There are two options for processing an asteroid:
It appears most likely we will choose option 2 because the equipment required to process asteroidal material is simple. The question is: How much we will process the material? Do we want to return only metal granules and ices? At what purity?
The next few sections cover only processing of asteroidal material. Transport of the asteroidal materials was covered in the web section on transportation, but it's notable that fuel propellants would be one product of asteroidal materials. If chemical rocketry is chosen then hydrogen and oxygen will likely be produced from water when needed. Likewise for alternative propulsion methods, e.g., hydrocarbon fuels, or instead of using chemical rocketry, just using steam rockets which consume only water without any further processing, heating the steam with nuclear or solar heat sources.
The environment for asteroid mining and processing
One should fully understand the environment in which we are working.
An asteroid the size of a domed football stadium filled with ore (roughly equivalent to 100 meters wide, tall and long), contains 2,000,000 tons of material. (In comparison, the space shuttle has maximum capacity of less than 30 tons.)
The surface gravity of this 100 meter asteroid would be practically zero -- less than one ten-thousandth that of the Moon. The excape velocity would be around 0.3 kilometers per hour (0.2 miles per hour), or 0.1 meter per second. Drop an object at arm's length and it would take 5 minutes to fall to the ground.
If you have an asteroid double the size and hence seven times the mass, the gravity only doubles at its surface (since the surface is further from the center of mass).
This means that you don't launch and land on an asteroid the same way you do on the Moon, and you use propellant differently. Landing and relaunching using just a locking spring is not a bad idea, for example.
Material can be moved around easily, and structures erected don't have to be concerned about gravity, only operational stresses.
Staying attached to and moving around the asteroid was discussed in an earlier section.
Some studies call for stopping the rotation of the asteroid if it's small in order to attach the solar-powered processing equipment to the asteroid and have it always facing the sun. A landmark early NASA study called for despinning the asteroid by anchoring a cable, wrapping it around the asteroid, and having a rocket-powered space jeep slow down and stop its rotation. For a 100 meter diameter asteroid rotating 4 times per day, about 29 tons of fuel would be needed to despin the asteroid. If the asteroid is fragile, the rocket could thrust gently for a week.
However, a better approach would probably be to use a yoyo-like gadget commonly used to despin satellites without using any fuel propellant. To do this, you wrap 2 lengths of string around the asteroid in the direction opposite to the spin, and tie a chunk of rock to each distal end. The other ends are attached to the asteroid so that they can let go once the yoyo has stopped its spin (i.e., at the part of the cycle when it would start spinning the asteroid back in the other direction). The centrifugal force pulls the chunks away, applying a torque to the asteroid. The length of string needed to despin depends on the size and mass of the asteroid and the mass of the chunks, but not on the initial angular velocity. The string can be very thin, although the needed strength does depend on the angular velocity.
For example, take a roughly spherical asteroid, 100 meters (yards) in diameter, massing about 1.6 million tons. If each string is 6 km long, enough to wrap around the asteroid 20 times, the two rock chunks needed would weigh about 20 tons each, i.e., only 1 part in 80,000 of the mass of the asteroid. If the thing is rotating initially at 4 revolutions/day, then ordinary 10 lb. test fishing line would be more than sufficient. The 20 ton chunks could come from the asteroid itself, e.g., bags of rock. (Calculations by Dr. Phil Chapman, personal communication.)
For bigger asteroids, or for a low-budget operation on a small asteroid, it's probably better to let the asteroid keep spinning and just attach the industry to the pole of rotation using a rotary joint.
Another method would be to chip off a small piece to de-spin, though this sounds a bit dirty and not very predictable. Just a few small explosives would do the job, though more complex nonexplosive methods could also be applied. Typically, they would take advantage of fracture cracks in the asteroid left over from eons-ago collisions. Once separated by explosion, big chunks could be easily nudged into orbit around the main asteroid, or pushed away at escape velocity.
Still another option is to have all processing equipment offsite, say, in orbit a kilometer away, and to send big bags of material off the asteroid to the facility, e.g., ballistically. "Pass another bag, Rogerů"
Mining and processing an asteroid is much less massive an operation than Earth or Moon mining. We do not need heavy mining and transport machinery, we don't need complex chemical processing as on the Moon in order to get valuable materials, and waste disposal is achieved by just putting all waste into a big bag. However, the near zero-gravity space environment has its unique challenges as well.
A typical asteroid would probably be crumbly, consisting of silicate dirt embedded with nickel-iron granules and volatiles. We can make this assumption for the purposes of this analysis, but should be aware that the consistency from asteroid to asteroid can vary from pure metal to pure powder, and could also entail a mix of consistencies.
Many different methods have been discussed for mining the asteroid. Conventional methods include scraping away at the asteroid's surface (i.e., strip mining), and tunnelling into the asteroid. Most Earth mining depends upon gravity to hold the cutting edge against the ore. (However, for many Earth mining operations this is not enough, and other means are employed, e.g., cables and reels.) Scraping away at the surface of the asteroid requires holding the cutting edge against the outer surface of the asteroid. This would require either local harpoons or anchors imbedded into the surface of the asteroid, or cables or a net around the asteroid for the cutter to hold onto.
Strip mining would result in a lot of dirt being thrown up. An unconventional space mining method sees this not only as a problem but also as an opportunity. A canopy around the mining site can be used to collect ore purposely kicking up, the canopy shaped and rotating to use the centrifugal force to channel the ore to the perimeter for collection, as this NASA artwork shows. If no canopy were put up, a lot of debris would cloud and cover the mining environment and probably interfere with mining operations. (Mining without a canopy would certainly be unacceptable in Earth orbit. Companies will most probably use a canopy also because the canopy would be quite profitable in terms of the amount of loose ore it would collect.)
A variation on this is to have a stationary canopy. A dust kicker goes down to the asteroid and just kicks up the ore at low velocity. When there's enough ore in the canopy, it's sealed off and moved to the processing site (where the ore can be collected by rotation or other mechanical means). It is simple and highly reliable, presenting minimal risk of breakdown of mining machinery.
Some studies adopted tunnelling to mine an asteroid. The cutter holds itself steady by the walls of the tunnel -- pushing against the walls or cutting into them. Tunnelling prevents consumption of the entire asteroid, but desirable ore veins or cracks can be followed.
Another candidate process for extracting volatiles from within near Earth asteroids which are dormant comets (currently estimated to be around 40% of near Earth asteroids) is to drill into the asteroid, much like we do for oil and natural gas. Geological and Mining Consultant David L. Kuck of Oracle, Arizona, proposes in a long paper entitled "Exploitation of Space Oases" some highly automated methods of drilling and producing volatiles without the need for extraction of materials and thus without dealing with the crushing, grinding and tailings disposal.
Mr. Kuck, an old name in the space resources community, has an unpublished database he has been building upon for many years which has not just orbital elements but also mission delta-V's at specific times and spectra along with compositional interpretations of the spectra. Much of this multifaceted database is published in the above paper. The abovementioned paper also goes into the likely geologies and classifications of comets based on a resourceful collection of scientific analyses of comets based on observations by spacecraft and telescopes, and his skills and experience in minerology and geologic processes.
Of particular interest may be near Earth asteroid 1979 VA, which was later realized to be comet Wilson-Harrington observed in 1949 with a sizeable coma tail just 30 years ago (a mere instant in astrogeologic time) and subsequently "lost" until unknowing rediscovery in 1979. Since we know for sure that this asteroid is a captured comet, we would be assured of getting all the volatiles we would need for a long, long time, and delta-v for later missions would not be nearly as much of an economic factor any more.
Processing the mining material
Asteroidal material in general is exceptionally good ore requiring a minimum of processing, since it has free metal already.
Only basic ore processing need occur at the asteroid, producing free metal and volatiles (usually stored as ices), and perhaps selected minerals, glasses and ceramics. The required equipment is quite simple.
The following is a sample ore processing system, but is not the only one proposed to date.
At the input chute, the ore will be ground up and sieved into different sizes as the first step of a basic ore processing system. Most asteroids probably offer far more crumbly material than we could consume in one mining expedition.
Simple mechanical grinders, using a gentle rocking jaw arrangement for coarse crushing and a series of rollers for fine crushing, could be arranged in a slowly rotating housing to provide centrifugal movement of the material. Vibrating screens are used to sift the grains for directing them to the proper sized grinders.
The streams of material are put through magnetic fields to separate the nickel-iron metal granules from the silicate grains. Alternatively, the streams can be dropped onto magnetic drums, whereby the silicates and weakly magnetic material deflect off the drum whereas the magnetic granules and pebbles stick to the magnetic drum until the scrape off point. Repeated cycling through the magnetic field and perhaps additional grinders can give highly pure bags of free nickel iron metal.
An optional additional piece of equipment is an "impact grinder" or "centrifugal grinder" whereby a very rapidly spinning wheel accelerates the material down its spokes and flings it against an impact block. Any silicate impurities still attached to the free metal are shattered off. It's feasible to have drum speeds sufficient to flatten the metal granules by impact. A centrifugal grinder may be used after mechanical grinding and sieving, and before further magnetic separation. In fact, most of the shattered silicate will be small particles which could be sieved out.
The nonmagnetic material is channelled into a solar oven where the volatiles are cooked out. In zero gravity and windless space, the oven mirrors can be huge and made of aluminum foil. The gas stream is piped to tanks located in a cold shadow of space. The tanks are put in series so that the furthest one away is coldest. This way, water condenses more in the first one, carbon dioxide and other vapors in the tanks downstream.
Rocket fuel for the delivery trip to Earth orbit can be produced by separating oxygen and hydrogen gases from the mix, or by electrolysis of water. Alternatively, the hydrogen could be chemically bonded with carbon to produce methane fuel. On the simpler end, simple steam rockets could be used. This is all discussed in chapter 3 on transportation in space.
Thin, relatively lightweight spherical tanks could be sent to store the frozen volatiles. Ultimately, tanks for storing frozen volatiles for sending to Earth orbit can be manufactured by some of the nickel iron metal, by use of a solar oven for melting the nickel iron metal. A cast can be made from sand or glass-ceramic material from melted leftover ore.
Some silicate material from the asteroid can be shipped back to Earth orbit to be used for making glass, fiberglass, ceramics, "astercrete", dirt to grow things in, and radiation shielding for habitats and sensitive silicon electronics.
Processing of glasses, ceramics, "astercrete" and the like is not discussed here, because it is discussed in the chapters on lunar material utilization and space manufacturing. If we were to not use lunar materials but use only asteroidal materials, processing asteroidal material to make glasses, ceramics and astercrete is analogous to the discussion on processing lunar materials for the same feedstocks and products.
Undesired material can be put in a big wastebag container, or "sandbags", or cast into bricks by a solar oven, used for shielding the habitat from space radiation, creating more cold shadows, or just removed from the mining operation's space. (If waste were simply ejected at escape velocity, it would not significantly increase the number of meteors in interplanetary space. However, it's cheaper to skip the ejector equipment and just bag it all.)
Finally, I should add that some studies consider processing all the asteroidal material by solar oven, skipping the magnetic separators, impact grinders, etc. This approach would utilize giant superlightweight mirrors to concentrate sunlight onto a cavity containing any matrix of material, to first extract the volatiles, and then raise the temperature to more than 1600C (2900F). Only the free metal would melt at the latter temperature. However, separating the molten metal from the silicate matrix seems a little tricky. Thus, I don't review that alternative here. Similar methods, "vacuum vapor distillation" as well as high temperature electrolysis, are discussed in chapter 4 on industrial processes.
Orbiting an asteroid
Issues of orbiting an irregularly shaped body with odd mass distributions, and landing on it, are technically beyond discussion here, but we have achieved a lot of experience from the NASA NEAR-Shoemaker and Japanese Hayabusa probes which orbited two asteroids and landed on both.
After the asteroid is entirely consumed, the equipment can be moved to the next asteroid to mine.
Overall, the equipment is capable of producing at least several hundred times its own mass, and perhaps thousands of times its own mass, per year.
Scientists and technicians are confident in their ability to mine asteroids robotically with the assistance of humans.
However, if a decision is made to send humans, then the equipment can be sent to the asteroid in advance of any people, on a slower and more fuel-efficient trajectory. Once in place with all vital systems appearing OK, the humans can be sent. Their first task will be to set up camp in a radiation-protected environment.
A few on-site general purpose engineers can speed up processing, help set up the equipment, teleoperate equipment without a time delay (being that communications from Earth experience a time delay of minutes due to distance and the speed of light), and handle any repairs and glitches. The workers would likely live in artificial gravity produced by connecting the habitat to a counterweight (e.g., the solar power plant and equipment) by cable and rotating the system. One-tenth to one-third Earth gravity is probably healthy for a long stay. All the chemicals necessary for breathing and drinking are abundant on asteroids, but a reserve of air and water, and getaway fuel, would always be kept on hand in case of major problems or an emergency.
People have spent months upon months, even more than a year, orbiting Earth. Why not send them and their space station somewhere else? We could also attach two such space modules by a cable and rotate them for some artificial gravity.
On a good budget, a standard and well proven Russian Salyut module could be sent. Alternatively, a Bigelow inflatable could be sent.
Even before any asteroidal material is returned, it could be a profitable private sector event, and get a company ahead in the new space race between private sector companies, as discussed in the PERMANENT section on mission plans and concepts, specifically A Near-Term Mission to a Near-Earth Asteroid.