Section 4.9 - Processing Factory
The Orbital Assembly step used pre-made components supplied from Earth. This step adds the capability to convert raw materials gathered in the Orbital Mining step into useful inventory and supplies. These either feed into more assembly, or are sold to others, for example fuel and oxygen sold to other projects. The processing factory will not be able to produce 100% of every item needed. Rather, the goal is to produce as much as possible with the least initial equipment so as to reduce the amount that has to be launched from Earth.
Desired Factory Outputs
The output of the factory can be grouped into the following categories by type and complexity. The ones listed are expected to be needed in large amounts, and not too difficult to extract, but a good deal of research and engineering will be needed before you can produce a definitive list of what to make, and in what order.
- Shielding and counterweights - This requires minimal processing of mined ores. Shielding is needed for most locations beyond Low Earth Orbit due to natural radiation levels, thermal variations, and if man-made reactors are in use. Counterweights are needed for some artificial gravity and space elevator designs. For these uses, sifting ores for composition and density and packing into containers, or compressing/sintering into uniform blocks may be all that is needed. The leftovers after extracting other materials, known as tailings or slag in industrial ore extraction, can be used as shielding, as can unprocessed inventory.
- Oxygen - The obvious use is for breathing, but oxidation is useful for other chemical processes, especially high thrust chemical rockets. Electric thrusters can be designed to operate with Oxygen as the fuel, greatly reducing the amounts required for a given mission. Oxygen can be produced from a variety of ores via thermal roasting or electrolysis methods.
- Water - Again, water has an obvious use for humans, and for plants in a habitat. It also serves as a convenient way to store O2 and H2 propellants, needing just electrolysis to separate into it's components. Water is also a good shielding material. It also can be used in suitably designed electric thrusters. Some C-type asteroids contain water, and merely require mild heating to extract it.
- Iron and Steel - A basic construction material. About 6% of meteorites, and presumably a similar fraction of NEOs, are metallic. They consist of primarily an Iron-Nickel alloy, with 5-10% of the latter, and a small amount of Cobalt. The remainder is rocky components. Iron-Nickel as-is should be a ductile structural metal. With the addition of a small amount of carbon (up to 4%) it should become a reasonable steel or cast iron alloy. The more numerous C-type asteroids contain carbon, so obtaining that should not be hard.
- Other Metals - Magnesium and Silicon are common elements in the rocky portions of asteroids. These are not present as native metal like iron, but rather as oxides such as the mineral Olivine. Therefore to extract the metal requires removing the Oxygen, a process called Reduction. If Oxygen is being extracted for it's own sake from rocky material, then some metal reduction will occur as a side effect. Other elements may be present in useful amounts, but it requires more exploration to determine quantities.
- Glass - For observation and habitat windows, and industrial processes that need concentrated light in an enclosed space. Silicon dioxide, or quartz, is an excellent transparent material, and silicon and oxygen are abundant in rocky asteroid material. Virtually any regolith can be solar melted into a glassy obsidian, which is black due to the included magnesium and iron, but can be reliably created with very little infrastructure.
- Fibers - Fibers are useful for their high tensile strength, for cables or reinforcing. The two likely types to be made from NEO sources are fiberglass, which can be made from melted rock, and carbon fibers, which can be made from the organic component of C-type asteroids.
- Soil and nutrients - Although hydroponics can be used to grow things without soil, soil can serve a dual use as a radiation shield as both mineral grains and water in the soil are effective shielding materials. Research and testing is required to see if NEO-derived soils are will make a good growing medium and don't contain hazardous materials. Some experiments were done with Lunar soils from the Apollo program. Research is also required for what additives in the form of fertilizers and nutrients, and what seeding of soil with micro-organisms would be needed for a productive soil. After all that, soil-based plants need to be compared to hydroponics and aeroponics to get the overall best answer for food production and recycling of organics.
Factory Design and Development
There is a great deal of experience with industrial processes on Earth, but not much for in space or at other locations. Before planning major projects, then, a rational development program should be done to gain the needed experience. A progression of experiments would run roughly like the following example for extracting oxygen from NEO regolith:
- Simulate the processes via mathematical and computer models
- Demonstrate extraction with prototype hardware and simulated regolith made from similar minerals
- Optionally demonstrate with meteorite material on Earth
- Demonstrate extraction with a prototype in Earth orbit using simulated regolith
- Demonstrate extraction with real NEO mined materials
- With experience from prototype experiments, design production equipment
The many industrial processes used on Earth are candidates to use in space, along with some that can uniquely be done in space. The selection of processes to use needs to take account of the differences of the space environment:
- Lack of gravity unless you design for it by rotation
- Easy access to vacuum and plentiful full spectrum sunlight
- Relatively high cost of supplies and materials from Earth
- Relative difficulty to dispose of waste heat
Suitable Industrial Processes
Many large scale industrial and mining processes on Earth, such as ball milling and gravity separation, assume the presence of gravity, and would require large motors to produce equivalent centrifugal effects in space. Froth flotation, very common in mineral ore beneficiation, requires both gravity and large amounts of volatile water.
In space, solar thermal processing is likely the simplest starting point for material processing, because solar reflectors require no moving parts. Clear candidates include roasting volatiles such as oxygen and water from ores. Many high-end industrial processes on Earth use vacuum furnaces based on electromagnetic induction or electric arc, heating mechanisms which a solar furnace may be able to augment or directly replace. Due to the high temperatures achievable in the vacuum of space, ore beneficiation could be performed by vacuum distillation, successively extracting metal oxides with increasing boiling points or differing relative volatility, such as silicon dioxide (boils at 3220K).
Direct electrolytic processing is also promising, using electrons from photovoltaic panels to directly drive chemical reactions. The electrolysis of water into hydrogen and oxygen is the simplest example, but many others are possible. On Earth, the most common industrial application of electrolytic reduction is in Hall–Héroult cells for the production of aluminum metal from aluminum oxide, which liberates oxygen gas. Aluminium is only a few percent of most asteroids, but direct electrolytic reduction of iron, silicon, and magnesium has been demonstrated, and electrolytic reduction of magnesium chloride dominates terrestrial industrial production. Electrolysis is typically performed in either a conductive liquid such as a molten salt; or in a solid oxide (solid oxide electrolysis or SOE), which can be a variety of electroceramics, frequently yttria-stabilized zirconia (YSZ).
For separating elements from ore, a variety of schemes have been proposed. Electrophoresis separates consitutent particles by size, and is commonly used in DNA analysis. Electromagnetic isotope separation in a calutron separates charged particles by mass, by ionizing the atoms in a near vacuum, and using a magnetic field to exploit the mass dependence of a charged particle's curved flight path. Very much like an ion drive, this process only produces milliamp ion currents due to the space charge effect, and requires high voltages and near-vacuum.
Native iron is available both on the Moon as impact debris from asteroids, and in metallic asteroids. Thus an iron-based starter factory is worth considering. On Earth, iron/steel is obviously a large part of the existing economy, especially in making manufactured items. Consider using metal scrap as a material in a starter factory on Earth. That will translate to space directly, where, for example, using trees as a starter material for wood construction would not. Iron is also available in oxidized minerals, but that requires more chemical processing than already reduced native metals.
An iron-based path would start by concentrating on items made from iron/steel. This would include structures, tanks and piping for chemical processing, reflectors for solar power (with an aluminum coating), electric generators to use the concentrated light, and, of course, machine tools for making more items with. Once the initial production is established, other types of products would gradually be added.
This path starts with extracting consumable supplies such as Oxygen and water.
Full Use Path
This path starts with the assumption that you want to use as much of the NEO starter material as possible, so you include whatever equipment is needed to do that. Since NEO composition varies by object, you would either need to know which object you are going to use, or bring back material from various NEOs to get the required composition mix.
A fundamental design feature of a space-based factory is it does not need to have a single fixed design like a car or a computer monitor on Earth. Rather, it can evolve and grow by several methods. One is to add new processing flows with new types of equipment. This expands the range of outputs it can produce. Starting with fewer types of equipment is partly for cost reasons - it would be too expensive to launch every kind of equipment at the start, and partly because all the various processing methods will not be adapted for space right away. In the longer term, new technology will come along which was not available at the start. Another growth path is increasing scale by adding more copies or larger versions of the existing equipment types. With a larger range of outputs and the ability to assemble items developed previously, the factory complex will be able to grow itself mostly from internal production rather than deliveries from Earth.
Likely early components for the factory will include:
- Various machine tools for making parts
- Smelter for extracting metals
- Thermal refinery for extracting liquids and gases
- Some robots for remote control, so it can operate before the people get there and prepare things.
Objects in space are inherently mobile. The industrial capacity does not need to stay as a single factory in a single location. As the processing capabilities and the set of users develop, you can expand from the initial factory location, likely in Earth orbit, to multiple locations with optimized capabilities for each location. Eventually you end up with a network of mining stations, ore processing stations, assembly stations, and supply depots/habitats, with some locations doing multiple tasks. A number of questions need to be answered about where to put the industrial elements:
- Should it start with a single location or multiple locations? It can split over time if the facility duplicates itself or builds specialized units for different locations.
- A location close to Earth minimizes cost of delivering the fraction of supplies and equipment that needs to come from there. It also allows for real-time control from the ground, either by computers or humans. On the other hand, a higher orbit has more exposure to sunlight, and is less delta velocity to deliver raw materials from NEOs. So where is the optimum location(s) for a given process?
- You can have a strategy of starting in low orbit at the start, when more material is coming from Earth, and then move to a higher orbit once a higher percentage of NEO material is used. Eventually you might move the processing factory all the way to the NEO, and only ship finished items back.
With the exception of living plants, most processes require a power source. In near-Earth space sunlight is abundant and higher intensity than on Earth due to lack of atmospheric absorption and weather, and is available a higher percentage of the time. Photovoltaic panels have been the primary method to generate power because they are relatively light, have no moving parts, and can be made in whatever reasonable size you want. For very high power levels the panel area becomes large and thus a design challenge. If you have a source of building materials, and the factory is not moving about much, then heavier generator types can be considered that use concentrated sunlight. That includes concentrated photovoltaic, where the panels come from Earth, and the reflectors to concentrate the light are made locally.
Many industrial processes require high temperatures. Solar furnaces can reach temperatures only limited by the surface temperature of the Sun, which is sufficient for many processes. For the few that require higher temperatures, electric driven devices can be used. It is also fairly easy to reach cold temperatures, by blocking sunlight and exposing items to the cosmic background at 2.7K as a heat sink.
Processing Factory Example
In this example we start with Earth Orbit mining, and expand outward in steps.
We use orbital scoop mining deployed from our previously built assembly station to collect air from the upper atmosphere and use it for fuel and breathing supplies.
Using fuel obtained in previous step, dispatch small electric tugs to collect orbital debris for raw materials, then feed it to the processing unit.
Send larger tugs with robotic capability to salvage satellites which have run out of fuel or otherwise can be repaired or bring back to have useful parts extracted. This will develop orbital maintenance skills, and allow sale of repaired items.
High Orbit Station
When sufficient fuel has been collected, move the processing plant or (part of it) to high orbit location. For our example wee choose a 13.66 day elliptical orbit for ease of access to the Moon and NEO orbits. This is half the Moon's orbital period. From the orbit period formula, this gives a semi-major axis of 241,400 km. We place the high point near the average distance of the Moon, but 90 degrees offset in position, so the Moon will not change the orbit significantly. The orbit then becomes approximately 132,800 x 350,000 km in radius. This orbit is chosen to for access to other locations. To depart for Near Earth Asteroids, vehicles will add velocity so that they encounter the Moon and get a gravity assist. To deliver items to lower orbit, they subtract velocity until they can aerobrake. To reach the Moon itself, They do a mild gravity assist to raise the orbit to near Lunar, then use propulsion to enter Lunar orbit.
A radiation-hardened solar array will be needed for the orbital tug to get through the radiation belt. Lightweight arrays can be used beyond that region. They are delivered folded and protected from radiation to maximize their later performance. The High Orbit Station will serve multiple purposes: laboratory/research park, processing plant, and assembly and repair.