Section 3.4 - Resource Extraction
This section discusses how to extract resources from their natural state. For matter and energy these are commonly called mining and energy production, but the latter is a misnomer. Energy cannot be created, it can only be converted from an existing source, so we prefer the term Extracted.
Mining is the process of extracting, or in the case of dispersed materials, collecting, physical materials for direct use or further processing. A Deposit or Ore is a naturally occurring material of sufficient size or concentration to be mined economically. Mining techniques will vary according to what you are extracting and where you are doing it. So this section organizes the methods by physical state and type of natural body being mined. Some processing may occur at the mining site to concentrate the ore or prepare it for transport. Concentration is called Beneficiation, and there are a number of other processes which can be applied, such as crushing and sintering.
Most of the baryonic mass in the Universe is inconveniently located in the interior of large bodies, where it hard to get to. In fact spheres, the shape which many large objects approximate, have the least surface area for a given volume. In other words, the ratio of relatively inaccessible material in the interior to accessible material on the surface is a maximum. Another problem is that useful metals, such as Iron, tend to collect in the center of large objects due to their density, where they are inaccessible due to surrounding layers of rock. Besides physical inaccessibility, much of the matter is in a plasma state (e.g. in stars) or very low density (e.g. in molecular clouds). So the challenge for mining is accessing the small fraction of currently accessible material, and developing techniques to increase the range of such materials.
Even conventional mining on Earth literally only "scrapes the surface". So-called deep mining and drilling typically only reach the top 0.1% of the planet's radius. This limits our ability to obtain rare materials, or enough of common materials for future large projects. Mining in space is one method to increase the range of accessible materials, because it is new accessible area with different history and composition than Earth, but it should be considered a part of the total challenge for future mining.
Mining Solid Bodies
Mining by G-Level
We can divide solid bodies into groups by size. Small bodies are such that typical mining equipment forces and velocities are larger than local gravity. Therefore equipment anchoring to keep it from moving itself is needed, or special design to contain gravel and dust from traveling long distances, or even leaving the body entirely. By extrapolation from Earth construction equipment, local gravity below 2 m/s2 (0.2 g) will require lower equipment forces or anchoring. Blasting is the most energetic mining operation. From blasting safety codes we estimate that gravity below 0.3 m/s2 (0.03 g) would require safety distances of over 1 km from shrapnel and debris, which becomes unreasonable. That value will likely need updating from actual experience. For now we will divide bodies into low, medium, and high gravity for mining purposes at 0.3 and 2 meters/s2. For the very smallest objects you do not mine them in the sense of removing material from a larger body, but capture and transport them to a processing location if needed.
Mining by Depth
We can also divide mining techniques by depth. These are surface, sub-surface or underground, and deep mining. Sub-surface is anywhere an open pit or angle of repose is not sufficient, and columns or walls are needed for support. The support can be part of the natural materials left in place, or artificial supports installed as part of the mine development. Deep mining is when the gravity loads on the natural materials from what is above it start to cause shifting or collapse. This requires fully enclosed tunnels or pipes or other special techniques. The strength of the surrounding material will determine when you reach a deep condition, and this will vary from place to place. As a guide, for rocky locations, deep can be considered more than 15km /(local acceleration of gravity in m/s2). Gravity falls as you move to the center of a body, so aside from the few largest asteroids, the whole interior may not reach a deep condition from lack of sufficient gravity load. Conversely, on a larger body such as the Moon, you can reach a deep condition within 0.5% of its radius due to higher gravity and larger radius.
We will take as an example of deep mining to excavate the core of the second most massive asteroid, 4 Vesta, for Iron. Vesta melted in its early formation from radioactive decay, and formed an Iron core and rocky mantle. It has the following properties:
- Polar Radius = 229 km surface, 106 km core
- Equatorial Radius = 305 km high point, 114 km core
- Gravity = 0.329 polar, 0.1532 equatorial high point, and 0.257 core in m/s2. The much lower equatorial gravity is due to the larger radius and rapid rotation (5.34 hour rotation period).
- Mantle Density = 3115 kg/m3
As an approximation we take the polar and equatorial average acceleration from surface to core times the distance. This amounts to 123,000 m x 0.293 and 191,000 m x 0.205. The polar value is slightly lower, so we dig down from the pole. A column of mantle rock of that height, density, and acceleration sees a pressure at the bottom of 112.26 MPa ( 16,300 psi ). This is higher than the likely strength of the rock, so an unsupported hole will likely fail, and we are indeed in a deep mining condition. Therefore the hole will need a lining such as steel obtained elsewhere, at least for the deeper parts. The core itself, being made of mostly Iron, will be self-supporting until too much of it has been extracted and it undergoes core collapse. About half of the core can be extracted safely, amounting to 23.3 million gigatons, or about 15 million years of Earth production at current rates.
At some point, even deep mining techniques will become impractical. Many bodies have hot interiors or liquid layers, and even without those problems, pressures would require tunnel or pipe walls too thick to be worth installing. The only approach to reaching these very deep resources is to remove all the overlying material first, which amounts to disassembling the body or a large portion of it.
The energy holding a large object together due to its gravity is called the Gravitational Binding Energy - . For a uniform sphere it is found by the formula
where G is the gravitational constant, M is the mass of the sphere, and r is its radius. Thus to extract all the matter from the object requires at least this amount of energy input at 100% efficiency. This assumes you don't do any processing, just physically remove all the material. As an example, for the Earth, allowing for the actual distribution of mass and density by depth, U = 2.487 · 1032 J.
Dis-Assembly Time - This is a characteristic time found by dividing the binding energy U, by the solar flux falling on the object. There are other energy sources which could be used, but sunlight is generally available in space and so gives a natural value for the time to dismantle the object using the solar energy falling on it at 100% efficiency. From the characteristic value, you can make estimates of the actual time for a large mining project by multiplying by the fraction of the object you intend to extract, and dividing by the solar conversion efficiency. The solar flux for the Earth averages 1360 W/m^2, and the radius is 6378000 meters, thus the total flux is 1.74 · 1017 J/s. This gives a characteristic dis-assembly time of 45.3 million years.
For bodies with appreciable surface pressure and a distinct solid or liquid surface, bulk mining of the atmosphere is relatively straightforward. The main requirement is a pump and a storage tank. If a particular component needs to be separated out it becomes more complex, needing liquefaction or freezing to separate the different compounds. For a low pressure atmosphere, methods like selective ionization or mass spectrometry may work to separate particular components. For bodies without a distinct surface, or in cases where you don't want to land, the orbital scoop mining method can be used.
This method involves skimming the upper atmosphere of an object to collect gas, then ejecting part of the collected mass as propellant to make up for drag. The altitude to collect gas is selected based on drag and heating levels for the scoop, and available thrust. The scoop is shaped as an inverse nozzle, converting a high velocity, low pressure stream into a low velocity (relative to the scoop) high pressure gas. The collected gas is pumped into a storage tank. Since collection velocity for objects with atmospheres is typically higher than a chemical rocket exhaust velocity, the portion ejected to make up for drag will need to be at electric thruster velocities (30-50 km/s). To maintain orbit, the average thrust must equal the average drag. The ratio of scoop velocity / exhaust velocity determines what fraction of the collected gas needs to be used for thrust. The concept will work in theory for any body where the exhaust velocity is sufficiently higher than the orbit velocity.
Intermittent Operation - Take as an example mining the Earth's atmosphere from orbit. At an altitude of 150 km, the density is 3 x 10-9 kg/m3. A vehicle traveling at 7800 m/s relative to the equator, which is 450 m/s above circular orbit velocity at that altitude, will encounter 23.4 micrograms of air per square meter per second, and see a drag of 0.18 N/m2. The stagnation pressure if the scoop fully brings the airflow to a stop is 1350 Pa (0.2 psi). A vacuum pump pulls this gas into a storage tank at higher pressure. Assume the scoop system has a mass of 100 kg/m2. Therefore it will lose 400 m/s of velocity in 220,000 seconds (2.6 days). The orbit is elliptical, so the collection will only be at full pressure at the lowest point, and actually take about 4 times longer. The scoop will have collected 5.2 kg of air per square meter over this time. Before too much drag happens, the scoop raises its perigee by 50 km using an electric thruster to stop the collection. This requires 15 m/s of velocity. Raising apogee by another 400 m/s, and then dropping perigee by 50 km to start the collection cycle gives a total velocity of 465 m/s required. At an exhaust velocity of 50 km/s, performing the velocity changes will consume 0.98 kg of propellant, which we assume to be some of the collected air. This leaves us with a net of 4.2 kg collected.
Continuous Operation - For a second example, assume a VASIMR type thruster which uses 200 kW power to produce 5.7 N thrust. The solar arrays need to be about 13 x 42 m in size, broken into 4 smaller arrays of 13x10.5 m which each follow the Sun. They are arranged lengthwise along the orbit direction, with the thruster in the back. Assume the thruster works no more than 30% of the time while mining, to allow margin to eventually raise orbit. Therefore the average drag can be 1.7 N. Assume a 13x10.5 m scoop at the front, which matches the solar array dimensions, thus a 136.5 square meter area. The atmospheric density must then be 2.3 x 10-10 kg/m3, which occurs at 200 km altitude. The scoop will collect 0.23 grams/sec, and the thruster will consume 0.034 grams/sec, leaving a net of 0.196 g/s (16.9 kg/day). Eventually whatever storage tanks you use are full, and the mining ship increases thrust and climbs to an orbit where it is not seeing significant drag. If the tanks hold 5 tons of air, the mining ship can collect it in 300 days. The question is then what the mass of the mining ship, to determine a mass return ratio.
Design Concept - The illustration shows a general concept for the scoop miner. The scoop is shaped as a hyperbolic cone and functions similarly to a turbo-molecular pump by bouncing incoming molecules off its surface. Once the density is sufficiently high for the air to act as a fluid instead of individual atoms, a stagnation region will form at the narrow end, which a conventional vacuum pump can collect from. Solar arrays are mounted behind the scoop so as to not increase drag, and are pivoted to follow the Sun as the vehicle orbits. At the rear are a storage tank, and the electric thruster powered by the arrays. As a safety measure, the vehicle should carry conventional thrusters and fuel to raise orbit if the main electric thruster fails.
Trolling for Air - Trolling is meant in the fishing sense, and not the annoying Internet person sense. In this version, the electric thruster and main solar arrays are at a higher altitude, and so not limited in size and power by drag limits. The scoop, pumps, smaller solar array to power the pumps, and storage tank are lowered on cables to an optimum altitude to collect air. If the same size as the continuous operation version above has an empty mass of 1 ton, then it needs to be at least 1 km below the center of mass for the gravity gradient (tide, or differential gravity) to be larger than the drag force. It will then hang at a trailing angle balancing gravity and drag. To have enough difference in air pressure to be worth lowering the scoop, the cable will be much longer than 1 km. When the tank is full, the cables are reeled in and the tank is unloaded or swapped. This saves using propellant for climbing up and down from the scoop altitude, and allows more powerful thrusters. Whether it is an overall advantage requires a more detailed study.
- Comment by Wicked_Inygma on Reddit, 4 Jun 2013: "Terrestrial air scoops were studied many years ago by R.H. Reichel at Boeing. The work is published in Electric Propulsion Development Vol. 9, a paper from the 1963 Electric Propulsion Conference in Berkeley and in a 1978 JBIS article. The operation of the scoop is considered in continuum flow (110 km altitude) and in a free molecule flow environment (160 km altitude). The work examines power requirements for liquefaction as well as radiator area requirements. The famed Michael Minovitch also studied terrestrial air scoops in detail. His 1988 patent for a self-refueling space propulsion system deals with a scoop that would operate in working fluid environments and operating between 50 and 100 km altitude."
A few bodies, the Earth being a notable one, have surface liquids, and others are known or suspected of having subsurface liquids. Collecting surface liquids in bulk is again a straightforward matter, needing a pump and storage tank. If the liquid is a mixture of compounds and a particular component is desired, then physical or chemical processes need to be applied to separate the desired component.
Drilling into a body with a submerged ocean, like Europa, might be hard, but seismic measurements should not be. Place one or more seismic detectors on Europa's surface, and then smack it really hard, like with the upper stage that launched the probe, and look at the vibrations you get. That's how we know about the interior of the Earth and prospect for underground resources. Boundaries like ice/water tend to reflect vibration waves (ie sound).
There will likely be natural vibrations from Europa flexing and moving (Europaquakes), but you can't be sure of them, so better to have your own source of vibration from a high speed impact.
Mining Gas Giants
Gas Giants have no solid surface. At a particular depth their atmospheres gradually become supercritical fluids, rather than a distinct layer, so different mining techniques are needed, especially if you want to access deeper materials. Scoop mining has been described above, for skimming the upper atmosphere. Buoyant mining equipment is possible but difficult. Gas giant atmospheres typically are mostly Hydrogen and Helium, so it is difficult to design equipment lighter on average than the lightest two elements. Reaching orbit is also difficult due to the thickness of the atmosphere and depth of the gravity well. To access deeper materials, large scale and rather drastic methods would be needed such as spin-up, boil-off, and disruption. The last is not recommended for inhabited stellar systems.
If scoop mining is insufficient in volume, and buoyant mining is too difficult, this method increases the typically fast rotation rate of gas giants until the equator is moving closer to orbital velocity. This makes removal of material from the equator to orbit easier. There are a number of techniques for increasing the rotation rate:
- Aerobraking momentum transfer: - A vehicle accelerates while in orbit around the gas giant, then deposits excess velocity by braking in the upper atmosphere, adding momentum to the planet.
- Kinetic deposition - Very high speed objects are directed at the equatorial region of the gas giant, such that the average rotation rate increases. Depending on impact velocity, the impactor may be absorbed by the planet, or material may be kicked out as ejecta.
- Tidal coupling - Intentionally place one or more sub-synchronous satellites in low orbits to raise tides which will accelerate the gas giant's rotation. This method is very slow.
- Magnetic coupling - If the gas giant has a magnetic field, react against that field to spin up the core, and eventually the rest of the planet.
- Reaction motor - A large high velocity fusion engine, powered by Hydrogen from the gas giant's atmosphere, is mounted in the upper atmosphere, and accelerates itself against atmospheric drag, which ends up adding to the planet's rotation. For this to function, the exhaust plume needs to be able to leave the planet.
This method involves reversing the way the planet formed in the first place. Planets form by collapse of a gas cloud as it radiates away energy. The largest Solar System gas giant, Jupiter, is apparently still radiating away excess heat today, after 5 billion years. If excess solar energy is directed at a gas giant, it will heat up and reverse this process. Assuming the outer layers heat up faster than the interior, this would puff up the atmosphere into regions where orbital and escape velocities are lower.
This is a brute-force method. One approach involves directing a large body at high speed at the planet. The other approach is to collect hydrogen, deuterium, or helium-3 and use them to make a very large thermonuclear device. The end result to to throw large amounts of material into planetary orbit or entirely disrupt the planet. It will be difficult to get material to only go into orbit and not escape completely, which is why this method is not recommended for systems that are already inhabited.
Stellar systems and interstellar regions contain small objects, particles, and gases in low density distributions. These include:
- Asteroid rocks smaller than 6 meters diameter and mass below 200 tons, for which collecting is a more apt term than mining.
- Smaller particles such as those which create the Zodiacal Light, coming from asteroid and comet debris, which range down to 10 micrometers in size.
- A flux of particles called a stellar wind emitted from stars.
- The interstellar medium, consisting of very low density gas and dust grains
- Interstellar gas clouds and nebulae of higher density than the general interstellar medium, but still low density by ordinary standards.
The challenge for mining these types of materials is their low density, forcing large collection volumes in order to collect significant amounts.
If a civilization's material needs are large enough, it may consider mining the stars themselves, where most of the mass of a stellar system is located. The temperature of stars is an obvious difficulty in doing this. The following methods are speculative at this time:
Artificial Red Giant
Class M stars have surface temperatures below the melting point of the most refractory compounds, and so might be mined directly. If you surround a star with reflectors such that most of the energy is trapped, the outer layers of the star will heat up and expand, creating an artificial red giant. When sufficiently expanded, you can skim off material and take it elsewhere to be used
Artificial magnetic fields might be used to create flares or jets from the star. When sufficiently far away the material is collected and used.
A sufficiently large planet is placed in a highly elliptical orbit which grazes a cool star. The planet will collect an atmosphere by gravity during the grazing. In the outer part of its orbit it will cool down and can be mined, and the process repeats. The required size of the planet is such that it does not lose mass from evaporation during the close pass.
There are numerous energy sources in the environment near stars. They are listed below in approximate order of available power and ease of extraction. There are fewer significant sources in the dark regions between stars, but we will consider what is available.
Photovoltaic conversion has been by far the most common way to extract energy in space. This is due to simplicity, scalability, mass, availability, and durability. Photovoltaic cells generate electricity directly without additional conversions or devices, They come in typical units of a few watts each, which can be scaled by simply using more of them. They are fairly low mass per power produced, and do not consume fuel. Except in the shadow of large bodies they can produce power 100% of the time, and have operating life measured in decades except in high radiation environments.
Cells for use in space operate on the same principles as ones made for use on Earth. The operating conditions are different, so they are somewhat modified. Sunlight is not filtered by the atmosphere, so they work with a different spectrum. Temperature ranges, vacuum conditions, and higher UV and other types of radiation all have to be accounted for. If higher power levels are needed, and launched from Earth, panels of multiple cells need to be folded for launch, and then deployed. Spacecraft pointing needs are typically different than the Sun's direction, so the panels have to be articulated so they can follow the Sun. If used in the shadow of a large body, some storage in the form of batteries or other devices is needed, or power must shut off temporarily.
The potential and kinetic energy of bodies in orbit is a potential source of energy.