Asteroids are a potentially huge and as yet unexploited source of many materials, especially metals. There are about 200,000 near Earth asteroids of size 100 metres diameter and larger. Each one contains about 3 million tonnes of material. This is a very useful resource that so far we are not taking advantage of.
We know what asteroids are made up of from meteorites that have fallen to Earth’s surface, and telescopic studies. Meteorites can be divided into four categories.
· ‘Iron meteorites’ are usually made up of iron-nickel (Fe-Ni) metal.
· ‘Stony irons’ consist of mixtures of Fe-Ni metal silicates and other minerals.
· ‘Achondrites’ are silicate rich and consist of a broad range of minerals.
· ‘Chondrites’ probably came from parent bodies that were too small to undergo a large degree of gravitational difference. They have tiny pellets of rock embedded in them and consist of variable percentages of Fe-Ni metal, usually between 0.3% and 35%.
Asteroids are rich in precious and ‘strategic’ metals, such as cobalt and the platinum group. They also contain many volatiles and mineral oxides suitable for refining on DaedalusaL4.
The Moon
As discussed in 4, Materials, the Moon is also rich in metals (mainly titanium, aluminium, magnesium and iron), but not in as high percentages as asteroids (see table 2.2b). It consists of many volatiles and supports large amounts of water in the form of ice. There are also substantial amounts of silica available, which is needed for window and solar cell production.
11.2 Refining
Asteroids
Much of the processing can be done on the asteroid itself. Temporary solar ovens can be easily constructed to melt the metals, which can then be moulded for many uses both in space and on Earth. Precious metals are plentiful on asteroids, and there would be a good market for these on Earth. As another of our industries is the production and maintenance of solar power satellites, we can use products of our mining to build and repair such satellites. The volatiles can be processed and reacted to make fuel propellants, and also to replenish oxygen supplies and other atmospheric requirements in the space station.
The Moon
Solar ovens will be used to melt the silica mined from the Moon to make windows and solar cells. This processing will take place on DL4. The addition of sodium oxide (Na2O) – which is used to lower the melting temperature that has to be generated by the furnace – will not be needed as the solar ovens are able to reach much greater temperatures than possible on Earth. Waste products left over will be used to shield the station from radiation and meteor impact. Waste products can also be used as aerobrake shields for the planetary bound cargoes.
There are five possible ways of element and mineral separation:
· magnetic separation of free metals
· thermal extraction of volatiles
· separating minerals by electrostatic benefaction
· separating minerals by vibration or flotation
· separating minerals by electrophoresis
On DL4 we will use magnetic separation, thermal extraction of volatiles and electrophoresis, as we found these to be the easiest and most economic of all the options.
Magnetic Separation of Free Metals
This is a relatively simple process where material is ground and then passed by a powerful magnet to separate the Fe-Ni granules from the silicate grains. A possible method is to drop a stream of material onto a rotating magnetic drum. The metal is then scraped off and passed through an impact grinder. (This is a piece of equipment where a rapidly spinning wheel pushes the material down its spokes and flings it against an impact block. Any silicate impurities are shattered off.) Then the material is once again dropped onto a magnetic drum and the result is highly pure free metal.
Thermal Extraction of Volatiles
Here the material is channelled into a solar oven where the volatiles - water, hydrogen, carbon, sulphur etc. – are cooked out. Lightweight aluminium foil mirrors can be used as a solar oven in space. The gas stream is piped out into tanks located in a cold shadow of space. These tanks are placed so that the furthest one away is the coldest. Therefore the different volatiles, with different freezing points, freeze in different tanks, i.e. - water freezes in the first tank, but the rest of the gases carry on down the pipe to the next tank, where another of the gases freezes. Solar ovens can also be used to melt the Fe-Ni metal, which can then be used to make tanks to store the frozen volatiles.
Electrophoresis
Electrophoresis is a high performance separation process that is ideal to space as it performs best in a micro-gravity environment while it has still been used on Earth for decades in medical and biological fields. An electric field is created across a tank filled with a fluid by oppositely charging two parallel walls. The ground mineral grains to be separated are put into the fluid where they are suspended due to zero gravity. Each mineral will collect different electric charges. The minerals then move to certain positions between the two walls, where they form planes of pure minerals parallel to the walls.
11.4 Solar Power Satellites
Another potentially lucrative industry is the construction and operation of solar power satellites. Such a system will also be used to power the station itself (see 2.2 Power Generation Systems).
This idea would place solar collectors in orbit, to collect the sun’s power, convert it to microwave energy and beam it to large rectifying antennas (rectennas) on Earth for distribution in the electric power grid.
There have been many designs for solar power satellite systems. The first, the SPS reference system involved the construction of an enormous single platform, assembled in space from large, compression-stabilised struts and joints as the fundamental building block. These were designed to measure about 10km by 5km and about 0.5 km deep and to eventually provide three major functions: power collection, platform support systems, and radio frequency power generation and transmission. Power collection involved the use of photovoltaic cells. When sunlight hits these cells, it generates DC electricity. The electricity is then passed through a converter that turns the electricity into 10cm microwave beams, which are transmitted to Earth to be picked up by rectenna and converted to AC electricity. The projected cost was $250 billion before the first Kilowatt-Hour was produced.
Between 1995 and 1997, NASA conducted a re-examination of the SPS system. This ‘Fresh Look Study’ resulted in the design of a number of SPS systems that could “deliver energy into terrestrial electrical power grids at prices equal to or below ground alternatives in a variety of markets, do so without major environmental drawbacks, and which could be developed at a fraction of the initial investment projected for the SPS reference system of the 1970’s”.
The two most likely suggestions are the ‘Sun Tower’ and the ‘Solar Disc’.
This
concept is an array of radio frequency (RF) transmitting space solar power satellites.
The satellites have been described as resembling large Earth-pointing sunflowers,
where the face of the flower is the transmitting array and the leaves on the
stalk are solar collectors. They would be placed in a sun-synchronous orbit.
The transmitter design consists of 0.5cm transmitting elements, having a total
diameter of 260m, and being approximately 0.5 – 1m thick. The primary technology
option for the sunlight-to-electrical power conversion is a gossamer structure
based reflector with non-dynamic conversion at the focus (advanced photovoltaics).
These collection systems are presumed to be always Sun facing, attached regularly
in pairs and to be about 50 – 100 meters in diameter. Each satellite is capable
of producing 250MW. The ground receiver for the Sun Tower is a 4 km diameter
site with direct feed into the power grid. A ground based energy storage system
would be required, in particular in the early phases of deployment in which
only one Sun Tower will be operational.
The predicted cost is approximately $12 billion for one sun tower, before the first Kilowatt-Hour is produced (using Earth materials), however this would be significantly decreased if lunar or asteroidal materials were used. About 18 of these satellites would be needed.
The Solar Disc is a large Sun-pointing
disc (of approx. diameter 5km) in GeoSynchronous orbit. The sunlight to electrical
energy conversion is via thin-film photovoltaic array. This energy is transferred
along two redundant structures to an Earth-pointing phased transmitter array.
This array is designed to be of diameter 1km, and thickness 2m. It will be ‘tiled’
in transmitting elements. Each element is hexagonal in shape and about 5cm in
diameter. Each satellite is capable of transmitting at 5.8 GHz. The receiving
ground segment is designed to be a 5km site with direct feed into a power grid.
A ground based energy storage system would not be required.
The predicted cost is approximately $40 billion for the production of one Solar Disc before the first Kilowatt-Hour is produced. However, as above with the Sun Tower, this figure was calculated as using Earth materials, and will be significantly reduced with the use of asteroidal and lunar materials. The current market would be sufficiently satisfied with the production of 3-4 satellites.
On DL4 the Solar Disc will be used for the following reasons:
1. Though the cost is greater per Solar Disc, the overall electricity production is a lot greater.
2. The Sun Tower is designed for SunSynchronous orbit, which is harder to obtain from DL4 than GeoSynchronous orbit, which the Solar Disc is designed for.
3. Because of DL4’s location, the workers will be closer to the position of the solar disc, so it will be easier to build the satellites in the beginning, and perform repairs when it is in operation.
The more of these goods that we can produce on the station, the cheaper the cost to the residents and the more self sufficient DL4 will be. While taking this approach, to manufacturing onboard DaedalusaL4, we recognise that there will be some changes in the production and packaging of goods.
Firstly glass and aluminium along with many other materials (see 4 Materials) can be produced from lunar or asteroid sources and residents will be encouraged to use them to replace terrestrial equivalents. However, additional problems remain.
Plastics and Polymers
Today’s society has become so dependent on plastic goods that it is hard to imagine life without them, so we didn't. We investigated the areas of plastic production and resource reclamation from polymers, to find the cheapest way of producing and disposing of plastics on board DL4.
There are three options for the production or supply of plastics:
· The first is to produce plastics from scratch on DL4 using terrestrial raw materials. The one advantage of this that has been suggested is that in micro-gravity, higher quality plastics could be produced. This was ruled out due to the cost, and its impingement on the ideal of self-sufficiency.
· The second option is to import all finished plastics from Earth. Again, this would go against the policy of self-sufficiency and would also incur high costs. We realise that some will have to be imported, especially for durable goods, but we would aim to keep this at a minimum.
· The third and only viable option is biodegradable polymers. These cannot be recycled, but as the name suggests, they are suitable for disposal using the biological waste decomposition system, which will be in place (see 17 Life Support Systems). There is much research and interest in biodegradable polymers at the present time. To be useful, they must posses the ability to be fully broken down, these products must not be toxic to the living organisms in the environment that it is used in, and the polymer must possess the qualities that make it attractive in the first place, e.g. – it must be water resistant, strong etc.
Therefore biodegradable polymers, which are made by infusing polymers with starch, were chosen. There are two basic ways of adding starch to a polymer - adding an acrylic acid to a polymer chain or grafting a starch molecule with a polyethylene molecule. Various plant starches are being researched for use, including sorghum and soy, both of which will be grown on DaedalusaL4. The settlement will obviously use the more economical of the two.
The process requires the extraction of protein from the plant chosen. This will already be in place for soybeans (for TVP production) so, for convenience, it is the first choice. Additionally, biodegradable soy protein plastic has been documented at levels of 80% volume of soy protein, which is higher than any we know of for sorghum. Various different chemicals may need to be added to create the finished product, but even supplying these the costs of production remains lower than the import of terrestrially produced plastics.
Biodegradable plastics made from plant protein, grown onboard DaedalusaL4, will fulfil the need for disposable plastics, and hopefully go some way to manufacturing durable goods also.
The other idea that will also be important to achieve self-sufficiency, will be the process of resource reclamation. There are a few problems on this front. All plastics react differently to heat and the various other methods used to return them to their ‘original’ states. (This is usually not possible, but they are often broken up into smaller, re-processible chains) There are three methods that we investigated.
The traditional method of recycling means that each individual type of plastic must be separated from the others, and then treated. This separation must be total, as one single PVC bottle in a batch of 10,000 PET bottles can ruin the lot. The method that is used at the moment, is identification of the plastics as they are made and then hand sorting of the plastics, as they are disposed of. This can be time consuming and it is not always reliable. Infra-red light can also be used to separate the plastics, and this is significantly more accurate.
Traditional methods were ruled out on the grounds that the finished products were not of a high enough standard to warrant that amount of time and energy in reprocessing them. Also, the amount of plastic waste that would be generated by a community of ten thousand is too small to have individual systems for recycling and reprocessing each type of plastic separately.
The option that we chose is solid state shear extrusion pulverization. This process subjects polymers to high shear and high pressure, while rapidly removing frictional heat. It can be used on unsorted plastics, and it produces a uniform powder that can be used to make everything from appliance parts to furnishings (ref. 102). The disadvantage is that the products are only suitable for non-food applications and that any dyes or other additions that need to be added to the powder to create the finished product will have to be imported. This process will be suitable for recovering resources from any plastic goods that residents choose to bring to the station.
Paper
Paper will be required, in some shape or form, but ideally, it will be kept to a minimum. Having researched the quantities required for paper production (trees, water) and taking into account the chemicals used to make it ‘pleasing to the eye’ (chlorine), it was decided that it would not be feasible or profitable to have full-scale paper production on DaedalusaL4. There would also be many waste products associated with the production, and the trees, grown in recreational areas, will not be sufficient in number or growth rate to support a paper industry. We will however, attempt to recycle as much of the paper that is used as possible. Paper food packaging and bags are ideal, as the colour is not very important, and no dyes that would be hard to produce or recover from the waste would have to be used.
Clothing
Finally, materials such as cotton and linen, which can be produced from plants, will be used for clothing and other soft goods. They have the advantage of being readily broken down biologically. To add a little variety, dyes will also be used, but these will have to be restricted to natural plant based ones to avoid harmful effluent, and excessive transportation costs.
