To Mars and Beyond: on Becoming an Interplanetary Civilization
(page 2) Phases 4-6: Space Locations
This illustration is an early concept for a rotating space station, made 5 years before the first orbital launch by Sputnik 1. Note the ring-shaped solar collector, which would heat a fluid to produce power, since solar cells were not well developed at the time. The reason for including it is to remind us that we can only design using what we know today. The approach we describe below may look equally out of date 60 years in the future. Despite that, we need to start somewhere if we are to make progress.
7.0 - Orbital Locations
The previous phases developed self-expanding factories, and used them to improve life on Earth and solve some pressing problems. The next phase tries to solve some longer-term problems by continuing the expansion beyond the Earth.
Current & Near Future Space Industry - Long-term problems don't have an immediate economic return, so the first reason to use seed factories and other advanced technologies in orbit is to support existing space industry. This includes about 1400 active satellites around Earth, and a few beyond it. Total space industry as of 2015 was US$ 334 billion of economic activity, split between government and private projects. They provide a ready market to justify the first investments in mining and manufacturing in orbit. Lower cost transport to orbit is being actively pursued, and should open up some new markets like low-orbit Internet relay, private space stations, and space tourism.
Sustainability Problems - We currently depend on fossil fuels for 86% of civilization's energy sources. While they have enabled growth from pre-industrial levels to our current state, they have the side effect of adding greenhouse gases, mainly CO2, to the atmosphere. These gases reduce infrared radiation to space, and thus alter the balance between incoming sunlight and outgoing heat. Venus' surface temperature of 462C is an example of why this is a bad idea. Only 50C of Venus' higher temperature is due to being closer to the Sun. The rest is primarily heat trapped by a thick CO2 atmosphere. We must transition this century to renewable non-carbon energy sources, lest we make the Earth more like the hell that Venus is.
Ironically, a lack of sufficient energy means we cannot afford, as a civilization, to capture and reprocess all waste materials, nor extract new materials from abundant low-grade sources. Rather, we get new materials from ores, which are natural concentrations that take less energy to process. However, high-grade ores are in finite supply, and if we do not reprocess all used materials, those ores will eventually run out. So the current course of civilization is unsustainable. We will either cause severe problems by increasing temperature, run out of necessary materials, or both.
Environmental Stress - At the same time, the world's population is growing, and not everyone has obtained the benefits of full development. Such development demands even more energy and materials, and it isn't fair to deny these benefits just because some people were not the first to get them. A growing population with a high standard of living will put more stress on the remaining natural world, and deplete accessible resources faster. Using seed factories to build more renewable energy and reprocess more wastes has less impact on Earth than current methods, but it does not have zero impact.
Using Space Resources - Eventually it becomes easier to access the abundant resources of space than, for example, putting more solar panels in climates unsuited to them, or digging ever deeper underground to find quality ores. There is seven times as much solar energy in nearby space as the average location on Earth. Atmospheric absorption, night, and weather account for the difference. There are also a wide variety of material resources in space, some of which are quite scarce on the Earth's surface. These resources can be mined with high mass ratios in terms of tons mined vs tons equipment. The combination of large amounts of energy and raw materials would allow rapid bootstrapping of space industry, and relieve the sustainability and environmental problems of a finite Earth.
Civilization-Level Risks - A final reason to expand into space is protection from civilization-level risks to Earth, and backup of civilization if we can't. One such risk is runaway greenhouse warming due to positive feedback loops. One such loop is melting ice caps exposing darker water or land, increasing the absorbed sunlight and causing further melting. Another is release of methane from hydrates and organic matter in permafrost. Methane is a strong greenhouse gas, and thus could lead to accelerated release. If terrestrial solutions prove inadequate, one answer is orbiting sunshades. They block part of the Sun's light from reaching Earth, and would help cool the planet.
Large asteroids fly past Earth fairly frequently, and occasionally hit us, causing widespread damage. There are enough craters, recorded impacts, and near misses to know it is a low probability event, like your house burning down, but we would like to have some insurance against the risk. Diverting or dispersing large objects headed towards Earth would reduce or eliminate that risk. Note that small objects either burn up or cause limited damage, and don't require action to prevent. There are any number of other existential risks to civilization, besides ones coming from space. Examples would be genetically engineered plagues and nuclear war. An active civilization beyond Earth then serves as a backup.
- Cost of Space Operations
Supporting space industry and dealing with the problems listed above depends on making the necessary tasks affordable. Currently it is very expensive to get to even the lowest orbit, much less distant parts of the Solar System. That is an engineering problem, though, not a fundamental limit due to nature's laws. For example, potatoes at retail cost about USD $1000/ton, and the wholesale electric energy to put those potatoes in Earth orbit (8.7 MWh) only costs $450, less than the potatoes do. Sending even cheap bulk commodities to space would be affordable if we could do it with high efficiency. Current launch costs are quoted at a minimum of $1.7 million/ton, or 3750 times the wholesale energy cost. This represents how much room there is for improvement.
There are a number of ways current costs can be reduced. Reaching space requires a good deal of equipment on Earth, historically rocket factories, rocket hardware, and launch sites. Seed factory type automated production that makes more of itself can lower the cost of this equipment. As with other industries, you can begin with a seed factory, and grow it until you have mature factories that produce the space hardware you need. Currently, rocket and satellite hardware is mostly used once and thrown away. That is a major contributor to the high cost. Improved technology can allow using the vehicle hardware multiple times, lowering transport cost. Conventional rockets are inherently inefficient. They require about 40 times the payload mass in fuel (LOX/RP-1), and thus 387 MJ/kg of fuel energy. The payload ends up with 32 MJ/kg of kinetic and potential energy, so the efficiency is only 8.3%. Alternate launch methods can dramatically improve on this value, but there must be sufficient traffic to justify their development.
Automated production in space, using materials and energy already there, can reduce how much has to be launched from Earth as much as 98%. The launch cost component of a finished product in space is then reduced by that ratio. If the production costs in space are less than the launch savings, then overall costs go down. More efficient space technologies, like electric propulsion and closed life support, further reduce the mass needed for fuel and supplies. The closer we can source items to the desired orbital destination, the less effort is required to move them there. So a Lunar or Mars base would ideally get most of what it needs locally, or from not too far away, and only get from Earth what can't be found otherwise. The combination of all these methods would bring us much closer to "potato cost".
- Space Expansion Process
The seed factory approach applies to developing space, just like on Earth. In general the path used for difficult and extreme locations on Earth is followed in orbit, although the details differ. Space is nearly undeveloped and barely populated, so we must first deliver some finished equipment to supply energy and gather raw materials. We also deliver core machines to process metals and other basic materials. These are used to build parts for more equipment, and habitats for people and food production. Other materials are then processed with the added equipment to produce products for other industries, in an expanding sequence. Once a given location has matured enough, it can build a new seed factory starter set and more finished equipment. These are sent off to the next location. This creates an expanding wave of development as far as people wish to carry it.
In the early stages there are fewer production machines available at a given location. So a larger percentage of parts and materials must be delivered from previous places, or from Earth. As more equipment accumulates, a higher percentage of products can be produced locally. Eventually a location can start exporting a surplus of items it can make. These are traded for those it can't, or that require materials that are locally rare. By export and trade it becomes economically self-supporting, and therefore no longer a cost burden. This is in contrast to a station or base that can't make things locally, and has to be supported from Earth at a continuing cost. Self supporting locations become part of an expanding civilization growing throughout the Solar System, and in the long term, beyond it.
- General Orbital Features
The orbital locations in Phase 4 begin 160 km altitude or more above the Earth, where stable orbits are possible. They range beyond that to the limits of the Earth's gravity, then interplanetary orbits at all distances from the Sun. The environment conditions, raw materials, and available energy vary widely across the different orbit regions. We therefore divide this phase into six sub-phases by region, and expect to develop them in order by distance. Orbital locations include smaller bodies like asteroids, but larger bodies with significant gravity wells are in the next major phase. The environment parameters for Phase 4 are generally more difficult than extreme ones (Phase 3B) on Earth. The added difficulty is a matter of degree, not orders of magnitude, and not in all parameters. Proper design and operation can deal with most of these difficult conditions. They include:
- Temperature - Distance from the Sun and percent field of view of the Cosmic Background at 2.7K are the main determinants of temperature. It can range anywhere from above 700K to below 50K (> 425 C to < -225 C). Orbits in shadow, or night on a surface, can produce wide temperature swings. Reflection from the surface of a body, or underground surroundings, will modify the ambient temperature.
- Water Supply - Orbital locations don't have a weather system that delivers a renewable water supply like Earth. But some bodies have a fixed amount of water in the form of hydrates or ice. Liquid water layers may exist inside dwarf planets, and water in general is abundant beyond the "frost line" (2.8 AU), where temperatures are low enough for ice to be stable in a vacuum.
- Atmosphere Pressure - Vacuum is the normal condition in open space and on the surface of most bodies. Very low but not zero pressure (~0.001% of Earth) occurs on some of the larger dwarf planets.
- Ground Pressure - This doesn't exist in open space, and is generally low even for larger asteroids, either because of low gravity or low surface strength. Soil pressure can become significant if you go deep enough into larger asteroids.
- Energy Supply - Varies from extremely high (> 10 kW/m^2) in close solar orbits to very low (<1.5 W/m^2) beyond Neptune. Asteroid and dwarf planet surfaces reduce this by 50% or higher, due to night sides and geography like mountains and craters. Orbits through a body's shadow will temporarily block sunlight and reduce the percentage time it is available. Nuclear power is possible using fuel delivered from Earth or mined from places like the Moon. Power can be transmitted over moderate distances using various frequency beams, or even simple reflectors.
- Gravity Level - Natural gravity ranges from zero in free orbits, up to ~3% of Earth's on dwarf planets. Artificial gravity by rotation can be used where biology requires it, or industries function better with it. This can range up to full Earth gravity, and the scale is only limited by structural materials.
- Radiation Dose - Most orbital regions have high natural levels of radiation from galactic cosmic rays, solar wind, solar particle events (flares), and the Van Allen radiation belts around Earth. These can be lowered to safe levels for people and other living things by using bulk mass as shielding. Solar panels and electronics are less sensitive to radiation than living things, but can still be damaged or temporarily upset. Shielding mass can come from asteroids or be imported from moons. Transport vehicles can use fuel, water, or other supplies as shielding. Large habitats may get enough shielding from their outer structure, equipment, and storage tanks.
- Ping Time - Round trip communication time varies greatly from milliseconds in low Earth orbits to hours or even days in the outer Solar System. Essentially all the communication time is due to speed of light delays. On Earth, fiber-optic cables transmit at about 2/3 the speed of light, and follow indirect paths from geography and a spherical planet.
- Travel Time - This also varies greatly according to orbit region. It can take a few hours to reach low Earth orbit or return from it to the ground, plus some travel time to other population on the Earth's surface. Travel to the outer Solar System using known propulsion systems will take a number of years or even decades.
- Stay Time - This is currently very short because there are no permanently inhabited locations in orbit. Astronaut crews typically stay about 6 months aboard the International Space Station. With the development of larger, more permanent locations, with artificial gravity, food supplies, etc. the stay times can increase to a number of years.
- Transport Energy - Much less than 5% of civilization is in space, so transport energy is measured from the Earth's surface to an orbital location. This is a minimum of 31 MJ/kg, set by the physics of reaching low Earth orbit. It is a lot higher using current rockets, because their operating efficiency is low. More distant orbital locations require additional energy to reach, and currently this reduces cargo mass dramatically, producing even higher transport energy/kg.
Sections 7.1 to 7.6 cover phases 4A to 4F. They describe the orbital regions in more detail, how they would be developed by following the general expansion pattern, and some of the methods adapted to local conditions. The start of each successive phase is staggered in time, but a region does not have to be fully developed before starting on the next one. They grow in parallel once started.
7.1 - Phase 4A: Low Orbit Locations
- Low Orbit Features
Earth orbits form a continuous range from the minimum set by atmospheric drag to the maximum set by solar gravity. We divide that range in half by the transport energy required to reach it, at 2700 km average altitude. Conditions are different enough between low and high orbits to get their own phases. Orbits can be elliptical and constantly vary in altitude, so we use the average between the high and low points, which is called the semi-major axis. Low orbits are about 22-40% in the Earth's shadow, which reduces available energy. The Earth fills a large part of the field of view, which affects the thermal balance, and lighting on the sunlit side. Orbit periods are 2.5 hours or less, and thus travel time from the Earth's surface is fairly short. Ping time by way of ground stations is under 20 ms, and thus not difficult. However low orbits have a limited view of the Earth's surface at one time, so communications are often relayed via a higher satellite. This adds about 500 ms to round-trip communications, which is a noticeable delay. The Earth's magnetic field traps particles into radiation belts that begin at low orbits and extend past them to high orbits. Unprotected people and electronics are damaged by their high radiation levels. So satellites need to avoid these belts, cross them quickly, have sufficient shielding, or deplete the belts artificially. The material resources of low orbit include the upper edge of the Earth's atmosphere and debris from inactive satellites and rocket stages. Other materials have to be imported from Earth or higher locations.
- Economic Uses
We want all phases of our program to become economically self-supporting. Low orbit is already actively occupied by a number of satellites. Current uses include Earth observation, such as weather, mapping, and agricultural monitoring. Other satellites are used for communications relay from low orbit, and that is likely to increase in the future. Government uses include research, such as the Space Station and Hubble Telescope, and national security. Future uses include tourism, orbital assembly and maintenance, and a transfer and refueling point to other destinations. Development of low orbits would thus start with the existing markets, and expand to new ones as costs are reduced and local industry built up. We discuss some options for development in this section, but detailed plans are yet to be done. We don't expect large scale habitats in low orbits, because more resources and energy are available higher up.
- Transport from Earth
Low orbit does not have many raw materials, so most things have to be imported. Rockets and space hardware have existed for decades, but they are too expensive for many future uses. As noted in section 7.0 Cost of Operations, seed factories on Earth can help reduce costs by building and running aerospace factories and launch sites more cheaply. Reusing rocket hardware can improve costs substantially. However, conventional rockets are only 8% efficient in terms of fuel energy to delivered payload energy. The efficiency is limited by chemistry and the mass of the Earth, and will not change. A shift to different launch technologies can get around the efficiency limit, but there must be enough traffic at sufficiently lower cost to justify the R&D investment.
Hypersonic Guns - There are many possible launch technologies - see part 2 of ST&EM for an extensive list. One promising example is the hypersonic gas gun, which has been used in research for decades (Figure 7.1). They are inexpensive to build compared to most aerospace hardware. This is because they don't have to fly, and can therefore be made of heavy industrial parts, and guns are basically simple devices. A larger version of such a gun, on a mountainside with the correct slope, can supply 50-70% of orbit velocity for rugged cargo that does not care about high g-forces. Those include bulk fuel, water, structural parts, even frozen food. More delicate cargo and people would use gentler transportation. Industrial factories grown from starter kits can supply the construction equipment to build on the mountain, the high pressure pipe and other parts for the gun, and an energy source to compress and heat the gas for launching.
Muzzle velocities much higher than ~4 km/s (half of orbit) become increasingly less efficient. This is because of limits on expansion rate of the working gas, and increased drag and heating while climbing through the atmosphere. The remaining part of orbit velocity is then supplied by an internal rocket engine on the projectile. Because it only supplies half or less of the velocity, it is many times smaller relative to the cargo. The efficiency is in the range of about 66% in fuel energy to added payload energy. The gun also consumes energy, but is comparatively cheap to operate because it is stationary on the ground. The projectiles are rugged and durable, and can be used many times. Re-entry is much gentler than the initial launch. The gun barrel and other parts are a heavy industry design, and also durable. So the construction cost of the gun is spread over many launches. The operating cost per ton to orbit should therefore be lower than conventional rockets. Humans and more delicate hardware would still have to travel by low-g methods, but a gun can offload a large portion of the mass to orbit, and there is no requirement that everything has to travel the same way, any more than it does here on Earth.
Other Launch Methods - Other technologies to get payloads into orbit include high-speed air-breathing engines for the early part of flight, and orbital structures with suborbital landing platforms for the later part. Both are more efficient than chemical rockets in their respective velocity ranges. Unfortunately neither can easily serve the whole job of transport to orbit, so a combination of systems, each operating where it works best, is likely the answer. High speed jet engines and orbital platforms would require substantial R&D, and are therefore not likely to be the first things you build. Instead, making current rockets cheaper to produce and used multiple times is the first thing to do, then supplement with a low R&D system like a hypersonic gun. Once new markets are opened up by the lower costs, the more advanced technologies that need more R&D can then make economic sense.
- Mining and Production
Low orbits are not ideal places to process raw materials, because that requires a lot of energy and sunlight is blocked a good percentage of the time. However there is sufficient energy to fabricate parts and assemble them to finished items. For example, spools of high strength fiber and metal wire are rugged enough to be launched by a hypersonic gun. These can be wrapped/plasma-sprayed in layers around inflatable/collapsible forms, to build up large, lightweight structures. Large pressurized volumes built this way can then house other production and assembly equipment and people to work there.
There are some material resources in low orbit that don't require a lot of processing. At altitudes of around 200 km it is possible to "scoop mine" the upper atmosphere. A collection scoop funnels incoming air, which is very thin at this altitude, to a vacuum pump and compressor. A portion of the air is expelled through electric thrusters at much higher velocity than the incoming flow. This makes up the drag from the scoop. The remaining air is stored in tanks. When the tanks are full, the mining ship climbs higher and unloads to storage tanks at a depot. The reason to mine like this, rather than launching gases directly from Earth, is solar arrays that power the mining ship can produce 180 times as much energy as used in a rocket to deliver a given payload mass. So launching solar arrays rather than tanks of oxygen results in much more usable product in the end.
A steady supply of air (or nitrogen and oxygen if separated) obviously has use to keep humans alive in orbit. It can also fuel tugs that collect dead satellites and debris from the Earth's "debris belt" - the region around Earth where they have unintentionally accumulated. The collected materials can be scavenged for parts or recycled into new products. Since dead satellites and empty rocket stages are made of aerospace materials, they don't require a lot of processing to reuse. At the same time, collecting these objects reduces the orbital collision hazard from them. The collection is only feasible with a cheap source of propellant and electric thrusters. The debris is in random orbits, and would consume too much propellant to gather otherwise. Air and debris mining can provide enough materials to support some production in low orbit. The larger sources from the Moon and asteroids, and the full time solar energy in higher orbits leads to the majority of production being done there. Low orbit industry then ends up supporting local end users, and as a transfer point from the ground to elsewhere.
7.2 - Phase 4B: High Orbit Locations
- High Orbit Features
We define high orbits as extending from 2700 km average altitude to the limit of the Earth's dominant gravitational influence (Hill Sphere), which is about 1.5 million km. Although this is a large range of distances, it only represents the upper 25% of energy between the Earth's surface and escape. That's because gravity is an inverse square force and weakens rapidly as you increase distance. The Moon is the most prominent feature in high orbit. It has its own area of dominant gravity, with a radius of about 60,000 km. Additional energy is needed to descend through the Moon's gravity field to low orbits or the surface, and conditions are different there. So distances within 35,000 km of the Moon are assigned to the later Phase 5A - Lunar Locations.
High orbits are in sunlight 85-100% of the time, reaching the highest values when farther from the Earth and Moon. Temperature is determined mostly by the Sun and the cold Cosmic Background, but at the lower altitudes the Earth contributes a significant amount of reflected light and infrared heat. Orbit periods range from 2.5 hours to 7 months, so travel times by the most efficient routes can be long. Direct paths can be much faster, 12 days or less, at the expense of additional energy. Ping time varies from as little as 25 ms, which is not difficult, up to 10 seconds, which has a large impact on voice, real-time control, and electronic data. The upper part of the Earth's radiation belts, solar, and cosmic radiation create high to dangerous levels for people, without added shielding. Energy resources are abundant in this region, but material resources are low in their natural state. The Moon and Near Earth Asteroids can supply materials with fairly low transport energies.
- Economic Uses
The most popular satellite orbit, geostationary, at 35,000 km altitude, is in this region. This orbit has a period of 24 hours, which matches the Earth's rotation. Therefore satellites stay above a fixed ground location, and ground antennas can be stationary rather than having to track satellite motion. Synchronous orbit is in the outer fringe of the radiation belts, so manufacturing and human habitation tends to want to be higher up. Delivery, refueling, and maintenance of high orbit satellites is the main current market in this region. Likely the next step is supplying fuel and other supplies back to low orbit and for early interplanetary locations. Future industries are numerous, but depend on bringing costs down to affordable levels. This would happen incrementally as production for early markets bootstraps to larger levels. The total solar flux through this region is 500 million times what our civilization uses in 2015, and just the Moon can support a billion years of mining at the whole world's current rate. A small fraction of these resources can make our civilization sustainable for a very long time.
- Transport from Other Orbits
Since high orbits are low in materials, they must be imported from elsewhere. The current method for transport from Earth uses a rocket to reach low orbit, then another rocket or electric propulsion to reach higher orbits. Electric propulsion is about ten times more fuel-efficient than chemical rockets, and is being used more in recent years. Electric thrusters require large amounts of solar power to operate, but efficient and lightweight solar panels have been developed in the last few decades (see NREL Efficiency Chart, Dec 2015 but frequently updated). However they are low thrust, and would expose unprotected humans to high radiation levels while slowly crossing the radiation belts. So some transport will have to be by alternate methods. Large amounts of propellant obtained in space relieves the penalty of doing this.
Bulk materials mined from Near Earth Asteroids are not time- or radiation-sensitive. They can be transported entirely by electric thrusters on tugs that make multiple trips. Since part of the product from these asteroids is more fuel for the tugs, the transport becomes self-sustaining once started. A tug can return about 750 times its hardware mass over a 15 year working life, while consuming about 17 times its mass in fuel over the same period. Tugs can also deliver hardware and finished products to other orbits as needed. The Moon is small enough that bulk materials can be tossed directly into orbit by an electric centrifuge. At 50% efficiency and 50% duty cycle from lunar night, a solar panel can power throwing 1000 times its own mass per year for 15 years. If the centrifuge is not too massive relative to the loads it throws, the overall mass return ratio is high. From low Lunar orbit, electric tugs take over and deliver the materials for processing. We want to source raw materials from both the Moon and Near Earth Asteroids, because they have different compositions.
- High Orbit Production
High orbits have abundant solar energy. It can be converted to electricity by solar panels or thermal generators, and used directly for heating using concentrating reflectors. Modern space solar panels and reflectors are very light weight relative to their power output, because they don't need to withstand gravity or weather. An initial stock of these power sources is enough to get early production going. Later expansions would be mostly self-built. The simplest product of all is radiation shielding for human crew. This only requires some crushing and sorting, then packaging into suitable containers around crew modules. Shielded modules allow extended crew stays in high orbit. The crews can operate a satellite maintenance and refueling station, and assist with early materials processing and production. To some extent the crew will be helped by remote control from Earth.
Next in difficulty are water and carbon compounds, from Carbonaceous-type asteroid. This requires 200-300C heat, which reflectors can supply, and a container and condenser to capture the vapors. Water and carbon can be chemically reformed to Oxygen and Hydrocarbons, which is a common high thrust rocket fuel. This is useful when transporting people through the radiation belts or for landing on the Moon. Water, carbon, air mined in low orbit, and possibly rock for soil can supply greenhouse modules, so that crews can produce their own food and recycling life support.
A higher temperature furnace can melt metallic asteroid pieces, add carbon to make steel, and then cast into basic shapes. With a supply of basic metal shapes, a seed factory that includes machine tools can then start making parts for additional machines. Basalt fibers made from Lunar basalts, and carbon fibers made from asteroid carbon compounds are very high strength. Other products would be vapor-deposited reflector sheets and parts for radiator panels. These are combined with high concentration solar cells from Earth to supply electricity at lower launch mass than complete panels. The same parts can be used to make furnaces and cooling systems for thermal processing of materials. Early production would therefore use a mix of pre-made processing equipment, like furnaces, and a growing set of equipment made on orbit. These will output a growing range of products, starting with fuel and other bulk supplies, and basic construction materials. Orbital industry can transition from importing modules and other station parts to building them locally, then exporting habitats to other destinations.
Ultimately large, comfortable space habitats can be built as permanent living space. These can be grown in layers, like an onion. Each layer adds a new compartmentalized pressure shell outside the previous ones. The outer few shells are in vacuum, and provide radiation, meteor impact, and thermal shielding. Inwards of that are pressurized areas with storage and mechanical equipment. Then comes living quarters and a central open space. As new layers are added, items are moved outwards to fill the larger space. Compared to building a large habitat all at once, this spreads the construction cost over time, and the habitat is only expanded when extra space is needed.
7.3 - Phase 4C: Inner Interplanetary Locations
- Inner Interplanetary Features
These locations are detached from the Earth's dominant gravity and orbit the Sun instead, although they may pass close to the Earth at times. They range from as close as equipment can function near the Sun to 1.8 AU, which is just beyond Mars' greatest distance from the Sun and where the Main Asteroid Belt starts. It excludes the four inner planets (Mercury, Venus, Earth, and Mars) and close orbits around them. Solar power is available 100% of the time in these orbits, but the intensity varies from 31% to many times that near Earth, depending on Solar distance. Ambient temperature correspondingly varies from very hot to 244K (-29C) for dark objects, less for bright or reflective ones. Travel time from Earth can range from months to years depending on orbit and propulsion method, and whether gravity assists from the planets are used. These save fuel, but usually require extra time. Solar and cosmic radiation are a moderately high background, with occasional flares/solar particle events that are much more intense, up to lethal human levels without shielding. Ping time ranges from a few seconds for orbits crossing near Earth, to over 45 minutes at 1.8 AU on the far side of the Sun from Earth, by way of a relay satellite. The Sun interrupts direct communication to the opposite side.
As noted in section 1.0, there are over 13,500 known asteroids closer than 1.3 AU, and several thousand more out to 1.8 AU. The largest is over 30 km in diameter, with about 100 times the mass of all the rock ever mined on Earth. So total material resources are very large. Asteroid orbits vary in size, are typically not circular, and somewhat tilted with respect to Earth's, so the energy required to reach a particular one varies. Timing matters also, since everything moves at different speeds in solar orbits. Efficient travel depends on your vehicle and the target being at the same place at the same time. The composition of asteroids vary across about a dozen spectral classes, indicating different chemical compositions. Only ten asteroids of the size we might mine have been visited by spacecraft. That does not count Vesta and Ceres in the Main Asteroid Belt. So most of our knowledge is telescopic and from examining meteorites that have fallen to Earth.
- Economic Uses
There are not that many spacecraft currently in this region. They are mostly scientific probes in transit to other planets, or stationed at the Earth-Sun Lagrange points 1 and 2 (ESL-1 and ESL-2). Future use is likely to start with asteroid mining and delivery to high Earth orbit with electric tugs. Most known asteroids in this region are too large to move as a whole. This is more because we can't find very small ones than because they don't exist. So mining would involve scraping material or grabbing a boulder from the surface of these larger objects. Prior to mining, a prospecting mission should visit multiple candidate asteroids and find out what they are made of in detail. As high Earth orbits become more developed, they can start to send equipment and seed factories to this region in addition to mining tugs. Since raw materials and full-time solar energy are available, the seed factories can grow into full scale factories and produce habitats, vehicles, and whatever else is needed.
- Interplanetary Transport
The main transport system in this region is slow but efficient electric tugs. They can haul large loads of rock relative to their mass, up to 1000 tons for a 10 ton vehicle and 23 tons of fuel, but this depends on the orbit destination and velocity changes needed. They can also move crew habitats faster with a lighter load. Chemical rockets are used when fast velocity changes are needed, and solar sails may be effective in moving things even more slowly, but with no propellant use, once large lightweight reflectors can be made in orbit. Over time, a network of "transfer habitats" are built up. These are stationed in repeating orbits to particular destinations, and save having to move crew habitats each time for multiple trips. Centrifugal platforms can also be built over time to both provide comfortable gravity and fast velocity change with efficient propulsion. The platform mass serves as energy storage which can be transferred to payloads. Since the rotating structure can be relatively massive, it depends on large amounts of traffic to justify it economically, and production of high strength materials in orbit.
- Interplanetary Production
Worldwide energy use on Earth is about 18 TeraWatts, and includes mining, processing, and manufacturing about 2 million kg/s of materials. Thus the energy intensity of Earth civilization is 9 MJ/kg on average. We will double this to allow for recycling of materials in space, and add 8 km/s of orbit velocity change, requiring 300 MJ of electric tug power. That covers a reasonable amount of interplanetary orbits. Transportation is thus the dominant energy use for new materials that need delivery. A space solar panel today produces 177 W/kg at the Earth's orbit, and produces the required 318 MJ in 20.8 days. Given an average life in space of 15 years, their total energy output is 260 times that needed to transport their own mass in raw materials and run the rest of civilization, including making replacement panels. Concentrating reflector and nuclear power sources are not yet developed enough for space to do calculate energy return ratios. They may turn out better or worse than solar panels, but as long as we have one known energy source with a high return ratio, we can base space industry on it.
The same process of bootstrapping production in high orbits can be used in interplanetary space. This starts with mining for export, then simple products made locally, and gradually bootstrapping to more complex ones. As distance increases, fewer raw materials would be brought from the Moon, and more from nearby asteroids. These asteroids are of different types, which provides a reasonable variety of materials to work with. If we restrict ourselves to within 20 degrees of the ecliptic plane, to maintain access to the planets and keep velocity changes lower, we have access to 1/3 of the Sun's total energy, or 1.3 x 10^26 Watts. This is 7 trillion times our current energy use, a number so large it is hard to imagine it could not sustain civilization.
7.4 - Phase 4D: Main Belt and Trojan Locations
- Main Belt and Trojan Features
These orbits extend from 1.8 to 5.2 AU in size (see Figure 7.2). They include the Main Asteroid Belt, the Hilda family which are in 3:2 resonance with Jupiter, and the Jupiter Trojans which occupy the Lagrange regions ahead of and behind Jupiter. It does not include Jupiter itself and the region within 20 million km of it (see Section 8.4). Solar power is available 100% of the time except shadowed areas around and on asteroids. Intensity varies from 31 to 3.7% of that near Earth. Ambient temperature varies from 244 to 217K (-29 to -56C) for black objects, and less for lighter colored ones. Travel time from Earth is typically years, with high to lethal radiation levels for unprotected people. Ping time varies from 13 to 120 minutes, including a relay to avoid a direct path through the Sun.
Of the 700,000 known Solar System objects graphed in Figure 1.1, 98% are in this region, including the dwarf planet Ceres. Total mass is about 3 billion billion tons, which far exceeds the Earth's total mining output of about 60 billion tons/year. About half the total mass is in the four largest objects: Ceres, Vesta, Pallas, and Hygeia. Composition varies considerably between asteroids due to differences in their formation and history. Velocity to reach orbit from the largest body, Ceres, is only 270 meters/second, or 860 times less kinetic energy than from Earth. So all these objects are easy to access once you are near them. The main energy cost is in adjusting your orbit around the Sun.
- Economic Uses
This region is nearly devoid of spacecraft at present, so most uses are in the future. Abundant raw materials of diverse composition, and adequate amounts of energy when concentrated, will enable mining to start with, with materials shipped to earlier locations which are more developed and have higher solar intensity. When it makes sense to do so, seed factories can help bootstrap a full range of local industry, and eventually large scale habitation. There is enough material and energy in this region to support a full civilization.
- Main Belt Transport
The same transport methods can be used in this region as for the inner interplanetary region. The main difference is adding reflectors to solar panels, or larger reflectors to thermal power units, to make up for the lower solar intensity. Electric centrifuges are somewhat more efficient for injecting bulk cargo to transfer orbits, because they do point acceleration rather than spiral orbits. If a large asteroid absorbs the reaction force, they also don't need propellant.
- Main Belt Production
The inner parts of the Main Belt have enough sunlight for solar panels to produce power directly. In the outer regions, solar panels benefit from reflectors to increase the light intensity. Concentrating reflectors can produce higher temperatures at all distances, either for industrial processes or habitats. Increasing amounts of reflectors are needed as you get farther from the Sun, but they are inherently lightweight in a zero gravity environment with no weather. Note that the total amount of solar energy available in this region is no larger than for the Inner Interplanetary region. It is the same photons, only more spread out. The difference is access to larger amounts of raw materials.
Asteroids are covered in a mixture of rocks and dust of varying sizes. This is the result of repeated impacts over their life and gravitational attraction. In fact, some asteroids are so low in density that they must be "gravel piles", with no solid central body. Since most asteroids are small, the rocks and dust are easily disturbed and can become a hazard to mining and production operations. So attention has to be given how to carefully remove materials without too much disturbance. They are then moved elsewhere by a tug, or to a nearby processing plant out of range of any dust clouds created. For larger operations, an inflatable or assembled shell can surround the whole asteroid, keeping dust contained. Processing equipment can then be attached to the outside of the shell, and materials delivered continuously until the asteroid is consumed. Because dust and debris is contained, more vigorous mining methods can be used.
7.5 - Phase 4E: Outer Interplanetary Locations
- Outer Interplanetary Features
These orbits extend from 5.4 to 50 AU in size, and includes about 225 known Centaur class objects, which cross gas giant orbits. These range up to 220 km in size for 2060 Chiron (asteroids are numbered and sometimes also named). It also includes about 1500 known objects beyond Neptune, from 30-50 AU, known as the Kuiper Belt. This includes Pluto and several other dwarf planets from 850 to 2400 km in size. The remainder are in the range of 15 to 850 km, with the lower end set by our current telescopes' ability to find them. There are undoubtedly smaller ones that are undiscovered. We do not include regions close to Saturn, Uranus, and Neptune, which are accounted for in Phase 5E. Available solar power is low, from 3.7% to 0.04% of near-Earth values, and would require large reflectors to increase intensity, or using nuclear or other power sources. Ambient temperatures are very cold, from 217 to 70K for black objects, and lower for lighter ones. Travel time from Earth is typically many years, with high to occasionally lethal radiation levels for unprotected people. Ping time is 1.15 to 14 hours on a direct path.
Although the number of known objects is currently much smaller than the Asteroid Belt and Jupiter Trojan region, the mass is much larger. Pluto alone accounts for 13 billion billion tons, or more than 4 times that of the Main Belt/Trojan region, and the total Outer Interplanetary mass is 20-50 times that of Pluto. With increasing distance from the Sun, gases and ices with lower boiling points were able to condense from the original Solar Nebula, so this region has more water, ammonia, nitrogen and other frozen materials, along with rocks and metals.
- Economic Uses
This region is likely too far to use with present technology. When civilization has expanded through the previous regions, and better technology is available, the first use is likely to be mining of the large sources of raw materials, and bringing them back to inner regions where there is more energy to process them. This is far enough in the future that technology is likely to change dramatically in unexpected directions. Therefore we can't yet make intelligent estimates for how else these regions can be used.
Outer Interplanetary Transport
Due to weak sunlight in this region, we expect that nuclear powered propulsion, and gravity assist from the larger bodies, would be major ways to get around. If nuclear fusion has not been sufficiently developed, fission would be the only available nuclear source. There is a finite known supply of suitable radioactive elements on Earth and the Moon. To supplement them, artificial radioactives can be produced near the Sun, where abundant energy can power accelerators to convert non-radioactive starting materials. If nuclear fusion is well developed, there is abundant hydrogen from which fusion fuels can be extracted. As distance increases from the Sun, orbit velocities, and thus required orbit velocity changes, decrease as the square root of distance. Solar flux decreases faster, as the inverse square of distance. So solar sails become less effective than for closer regions.
Outer Interplanetary Production
We don't expect a lot of production in this region until technology improves. Ices like water and nitrogen are very useful to people, and found in large amounts in the outer regions. So mining and transport of them to inner regions is a possibility. Transport would be slow, taking many years, so there would need to be enough demand to set up a "pipeline" of cargo in transit, with vehicles at each end to set it on course and collect it at the end. The cargo can travel unattended in between, saving on vehicle time. Once the pipeline is filled, then cargoes arrive on a regular schedule. If fusion is well developed, a fusion-based economy may develop, with full production and habitation. We don't see a strong reason to live this far out rather than the warmer and brighter inner regions, but such reasons may develop.
7.6 - Phase 4F: Scattered, Hills, and Oort Locations
- Distant Orbit Features
These orbits extend from 50 AU to the limits of the Sun's dominance at about 100,000 AU. It includes the Scattered Disk objects (about 225 known so far), whose orbits lie entirely beyond Neptune, and are therefore not strongly affected by the Gas Giant's gravity, and range from 50 to 2000 AU maximum distance. The Hills Cloud ranges from 2000 to 10,000 AU, and the Oort Cloud beyond that. Currently only two objects are known to belong to the Hills Cloud, and indirect evidence for the Oort Cloud comes from long period comets whose orbits originate there. Our ability to detect objects is currently limited to ~80 AU, so we can only find objects from this region whose closest orbit point (perihelion) is in the 50-80 AU range and are currently at that end of their orbit. We therefore expect to find many times more objects in the future. Their total mass is poorly known at present, but is estimated to be 4-80 times that of Earth, which is a vast reservoir of materials. Comets are merely such objects whose orbits have changed to come close to the Sun and their icy components evaporate. This gives us some information on their frozen composition. Solar energy is quite weak in this region, below 0.04% of that near Earth, and ambient temperatures are below 70K down to near 2.7K. Travel time with current propulsion technology is many years to centuries. Ping time ranges from 14 hours to 3 years.
- Economic Uses
We don't have enough information about objects in this region, and they are too far away to use with current technology. So any uses beyond science and exploration are deferred to the far future. When that time comes, though, there is a very large reserve of materials that can be put to use.
- Distant Orbit Transport
To keep transport times within reason, very high energy propulsion would be needed, such as nuclear fusion. Since the light elements needed for fusion are common in these outer regions, this could be self-fueling once set up. Unfortunately, fusion is not yet a viable technology, so transport that uses it remains speculative at present.
Distant Orbit Production
Due to the low to nearly non-existent solar energy in this region, nuclear energy sources would likely be needed to even consider local production. Production must remain speculative at present.
8.0 - Planetary System Locations
The planetary systems of Phase 5 are distinguished in several ways from orbital locations, so require specific designs to accommodate the differences. First is their gravity fields, which require energy to traverse up and down, and create significant surface gravity. Second is the large size and diversity of conditions found on and around these bodies.
Reasons to expand to these locations include access to their raw materials, and relief of the Earth's biosphere by moving industry off-planet. Some people prefer natural gravity and stable horizons provided by a planet-sized body. As with orbital locations, we proceed from nearest to farthest, and each planetary region is developed after orbit transport is available to reach the region.
8.1 - Phase 5A: Lunar Locations
- Lunar Features
The Lunar region includes the Moon itself, and orbits within 35,000 km of the Moon's center, which are close enough to be relatively stable. Lunar orbits in general are somewhat unstable. The Moon has mass concentrations from past impacts that create an uneven gravity field. The Earth and Sun are much more massive than the Moon, and therefore have significant tidal effects on objects in orbit. The Moon has the same average distance from the Sun as the Earth, so base solar flux and ambient temperature is the same. That sunlight is partly blocked in lower Lunar orbits, and blocked 50% or more on the surface at monthly intervals. Surface gravity averages 1.625 m/s^2, or 1/6th of Earths, with a total variation of 0.0253 m/s^2 according to location (Figure 8.1). Escape velocity from the Lunar surface is 2380 m/s, or 21% of Earth. Therefore escape energy is only 4.5% of Earth. 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. Surface area of the Moon is 38 million km^2, or about one quarter of the Earth's land area, not counting the area added by vertical topography.
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 exactly 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 is not a point, and provides varying vantage points. Orbits around the Moon vary from 108 minutes close to the surface, to 6.8 days at the upper edge of the region. Travel time from Earth is 3-4 days for people, by direct transfer orbit. Cargo delivery by electric tug is much more efficient, but also much slower. Without shielding it would expose people to lethal radiation traversing the Earth's radiation belts. Ping time 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 it's orbit. This includes a satellite relay if you are communicating with the far side that can't be seen directly from Earth. Radiation levels on and around the Moon are high to lethal when unprotected, due to solar and cosmic background radiation.
The Moon has a somewhat variable, and reasonably well understood, geology. This is the result of a number of lander and orbital missions, some of which returned samples, and Lunar meteorites 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. Since the Moon retains no atmosphere, the surface is heavily cratered and broken up by repeated impacts of all sizes. The result is a Regolith, a mix of the original crust and impacting asteroids, thrown around many times during crater formation.
- Economic Uses
The Lunar region is embedded in the High Orbit region, so we can start to use it as soon as transport is available from there. The first economic use of the Moon comes from its relative closeness to populated satellite orbits, and low energy to reach these orbits. There are few volatile compounds left on the Moon, because it formed in a molten state, suffered many high energy impacts, and is too small to keep an atmosphere. A fully developed economy would therefore supplement Lunar materials with those from nearby asteroids and some from Earth. Beyond supporting satellite maintenance and refueling, the next stage is orbital production and construction of larger and more powerful satellites. Since these satellites are not constrained by launch mass and cost from Earth, they can use simpler and heavier designs. Further expansion of production may leads to power satellites, which beam energy to Earth for baseload power. If this can be done economically, it would likely be the largest export market to Earth. Solar-thermal with storage works in sunny climates on Earth, but many people don't live in such climates. Solar flux in space is 10 times higher than such poor climates, and may prove cheaper overall, despite the extra cost of building in space.
- Lunar Transport
Early landings on the Moon do not have the support of much infrastructure, so require high thrust chemical rockets to navigate the Lunar gravity field. 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, then an electric tug delivers them to low Lunar orbit for efficiency. A low Lunar fuel station 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%.
As mentioned earlier, an electric centrifuge can deliver 1000 times the solar array mass per year to Lunar orbit. This consists of an electric motor driving a rotor with a long and short arm that are balanced. During the day the solar panels feed power directly to the centrifuge until the tip of the long arm is moving at slightly above Lunar orbit velocity. The payload is released and coasts to a collection station in low Lunar orbit. At the same time, a counterweight is released from the short arm, which hits a hill behind the centrifuge. The reason for the counterweight is an unbalanced rotor would produce huge forces on the centrifuge structure. Whether the payload is sintered blocks of unprocessed surface material (regolith), or first separated to, for example, metals, is yet to be determined. An electric tug takes loads from the collection station to a higher orbit where further processing and production is done.
Lunar basalt and Carbon from asteroids carbon can be turned into high strength fibers to build a much larger orbital centrifuge. For the same tip velocity equal to orbit velocity, the tip acceleration is much lower at a larger size. If it is set to 1 Earth gravity, a crew can live comfortably, and make trips to the Lunar surface with very low fuel use. The tip velocity cancels orbit velocity at the low point, so a lander can be dropped off at low altitude. It would not be dropped directly on the surface because of the variable gravity field and Lunar mountains and crater walls. The lander therefore uses a small amount of fuel to cover the last 10-20 km to and from the surface. Such an orbital centrifuge makes sense if the traffic to the Moon is large enough. There is a one-time cost in mass to build it, but afterwards it saves most of the required lander propellant. At the other end of the rotation, the centrifuge 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 centrifuge, but if traffic is balanced in direction it is a temporary change. If traffic is more in one direction than the other, the difference can be made up by electric thrusters. 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 ping times. At the same time, the centrifuge can provide easy access to the surface when needed.
- Lunar Production
Early Lunar production can start with mining bulk regolith, and separating oxygen and metals for launch to orbit. Once a surface station and seed factory equipment is set up, this can be expanded to other products for local use and export. As better transport systems are installed in sequence, the cost of export will decrease. The advantage of the Moon is relative closeness to high orbit markets, and low energy to move cargo. However the Moon lacks some needed some needed materials, which can be supplied from asteroids or Earth. Surface factories can produce some finished products, but others, especially in early growth stages, would also come from Earth. Thus a two-way trade would develop for Lunar operations to support themselves.
Some regions of the Lunar surface contain ~10 parts per million Uranium and Thorium. The ore thus has an energy content of about 800 MJ/kg (20 times that of coal on Earth). Helium-3 has been proposed as a fusion fuel to be mined from the Moon. Although the energy content of pure He-3 is 200 TJ/kg, the concentration is only 15 parts per billion or less, resulting in an ore content of only 3 MJ/kg. Fissionable elements are thus a better energy source from the Moon. Uses would be for areas that have low sunlight (the outer Solar System) or long nights (like the Moon itself). Alternate energy sources for power-deficient regions on the Moon include microwave or laser beamed power or large and lightweight reflectors. Thermal energy storage is also an option, using 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.
Low gravity is known to be harmful, so long-term habitats on the Lunar surface may require centrifuges to create artificial gravity. An example would be a large habitat dome for spaciousness, and a centrifuge built around the rim for living quarters. 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.
8.2 - Phase 5B: Mars Locations
- Mars Features
The Mars region includes the planet, two small moons, and orbits within 340,000 km (100 radii) of Mars' center. Mars's orbit around the Sun is 9.3% eccentric, and varies from 1.38 to 1.67 AU in distance. Therefore solar flux varies from 494 to 716 W/m^2, or 36 to 52.5% of the Earth's value. Mars surface gravity varies from 3.683 to 3.743 m/s^2, a 1.6% variation, with a reference value of 3.711. Lower values are due to equatorial location and extremely tall volcanoes, while higher values are at lower altitudes in the north polar region and Hellas Basin. Escape velocity from the surface is 5,027 m/s, and is 502 m/s at the upper edge of the orbital region. Circular orbit velocities are 70.7% of escape. This ratio holds for orbits around any body. Escape energy is 20% that of Earth. Orbits around Mars vary from 100 minutes for low ones, to 70 days at the edge of the region. Surface area of Mars is 144.8 million km^2, or 97% of Earth's land area. The Martian day length is 24h 40m, slightly longer than Earth, and the Martian year is 1.88 Earth years. Travel times vary according to the relative positions of Mars and Earth, and the propulsion method chosen. When aligned, transfer orbits typically take 8 months one way.
The Martian moons Phobos and Deimos orbit radii are 9,377 and 23,460 km respectively. They are 22.2 and 12.6 km in diameter, but irregular in shape. They have a combined mass of 12,800 billion tons, or two centuries of Earth's total mining output, so represent a significant material resource in convenient orbits. Their composition is uncertain, but spectra are similar to Carbonaceous type asteroids, and thermal infrared suggest hydrated minerals similar to those on the Martian surface. Ping time to the Mars region varies from 6 to 45 minutes, depending on relative orbital position and need for a relay satellite to avoid the Sun. Unprotected radiation levels range from high to lethal, but the Mars surface and its moons provide ample material for shielding.
Mars has an atmosphere which is 96% CO2, a bit under 2% each Argon and Nitrogen, and an assortment of trace gases. Surface pressure varies from 30 Pascals at the top of Olympus Mons to 1155 Pascals in the Hellas Planitia basin. The high value is 1.14% of sea-level pressure on Earth. The pressure varies by 30% annually, as some of the CO2 freezes and evaporates at the poles. Surface temperatures vary from 120 to 293K (-153 to 20 C), depending on latitude and season. Typical day-night variation is 70K/C because the atmosphere does not have much thermal mass. Mars has quite a varied surface geology, as a result of internal melting and vulcanism, impacts, and much higher levels of water and atmospheric pressure earlier in it's history (Figure 8.2 and see USGS Map 3292 for a more detailed version). It includes significant amounts of water in the soil as hydrates and permafrost, and frozen in thick dusty ice caps.
- Economic Uses
The early use of Mars is as a human scientific outpost, which is an extension of the robotic science being carried out today. This can begin by using Phobos and a few relay satellites as remote control sites for surface robots. The robots perform science and prospecting at first, then added or different robots start to prepare a surface outpost for human crews. Once sufficient supplies and basic production like fuel and oxygen are in place, human crews can start to visit. As more production capacity is built up on Mars and in surrounding orbits, it can start to transition from purely science, to prospecting for unique resources, building up local habitation, and a full economy. Since Mars has almost the same land area as Earth, there is plenty of room to do this.
- Mars Transport
In Phases 4B and 4C (sections 7.2 and 7.3 above), electric tugs were put to use transporting asteroid materials back to high Earth orbit for processing. To expand towards Mars, we use the same type of tugs to move materials from Near Earth or Near Mars orbits to specific Mars Cycler orbits. These are orbits that do repeat flybys of Earth and Mars. Given roughly 20,000 known asteroids in this region, there will be a number of them that have low velocity change to reach the cycler orbit. A second tug delivers habitat modules and initial processing equipment from near Earth to meet the tug in the cycler orbit. The raw asteroid materials are distributed around the habitat for radiation shielding. At the next opportunity a human crew meets up with the new "Mars Transfer Station", and begins to process the raw materials into fuel, air, water, metals, etc. They also establish a greenhouse to produce food. When raw materials run low, a tug is dispatched to a nearby asteroid and brings back more. Whenever the transfer station is near Earth, new crew, equipment, and parts can be delivered. Crew aboard the transfer station are safe from radiation hazards, and can produce most of their own supplies. The Station can be used multiple times to bring new crews to Mars, saving trip mass over launching a new habitat for each mission. Cargo besides people can travel ahead of time to the Mars region on direct tugs.
When the transfer station has been built up enough, a set of crew detach and inject to the Mars region, then dock with Phobos and repeat the build-up process. Instead of mining raw materials from asteroids between Earth and Mars, they can now mine Phobos for what it can supply, and fetch other materials as needed from Near Mars Asteroids, which are closer. Eventually enough fuel is produced on Phobos to power landers to the surface, and intensive exploration and build up of a surface base can start. Early set-up of the surface base would be remote-controlled, until at least fuel production for return trips is possible. At that point human crews can start helping the robots in person.
Mars is large enough that a lot of fuel is consumed traveling back and forth from orbit, because independent landers have to use low efficiency chemical rockets. In the longer term, you would want to build an orbital centrifuge to provide part or most of the velocity change in both directions. Setting it up involves a large construction mass, but the fuel savings for each trip make that up. Carbon fiber from carbon on Phobos or Near Mars Asteroids can be used to build it.
- Mars Production
Like other phases of the program, the first production in the Mars region is mining raw materials for basic products, using pre-made equipment. Seed factory units are delivered later to Mars orbit and the Martian surface, and diversified production bootstraps from that to larger scale production. Our transport network extends back to Earth, and we can deliver parts and supplies as needed on a regular basis. Eventually export markets can develop, and to support them efficient transport back to orbit would be needed. Examples would be electromagnetic catapults on the large Martian volcanoes, or larger orbital centrifuges. It's too early to choose a particular technology, but chemical rockets are inefficient in the long run, and sufficient traffic would warrant building something better. Because of the gravity well of Mars, exports may not be economic right away, but rather a build-up period will happen first.
Terraforming Mars has often been suggested because of the similarity to Earth. However, doing this for a small population is excessive effort for the amount of use it would get. It makes more sense to just terraform the areas under habitat domes as needed. Since the internal pressure under the domes would be much higher than outside, they would need a lot of structure to anchor them to the surface and keep from being blown away. Alternately, they can be held down by the weight of bulk soil and rock, or thick glass, which double as radiation shielding. Once population on Mars reaches into the millions, they can decide if terraforming makes sense. The presence of large amounts of self-upgrading production can make the task easier.
8.3 - Phase 5C: Venus and Mercury Locations
- Venus and Mercury Features
The Venus and Mercury regions include those planets, and orbits within 600,000 and 100,000 km of their centers, respectively. This is the distance at which reasonably stable orbits are possible against the Sun's influence. There are fewer known asteroids close to the orbits of Venus and Mercury, and intercept velocities are higher. These planets also have no known moons. Therefore the main interest is the planets themselves. Venus' orbit is nearly circular at 0.723 AU, while Mercury's is 20% eccentric, and varies from 0.307 to 0.467 AU. Solar flux is therefore 1.9 times as high at Venus, and to 4.6 to 10.6 times as high at Mercury. Ambient temperatures in orbit are therefore higher without shielding. The surface temperature is 735K (462 C) for Venus, due to a thick atmosphere with a strong greenhouse effect. Mercury ranges from below 100K in shadowed polar craters, to as high as 700K at the sub-solar point at perihelion. A given location can vary nearly this whole temperature range due to a 58.6 day rotation period and lack of atmosphere, allowing the night side to cool dramatically. Venus' surface gravity is 8.87 m/s^2 (90% of Earth), and Mercury's is 3.7 m/s^2, the same as for Mars. Escape velocities are 10.36 and 4.25 km/s from their surfaces and 1,041 and 664 m/s from the outer edge of their regions. Orbits range from 90 minutes to 59 days around Venus, and 85 minutes to 15.5 days around Mercury. Ping times vary from 4.3 to 30 minutes for Venus, and 9 to 25 minutes for Mercury. Travel times by least energy Hohmann transfer orbits are 4.8 and 3.5 months respectively.
The thick atmosphere of Venus has restricted getting information about the surface geology. The atmosphere has a surface pressure of 9.2 MPa (90.8 times Earth), and is composed of 96.5% CO2 and 3.5% Nitrogen, with some trace gases. While it is very hot at the surface, moderate pressures of 0.5 times Earth and temperatures around 27 C exist at 55 km altitude. Mercury has an primary composition of 40% Oxygen, 25% Silicon, 11% Magnesium, 6% Aluminum, 4% each Calcium and Iron, and 2% Sulfur, with some variability by location.
- Economic Uses
Near-term use of Venus and Mercury is more difficult because of the added propellant to reach them and generally hostile temperatures. Scoop-mining gases from the upper reaches of Venus' atmosphere is a possibility. In the mid-term, floating habitats are possible at altitudes where temperature and pressure are reasonable. In the long term, on the order of 0.1 cubic km of asteroid iron or aluminum could be fashioned into sunshades to cool the planet. Reduced temperature lowers the scale height of the atmosphere, and preferentially lowers the pressure of the high altitude regions of the planet, making them more accessible. There is the possibility that low enough temperatures will promote carbonation of the volcanic surface minerals, further lowering pressures, and that this process can be enhanced artificially. If the surface conditions can be made more tolerable, then large scale access to raw materials plus high energy available in orbit could promote industries.
It takes 7 to 12 km/s velocity change to travel from near Earth to the Venus and Mercury orbit regions, and therefore 0.15 and 0.27 kg of propellant per kg cargo using electric thrusters, but without gravity assists. These require 263 and 474 MJ of solar array power respectively. If the cargo is also solar arrays, or equivalent thermal power generation, they will produce an additional 13.75 and 91.5 MJ/day added power output, and repay the extra energy use in 19 and 5.2 days respectively. So energy-intensive processes highly favor going closer to the Sun.
- Venus and Mercury Transport
Increased gravity as you get closer to the Sun implies more energy is needed to change orbits and reach Venus and Mercury. Solar flux increases faster then velocity changes, and this introduces the possibility of solar sails as a transport method in the inner regions. For example, reflected sunlight provides 15.5 Newtons/km^2 at Venus, and a 1 micron Magnesium-Aluminum sail material would mass 2400 kg/km^2. This generates 558 m/s/day acceleration for the bare sail, which is reduced by the remaining structure and cargo mass, and angling the sail to control thrust direction. This acceleration is comparable to that for electric thrusters vs solar array mass near Earth. The advantage of solar sails is they do not consume propellant. Their disadvantage in the outer Solar System is low solar intensity them very slow. Electric thrusters are quite viable closer to the Sun, and a combined system is possible to take advantage of the reduced propellant from the sail and wider thrust angles from the electric engine. Orbital centrifuges can assist with reaching the surface or escaping from Mercury and Venus, once enough traffic exists to justify their construction.
- Venus and Mercury Production
Bootstrapping production in the Venus and Mercury regions would likely start in orbit, using imported asteroid materials and scoop mining from Venus orbit. Habitats with artificial gravity and sufficient thermal and radiation shielding can be built in more developed regions and transported to orbit around Venus and Mercury. Centrifugal launch is possible from Mercury's polar regions, where temperatures are more moderate. The combination of materials delivered at least partly by solar sailing and abundant energy resources should allow production to grow rapidly.
8.4 - Phase 5D: Jupiter System Locations
- Jupiter System Features
The Jupiter System includes the largest planet in the Solar System (317.8 Earth masses), and orbits within 20 million km of the planet's center, which are reasonably stable. It includes four very large moons (more than 3000 km diameter), and 63 others found so far from 170 km down to 1 km in size. Some of the moons are farther than 20 million km in irregular orbits, and for our purposes we consider them loosely captured asteroids. The larger moons can support their own stable orbits, and can be used for gravity assists to change orbits in the Jupiter system. Solar flux varies from to 3.3 to 4.1% of that near Earth, so concentrating reflectors are useful in this region. Escape velocity is 59.5 km/s from low Jupiter orbit, and 3.48 km/s from the edge of the system. Orbit periods range from 174 minutes to 1.62 years. Travel time by low energy transfer orbit is 2.7 years, while gravity assists and other propulsion methods can increase or decrease the time. Ping time is 1.06 to 1.85 hours from Earth, depending on relative orbit locations. Jupiter has a strong magnetic field and therefore strong radiation belts. This ranges from high to immediately deadly levels for unprotected humans, and can rapidly degrade even shielded electronics. Outside these belts, the usual solar and galactic radiation is still a hazard. Ambient temperature in Jupiter system orbits are about 217K (-56 C) for black objects, and less for lighter colored ones.
Jupiter is a Gas Giant, and therefore has no solid surface. The atmosphere is 90% Hydrogen, 10% Helium, plus trace gases. Orbit minus rotation velocity is 29.5 km/s, which makes it very difficult to scoop-mine the atmosphere. The four large moons (Io, Europa, Ganymede, and Callisto) orbit 0.422, 0.671, 1.07, and 1.88 million km from Jupiter in nearly circular orbits. They have a combined surface area of 232.8 million km^2, or 1.56 times the land area of Earth. They have negligible atmospheres. All four are tidally locked to Jupiter, so their days are equal to their orbit periods of 1.77, 3.55, 7.15, and 16.7 days. Surface gravity varies from 1.23 to 1.80 m/s^2, or 12.5 to 18.4% of Earth. Io has a surface composed of volcanic deposits and sulfur compounds. The other three large moons are either entirely or partially covered in ice, with various minerals and frozen compounds making up the rest of their surfaces. Surface temperatures of the large moons range from 70-165K, except for volcanic hot spots on Io, and vary mostly by latitude and how close they are to Jupiter, and thus how much reflected light they get on the near side.
- Economic Uses
Use of the Jupiter System would likely follow from the Main Belt and Trojan locations in Phase 4D. The outer Jovian moons are likely captured asteroids, and are the same Solar distance as the Trojan group. So they don't represent new technical challenges. The Jovian moons together have 6.6% of Earth's mass, or 5.4 times that of Earth's Moon. They thus represent a very large source of materials, with some significant variations in composition. Early uses are likely to be mining-based, with return of materials to more developed regions. Even the largest moon, Ganymede, has a low enough gravity an electric centrifuge can throw cargo directly to orbit, after which an electric tug can transport it elsewhere. Water is widely available in the Jupiter System, so it can be used to fuel the tugs. High radiation close to the planet requires careful design for habitats and electronics.
- Jupiter System Transport
Transport from previous locations in the Asteroid Belt and closer to Earth would be via electric propulsion, and, for human crews, with shielded habitat modules. High thrust landers would be needed for early landings on the large moons, while later transport can use orbital centrifuges. Because of the high radiation levels close to Jupiter, a mix of heavily shielded habitats on cyclic orbits and remote control of equipment is likely. We don't expect to land on or mine Jupiter because of the extremely high energy required. The other Gas Giants are easier to access and have similar atmospheres.
- Jupiter System Production
Growth of local production follows the usual path of mining first, then seed factories to bootstrap other industries. The smaller outer moons can be an early source of fuel and water. Rocky and metallic materials may need to be imported from the surrounding regions, depending on the composition of the moons. Large reflectors would be a desirable early product to generate power and heat.
8.5 - Phase 5E: Outer Gas Giant Locations
- Outer Gas Giant Features
The outer Gas Giants are Saturn, Uranus, and Neptune, which average 9.55, 19.22, and 30.11 AU from the Sun. These planets and their surrounding orbital regions are embedded in the Outer Interplanetary region of Phase 4E (section 7.5). Saturn's region extends 20 million km from the planet center, and includes 62 known moons. Eleven of these moons are larger than 100 km, of which five (Tethys, Dione, Rhea, Titan, and Iapetus) are larger than 1,000 km, Titan being 5,150 km in diameter (75% that of Mars). Uranus' region is 12 million km in radius, and has 27 known moons, of which five (Miranda, Ariel, Umbriel, Titania, and Oberon) are considered major, ranging from 470 to 1575 km in diameter. Neptune's region is also 12 million km radius, and has 14 known moons, of which Triton is by far the largest at 2700 km diameter. As with Jupiter, the outermost moons are beyond the region limits in irregular orbits, and we consider them loosely captured asteroids. The larger moons can support stable orbits around them and perform gravity assists. All the Gas Giants, including Jupiter, have ring systems, but Saturn's is particularly significant. It is estimated to mass 30 million billion tons, almost as much as the 400 km size moon Mimas, which orbits nearby.
Solar flux is about 1%, 1/4%, and 1/9% of that near Earth in order of region distance. Large reflectors would be needed to bring sunlight to useful levels, so nuclear power may be more effective in these regions. Escape velocities are 35.5, 21.3, and 23.5 km/s from each planet, and 1,880, 965, and 1,061 m/s from the edge of their regions. Orbit periods are 250, 180, and 155 minutes close to the planets, and 2.1, 3.5, and 3.2 years at the edge of their regions. Travel times are 6, 16, and 30 years by minimum energy Hohmann transfer orbits, so it is recommended to use gravity assists and added propulsion to shorten the trips. Ping times average 2.65, 5.33, and 8.35 hours, plus or minus about 20 minutes for relative planetary positions. Radiation levels around Saturn are about as high as Earth's Van Allen Belts, requiring added shielding for people and electronics. Levels around Uranus and Neptune are lower, but still include the Solar and Cosmic background flux which is present in most parts of the Solar System.
Like Jupiter, the outer Gas Giants have no solid surface. Their atmospheres merely get denser with depth until they are beyond the critical point and shade into liquid density and higher. Compositions are 96% Hydrogen, 3% Helium for Saturn; 83% Hydrogen, 15% Helium, and 2.3% Methane for Uranus; and 80% Hydrogen, 19% Helium, and 1.5% Methane for Neptune. All three have trace gases below 1% levels. Orbit minus planet rotation velocities are 15.2, 12.5, and 13.9 km/s, so scoop mining their atmospheres should be feasible. The combined total of 103 moons and one major ring system around these planets have a mass of 1720 x 10^20 kg, or 2.34 times the Moon. They represent a very large source of materials, but their distance means mining of these bodies will happen much later than for inner regions. Titan has a thick Nitrogen atmosphere (1.4 times Earth pressure) with 1.4% Methane at upper levels and 4.9% at lower levels. Since low orbits are about 1.8 km/s, scoop-mining this atmosphere is particularly easy, and may be a good opportunity despite the distance.
- Economic Uses
The outer Gas Giants are too far to use at present. When civilization has expanded through the previous regions, the first uses are likely to be mining of the large sources of raw materials, and bringing them back to inner regions where there is more energy to process them. Helium-3 has been proposed as a low radiation fusion fuel. Fusion in general has not yet been solved, and the He-3 - He-3 reaction is harder than the Deuterium-Tritium one that is the main target of research. If the technical issues are overcome and a need develops, Uranus and Neptune have the highest ore concentrations of Helium in their atmospheres, and thus the He-3 isotope. At the same time, fusion reactors would be available to make trips to these planets in reasonable time. Any uses are far enough in the future that technology is likely to change dramatically in unexpected directions. Therefore we can't yet make intelligent estimates for how these regions can best be used.
- Outer Gas Giant Transport
The power source for outer system transport is still to be determined. Nuclear fuel has an energy content of ~80 TeraJoules/kg, while solar panels near Earth produce around 80 GigaJoules/kg. The relative output then depends on the conversion efficiency of nuclear reactor fuel energy to useful output, and the mass ratio of the reactor system to the fuel load. Fissionable elements are relatively rare compared to silicon used for solar panels or aluminum/magnesium used for concentrating reflectors. So for large-scale energy demands, the solar sources have better material supply. However, in distant regions where sunlight is weak, nuclear approaches may have the advantage. Although reactors emit harmful radiation, most parts of the Solar System are filled with harmful radiation anyway. The same shielding that protects people and equipment can do it from both sources.
- Outer Gas Giant Production
Until technology improves enough, we expect use of these regions to be mainly mining and scientific exploration.
9.0 - Interstellar Locations
Phase 6 is the last major phase of our program, and is far enough in the future that we can only speculate about it in general terms. Interstellar space, the cold regions between stars, is not much different from the environment of the outer parts of the Oort Cloud in Phase 4F. We know very little as yet about equivalents to the Oort Cloud around other stars, or wandering objects not attached to stars. We have better information about planets and disks around other stars. Their parent stars tell us where to look, and the stars themselves provide data about the planets from Doppler shifts and transits. The number of discovered planets is growing rapidly, from none before 1988 to about 2000 by the end of 2015. We expect there to be smaller objects in systems with planets, but we can only see them directly if they form a thick enough disk.
The key difference that warrants a new phase is the extreme distances in this phase. This mostly breaks the ability to deliver things from, and communicate with, the Solar System. Expansion of civilization to these regions would require high self-sufficiency in transport and good enough seed factories and starting materials to enable growth without assistance. The Sun acts as a gravitational lens, with a focus around 800 AU from the Sun, in the Scattered Disk region. Placing telescopes directly opposite a star of interest would allow much more detailed observations than otherwise possible, because of the 2 million km optical diameter of the Sun.
9.1 - Phase 6A: Interstellar Space Locations
- Interstellar Features
We define the Interstellar region as starting beyond 100,000 AU from the Sun, where nearby stars and the Milky Way galaxy as a whole begin to contest the Sun's gravity. There is no outer limit defined for this region beyond whatever travel distances are possible from future technology. Since we don't know what those future technologies will be, for now we will arbitrarily set a boundary of 20 light years from the Sun. Probably the most significant feature of this region is that stars are all in relative motion to each other, with an average velocity of 50 km/s. This is on top of the general rotation of the galaxy at about 225 km/s. There are about 85 stellar systems within the 20 light year "Solar Neighborhood". Given their average velocity, they will travel 20 light years in 120,000 years, so the membership of the neighborhood will change about every 1400 years on average. The local interstellar region is very low density gas, at ~0.3 atoms per cubic centimeter, or 1 gram per 564 km cube. That does not include cometary clouds or wandering objects. Solar radiation is not a factor in this region, but cosmic radiation still is.
- Economic Uses
We don't know enough about material resources and energy sources in this region to propose economic uses. The distance to the Sun detaches any industries from regular trade with the rest of civilization. Science, exploration, and seeding interstellar colonies are possible activities.
- Interstellar Transport
Interstellar transport can be divided into slow and fast types. The slow type is on the order of stellar velocities (5-500 km/s) using large habitats with large material reserves and fusion power as an energy source. These habitats subsist on the cometary clouds around stars and rogue objects between stars. When they get close enough to a selected star, they can enter orbit and travel with it. Travel times between stars would be 3000 years or longer. Fast interstellar puts much more energy into transportation, to reach higher velocity and shorten time to a destination. Possible methods include fusion powered engines and beamed power from the Sun. Rather than a large habitat with a full range of civilized activity, fast habitats operate more like ships on Earth, with a crew dedicated to reaching a destination and maintaining operations.
- Interstellar Production
We don't know enough about resources in this region to consider gathering raw materials. Therefore the only production we can consider is aboard transport vehicles.
9.2 - Phase 6B: Exostellar Locations
- Exostellar Features
We define Exostellar regions as those surrounding individual stars or multi-stellar systems, of which there are 85 within 20 light years, with 127 stars. The size of the regions are scaled to the square root of the system mass divided by Sun's mass, times 100,000 AU. This accounts for their region of gravitational dominance and any cometary cloud they have. Four of these systems currently have known planets, but ten more are suspected, and data is incomplete at present. Two of these star systems, epsilon Eridani and Tau Ceti, have known circumstellar disks. More study is needed with better telescopes before any attempt to plan travel to these stars.
- Economic Uses
Due to extreme distance, the only economic uses we see for now are science, exploration, and seeding independent colonies.
- Exostellar Transport
Transport between stars is covered under Phase 6A in the previous section. Travel within a given stellar region would use the same technologies as around the Sun, with modifications for available energy sources.
- Exostellar Production
As mentioned earlier, we would want to observe the nearby stars in more detail by using the Sun as a giant gravitational lens. Following that would likely be robotic probes to more closely examine whatever is found around these stars. A self-bootstrapping seed factory approach should work at other stars, since energy and matter are the same everywhere. However the details will depend on what resources are available.
10.0 - Directions for Further Work
It isn't in the scope of this report to provide detailed program plans and hardware designs. Rather we present a possible path that leverages self-upgrading production methods. In this section we will attempt to describe further work that can be done in the near term. This work would be a step along a path whose end we cannot define as yet.
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