# Space Transport and Engineering Methods/Resource Uses

Section (1.9) described existing space programs and projects. This section lists categories of space activities and mission objectives for future programs and projects. Parts 2 and 3 of the book will describe the transport and engineering elements to implement these activities. They also discuss how to design, build, and operate complete systems that perform the desired functions. A given project, location, or mission may include more than one of the items below. The multiple activities can be present but not otherwise interlinked, in the same way a large office building may house multiple unrelated tenants. In that case the project may share joint services like electrical power among the various activities. Alternately, the multiple activities may link to each other in parallel, such as retrieving asteroid materials and refueling the tug from those same materials. The activities may also be connected in a logical sequence, where one activity follows another, such as building new exploration or mining vehicles from previously produced inventory materials. We call any program with multiple different activities a Complex Program, and an extensive example is provided in Part 4. Many other such complex programs are possible, but our example is intended to be both a tutorial and a reasonable proposal for future activities.
The activities listed below are somewhat in order of idealized complexity. An actual program that implements multiple items would likely choose a different order which makes sense from a design and schedule standpoint. As civilization expands into space, eventually we will incorporate all the industries and activities that occur on Earth today. So this list is not comprehensive. Instead, it includes the more significant items that are particularly space related.

## Bulk Mass

Bulk mass is matter which is undifferentiated into manufactured components, either in the raw unprocessed state, or processed into a refined product. The quantity of bulk matter is a main variable, as all the bulk material of a given type is equivalent. The Earth has a deep gravity well, which requires 62.5 MJ/kg to escape. This is currently difficult and expensive. Local sources of bulk materials, that are already in space near to where you need them, can often be delivered and processed for less total energy. Solar energy is abundant in the inner Solar System in quantity, and available nearly everywhere. So the delivery and processing in theory can be low cost. So there is an economic incentive to obtaining bulk materials in space. Bulk materials can be used for propellants and fuels, serve as radiation shielding, or treated as mineral ores for extraction and further processing into useful products.
NASA has termed use of local materials in space In-Situ Resource Utilization (ISRU). They have set up an ISRU Project Office and are doing some early research on methods. The ISRU name is more palatable to some members of committees who decide NASA's budget than "space mining", which sounds too much like science fiction. Despite that, Title IV of Public Law 114-90, also known as the Space Resource Exploration and Utilization Act of 2015 makes provisions for private commercial recovery and ownership of space resources. Asteroids have negligible gravity wells, and by early 2016 over 14,000 of them in the Near Earth Asteroid (NEA) category have been discovered. Different asteroids also have a variety of useful chemical compositions. This combination has led to the most interest in Asteroid Mining relative to other space locations, but any location with available materials and energy is a candidate. NASA is also in the early stages of developing an Asteroid Redirect Mission to bring back a boulder from an NEA to a safe lunar orbit, where it can later be examined in detail.

### Propellants and Fuels

Propellants are the materials consumed and ejected in many types of propulsion systems. They often make up a large or dominant portion of space system mass. Propellant applications include space transport, ballistic transport, aerodynamic transport, surface transport, and portable power. Due to the wide range of applications, environments for their use, and sources of raw materials, there is also a wide range of potential propellants. Specific propellants for space transport are discussed in Part 2 of this book. The other applications are listed below.
Fuels are energy sources reacted with materials in the environment, such as air-breathing engines, or with an oxidizer in the case of chemical rockets. Nuclear fuels develop energy by internal reactions, and do not require another material to react with. Of particular interest for advanced uses are fission fuels such as Uranium or Thorium, and if fusion power is developed, light isotopes for those reactions. This is the energy released by nuclear reactions is very high relative to chemical ones.
Production of propellants and fuels from local resources is called In-Situ Propellant Production. To date, all propellants and fuels have been supplied from Earth. Using local resources will often prove less expensive. The mass overhead for delivery is reduced by using depots at intervals along a trip. If all the propellant mass is carried from the start, more of it has to undergo a velocity change. So you have the overhead of using some of it up to move the remainder. By refilling at intervals, the on-board mass is less, and so the mission overhead of moving it is less. The supply sources for the propellants are close to or at the depot locations, and delivery is by high-efficiency electric propulsion. Therefore the supply overhead is also low.
Besides conventional space transport, other propellant and fuel applications include:
* Ballistic Transport - On an undeveloped or heavily cratered terrain, attempting to drive across the surface will be difficult, and for long distances will be slow. Ballistic transport uses similar methods as space transport, but instead of going to and from orbit, or between orbits, is used in sub-orbital trajectories to go from one point to another. The lower velocities potentially allow for simpler propulsion or higher payloads. This method is especially useful on smaller bodies where the velocity requirements are lower.
* Aerodynamic Transport - On bodies with a sufficiently dense atmosphere, you can potentially use aerodynamic lift or buoyancy and a propulsion engine for transport, similar to how we use aircraft on Earth.
* Surface Transport - Solar power is long-lived, and can generate thousands of times the energy over it's life than a chemical fuel. However it does not have a very high power to area ratio, making it cumbersome for faster surface vehicles, and batteries can take a significant time to recharge. As an alternative, a fuel cell or combustion engine can provide more power in a compact device and be more suitable for such vehicles. Stationary fuel stations can refuel a vehicle quickly when distributed within the vehicle's operating range.
* Portable Power - Solar power and/or batteries are suitable for small portable devices which do not need high power levels. Higher power levels for short periods of time may be satisfied by propellant or fuel sources.

### Radiation Shielding

Varying levels of radiation occur throughout the Solar System, including on Earth. Many locations in space have levels that are hazardous to people, or can affect electronics and materials. Natural space radiation comes from four main sources: cosmic rays, solar wind and plasma events, trapped particles in magnetic fields, and concentrations of radioactive elements. Human-caused radiation can come from nuclear and other high energy devices. The effectiveness of shielding varies by composition, with light elements protecting better for some types of radiation, and heavy elements for others. Where mass is important, optimized and processed materials may be preferred for shielding, but unprocessed bulk material is easier to supply.
Habitats, vehicles, and equipment can reduce radiation levels by their own mass, before adding additional shielding. For example, large habitats with thick walls, agricultural soil, water, and atmosphere may provide sufficient shielding as is. For vehicles or lightweight habitats, additional shielding can be added bulk propellants, water, and food supplies. The surface or subsurface of large bodies usually has sufficient local material to provide radiation shielding. The approaches to using it are building underground, or surface construction by moving enough material around and over radiation-sensitive areas. A thickness of 25 cm or more of loosely packed unprocessed material can reduce exposure to reasonable levels for humans (see Miller et al, 2008). In open space, where bulk materials are being processed, the unprocessed raw material, stockpiles of processed materials, and wastes after processing (slag) can all be used for shielding with proper arrangement around habitats and equipment.

### Ore Delivery

An Ore is any natural material containing enough of a desired product to be economic to extract. On Earth, crude oil, iron ore, and crushed stone are all ores used to make other products, and by mass are the largest volume transported. In space, bulk ores and their products are also likely to be a major transport item by mass. Depending on economics and technology, bulk ore can be transported in it's raw state, concentrated in the desired components, called Beneficiation, and then transport the concentrate, or processed in place to a final material which is then delivered to where it is needed. Final destinations can be anywhere the ore or its products are needed. Some selected examples include:
* Delivery to Earth - Nearly all people and economic activity is on Earth, and that will continue to be true in coming decades. A large and economically developed population uses a lot of material resources. Some materials may become scarce enough that it is economically feasible to obtain them from space. Returning high-value materials like Platinum-group elements does not pose a significant transportation challenge, so we will look at the other end of the cost scale.
    Assume you want to import large amounts of Iron to the Earth, since high-grade iron ores are in finite supply, and M-type asteroids can supply nearly pure iron-nickel-cobalt alloy already in metal form. The simplest method is to aim pieces from metallic asteroids at a selected spot, and just collect the bits that make it to the ground. This requires no processing. Figure 1.10-1 shows a natural example of this. You can find other examples of surviving pieces in museum asteroid collections. Non-metal slag and volatiles will tend to burn off during re-entry. Choose a size, likely around 10-50 tons, for the pieces so that re-entry drag will slow them down, and you don't get a big crater. There are plenty of places on Earth with few people and decent access for shipping. The market for steel is about 1.5 billion tons/year. The challenge is delivering it to Earth for around $1/kg or less. You likely will need to redirect and chop up a megaton (60 meter) metallic asteroid or more, but the yield is worth at least $1 billion ( 1 million tons steel @ $1000/ton), which may be enough to cover operating costs for an efficient operation.
* Delivery to Space Locations - Because of the current high cost of launch to orbit, material in a desired space location is worth much more than most materials on Earth. It needs to be moved from where it naturally occurs to where it is needed. The Earth's gravity well requires a lot of energy to climb out of, and to date launch systems use inefficient rockets to do it with. Materials which are already nearby in space can use efficient electric thrusters for transport. On the surface of bodies, some materials can be obtained locally. Transport to and from bodies smaller than Earth can use mechanical and electric methods. These all are potentially much less expensive solutions once they are set up.
* Oort/Rogue Object Delivery - In the distant future, a large interstellar mission may require a lot of propellants and other supplies, due to the high velocity and long duration of the trip. In this concept, several comets or unbound rogue objects, or parts thereof, are intercepted by a propulsion unit that comes from the main vehicle, or is sent ahead from the launch point. The propulsion unit consumes part of the mass to bring the rest of the mass up to the speed of the main vehicle. The delivered mass is used to further accelerate the main vehicle and resupply other materials. This allows somewhat better velocities than starting with all the fuel at the start of the mission, since the main vehicle has less mass to accelerate. For this to work, you need to know where the objects are ahead of time, or trust that their density is sufficient to find them along your path as needed.

## Industrial Capacity

Simple bulk materials like propellants and radiation shielding are useful, but don't satisfy all future project goals. Space Manufacturing is the production of more refined or complex goods in space, from raw materials and available energy. It is distinguished from space industry on Earth, where complex goods like satellites and rockets are made, then delivered to space ready to use. Historically, a key feature of all manufacturing is using tools to make more tools. When applied to space manufacturing, a large industrial capacity can be grown from a smaller and simpler starter set. This avoids the cost of sending entire factories into space. A simple bootstrapping example is making pressure vessels from metallic asteroid material, which are then used in chemical processing of ores.
Some production will be for Final Goods in the economic sense. These are end products not used in further production. An example is an orbiting greenhouse that produces food for people in space. The remainder of production is either Capital Goods, lasting items used in further production, or Intermediate Goods, which are partly finished items between raw materials and final products. When the industrial capacity is intended for more than final products, and some of the capital goods are made in space, rather than delivered from Earth, then the production and growth sequence must be optimized for factors like design cost, growth schedule, and initial mass launched from Earth.

### Seed Factories

A "seed factory" is a starter set of equipment which is intended to grow to a mature industrial capacity. This is by analogy to a plant seed, which grows to a mature plant. Some portion of the factory output is directed at self-growth, and the remainder to desired final products to be used. An early NASA-funded study of the concept is reported in: Freitas and Gilbreath, eds. Advanced Automation for Space Missions, NASA Conference Publication 2255, 1982. Computer, automation, and communications technology was not good enough in 1980 for the intended use on the Moon, so NASA did not pursue the idea further. The concept was limited to "replication", making exact copies of the starter set until enough total capacity was reached. The study also assumed the seed factory made 100% of its own parts, was 100% automated, and could only be used in space.
The current seed factory concept includes two other methods of growth: "diversification", which is making new items not in the original starter set, and "scaling", making items of different sizes than what you start with. The assumptions of 100% self-production, 100% automation, and only used in space are removed. Therefore a seed factory may start out making only a percentage of its own items, with the remainder supplied from elsewhere, and use people directly or by remote control to do some tasks. This greatly simplifies the design of the starter set. Over time, the growing production capacity can make more of its own items, and need less supplied from outside. The idea of self-expanding production using local energy and materials applies everywhere, not just in space. It is a complex subject, and this wikibook is about space systems, so a separate book was started for it.
Self-expanding production can provide a large amount of leverage in terms of mass launched from Earth to final products and missions you can carry out. We therefore consider it an important concept, and will reference it extensively. Because space locations are not uniform in energy or material resources, the seed equipment and what it makes will likely be distributed in a trade network where components are optimized for location. The network will include some parts and materials that still are supplied from Earth. The various parts of the network may be owned and operated by different entities, leading to a self-sustaining space economy.

## Manufactured Items

Once you have set up an industrial capacity, you want to use it to make useful products. There are as many possible products to make in space as there are on Earth. Which ones make sense to make at a given location depends on mass, complexity of production, and economic value. The following items have been suggested for relative ease of production and significant mass savings:
* Structural Materials - A variety of structural materials can be made from local materials in space, reducing the amount of material that has to be brought from Earth. An example is Iron-Nickel shapes like columns or plates, made from metallic type asteroids. Another is cast or sintered rock, made from Lunar or Mars surface material, or stony-type asteroids. They would be melted with solar or microwave furnaces. A third example are high strength Carbon or Basalt fibers from respective local sources.
* Solar Sails from Metallic Asteroids - This is a combination of structural material and transport method. To recover large amounts of material from inner Solar System asteroids, Iron-Nickel alloy found in the metallic type can be rolled into foil, and then used to make solar sails. If what you want is the metal, then it sails itself to where you want it. If you want some other asteroid material, larger amounts of sail area can be used for a cargo tug. To make the sails, you need the functions of a rolling mill - a way to heat the material and a way to force it between two rollers to make thin sheets.
    Drawbacks to Iron-Nickel sails are their higher mass compared to light alloys like aluminum-magnesium, and their reflectivity is lower in the natural state. Solar sails are also somewhat limited in the directions they can apply thrust. Advantages are the raw material is readily available in large quantities from the asteroids themselves, and it does not need a lot of processing to make into a usable form. An Aluminum or Aluminum-Silicon alloy coating can be added to increase reflectivity if desired.
* Glass - Besides the obvious use as windows for greenhouses, glass can be used for fiber optic cable, and for inert reaction vessels, including those which concentrated sunlight is sent through. The chemically simplest glass is Fused Quartz, which is pure Silicon Dioxide (SiO$_2$). Silicon and Oxygen make up over 60% of Lunar soil, and about 50% of stony-type asteroids, so the components are very common. However they are usually bound in silicate minerals with other elements, and require chemical or thermal separation.
* Brick and Concrete - Brick and concrete can be used for lower-strength construction, such as radiation shielding, thermal shelters, and landing pads, when air leakage is not an issue. Conventional brick is made by heating a mixture of sand and clay until the particles partly melt and bond together, a process called Sintering. Building elements can be made the same way in space, provided a sufficient source of heat and right type of ingredients can be found. Its chief advantage is simplicity. Concrete is a class of artificial stone made from varying size crushed stone, called Aggregates, and a binder material to hold them together. On Earth the most common binder is Portland Cement, a mixture of shale and limestone heated to high temperature and then ground to a fine powder. Many other binders are possible, and some of them would be useful in space. The usefulness of concrete is based on it's relatively low cost, and the ability to be cast at room temperature in a variety of shapes which then harden.
* Chemical Products - This includes plastics, chemical reagents, lubricants, and many other items made by chemical processing. Lunar rocks are high in metal oxides, such as Silicon, Iron, Aluminum, Magnesium, and sometimes Titanium. These metals are useful for structures, solar cells, and electronics. Converting the oxides to metals and separating the elements is called Extractive Metallurgy. Processes include physical ones like crushing and grinding to separate mineral grains, and magnetic separation. They also include chemical ones like liquid solutions, thermal ones using high temperatures, and electrical ones like electrolysis. The chemical solutions and electrolysis require suitable reagents. These include compounds with non-metallic elements like Calcium, Potassium, Sodium, Phosphorous, and Sulfur. These elements can be found in some types of asteroids. Plastics and lubricants are typically Carbon compounds, which are also found in asteroids.
* Biological Products - This of course includes food, but also non-food items like wood, and chemical outputs of micro-organisms. The oldest example of the latter is alcohol from yeast, but modern biotechnology can produce a wide variety of items. Growing food typically requires water, Carbon Dioxide, fertilizers with Nitrogen, Phosphorus, and Potassium, and trace elements. Some asteroids have water or hydrated minerals. The Moon is deficient in these, because it formed in a molten state and has a low escape velocity, and they were baked out and lost. Mars and Venus have atmospheres with high percentages of Carbon Dioxide, and Mars has Nitrogen. Asteroids are a source for the elements in fertilizers, and the trace elements.

## Energy

Uses for energy in space are as ubiquitous as they are on Earth. Solar Panels have bee used on satellites from the earliest days, since they are modular, light-weight, reliable, and produce more energy over their life than batteries or fuel cells. Power levels have varied from a few Watts to approximately 100 KW on the Space Station. Future energy needs include larger amounts for propulsion and to operate industrial systems. Habitats, communications, and scientific equipment can be large energy consumers, and finally a major future use is exporting energy to Earth or other locations in space. Types of future energy production include:
* Solar-Electric - This includes existing photovoltaic solar panels, and solar thermal power systems. The latter concentrate sunlight onto a heat engine/generator combination. Solar flux is adequate in the inner Solar System, but large lightweight reflectors may be used in the outer Solar System to improve power density.
* Solar-Thermal - Some future uses require heating rather than electricity, such as an industrial casting furnace. Concentrating reflectors can achieve this rather easily. Thermal storage using local bulk mass can bridge night-time power needs. The material is heated during the day, and the stored heat used to operate a generator at night. This avoids needing large batteries for higher power demands.
* Nuclear Sources - Radioisotope decay heat coupled to thermoelectric generators has been used on a small scale for scientific missions to the outer Solar System, or when daytime sunlight and batteries are insufficient on a planetary surface. A number of small reactors using thermoelectric or thermionic generators have been flown. For future applications like higher-power propulsion or surface habitats, reactors with heat-engine generators, which are more efficient, have been proposed. If fusion rectors are developed, they would be very useful in space, since hydrogen is widely available as you get farther from the Sun.
* Beamed Power - Civilization on Earth has a large and growing demand for energy, but fossil fuels are unsustainable. Ground-based solar panels have become popular in recent years, but orbital locations can provide seven times as much energy per panel on average. This is due to night, weather, and atmospheric absorption on the ground. A large Space-Based Solar Power plant can send power to the ground using an efficient microwave beam. Advantages of orbital solar power are nearly 100% operating time, and lack of Carbon emissions or nuclear risks. A disadvantage is the size of the collector on the ground is governed by the transmission wavelength and distance of the orbital station, so there is a minimum size for it to function efficiently. This can be counteracted to some extent by using shorter wavelengths or lower orbits. To be feasible for Earth, the total system (orbital and ground collector) needs to be less than 7 times as expensive as solar panels on Earth, otherwise using terrestrial panels would be less expensive. Alternate uses would be to beam power to a Lunar surface base from orbit to supplement nighttime power.
    At current launch costs, it makes economic sense to beam power *up* to space by swapping the transmitter and collector locations, as power in orbit is worth more than power on the ground. In the form of visible light or microwaves this would supplement on-board power obtained from sunlight. For orbital tugs at low altitude, the supplement is especially useful as the Earth's shadow covers 40% of typical low orbits.
    Laser power transmission is a future possibility. Beam generation is less efficient, but it can be focused more easily over long distances due to the shorter wavelength. Uses range from powering launch vehicles from the ground, to interstellar missions using the Sun as a gravitational lens. Given a suitable atmosphere, for example Carbon Dioxide rich ones like Venus and Mars, the atmosphere might be used as a lasing medium to generate powerful beams. This concept has not been explored in detail as far as is known.

## Engineered Environments

All of space, and many parts of the Earth, have conditions not suited to humans, or higher life in general. On Earth we apply the various fields of engineering and technology to modify the natural environment in specific locations, such as buildings, ships, and aircraft. Common modified conditions include temperature, protection from weather, and pressure (in the case of aircraft and submarines). In space, we must modify additional conditions like atmospheric composition (or entire lack of atmosphere), gravity level for long term stays, radiation levels, and other parameters. In some circumstances the environment would be set up for plants (high CO2 ratio), or machines, rather than humans. To date, engineered space environments have involved short-term travel in vehicles, even shorter times in space suits, and up to a year and up to 6 or so people in space stations.
Future projects may include larger populations and longer stay times, construction of habitats in space rather than launching already-built units, and production and recycling of basic needs like air, food, and water. Reasons include longer-duration exploration and science, commercial and industrial activities, and the desire to live in interesting and unique locations. Future space environment projects include:
* Space Habitats - Humans evolved on Earth, so it is the only place we know of where we can survive, even for a short time, without the help of technology. Parts of the Earth, such as much of Antarctica, are lethal even in a short time without at least clothing. A Habitat in nature has the right conditions for particular species to live. Artificial habitats, such as homes and greenhouses, are purposely built artifacts which provide the right conditions. Space habitats provide these conditions, but are located in space, rather than on Earth. They are distinguished from space vehicles and stations by long-term occupancy and size. Theoretically a space habitat could be sized for a single person, but needed technical skills and psychology probably set a lower limit at about 6-8 people. At the other extreme, linked assemblies or close formations of rotating orbital habitats, or large non-rotating ones, could support planetary-scale populations. Surface habitats can range from individual pressurized modules, to permanent bases and cities. At the limit, an entire moon or planet can be converted to habitable conditions, which is known as Terraforming. Supporting planetary-scale populations won't be needed for a long time, so most current work is aimed at the lower end of the size scale.
Since the environment parameters that humans and agriculture prefer is fairly narrow, habitats will have similar functions regardless of size. These functions include:
   * Atmosphere Maintenance -
   * Temperature Control -
   * Artificial Gravity -
   * Lighting -
   * Radiation Level -
   * Food Supply -
   * Water Supply -
   * Waste Disposal -
    Artificial habitats are much less mass intensive than natural ones. For example, the Space Station uses roughly 100 tons to support each person, while the Earth uses about 5 trillion tons. So habitats constructed in the Solar System can either support vastly larger numbers of people, allow vastly larger living space and energy use for current population levels, or only use a small fraction of available resources.
* Closed Life Support - All space projects that include people require a Life Support System of some kind. To date, these systems have been "open" in the sense of needing outside supplies of oxygen and food. "Closed" systems recycle part or all of the materials used to sustain life, therefore the amount of stored or newly delivered supplies can be reduced. If coupled with local extraction of needed materials, outside supplies can be eliminated entirely. Water, air, and food are the principal items that would be recycled. Closed systems can be artificial, using machines and chemical processes, or ecological, using living things. For food at least, people have a preference for naturally grown items, and plants naturally produce Oxygen, so this tends to result in mostly ecological systems. Closed life support can be integrated with human living space, such as a habitat dome with both living quarters and farm areas. Alternately, a greenhouse might be optimized for plant growth conditions, including high CO$_2$ levels, and people use breathing equipment and remote control.
* Habitat Construction - Launch from Earth involves passing through the atmosphere, where large size increases drag, and very large objects are not mechanically suited to fit on launch vehicles with diameters of a few meters. The Space Station was therefore assembled from many smaller prefabricated components over time. Even larger future projects may use multiple orbital construction methods. Folded structures, such as solar panels, have been extensively used for decades. To contain an atmosphere, rigid pressure vessels have been used in the past. Inflatable structures, which are collapsed for launch, are currently being demonstrated. In the future, large habitats may be assembled from components launched in compact form, or manufactured locally in space.
* Recycled Vehicles - A conventional rocket takes the final stage, along with the payload, into orbit. By re-fueling the stage, or by converting the stage tanks and structures to another use (such as an occupied pressurized module), some payload weight and volume is saved. For example, the Skylab space station was made from a converted Saturn V 3rd stage. The conversion can be done before launch, as in the case of Skylab, preparations for later conversion can be installed, such as connections for refueling and attaching later hardware, or the stage can be entirely unmodified, and all the changed performed in orbit. A number of studies were done on re-using Space Shuttle external tanks for purposes like pressurized living space or raw aluminum for orbital manufacturing, but these did not progress to actual projects. Re-fueling of upper stages has also been studied, but not tested.

## Transport

Space transport began before reaching orbit, in the form of ballistic missiles. It continues to be needed today, for delivery of new equipment and facilities, cargo, and raw materials. The primary transport method used to date is chemical rockets, both for launch from Earth, and in-space missions. More recently, air-breathing propulsion using carrier aircraft, ion-electric, aerobraking, and gravity assists have been used. Solar sails, electrodynamic, and higher speed air-breathing engines are at an experimental stage. Existing and future transport methods are more fully described in Part 2 of this book. These methods will continue to be needed as long as space projects exist. Some future transport applications include:
* Hazard Removal - Artificial and natural hazards exist in space and on Earth, and transportation methods can remove them or place them in safe orbits. Types of artificial hazards include Space Debris from old satellites, empty rocket stages, and fragments thereof. Below about 2000 km altitude, this debris is denser than natural small meteoroids. Transporting nuclear wastes and very hazardous biological materials from Earth to safer locations in space has been studied, but is currently too expensive. Debris impacts can damage active satellites, where the latter are expensive for the same reason, so there is a stronger economic case for removing the debris.
    Asteroids and comets are known to hit large bodies everywhere in the Solar System, including Earth. Evidence includes impact craters on every solid body, a comet impacting Jupiter, and current falls on Earth. A future project is diverting asteroids and comets which are on dangerous paths. Extensive searches have been ongoing for Earth-approaching objects, and their discovery rate is increasing as better telescopes are built for this purpose. Lunar impacts are often neglected, but more mass can be tossed off the Moon, because it's smaller, to end up in Earth's larger gravity well nearby. You be killed just as well by a 1 ton Lunar fragment as by a megaton asteroid, but the deaths are more distributed in time and space. Methods to move or destroy dangerous objects is at an early experimental stage.
    Long period comets are undetectable with current technology until they get within about 10 AU of the Sun. If one was headed towards Earth, there is not time to arrange a shift in it's orbit, so the only reasonable way to deal with it is to use an interceptor with one or more large nuclear bombs to fragment or destroy it. Comet trajectories are hard to predict because they have natural rocket thrusters in the form of gas jets. Future projects may place search telescopes farther out, to find such comets earlier. They may also station interceptors farther out into the Solar System, so fragments have more time to disperse. If more time is available, dangerous comets can be diverted using their own material as propellant, or another, smaller, natural object can be diverted to collide with it.
* Interstellar Transporter - The energy to transmit the description of an object to another star, even at an atom by atom level, is about a million times less than the energy to physically move the object from one star to another. In the far future, after a first probe sets up a receiving/replication station at the other star, other objects are more efficiently scanned, transmitted, and reconstructed at the receiving end. Using atomic scale technology (such as scanning tunneling microscopes) it may be possible to eventually scan and send people this way. The subjective time to travel at the speed of light is then zero, although the actual transit time is still governed by the speed of light.

## Science and Exploration

Science and exploration have been the main motivations for missions beyond Earth orbit, once the Space Race of the 1960's wound down. They have also been important motivations in Earth orbit, but no longer the dominant ones. Most work in these areas is paid for by government agencies, so their pace is limited by available budgets. Humans have not traveled past Low Earth Orbit since the Apollo program, but numerous Solar System Probes have visited all the discovered major planets, some asteroids and comets, and five have left or are on trajectories to leave our home system entirely.
Future human exploration of the Moon and Mars has long been discussed, but has mostly been held up by lack of sufficient funding. Current development of the large Space Launch System (SLS) rocket, lower cost commercial vehicles, and better technologies, including use of in-space resources, is starting to change the situation. With limited funding, divisive arguments about "Moon vs Mars" and "Humans vs Robotic" have stalled program planning. In reality, these are false choices. With better technologies, multiple destinations are affordable. Robots can prepare the way for and assist humans. Leveraging commercial systems brings economies of scale which agency budgets can't reach on their own. The proper way to think of it is science and exploration pave the way for future commercial and industrial activity, and commerce and industry make the science and exploration more affordable. Part 4 of this book discusses both types of projects in the context of an integrated long-term program.
There are many planned astronomical instruments and interplanetary probes, and many more have been proposed, but are too far in time to be funded yet. Only a few human missions beyond Low Earth Orbit are in development or detailed planning. Detailed plans for Lunar or Mars missions have not yet been prepared, although technology work is in progress.

## Communication

The use of space for communications is well developed. It is the primary purpose of 52% of the 1261 operational satellites as of the end of 2014. Essentially all other satellites have some communications functions. If costs were lower, use of space communications would increase in areas like satellite broadband. This isn't used much today because of higher cost and technical limitations compared to ground services. A future large network of low-orbit satellites could overcome some of the limitations and provide world-wide coverage. To date, most satellite communications has been by microwave radio. Some experiments have been done with laser transmission, which can provide much higher bandwidth, especially for locations beyond Earth orbit.
* Gravity Lens Relay - Massive objects like stars bend light via gravity. If you travel a sufficient distance from the star, at least 550 AU for the Sun, that light reaches a focus. A far future project would be to use the star as a giant lens, to focus communications over interstellar distances. For optimal signal relay, you would use such gravity lenses between pairs of stars, and boost the signal at each star before passing it on to the next. Such a network could link the entire galaxy, although the speed of light presents a significant obstacle to practical use. Gravity lenses can also be used for astronomy, which requires the least effort as you only need equipment around the Sun, and for power beaming to distant spacecraft. The latter requires much more effort than communications, since the power levels are much higher.

## Entertainment

This category includes activities like Space Tourism, Zero-Gravity Sports, and other activities purely for entertainment. Tourism on Earth represents about 3% of worldwide spending, and involves over a billion annual travel arrivals. There is every reason to expect a lot of people will travel to space for entertainment, if only the extreme cost and significant risk were reduced. Spectator sports in zero gravity may come sooner than general travel, because top athlete income is already in the same range as astronaut launch cost to orbit. Racing or prize competitions are a possibility, since events like the America's Cup already draw huge budgets and involve high technology. Such competitions would also have the useful purpose of promoting improvements in space technology, as the Google Lunar X Prixe is demonstrating.

## [NOTES TO BE INCORPORATED]

The problem with the big rocket approach is that it is exactly as hard to make the next trip as the last trip. What we should be doing is building infrastructure to make future trips easier. My current favored approach, which is subject to change as new ideas come along is to:
* Use a hypervelocity gun on a mountain in Ecuador to launch small cargoes of supplies on a regular basis in tens to hundreds of kg per shot. Build up a manned base station there using robots to do the initial assembly work. People come later via ordinary rocket.
* At your low orbit station, assemble VASIMR type plasma rockets which can chug to nearby asteroids and return ~20x their weight in asteroid mass per trip. Choose a carbonaceous type asteroid, and use the carbon to start making space elevator cable. Use the metallic component to expand the orbital base. Use the oxygen component as more fuel for future
trips. You will need some chemical and metal processing equipment to do that, send it up in parts via your gun.
* As your space elevator cable grows, use the plasma rocket to raise the orbit gradually until it reaches about 30% of a full space elevator. That is the practical limit with carbon fiber we can make now (if we learn how to make better carbon fiber later, great, update the plans and upgrade the fiber). 30% is very useful. Take 30% off the velocity to reach earth orbit,and reuseable single stage to orbit vehicles become fairly easy to make. 70% of orbital velocity means 50% of kinetic energy relative to standard rockets.
* 30% above orbital speed on the upper end of the elevator cable is nearly escape velocity (which is 41% above orbital). So you are nearly able to get anywhere outside earth orbit. To start with that is grabbing more asteroid stuff to expand your system.
* Lunar orbit velocity is 21% of Earth orbit velocity. So if we can build a 30% Earth space elevator, we can build a full space elevator on the Moon. Now we can take off and land on the Moon with negligible fuel use.
* Since we already know how to make habitats and oxygen out of local materials, and use the O2 for fuel, we can pretty much spread out through all the nearby asteroids and the Moon. When we get to Phobos and Deimos, we can build a space elevator going down to Mars. Oh hey, Mars orbit velocity is 45% of Earth's, so our space elevator technology can take
care of 2/3 of the velocity to land, the rest is by some aerobraking and landing jets. Going up we can get our oxygen from the Martian atmosphere (its CO2), and only need to get to 1/3 of orbital speed to reach the elevator, so getting to and from Mars is relatively easy all around.
No, not necessarily a cycler orbit, that requires constant maintenance to keep it aligned at both ends. I mean something like an 18 month orbit which has a near point close to the Earth. Every second year you have a chance to hop aboard the habitat and ride it to near Mars orbit. If you put two of them in the same orbit, 180 degrees out of phase, you have a chance every year. The habitats are not sitting there doing nothing, they are collecting asteroid material from nearby, and extracting fuel, growing food in the greenhouse, making structural parts for landers, etc. So when a crew is heading to Mars, they pick up supplies on the way.
The far point of the orbit is close enough to Mars that it does not require a huge delta V to catch up, and if you refuel at the habitat, the normal rocket equation formula does not apply like going all the way from Earth on your own. You just have to set up the habitats so there is always one near the right place to meet Earth, and then drop you off near Mars. That requires several habitats whose near points are at different positions along the Earth's path.
One development plan concept
IDEF0 flow modeling
"Building Co-Op" for making homes, other stuff
Open Source Hardware & Telerobotics
Supersonic & Gun Launch,
Assembly Robots,
Leo station,
Plasma Thruster, Vex = 50,000 m/s F=ma=dm x v
Small NEA's,
    Candidates:1992 UY4, 2001 SK162, 2001 PM9
* Iron component of meteorites averages 90% Fe, 9.5 Ni, 0.65 Co
* Extract o2 for plasma fuel
Elevators
* at 1 g and 2400 m/s tip velocity, r = v^2/a = 588 km, p = 1540 s
Moon,
Eros + Ceres,
Deimos
Mars & Beyond