar region, and thus fills less than 0.01% of the sky, and makes a small thermal contribution. The remainder of the sky is the cosmic background at near absolute zero (2.7K), which serves as an effective heat sink. Since orbits can be elliptical and change over time, the resulting thermal environment can vary significantly. The albedo, emissivity, and thermal properties of an orbiting object will determine how much heat is gained and lost from environment conditions, and what equilibrium temperatures they will reach.

Atmosphere and Water - As noted under temperature, the Moon's Atmosphere barely exists. Its total mass is less than 10 tons, which is the Earth's atmosphere over a single square meter. If pressurized environments are needed for people or equipment, the gases need to be produced locally, or imported. About 50 kg/s of solar wind particles flow through the region at high velocity. Since this is spread over an area of 3.85 billion km2 it would be difficult to collect in useful amounts. An estimated 600 million tons of water ice has been discovered in permanently shadowed craters near the poles. The remainder of the Moon's surface has 10-1000 ppm of chemically bound water in minerals. The dry and airless state of the Moon is due to the low escape velocity, coupled with extended periods of early heating from impacts, tides, and radioactive decay.

Ground Loads - Ground strength for surface structures and transport is adequate to excellent. The surface consists of broken rocks and dust of various sizes, which compacts a few cm but then can support heavy loads. Excavation, mining, or drilling would reach the limits of overlying rock strength at ~38 km. Depths below this require support structures. Much of the shallower depths consist of fractured material, and will also need support against movement and collapse.



Gravity Level - Surface gravity averages 1.625 m/s2, or 1/6 of Earth's, with a total variation of 0.0253 m/s2 according to location (Figure 4.12-2). Free-fall conditions in orbit produce no effective acceleration. Structural support needed against gravity is therefore much lower than for Earth. The gravity levels required for long-term human health and plant and animal growth are not yet determined, but are likely to be more than natural surface or free-fall levels. Artificial gravity on both the surface and in orbit can be generated by rotation, otherwise stay times may be limited. Centrifugal forces from rotation will impose significant design loads on structures.

Radiation Level - Unprotected radiation levels on the surface and in orbit are in the range of 0.1-1 mSv/day, which could reach lifetime astronaut exposure limits in a few years. A single powerful solar flare can deliver lethal amounts of radiation, although most are much weaker. Radiation also causes long term damage and electronic faults in equipment. The radiation mainly comes from the Sun and cosmic background. A meter or more of cover by lunar soil or building underground can provide safe radiation levels on the lunar surface. Bulk mass of various kinds can provide shielding in orbit.

Communication and Travel Times - Round-trip (ping) communication times from Earth to the Lunar region varies from 2.2 to 2.94 seconds, depending on where in the region, and the Moon's distance in its orbit. This includes satellite relay time if you are communicating with areas behind the Moon that can't be seen directly from Earth. Travel time from Earth is nominally 3-4 days for people, by direct transfer orbit. Cargo delivery by electric tug is much more efficient, but also much slower. Slow transits without shielding or other mitigation would expose people to high levels of radiation while traversing the Van Allen belts.

Stay Time - Average stay times for people affect transport needs and the types of habitation required. Only nine trips with people have been made so far to Lunar locations, and those lasted only a few days each. These trips were made nearly 50 years ago. This data is too sparse and too old to establish an average for design purposes. Future projects will need to define stay times based on internal needs, until long-term habitation establishes an average.

Transport Energy - Reaching the Lunar region from the Earth's surface involves first getting to a high orbit that intercepts the Moon's location, then entering a stable orbit or landing on the surface. The first of these theoretically requires 62 MJ/kg, and entering orbit or landing requires up to 2.8 MJ/kg. These are ideal values for potential and kinetic energy. The actual expended energy depends on the details and efficiency of the transportation methods. The transport energies from Earth are high compared to the 10-20 MJ embodied energy of typical products or 15 MJ/kg of high energy chemical propellants. This favors local production where possible.

&emsp;Escape velocity from the Lunar surface is 2380 m/s, or 21% of Earth's. Therefore escape energy is only 4.5% of Earth's.  Low orbit velocity is 1680 m/s, so the difference to escape is an added 700 m/s. Orbit velocity at the upper edge of the region is 375 m/s, and escape an additional 155 m/s. These velocities and the associated energy are relatively low. Internal regional transport and reaching the High Orbit and Inner Interplanetary regions are then fairly easy from this region.

Available Resources
Energy Supply - The Sun provides a nearly constant flux of 1361 W/m2 at 1 AU. The Earth's orbit is slightly elliptical, and the Moon's orbit brings it closer to and farther from the Sun than the Earth. So the available energy in the Lunar region varies somewhat from this value. In full sunlight this energy is sufficient to process about 2 tons/m2/year of raw materials. Collecting the energy with solar panels or concentrating reflectors may only require 2 kg/m2 of equipment. So the primary energy return can be very high. Total solar energy flow through the region is 5.24 million TW, which is vastly greater than current world energy consumption of about 20 TW. Localized areas on the Moon contain up to 10 parts per million of Thorium and Uranium, which may be useful for energy production.

&emsp;Most of the Moon's surface is exposed to the Sun 50% or less of the time, depending on local topography. Higher latitudes get less sunlight per unit of surface area due to lower Sun angles. Orbits are exposed to sunlight 50% or more of the time, generally increasing with altitude. Dark periods are relatively short in orbit. Dark periods on the surface last half a Lunar day, or two weeks. Since direct sunlight isn't available for this time, alternate energy solutions are needed. Possible approaches include:


 * Ceasing Operations - High power operations can simply be stopped when sunlight isn't available, and minimal support provided by conventional sources like batteries. One of the following options can be combined with reduced nighttime operations, but not stopping them entirely.


 * Thermal Storage - This uses the vast amount of rocks and dust as the storage medium, and the natural vacuum as insulation. The material is heated during the day by sunlight, and the heat used to generate power during the Lunar night.


 * Nuclear Power - The surface is already bathed in high levels of natural radiation, so nuclear sources are not as much an issue as on Earth. A variety of nuclear designs are possible, with different power levels, and fueled from Earth or local sources.


 * Beamed Power - Lunar orbits receive sunlight when the surface does not. Energy can be directed to the surface using simple reflectors, microwaves, or lasers, with varying beam target size vs distance.


 * Transmission Lines - East-West transmission lines can deliver power from areas in sunlight to those that are dark. In the worst case at the Equator the lines would have to be 2700 km long to provide continuous power.  This is 1/4 of the Lunar circumference, with one line in each direction.  These are very long distances, and would only make sense when large-scale development is present.  Shadowed craters get no sunlight, but the crater rim may get adequate power levels.  A shorter transmission line or reflector can then deliver that power to where it is needed.

Materials Supply - Lunar orbits are essentially devoid of raw materials, so projects located there must import what they need. The Moon's surface has a reasonably well understood, Geology. See also the Lunar and Planetary Institute's Lunar Sourcebook (1991) for more detailed information. That understanding is from a number of lander and orbital missions, some of which returned samples for analysis, and from Lunar meteorites which were thrown to Earth by impacts. Broadly, the surface composition is oxide minerals of silicon, iron, calcium, aluminum, and magnesium, in order of elemental concentration, with 3-4% other elements. The Moon's surface is not uniform in composition (Lawrence et. al. 1998), with higher concentrations of Iron, Potassium, Phosphorus, Rare Earth, and Thorium found mostly in the Oceanus Procellarum region. There are few volatile (low boiling point) compounds left on the Moon. It formed in a molten state, then suffered many high energy impacts, and tidal and radioactive heating which kept it molten for extended periods. The Moon is too small to keep an atmosphere, so the volatile compounds mostly escaped. While molten, the denser materials sank to the interior, and lighter minerals accumulated near the surface. As the Moon cooled, the lighter minerals crystallized according to melting point, then further sank or rose according to density. Radioactive elements were preferentially concentrated near the surface, leading to additional melting or prolonged cooling. All of this resulted in a crustal layer about 50 km thick.



&emsp;Since the Moon retains no atmosphere, it does not slow incoming objects, and weathering does not occur as on Earth. The original crust which solidified by cooling was later heavily cratered and broken up, but fairly unmodified in composition. The surface is covered by a 2 to 8 meter thick Regolith (Lunar soil) made from the original crust plus impacting asteroids. This has been thrown around and mixed during crater formation. The fine regolith is interspersed with larger rocks and boulders (Figure 4.12-3). Because of the broken nature, the regolith is easily collected and moved. Since the layer is global on the Moon, roughly 380 trillion tons of material is available from this source. If this is insufficient, or localized but deeper ores are needed, conventional mining techniques can be used. Blasting is commonly used on Earth for bulk mining, but uses nitrogen-based explosives. That element is in short supply on the Lunar surface. Alternatives include supplying nitrogen compounds or ready explosives from elsewhere, artificial cratering by directing impacts from orbit, small scale fracturing using local rock and accelerators, or Plasma Torch cutting using oxygen as the carrier gas, where oxygen is the most abundant element in the Lunar crust. Maximum practical mining depth is limited by overlying rock pressure and crustal temperature gradients, but should be on the order of multiple km. It therefore would be on the order of a thousand times more raw material than the regolith layer.

Industry Survey
We can develop a list of possible Lunar activities by looking at all Earth industry categories, and identifying ones that can potentially operate there in the future. To these we can add any activities that are unique to the Lunar region. Existing industries are classified for statistical purposes by the North American Industry Classification System (NAICS). We will use their numbering system and sequence for our survey, and insert unique Lunar items where most appropriate.

11 - Agriculture: Local agriculture will be useful to the extent that people and other living things in the region need food, and it avoids transport from Earth or other space locations. Plants can also recycle waste products from people while producing food and other useful products. Filtered sunlight is widely available in Lunar orbit, but the long Lunar night on the surface, and lack of sunlight underground, may require artificial lighting for higher plants. Microorganisms may be more tolerant of long periods of darkness. Carbon (100-160 ppm), hydrogen (30-60 ppm), nitrogen (60-120 ppm) and water (10-1000 ppm) are rare in Lunar soils, and may need to be imported from other locations ( Fegley and Swindle, 1993 ).

21 - Mining: The Moon's surface is covered by an average of 10 million tons/km2 of already broken up rocks and dust, and 1.36GW/km2 of daytime solar energy is available to power the mining and processing operations. Mining appears feasible for local use, and to supply locations which take less effort to reach than from the Earth. Delivery of mined products to the Earth itself would be limited. The mineral oxides making up most of the Moon's surface are also widely available on Earth at low cost. There may be some rare materials that will be worth transporting that far, especially if very low cost transport is developed.

22 - Utilities: All activities in the Lunar region will need local power, which at first can be supplied by equipment such as solar panels and batteries brought from Earth. When higher power levels are needed, simple systems like concentrating reflectors and thermal storage can be made locally. As local manufacturing develops, more complex power systems can be built. Our civilization consumes a large and growing amount of energy. 4-15 times as much solar energy is available in the High Orbit region as locations on the Earth's surface, and it is not interrupted by night and weather. If that energy can be delivered at a low enough cost, there would be a large demand for it. The Lunar region is likely too far to efficiently generate and transmit energy to Earth, but can be a source of materials and products to do so in the High Orbit region. The available quantity of solar energy within reasonable transmission range (80,000 km orbit radius) is 27.4 million TW, or over a million times Earth's current total energy use.

23 - Construction: The Earth is large enough to support its human population from a purely physical space standpoint for the foreseeable future. At the density of Manhattan, a population of 10 billion would fit on 0.25% of the world's land area. Even though more space is not necessary, it would be desirable for a number of reasons. The Lunar region can supply some of that physical space. Activities in the Lunar region that involve people and other living things will require habitats to support them, since the natural environment cannot. Large orbital habitats and industrial facilities would be difficult to deliver in a single piece. Either on-site construction is needed, or the orbital equivalent of shipyards and tow ships. Large surface habitats and other facilities would be too difficult to move, and need local construction.

31-33 - Manufacturing: The range of possible manufactured products in the Lunar region is vast. The first to be made are likely to be those that use local resources, displace significant mass otherwise brought from Earth, and are fairly simple. Examples include bulk shielding, thermal storage mass for nighttime power, basic chemicals for propulsion and life support, and mineral products and metals for construction. Once starter sets of production equipment are delivered to the region, they can begin to bootstrap their own expansion, and increase the range of products made locally. Transport energies to and from nearby regions are low, so a wider range of material inputs and product exports for local manufacturing may be feasible.

42 - Wholesale Trade: Individual operations in the region will trade with each other and with other regions, according to the economic principle of comparative advantage. The Moon has large amounts of easily extracted raw materials. So those materials, or products made from them, are likely to flow to Lunar and Earth orbits where they are lacking, and where transport energy is lower than from Earth. Ownership and control of trade may be from Earth rather than local. Communicating with the ground is fairly easy from this region.

44-45 - Retail Trade: We do not expect significant retail trade to develop in the region for a considerable time. Early populations in the Lunar region will be there for scientific and industry purposes. Their employers will likely provide for their basic needs, and even budget for some optional items like entertainment media. As permanent habitation develops and a non-working population accumulates, there will be more time and resources that can be applied to personal choices, and non-essential specialties. Retail trade may then develop.

48-49 - Transportation and Warehousing Transport is required from previously developed regions in order to start any activity in this region. Such transport is accounted for in earlier phases from their starting locations on Earth or in Earth Orbits. Transport within the Lunar region and to farther destinations is accounted for in this phase. At first the transport equipment is all made elsewhere, but over time propellants, structures, and more complex items can be made locally. Transport includes infrastructure such as spaceports and surface vehicle hangars. Warehousing includes all types of storage, which will start with bulk items like unprocessed rock and basic chemicals.

51 - Information Transmitting information through space requires no mass and little energy. So many satellite systems already exist in Earth orbit purely for communications, and nearly all have communications systems to support their primary purpose. Communications networks will be extended to the Lunar region once significant operations begin there. Networking, communications, and information technology for internal use within the Lunar region, and to Earth and other regions, will be extensive from the start. The equipment used is complex and fairly low mass. So it is likely to continue to come from Earth.

52 - Finance and Insurance This will most likely be supplied from Earth, since ownership rights, contracts, and money are all non-material relationships and can be transacted remotely.

53 - Real Estate, Rental, and Leases The Outer Space Treaty prohibits claiming celestial bodies but allows peaceful uses in space. International agreements for the Space Station, and orbital slots and frequencies for communications satellites have set precedents for ownership and use of equipment in space. What is not settled is territorial rights to less than entire bodies. For example, if someone builds a mining operation on the Moon, or a colony in Lunar orbit, will they own a territory around it in the sense of land rights on Earth? A reasonable answer is exclusion of others for technical and safety reasons. Thus, someone else cannot build so as to shadow your solar collectors or damage your equipment with rocket exhaust. Until the legal questions are settled, we cannot say what the scope of this industry category will be. Like Finance and Insurance, whatever the scope is, it will likely be handled from Earth for similar reasons.

54 - Professional, Scientific, and Technical: Some research and development, especially scientific, is best carried out locally in the Lunar region. Such activity began in the 1960's, to a large degree for political reasons, and continues to the present. Scientific and increasingly commercial activity is expected to continue in the future in the Lunar region. Most of the people involved in space activity so far have been on Earth, building the equipment and operating it remotely. This is likely to continue for some time, but increasing numbers of people will visit and work in the Lunar region. They will be concentrated in scientific and technical fields at first.

55-56 - Management and Organizational Support: Business management and administration will likely be mostly handled from Earth. Support activities like orbital debris collection and radiation remediation are tasks for Earth orbits. These in turn may be supplied from the Lunar region.

61 - Education: This will likely be done in advance or remotely from Earth at first. Local education will be mostly limited to training for industrial operations. General childhood and higher education will be deferred until permanent habitation develops with populations of young people to teach.

62 - Health and Social Services: Health monitoring and first aid capacity are needed for people in the Lunar region from the start. At first, this will be by training the crews themselves, with remote monitoring and advice. As the number of people grows in the region, more dedicated equipment and health specialists can be supplied. Telepresence, artificial reality, and haptic robots are not currently good enough to do health care remotely, and may not ever be so from Earth, due to speed of light delays. Automated health care carried out locally may be possible. These depend on future R&D, so for now health care concepts would involve existing methods. Nursing and residential care is assumed to be provided on Earth at first, by returning people there. As population grows in the region, local facilities may be established. We assume social assistance will be done remotely or not be needed. Basics like habitation and food need to be provided to everyone in the region for them to live at all. Return to Earth is an option for anyone who needs social support.

71 - Arts, Entertainment, and Recreation: Entertainment will start out as remote delivery or software for people in the region, because their energy and mass are low. Active recreation would start with necessary exercise for health maintenance. Early Lunar landing sites may be preserved as historical locations, and unique sports may develop in the Lunar region, such as rover racing or low-g gymnastics, but these are speculative at present.

72 - Accommodations and Food: Living space, food, and drink must always be provided for people in the region. At first these will be provided by sponsoring organizations and self-operated. As local capacity grows and people establish long-term residences, there will be room for temporary travel accommodations, tourism, and specialty food and drink locations. Space tourism to Low Orbit has already happened, and proposed to the Lunar region, but it is a very limited market due to extreme cost. It does demonstrate there is an interest. The existence of the Adventure Travel market makes it likely there will be more tourists once costs become more reasonable.

81 - Other Services: This category covers miscellaneous activities not covered elsewhere. Repair and maintenance is highly desired for the Lunar region from the start. This is because the equipment is either expensive to replace or life-critical. Personal services will start out self-provided until populations are larger. Private and civic organizations are not needed at first, can be extensions of Earth organizations, or self-organized locally.

92 - Public Administration: At first, most of this industry category will be handled from Earth. Fire and public safety will start out self-provided, and develop as specialties as populations grow. Environment quality and monitoring would be designed into Lunar systems as necessities. Publicly funded civil and national security space activities have been the only ones carried out so far in the region, and they are likely to continue to be important. Government budgets for these activities are finite, while business and private ones are not limited in this way, so we expect the latter to dominate eventually. For the sake of efficiency, public and private projects should be integrated so as to support each other.

Project Drivers
Which of the above activities make sense for the Lunar region, and when, is a function of a number of factors. They include project motivations, their economics, available technology, and prior and parallel development. These drivers will change over time, affecting which of these activities get started and when.

Motivations
Motivations to develop the region can be personal, organizational, or social. Curiosity drives scientific exploration of the Moon itself, and support of other science in the future. Safety from natural and human-created hazards is another motivation. It has driven orbital weather and defense systems in the past, and may drive asteroid deflection and solar blocking systems in the future. The natural Lunar environment is devoid of life and full of radiation. So moving hazardous activities off the Earth is another safety-driven motivation. The profit motive is ever-present in the business world. The unclaimed material and energy resources in the region are a potential source of profitable activity. [add list of human motivations and those that apply]

Economics
What projects are feasible, and when, is partly driven by available technology and support from previous phases. But it is also driven by the economics of the projects themselves. Economic principles like comparative advantage and returns on investment will still apply in the Lunar region. The first advantage of the Moon is relative closeness to the Earth, and to Earth Orbits which are already in use. Communications and transport times are shorter than for farther regions. Next is the low energy to reach nearby orbits, due to the Moon's smaller mass and position high in the Earth's gravity field. Transport "downhill" (towards the Earth) is eased by slow aerobraking with the Earth's atmosphere. Slow aerobraking uses multiple passes to lower heating and avoid damage. Transport "uphill" (away from the Earth-Moon system) is eased by using the Moon for gravity assists, when both departing and arriving. Lastly, large amounts of raw materials and solar energy are available in the Lunar region to enable projects.

&emsp;While large amounts of materials are available in the Lunar region, they are not a complete source for all possible needs. The Lunar surface is deficient in volatile compounds, which escaped, and denser minerals and native iron, which sank to the interior. A fully developed economy would need to supplement local materials from elsewhere, driving trade. This includes Near Earth Asteroids, and from lower orbits or Earth. Near Earth Asteroids are a good choice for bulk materials because of the low energy to reach and return from them. Lower orbits and Earth are the likely source for higher value and finished goods, because of the more established industries there. In the long term, other sources may end up being economic. Costs tends to increase with the energy consumed in a task. So trade can make sense when the transport energy from elsewhere is less than the mining and processing energy from scarce local sources. Trade, in turn, drives a need for low cost transport.

&emsp;Earth already maintains a complete civilization. So physical products from the Lunar region are not likely to compete with Earthly ones on cost. Exceptions include scientific and collectible samples, where their value specifically comes from their Lunar origin. Information is low mass and fairly easy to deliver, so may have significant economic value when coming from the Moon. Energy is a very large market on Earth, and solar energy is much more available in Earth orbit. If it can be delivered economically, that would be a spur to space industry in general, and for exports from the Lunar region to support it.

Technology
As of 2016, launch costs from Earth to the Lunar region were about $55 million/ton. This is somewhat higher than the price of gold/ton, which severely limits what kinds of projects are feasible. The raw wholesale energy to reach the Moon is only $1000/ton, which shows how much room there is for improvement. A number of new transport systems are in development. The SLS may eventually reach a cost of $1 billion/flight for 55 tons to the region using all chemical propulsion, or $18 million/ton. The SLS and an efficient electric tug from low orbit could deliver about 105 tons for $1.2 billion, or $11.5 million/ton. The Falcon Heavy can lift 64 tons to Earth orbit, and with an electric tug could deliver 50 tons to the Lunar region for about $200 million, or $4 million/ton. These are improvements, but still leave costs very high. For comparison, the mid-2017 price for silver was $0.5 million/ton. Any transportation measured in multiples of a precious metal price is still an impediment to significant development.

&emsp;We therefore have a strong incentive for two kinds of technical improvements: systems with even lower transport costs, and use of local resources to avoid transportation. Both require extensive R&D to get the new technologies ready before attempting large-scale lunar development. Both improvements are useful for other locations. So we expect their development to be started in earlier program phases, and upgraded as needed for Lunar development.

Prior and Parallel Development
The technology improvements noted above will not all happen at once, so different lunar activities and projects will become feasible at different times. This will be in parallel with other program phases on Earth and in space, and other development in the Lunar region either before or in parallel. To the extent that transport costs to all parts of space are still high, it favors high value/low mass activities, and those where economic returns do not matter, such as publicly funded research and exploration. Low and high Earth orbits will likely continue to be more developed than the Moon, and therefore drive the improvements in transport and resource use. High thrust systems may be adapted from other regions for initial Lunar surface access. Improvements like Skyhooks, which are initially developed for Earth orbits, can later be adapted for Lunar surface access. Asteroid processing may start at L2, which is outside the Lunar region, but additional locations near or on the Moon may be added later. These locations can use the same or upgraded equipment.

Development Projects
We can now start to combine the above information into a general approach for Lunar development, and identify specific projects. We can provide initial concepts for these projects, which will gives a sense of their scale and main features. But this is merely a starting point, and does not exclude alternate ideas. It also barely begins optimization and integration of the projects to each other and other projects elsewhere.

General Approach
The Moon's surface is the size of two continents on Earth, and the orbital region has a cross section of 3.85 billion km2, or 7.54 times the Earth's total surface. This region is far too large to develop all at once, or by a single project or organization. Our general approach is then to identify a number of smaller tasks and projects. They can be individually carried out by different organizations and put into a logical sequence. They would interact with each other when they exist at the same time, with other program phases, and with the rest of civilization.

&emsp;The tasks can be grouped by start times into preparation, orbital, and surface development. There are already a number of current and near-term projects aimed at the early tasks. We note these next, and their existence should be integrated into plans for later projects. We don't have an integrated sequence yet for specific longer-term projects. So they are listed by primary function (production, habitation, transport, or services) and location (orbital or lunar surface) instead. Within each category we list them them in approximate start time order. We expect earlier projects to overlap later ones in the same category, rather than be completely replaced.

Preparation - Planning and designing future Lunar projects requires understanding the relevant features of the region in general, and of specific operating locations. Preparation for Lunar development therefore involves tasks like exploration, surveys, prospecting, and site investigation. Scientific exploration of the Moon began as soon as telescopes were available to observe it in detail. It accelerated once rocketry enabled placing instruments in orbit and on the surface, and bringing back samples for analysis. The orbital region is now well enough understood to begin projects there. For the surface, surveys and prospecting of the Moon as a whole are ongoing, but detailed work for future surface activities is incomplete. We assume devices like surface rovers will be sent before substantial development of a particular location. They would do detailed surveys, prospecting, and subsurface investigation.

Orbital Development - Lunar orbit is easier to reach than the surface, especially at first. So we assume orbital development begins earlier than the surface. Orbital locations generally don't need physical preparation, as they are devoid of natural materials. Use of a given orbit can thus begin with delivery of equipment. Smaller satellites, such as for communications relay, can be delivered as a complete unit from Earth. Larger installations like an inhabited station can use sections produced on Earth, and assembled in Earth orbit or in their final location. In either case, an electric tug can deliver it slowly but efficiently. People would first travel by faster chemical rockets to avoid radiation exposure. The same electric tugs can fetch asteroid rock from nearby orbits. It can first be used as-is for bulk radiation shielding. Processing equipment brought from Earth can then start making simpler products, like propellants, oxygen, and water, out of the asteroid materials.

&emsp;We don't yet know the best path for further development. A lot more R&D is needed on this subject, but for example we can follow history and use iron and steam to bootstrap a growing economy. Metallic asteroids contain mostly iron, alloyed with varying amounts of nickel and cobalt. Chondrite asteroids have up to 20% water and carbon. The carbon can be added to the iron to make steel, and the water heated with focused sunlight for steam power. A starter set of production tools would convert the metal stock to parts for power units and more production machines, bootstrapping their own expansion. They can also make structural parts, pressure vessels, and other items to widen the range of production processes. This may be a simpler way to bootstrap than producing solar panels at first. Later mining and transport from the Lunar surface would increase the quantity and diversity of source materials. Rare materials and hard-to-make items will continue to come from Earth, but a decreasing percentage as production builds up. New people, remote control of Lunar systems, and support services are still be provided from Earth.

Surface Development - Lunar surface work starts later than in orbit, but continues in parallel afterwards. Sites on the surface are typically not ideal in their natural state. So construction sitework can begin after prospecting and site investigation. This would use some combination of robotic, automated, and remote controlled machines delivered from Earth. Propellants produced in orbit make their delivery easier. Some kinds of projects don't need large numbers of people. They can continue with remote operation, and perhaps short visits by crew for maintenance. Short visits and smaller long-term populations can feasibly be supported by deliveries from elsewhere. Delivering everything for larger populations and industrial operations is inefficient. Local mining, processing, and fabrication would be built up over time to support them.

&emsp;Like for orbit, the best path for growth on the surface needs extensive R&D. It could begin with delivery of ready production equipment, then use starter sets to bootstrap further expansion. Depending on size and cost, local production of simple products may be started as early as the remote operation stage. These can be stockpiled for later development. Even after people are supported part or full time on the surface, you can continue to use machines to augment their work, using self- or remote-control. Depending on the level of automation, there may be high numbers of such machines per person. This approach will leverage the limited early human population.

Current and Near-Term Projects:
Transport from Earth - Direct transport from Earth is being worked on by NASA through their Space Launch System (SLS) and Orion Spacecraft projects. The SLS provides the capacity to deliver large payloads to the Lunar region (and other destinations), and Orion for carrying people. Other Chemical Rockets, which are in varying stages of operation and development, can also deliver payloads to the region. Only the larger ones can deliver significant payloads unaided. The Scaled Composites Stratolaunch carrier aircraft is part of a hybrid turbofan/chemical rocket transport system and can potentially support Lunar missions. The combined cycle SABRE airbreathing/rocket engine for the Reaction Engines Skylon spaceplane is in early stages of development. Skylon is intended to carry 15 tons to low orbit, enough support Lunar missions.

Transport from Low Orbit - Chemical rocket performance is limited by the available energy of the propellant. It is used for the Earth to Low Orbit transport segment because it provides high enough thrust to overcome the Earth's gravity and prevent the trajectory from intersecting the surface again before reaching orbit. Once Low Orbit projects are built in Phase 4A, slower but much higher performance alternatives can be used, and transport infrastructure built up from Low Orbit to the Lunar region. Even with chemical propulsion, system cost can be lowered compared to the current practice of single-use transport. This is possible if a cheaper propellant source is available or the hardware can be used multiple times. Multi-use orbital transport vehicles are generally referred to as Space Tugs by analogy to tugboats, which push ships and barges around.

Missions to the Lunar Region - There have been 64 successful public Missions to the Moon or involving the Lunar region since 1959. Notable among them were the Apollo missions which carried people and brought back 382 kg of samples. About 20 more public and private probes are under development, intended to launch between 2017 and the 2020s. Two short-term crewed missions are also in development. A dozen more probes and crewed missions are currently proposed but not yet in development. The NASA Deep Space Gateway is a crew-tended station planned for the 2020's in a halo orbit near the Moon. The European and Chinese Space Agencies have proposed Lunar surface bases by the 2030s. Both may involve international and private participation. We expect public projects in the region to continue to evolve, but be limited by available funding.

Long-Term Projects:
As noted in the general approach above, a lot of research and development is needed before we can make definite plans for long term Lunar development. What we can do now is identify potential projects and what work is needed to prepare for them. They can be put together into a plan, but this should be considered preliminary and very likely to be revised as time goes by. Projects are grouped by function and location for convenience, then in approximate time order.

Lunar Orbit Production
Lunar orbits require less energy to reach than the surface, and are easy to reach from the High Earth Orbit region because the Lunar region is embedded in it. So we expect orbital production to precede surface production. Early production can be an outgrowth of public projects such as the Deep Space Gateway. These projects will begin with assembly of pre-made components brought from Earth. Local production can then be added incrementally. Production can start with pre-made tools and equipment, then bootstrap its own expansion by using the seed factory approach. A starter set of equipment is used to make some parts for its own expansion, with the remainder imported. Over time, more can be made locally, to the limit of what makes economic sense. Production outputs can be used locally in orbit, delivered to the surface, exported to existing markets in the High and Low Orbit regions, and exported to farther destinations. We do not expect much export to Earth from the region because civilization is well developed there and costs are likely lower. Major production steps include:

Materials and Energy Sources - There are no significant materials sources in Lunar orbit, so they must be imported. Despite greater physical distance, Near Earth Asteroids are the easiest source of materials at first. This is because electric propulsion can be used, which is very efficient, and the Moon can be used for gravity assists, reducing propulsive delta-V. Neither is available for early transport to and from the surface. Asteroids also have more varied compositions than the Lunar surface, allowing a wider range of products. When transport infrastructure such as catapults and skyhooks become available (see Lunar Transport below), the lower cost, higher volume, and shorter delivery time from the Lunar surface becomes an advantage, shifting the balance to more local materials supply. Some materials are rare in both asteroid and Lunar ores, and would still need to be supplied from Earth.

&emsp;Orbital locations have solar energy available 50-100% of the time, mostly as a function of altitude, while surface locations have it 50% or less, decreasing with latitude. Since the embodied energy to make products is typically larger than the kinetic energy to move materials from the Lunar surface to orbit, production can generally be faster in orbit. Most of the markets for orbital production will be local or other orbital regions, and require less energy for delivery than from the surface. Orbital production should continue to be favored for these reasons.

Materials Processing - Some products, like bulk radiation shielding, don't require further processing, just delivery. But most require conversion of raw materials to finished stock by mechanical, thermal, chemical, electrical, or other methods. Processing can start with the easiest and simplest methods. An example is water extraction from Chondrite type asteroid rock, which only requires heating a container using concentrated sunlight, and a shadowed condenser to collect the evolved vapor. Metallic asteroids are dominated by native iron-nickel-cobalt alloy, which makes up more than 95% of their mass. However individual samples vary in composition, and may have undesirable trace elements, lack other desirable alloying elements, or contain rocky inclusions. The Stony-iron group have higher percentages of silicates and less native metal. Processing is therefore generally needed to produce uniform alloys in desired sizes and shapes. The stony fraction of stony-irons, and the pure Stony or S-Type asteroids are typically silicate minerals which contain various metallic elements. These require extensive processing to extract desired products.

Orbital Fabrication - There are many known fabrication processes to convert stock materials into finished parts. These include all the known methods used on Earth. To these we can add some unique methods which take advantage of zero gravity, vacuum, and full-spectrum solar energy. Which ones are suitable for Lunar orbit and in what sequence to develop them will requires a lot of R&D. At a minimum, processes known to work on Earth can be used unchanged in space by providing a normal atmosphere and artificial gravity. However, these may be quite massive and inefficient in orbit.

Orbital Assembly and Construction - This can begin with prefabricated elements delivered from Earth and assembled in orbit, such as space stations and larger and more powerful satellites. As local production develops, the orbital products are less constrained by launch mass and cost from Earth, so they can use simpler and heavier designs. The Lunar orbit region is a useful location to combine raw materials supply, energy supply, and parts delivery from Earth or lower orbits. It may therefore develop as a major assembly and construction site. Large projects like space colonies may be too massive to move once constructed. In that case, smaller elements may be produced in Lunar orbit, then delivered to their final location for construction and installation.

Lunar Surface Production
Production systems on the surface can make some items without using imported materials. The relatively limited variation in Lunar ores means a wider range of products will need significant outside supplies. Landing on the Moon requires significant propellant at first, so imported supplies have a higher cost. So early production will favor those items that can be mostly or entirely made locally. Once efficient two-way transport is available (see Transport below), large amounts of raw materials and unfinished goods can be exported for orbital production. More finished goods and materials can also be imported for surface production. This allows a wider range of items to be made on the surface, and results in a trading network that benefits both locations. The preferred locations for surface production depends on availability of energy and material sources, local conditions, transport capacity, and the intended destinations and uses for the products. For example, a scientific outpost may not want industrial operations nearby if they cause unwanted disturbances. Possible products include:

Minimally Processed Regolith - Early Lunar mining and construction would be in support of near-term public scientific and exploration projects such as a surface station. This would include clearing and leveling building sites and access paths; then excavation, placement, and covering of station elements with unprocessed local regolith for radiation, thermal, and debris protection. Debris protection is both from natural meteoroids, and materials thrown by lander exhaust. Covering may be by simple loose piling of material over a support structure, and not require processing beyond sorting for rock size. Paving and blocks may be produced by melting the soil with concentrated sunlight.

Water Extraction - The Moon is devoid of known liquid water, but Lunar Water is known to exist in two main forms. The first is chemically bound as hydrates and hydroxide minerals. High temperatures are required to extract the water from this source. The concentrations are ~10-1000 parts per million. The second is water ice trapped in permanently shadowed craters near the poles, where temperatures are low enough to preserve it. Asteroids can contain up to 20% water, and Hydrogen can be combined with abundant mineral oxygen to produce 9 times its mass in water. So import is an alternative to the low concentrations and limited locations of local sources. In the long term, water is abundant beyond the mid-Asteroid Belt Frost Line, where temperatures have stayed low enough to retain solid ice. If low cost transport is available, they would be a preferred source over the limited Lunar sources. Of course, Earth has a great deal of water, but transporting it requires a lot of energy.

Native Iron Production - An early production path may be to mine and process native Lunar iron, which is present at about 0.5% of Lunar soil. If this process works, it may be feasible with fairly simple and low mass equipment, giving an early return on hardware delivered to the Moon. The soil is sieved for small grains, and iron-bearing particles are selected by magnetic separation. The remainder are used to create sand molds. A robot smooths a patch of ground, spreads the non-iron grains over them, then compacts and makes depressions for molds. The iron-bearing particles are placed in the mold cavities, and a large concentrating dish focuses sunlight on the cavities, scanning them sequentially if needed. The iron particles are often attached to other mineral grains, but being denser it will sink to the bottom of the mold, and glassy slag will rise to the top. The result would be basic shapes like plates and bars, with slag stuck to it on both sides from melting.

&emsp;Depending on the melting point of iron vs other minerals, it may be necessary to select for refractory ones for the molds. One way to do this is to heat samples until the less refractory components sublimate. Another is to prospect for suitable refractory source rocks. A third approach is to bring a highly refractory crucible in which the grains are melted, and pouring off the slag and iron separately. In that case the mold is hot for a short period and will not melt much. R&D is needed to determine which, if any of these approaches will work. The cast metal stock will need to be sand-blasted to clean off adhered slag and mold grains. Fortunately there is no shortage of mineral grains on the Lunar Surface to do that with. You now have an inventory of iron stock to use for all types of construction and parts making.

Chemical Processing and Metallurgy - More complex processing may require more equipment, but yield a variety of stock materials from a given amount of Lunar ore. There is generally about 43% oxygen in any given soil, so any of several reduction processes can be used to extract it. Oxygen is useful as a propellant, and for life support. The soil averages about 21% silicon, which can be used for solar panels and in steel for transformers and motors. Combined with oxygen it makes quartz, and with other elements makes common glass for mirrors and windows. The soil averages about 24% iron, aluminum, magnesium, and titanium, which are useful structural metals. Lunar basalt, which makes up most of the maria regions, can make high strength basalt fiber. So with full processing, nearly all of the soil can be turned into useful products. However, the soil is low in certain elements like carbon, hydrogen, and nitrogen. The first is used in making steel, and all three are needed for anything organic. These may be imported rather than extracted from low-grade ores. Early markets will likely be to previously developed regions, and more solar energy is available for production in higher Lunar orbits. So we expect early production will mostly be for export to Lunar orbit and beyond, with production for local use gradually increasing.

Helium-3 Mining - Mining of Helium-3 from the Moon has often been suggested for terrestrial power, because it has low radiation by-products in fusion reactions. There are several problems with this idea. The first is that Deuterium/Helium-3 fusion is about ten times harder than Deuterium/Tritium. We don't yet know how to sustain D-T fusion, much less do it economically, and we are even farther from doing the harder D-He3 fusion. So we simply don't need it right now. The second is He3 is implanted on the Lunar surface from the Solar wind. However this source is diffuse, and this isotope is extremely volatile. The net abundance is therefore only 3-15 parts per billion on most of the Moon. We would therefore have to process a billion tons of Lunar soil to get 3-15 tons of product.

&emsp;The abundance is 1000-6000 times higher in outer gas giants because their atmospheres contain 15% (Uranus) and 19% (Neptune) Helium, and therefore proportionally more of the He3 isotope. The higher ore concentration requires proportionally less tonnage of mining and processing to extract a ton of product. Although the outer planets are much farther than the Moon, if we need He3, then D-T fusion would already be solved, and it can power ships to reach to reach those planets. Since those atmospheres are mostly hydrogen, the Deuterium isotope is widely available. Tritium can be made from He3 by neutron bombardment. The bulk hydrogen and helium from the atmospheres can be used as propellants. Thus mining the outer planets can be self-sustaining. The third problem is relative energy content. Pure He3 can supply 200 TJ/kg, but at Lunar concentrations the mined soil can only supply up to 3 MJ/kg in reaction energy. This is only about a tenth the energy in fossil fuels and a fifth that in wood. So it would hardly be worth mining on Earth, much less the Moon.

&emsp;Silicon and aluminum make up 28% of typical Lunar soil. Assume only 10% of the soil mass is extracted to these elements and made into solar cells and structure for power satellites. The satellites will produce about 100 W/kg, so the net power per mined kg of soil is 10 W/kg. A nominal life for such cells is 15 years in orbit, during which they would produce 4.75 GJ/kg of energy. This energy can be beamed to Earth around the clock. So if you wish to mine the Moon for terrestrial energy supply, you can generate 1500 times as much per kg via solar power than He3 fusion, even at low elemental extraction efficiency. Space solar cells degrade due to radiation damage. However the damaged cells represent a high quality ore for reprocessing. It should take less energy to do this than the original extraction of silicon from rock, and the cells already produce much more energy than used to produce them. So in principle we can extend the life of the satellites indefinitely. In that case their energy production is only limited by the life of the Sun.

&emsp;As noted above under Available Resources, some regions of the Lunar surface contain up to 10 parts per million Uranium and Thorium. These elements were concentrated in the crust by differential processes as the Moon cooled from a molten state. The mined ore has an energy content of about 800 MJ/kg, or hundreds of times the He-3 energy content. So even for nuclear power on the Moon or other space locations, He3 is not the most efficient source. Solar is even higher energy content, but there are situations where nuclear power is useful. These include spanning the long Lunar night, in shadowed craters, or for portable power, and for destinations far from the Sun where solar power is weak. The conclusion is mining He3 from the Moon does not make sense for any purpose we can see.

Surface Fabrication and Assembly - Like for other program phases, we use a bootstrapping process to build up our production capacity. We begin by importing ready-to-use tools and equipment, such as a mobile solar furnace for paving and block-making. We then add a starter set of machines (seed factory) capable of fabricating parts for additional production machines, as well as other end products. Machine tools such as lathes and milling machines are designed to make metal parts from metal stock. Modern ones are computer controlled, and are themselves mostly made out of metal. So an initial set of machines, when supplied with local stock materials, can make many of the parts for more machines. Given plans for a wide variety of machines, the initial set can increase in both size in complexity. For example a forging press or rolling mill are useful for making some types of metal parts, but those are likely too heavy to bring to the Moon to start with. But if you have a good supply of local materials, you can build them later when needed.

&emsp;The starter set cannot make all types of needed parts, so the remainder continue to be imported. As the collection of machines grow, imported parts and materials decrease as a percentage, though they may increase in absolute amount. Parts are then assembled using some combination of robotics, remote-control, and direct human labor. Early assembly may be as simple as stacking blocks for a radiation shelter, but more complex products will need dedicated assembly space. Robots and factory buildings are themselves products which can be fabricated and assembled, so the entire production system can grow itself over time.

Electric Power - Energy is needed to operate all kinds of production on the Lunar surface. This includes electrical power and process heat. Early electric power systems can be brought as complete systems from orbit. This may be photovoltaic solar panels or nuclear. Mass is not an issue for a stationary facility, so solar-thermal using local materials is an expansion option. Complex equipment such as a steam turbine-generator set can be delivered. Reflectors to provide concentrated sunlight can be made locally. These can be sheet metal with an evaporated aluminum coating to make them more reflective, or in a simpler version pre-made reflective sheets are delivered and only the support structure is made locally. The percentage made locally can increase over time as the range of production processes grows. Water and rock from local sources supply the working materials. A container is filled with Lunar rock and a little gas for heat transfer. This is surrounded by sized regolith particles for vacuum insulation. During the day the container is heated with concentrated sunlight, and the heat transfer gas directly used to boil water and run the generator. At night, the stored heat in the rocks continues to heat the gas. Large amounts of rock are available on the Surface, so we can provide two weeks worth of storage, and a crater can supply a convenient place to put the container and insulation.

Process Heating - Many industrial processes need heating in some form. This can also be produced with concentrated sunlight the same way as for electric power, but substituting a suitable heater or furnace at the focus rather than a thermal storage device. Different processes need different amounts of heat, and the temperature for a given process can vary over time. This can be provided with an array of steerable mirrors directed to chosen targets as needed, and a blocking shutter for fine control. The furnaces and heaters would likely want to be stationary for a number of reasons. Since the Sun moves in the sky, the mirrors have to be steerable anyway, so directing them to different targets adds little complexity to the design.

Lunar Orbit Habitation
[TBD]

Lunar Surface Habitation
Habitation in general is the facilities that allow people to occupy a given place. On Earth this includes homes, and the parts of commercial and industrial facilities designed for people's needs. For example, rest rooms and break areas in a factory are designed for people, while the production machines are designed for whatever products the factory makes. Since the Lunar surface is hostile to human life, habitats there must protect from the natural environment and provide all of people's basic needs. This includes air, water, food, temperature control, sleep, sanitation, and others.



Near Term Habitats - Habitats for near-term science and exploration missions to the Lunar surface will likely be based on pressurized modules, like the current International Space Station uses. They would be pre-fabricated and delivered from Earth, then assembled on-site. This will require equipment to prepare the site and move the pieces from where they land to their final location. These may be either rigid-shell or inflatable design. Modules and other equipment may need protection from daily temperature variations, radiation, meteorites, mechanical and electrostatic dust transport, and human-caused hazards such as lander rocket exhaust. A general approach is to use arched structures similar to ones used on Earth (Figure 4.12-4). These are covered by Lunar soil and rocks, either in natural form or formed into blocks. The structure allows access to the outside of modules for maintenance and additional storage and work space for movable items. Since low gravity is known to be harmful over extended periods, crew time would be limited to a year or less. For small numbers of people it is feasible to supply them from orbit.

Long Term Habitats - Habitats involving longer stays or permanent residence, and larger numbers of people, will need different designs. People can withstand cramped quarters for short times, but for psychological and personal reasons they will want more space for long-term stays. To maintain health they may need centrifuges to create artificial gravity. An example design would be a large habitat dome for spaciousness, and a centrifuge built around the rim for living or exercise needs. Residents would spend enough time in the centrifuge to maintain health, but could work and enjoy the low gravity the rest of the time. We have essentially no data on how much gravity is enough between zero and 1.0. We know the body deteriorates over time in zero gravity. So as a worst case, people would need to spend most of their time in a one gee centrifuge of some type, but this subject needs more research.

&emsp;Large habitats can be assembled from pre-made sections delivered from elsewhere, or use local production of structural elements. Until sufficient capacity for people is in place on the surface, it makes sense to do much of the work via remote-controlled robots. Stations in Lunar orbit or at one of the nearby Lagrange points would have short communications time. This may allow better real-time control than from Earth. Since the natural environment is hostile to people, safety is a critical design factor. This would include multiple layers and compartments to contain atmosphere, and emergency breathing apparatus in multiple locations. It would also include failsafe or multiple life support systems. Failsafe design would still support residents in case power or equipment fails, though at reduced capacity or duration. Growing plants has been demonstrated in zero gravity, so it may be possible to grow food in natural Lunar gravity. Research is needed to make sure it is feasible and does not create problems in the food products. We also need to determine if Lunar soil can be used or is even desirable, lighting for the plants with an abnormal day length, and how to supply organics and other plant nutrients.

Lunar Orbit Transport
Lunar orbit transport includes projects and systems based in this orbital region. Ones which are based in previous regions, such as Earth orbits, are assigned to their respective program phases.

Reusable Landers - Early landings on the Moon do not have the support of much infrastructure, so they require high thrust chemical rockets to navigate the Lunar gravity field. For transport down to the surface, Carbonaceous type asteroids contain up to 20% carbon compounds and water. This can be reformed chemically to hydrocarbons and oxygen, which is a common rocket fuel combination. They would be produced at a high orbit location where there is full-time sunlight for power. An electric tug delivers them to low Lunar orbit for efficiency. A propellant depot then fills a reusable lander which delivers people and early cargo to the surface. Oxygen is the most common element on the Moon, and a number of ways of extracting it from oxide minerals have been studied. An oxygen plant to refuel for the return trip can reduce the round-trip mass ratio from 2.9 (LOX/CH4) to 1.96. This increases payload per trip from ~25% to ~40%.

Orbital Tugs - These are needed for efficient transport within Lunar orbits, and to and from these orbits to other regions. Transport to the High and Low orbit regions can be relatively low energy. In some cases the Moon can be used for gravity assists to lower orbits, and aerobraking used to lower apogee. Gravity assists can also be used to escape from the Earth-Moon system, but upwards transport generally requires more propulsion. Gravity and drag do not help in changing orbits around the Moon, but total velocity requirements are fairly low. We expect most of the propulsion will be electric. Orbital tugs will likely be developed first for Earth orbits, and just copied for Lunar uses.

Lunar Skyhooks - Lunar basalt and carbon from asteroids can be turned into high strength fibers. These can be used for a large rotating structure known as a "Skyhook", where the rotation at the tips cancels part or all of the orbit velocity. For an orbit velocity of 1564 m/s, which corresponds to an altitude of 262 km, a radius of 250 km produces 1.00 gees at the tip while canceling the orbit velocity. The low point of rotation is then 12 km above the surface, which misses any mountains and allows for the irregular gravity field. Vehicles can then arrive and be dropped off at zero horizontal velocity and 12 km +/- the elevation of the landing point above the surface. This will require minimal propulsion to land and take off. 1.00 gees at the tip allows a crew to live comfortably in orbit, and make trips to the Lunar surface with very low fuel use. A "frozen" orbit at 86 degrees inclination minimizes orbit perturbations and allows access to nearly all of the Lunar surface. However it limits the times when a particular location can be accessed. An equatorial orbit would have more frequent access but more limited surface coverage. Unlike Earth, the Moon rotates so slowly that the benefit of its own rotation in reaching orbit is negligible.

&emsp;An orbital Skyhook only makes economic sense if the traffic to the Lunar surface is large enough to justify it. This will not be true at first, and bulk material from the surface can aid in its construction, so it would not be built right away. There is a one-time cost in mass to build a Skyhook, but afterwards it saves most of the required lander propellant. A Skyhook can be built incrementally, and provide partial savings before completion, which affects the economics. At the other end of the rotation, the tip is moving faster than Lunar escape velocity, and can therefore send vehicles to a large range of orbits by choosing the radius and time of release. Catching and releasing vehicles affects the orbit of the structure, but if traffic is balanced in direction it is a temporary change. Ballast mass can be provided at the center to decrease the orbit changes, and zero-gravity activities may accumulate there in any case. If traffic is more in one direction than the other, the difference can be made up by electric propulsion. Since low gravity is known to be harmful, an orbital centrifuge can allow crew to mostly live in normal gravity, and operate equipment by remote control with short short communication times. At the same time, the centrifuge can provide easy access to the surface when needed. It is not purely a transport system, but more like a spaceport in the sense of an airport on Earth is a hub for surrounding activity.