Original:  These missions are complex endeavors and return a small amount of material (<1 mg Hayabusa, 0.1 g Hayabusa2, 60 g planned OSIRIS-REx) for the size and expense of the project ($300 million Hayabusa2, $800 million- $1.16 billion OSIRIS-REx), though these small samples are enough for researchers to study and analyze. There are major technical hurdles in the way of potential asteroid mining. Asteroid mining shifted to a more distant long-term goal and some 'asteroid mining' companies have pivoted to more general-purpose propulsion technology. Minerals in space As resource depletion on Earth becomes more real, the idea of extracting valuable elements from asteroids and returning these to Earth for profit, or using space-based resources to build solar-power satellites and space habitats, becomes more attractive. Hypothetically, water processed from ice could refuel orbiting propellant depots. Although asteroids and Earth accreted from the same starting materials, Earth's relatively stronger gravity pulled all heavy siderophilic (iron-loving) elements into its core during its molten youth more than four billion years ago. This left the crust depleted of such valuable elements until a rain of asteroid impacts re-infused the depleted crust with metals like gold, cobalt, iron, manganese, molybdenum, nickel, osmium, palladium, platinum, rhenium, rhodium, ruthenium and tungsten (some flow from core to surface does occur, e.g. at the Bushveld Igneous Complex, a famously rich source of platinum-group metals). Today, these metals are mined from Earth's crust, and they are essential for economic and technological progress. Hence, the geologic history of Earth may very well set the stage for a future of asteroid mining. In 2006, the Keck Observatory announced that the binary Jupiter trojan 617 Patroclus, and possibly large numbers of other Jupiter trojans, are likely extinct comets and consist largely of water ice. Similarly, Jupiter-family comets, and possibly near-Earth asteroids that are extinct comets, might also provide water. The process of in-situ resource utilization—using materials native to space for propellant, thermal management, tankage, radiation shielding, and other high-mass components of space infrastructure—could lead to radical reductions in its cost. Although whether these cost reductions could be achieved, and if achieved would offset the enormous infrastructure investment required, is unknown. Ice would satisfy one of two necessary conditions to enable "human expansion into the Solar System" (the ultimate goal for human space flight proposed by the 2009 "Augustine Commission" Review of United States Human Space Flight Plans Committee): physical sustainability and economic sustainability. From the astrobiological perspective, asteroid prospecting could provide scientific data for the search for extraterrestrial intelligence (SETI). Some astrophysicists have suggested that if advanced extraterrestrial civilizations employed asteroid mining long ago, the hallmarks of these activities might be detectable. Asteroid selection + Comparison of delta-v requirements for standard Hohmann transfersMissionΔvEarth surface to LEO8.0 km/sLEO to near-Earth asteroid5.5 km/sThis is the average amount; asteroids with much lower delta-v exist.LEO to lunar surface6.3 km/sLEO to moons of Mars8.0 km/s An important factor to consider in target selection is orbital economics, in particular the change in velocity (Δv) and travel time to and from the target. More of the extracted native material must be expended as propellant in higher Δv trajectories, thus less returned as payload. Direct Hohmann trajectories are faster than Hohmann trajectories assisted by planetary and/or lunar flybys, which in turn are faster than those of the Interplanetary Transport Network, but the reduction in transfer time comes at the cost of increased Δv requirements. The Easily Recoverable Object (ERO) subclass of Near-Earth asteroids are considered likely candidates for early mining activity. Their low Δv makes them suitable for use in extracting construction materials for near-Earth space-based facilities, greatly reducing the economic cost of transporting supplies into Earth orbit. The table above shows a comparison of Δv requirements for various missions. In terms of propulsion energy requirements, a mission to a near-Earth asteroid compares favorably to alternative mining missions. An example of a potential target for an early asteroid mining expedition is 4660 Nereus, expected to be mainly enstatite. This body has a very low Δv compared to lifting materials from the surface of the Moon. However, it would require a much longer round-trip to return the material. Multiple types of asteroids have been identified but the three main types would include the C-type, S-type, and M-type asteroids: C-type asteroids have a high abundance of water which is not currently of use for mining but could be used in an exploration effort beyond the asteroid. Mission costs could be reduced by using the available water from the asteroid. C-type asteroids also have a lot of organic carbon, phosphorus, and other key ingredients for fertilizer which could be used to grow food. S-type asteroids carry little water but look more attractive because they contain numerous metals, including nickel, cobalt, and more valuable metals, such as gold, platinum, and rhodium. A small 10-meter S-type asteroid contains about of metal with in the form of rare metals like platinum and gold. M-type asteroids are rare but contain up to 10 times more metal than S-types A class of easily recoverable objects (EROs) was identified by a group of researchers in 2013. Twelve asteroids made up the initially identified group, all of which could be potentially mined with present-day rocket technology. Of 9,000 asteroids searched in the NEO database, these twelve could all be brought into an Earth-accessible orbit by changing their velocity by less than . The dozen asteroids range in size from . Asteroid cataloging The B612 Foundation is a private nonprofit foundation with headquarters in the United States, dedicated to protecting Earth from asteroid strikes. As a non-governmental organization it has conducted two lines of related research to help detect asteroids that could one day strike Earth, and find the technological means to divert their path to avoid such collisions. The foundation's 2013 goal was to design and build a privately financed asteroid-finding space telescope, Sentinel, hoping in 2013 to launch it in 2017–2018. The Sentinel's infrared telescope, once parked in an orbit similar to that of Venus, is designed to help identify threatening asteroids by cataloging 90\% of those with diameters larger than , as well as surveying smaller Solar System objects. Data gathered by Sentinel was intended to be provided through an existing scientific data-sharing network that includes NASA and academic institutions such as the Minor Planet Center in Cambridge, Massachusetts. Given the satellite's telescopic accuracy, Sentinel's data may prove valuable for other possible future missions, such as asteroid mining. Broad, William J. Vindication for Entrepreneurs Watching Sky: Yes, It Can Fall, The New York Times website, February 16, 2013 and in print on February 17, 2013, p. A1 of the New York edition. Retrieved June 27, 2014. Mining considerations There are four options for mining: In-space manufacturing (ISM), which may be enabled by biomining. Bring raw asteroidal material to Earth for use. Process it on-site to bring back only processed materials, and perhaps produce propellant for the return trip. Transport the asteroid to a safe orbit around the Moon or Earth or to the ISS. This can hypothetically allow for most materials to be used and not wasted. Processing in situ for the purpose of extracting high-value minerals will reduce the energy requirements for transporting the materials, although the processing facilities must first be transported to the mining site. In situ mining will involve drilling boreholes and injecting hot fluid/gas and allow the useful material to react or melt with the solvent and extract the solute. Due to the weak gravitational fields of asteroids, any activities, like drilling, will cause large disturbances and form dust clouds. These might be confined by some dome or bubble barrier. Or else some means of rapidly dissipating any dust could be provided for. Mining operations require special equipment to handle the extraction and processing of ore in outer space. The machinery will need to be anchored to the body, but once in place, the ore can be moved about more readily due to the lack of gravity. However, no techniques for refining ore in zero gravity currently exist. Docking with an asteroid might be performed using a harpoon-like process, where a projectile would penetrate the surface to serve as an anchor; then an attached cable would be used to winch the vehicle to the surface, if the asteroid is both penetrable and rigid enough for a harpoon to be effective. Due to the distance from Earth to an asteroid selected for mining, the round-trip time for communications will be several minutes or more, except during occasional close approaches to Earth by near-Earth asteroids. Thus any mining equipment will either need to be highly automated, or a human presence will be needed nearby. Humans would also be useful for troubleshooting problems and for maintaining the equipment. On the other hand, multi-minute communications delays have not prevented the success of robotic exploration of Mars, and automated systems would be much less expensive to build and deploy. Technology being developed by Planetary Resources to locate and harvest these asteroids has resulted in the plans for three different types of satellites: Arkyd Series 100 (the Leo Space telescope) is a less expensive instrument that will be used to find, analyze, and see what resources are available on nearby asteroids. Arkyd Series 200 (the Interceptor) Satellite that would actually land on the asteroid to get a closer analysis of the available resources. Arkyd Series 300 (Rendezvous Prospector) Satellite developed for research and finding resources deeper in space. Technology being developed by Deep Space Industries to examine, sample, and harvest asteroids is divided into three families of spacecraft: FireFlies are triplets of nearly identical spacecraft in CubeSat form launched to different asteroids to rendezvous and examine them. DragonFlies also are launched in waves of three nearly identical spacecraft to gather small samples (5–10 kg) and return them to Earth for analysis. Harvestors voyage out to asteroids to gather hundreds of tons of material for return to high Earth orbit for processing. Technology is being developed by TransAstra Corporation to locate and harvest asteroids with the Apis family of spacecraft, which comprises three classes of flight systems: Mini Bee is an experimental technology demonstration vehicle designed to showcase the company's patented approach to asteroid mining using concentrated solar energy known as optical mining Honey Bee is a mid-sized spacecraft designed to utilize optical mining technology to harvest asteroids up to 10 meters in average diameter Queen Bee is the largest spacecraft in the Apis family, an evolution of the Honey Bee that is scaled to enable capture and mining of asteroids up to 40 meters in average diameter Asteroid mining could potentially revolutionize space exploration. The C-type asteroids' high abundance of water could be used to produce fuel by splitting water into hydrogen and oxygen. This would make space travel a more feasible option by lowering cost of fuel. While the cost of fuel is a relatively insignificant factor in the overall cost for low earth orbit manned space missions, storing it and the size of the craft become a much bigger factor for interplanetary missions. Typically 1 kg in orbit is equivalent to more than 10 kg on the ground (for a Falcon 9 1.0 it would need 250 tons of fuel to put 5 tons in GEO orbit or 10 tons in LEO). This limitation is a major factor in the difficulty of interplanetary missions as fuel becomes payload. Extraction techniques Surface mining On some types of asteroids, material may be scraped off the surface using a scoop or auger, or for larger pieces, an "active grab." There is strong evidence that many asteroids consist of rubble piles, potentially making this approach impractical. Shaft mining A mine can be dug into the asteroid, and the material extracted through the shaft. This requires precise knowledge to engineer accuracy of astro-location under the surface regolith and a transportation system to carry the desired ore to the processing facility. Magnetic rakes Asteroids with a high metal content may be covered in loose grains that can be gathered by means of a magnet. Heating For asteroids such as carbonaceous chondrites that contain hydrated minerals, water and other volatiles can be extracted simply by heating. A water extraction test in 2016 by Honeybee Robotics used asteroid regolith simulant developed by Deep Space Industries and the University of Central Florida to match the bulk mineralogy of a particular carbonaceous meteorite. Although the simulant was physically dry (i.e., it contained no water molecules adsorbed in the matrix of the rocky material), heating to about 510 °C released hydroxyl, which came out as substantial amounts of water vapor from the molecular structure of phyllosilicate clays and sulphur compounds. The vapor was condensed into liquid water filling the collection containers, demonstrating the feasibility of mining water from certain classes of physically dry asteroids. For volatile materials in extinct comets, heat can be used to melt and vaporize the matrix. Mond process The nickel and iron of an iron rich asteroid could be extracted by the Mond process. This involves passing carbon monoxide over the asteroid at a temperature between 50 and 60 °C for nickel, higher for iron, and with high pressures and enclosed in materials that are resistant to the corrosive carbonyls. This forms the gases nickel tetracarbonyl and iron pentacarbonyl - then nickel and iron can be removed from the gas again at higher temperatures, and platinum, gold etc. left as a residue.Lewis, John S. "Extraction of volatiles and metals from extraterrestrial materials." (1992). Self-replicating machines A 1980 NASA study entitled Advanced Automation for Space Missions proposed a complex automated factory on the Moon that would work over several years to build 80\% of a copy of itself, the other 20\% being imported from Earth since those more complex parts (like computer chips) would require a vastly larger supply chain to produce. Exponential growth of factories over many years could refine large amounts of lunar (or asteroidal) regolith. Since 1980 there has been major progress in miniaturization, nanotechnology, materials science, and additive manufacturing, so it may be possible to achieve 100\% "closure" with a reasonably small mass of hardware, although these technology advancements are themselves enabled on Earth by expansion of the supply chain so it needs further study. A NASA study in 2012 proposed a "bootstrapping" approach to establish an in-space supply chain with 100\% closure, suggesting it could be achieved in only two to four decades with low annual cost. A study in 2016 again claimed it is possible to complete in just a few decades because of ongoing advances in robotics, and it argued it will provide benefits back to the Earth including economic growth, environmental protection, and provision of clean energy while also providing humanity protection against existential threats. Failed mining projects On April 24, 2012 a plan was announced by billionaire entrepreneurs to mine asteroids for their resources. The company was called Planetary Resources and its founders include aerospace entrepreneurs Eric Anderson and Peter Diamandis. Advisers included film director and explorer James Cameron and investors included Google's chief executive Larry Page. Its executive chairman was Eric Schmidt. They planned to create a fuel depot in space by 2020 by using water from asteroids, splitting it to liquid oxygen and liquid hydrogen for rocket fuel. From there, it could be shipped to Earth orbit for refueling commercial satellites or spacecraft. In 2020, the scheme was wound down and all hardware assets were auctioned off. Proposed mining projects Another similar venture, called Deep Space Industries, was started in 2013 by David Gump, who had founded other space companies. At the time, the company hoped to begin prospecting for asteroids suitable for mining by 2015 and by 2016 return asteroid samples to Earth. Deep Space Industries planned to begin mining asteroids by 2023. At ISDC-San Diego 2013, Kepler Energy and Space Engineering (KESE, llc) also announced it was going to mine asteroids, using a simpler, more straightforward approach: KESE plans to use almost exclusively existing guidance, navigation and anchoring technologies from mostly successful missions like the Rosetta/Philae, Dawn, and Hayabusa, and current NASA Technology Transfer tooling to build and send a 4-module Automated Mining System (AMS) to a small asteroid with a simple digging tool to collect ≈40 tons of asteroid regolith and bring each of the four return modules back to low Earth orbit (LEO) by the end of the decade. Small asteroids are expected to be loose piles of rubble, therefore providing for easy extraction. In September 2012, the NASA Institute for Advanced Concepts (NIAC) announced the Robotic Asteroid Prospector project, which will examine and evaluate the feasibility of asteroid mining in terms of means, methods, and systems.Robotic Asteroid Prospector (RAP) Staged from L-1: Start of the Deep Space Economy nasa.gov, accessed 2012-09-11 Being the largest body in the asteroid belt, Ceres could become the main base and transport hub for future asteroid mining infrastructure, allowing mineral resources to be transported to Mars, the Moon, and Earth. Because of its small escape velocity combined with large amounts of water ice, it also could serve as a source of water, fuel, and oxygen for ships going through and beyond the asteroid belt. Transportation from Mars or the Moon to Ceres would be even more energy-efficient than transportation from Earth to the Moon. Potential targets According to the Asterank database, the following asteroids are considered the best targets for mining if maximum cost-effectiveness is to be achieved (last updated December 2018): Asteroid Est. Value (US$ billion) Est.
Revised:  These missions are complex endeavors and return a small amount of material (<1 mg Hayabusa, 0.1 g Hayabusa2, 60 g planned OSIRIS-REx) for the size and expense of the project ($300 million Hayabusa2, $800 million- $1.16 billion OSIRIS-REx), though these small samples are enough for researchers to study and analyze. There are major technical hurdles in the way of potential asteroid mining. Asteroid mining shifted to a more distant long-term goal and some 'asteroid mining' companies have pivoted to more general-purpose propulsion technology. Minerals in space As resource depletion on Earth becomes more real, the idea of extracting valuable elements from asteroids and returning these to Earth for profit, or using space-based resources to build solar-power satellites and space habitats, becomes more attractive. Hypothetically, water processed from ice could refuel orbiting propellant depots. Although asteroids and Earth accreted from the same starting materials, Earth's relatively stronger gravity pulled all heavy siderophilic (iron-loving) elements into its core during its molten youth more than four billion years ago. This left the crust depleted of such valuable elements until a rain of asteroid impacts re-infused the depleted crust with metals like gold, cobalt, iron, manganese, molybdenum, nickel, osmium, palladium, platinum, rhenium, rhodium, ruthenium and tungsten (some flow from core to surface does occur, e.g. at the Bushveld Igneous Complex, a famously rich source of platinum-group metals). Today, these metals are mined from Earth's crust, and they are essential for economic and technological progress. Hence, the geologic history of Earth may very well set the stage for a future of asteroid mining. In 2006, the Keck Observatory announced that the binary Jupiter trojan 617 Patroclus, and possibly large numbers of other Jupiter trojans, are likely extinct comets and consist largely of water ice. Similarly, Jupiter-family comets, and possibly near-Earth asteroids that are extinct comets, might also provide water. The process of in-situ resource utilization—using materials native to space for propellant, thermal management, tankage, radiation shielding, and other high-mass components of space infrastructure—could lead to radical reductions in its cost. Although whether these cost reductions could be achieved, and if achieved would offset the enormous infrastructure investment required, is unknown. Ice would satisfy one of two necessary conditions to enable "human expansion into the Solar System" (the ultimate goal for human space flight proposed by the 2009 "Augustine Commission" Review of United States Human Space Flight Plans Committee): physical sustainability and economic sustainability. From the astrobiological perspective, asteroid prospecting could provide scientific data for the search for extraterrestrial intelligence (SETI). Some astrophysicists have suggested that if advanced extraterrestrial civilizations employed asteroid mining long ago, the hallmarks of these activities might be detectable. Asteroid selection + Comparison of delta-v requirements for standard Hohmann transfersMissionΔvEarth surface to LEO8.0 km/sLEO to near-Earth asteroid5.5 km/sThis is the average amount; asteroids with much lower delta-v exist.LEO to lunar surface6.3 km/sLEO to moons of Mars8.0 km/s An important factor to consider in target selection is orbital economics, in particular the change in velocity (Δv) and travel time to and from the target. More of the extracted native material must be expended as propellant in higher Δv trajectories, thus less returned as payload. Direct Hohmann trajectories are faster than Hohmann trajectories assisted by planetary and/or lunar flybys, which in turn are faster than those of the Interplanetary Transport Network, but the reduction in transfer time comes at the cost of increased Δv requirements. The Easily Recoverable Object (ERO) subclass of Near-Earth asteroids are considered likely candidates for early mining activity. Their low Δv makes them suitable for use in extracting construction materials for near-Earth space-based facilities, greatly reducing the economic cost of transporting supplies into Earth orbit. The table above shows a comparison of Δv requirements for various missions. In terms of propulsion energy requirements, a mission to a near-Earth asteroid compares favorably to alternative mining missions. An example of a potential target for an early asteroid mining expedition is 4660 Nereus, expected to be mainly enstatite. This body has a very low Δv compared to lifting materials from the surface of the Moon. However, it would require a much longer round-trip to return the material. Multiple types of asteroids have been identified but the three main types would include the C-type, S-type, and M-type asteroids: C-type asteroids have a high abundance of water which is not currently of use for mining but could be used in an exploration effort beyond the asteroid. Mission costs could be reduced by using the available water from the asteroid. C-type asteroids also have a lot of organic carbon, phosphorus, and other key ingredients for fertilizer which could be used to grow food. S-type asteroids carry little water but look more attractive because they contain numerous metals, including nickel, cobalt, and more valuable metals, such as gold, platinum, and rhodium. A small 10-meter S-type asteroid contains about of metal with in the form of rare metals like platinum and gold. M-type asteroids are rare but contain up to 10 times more metal than S-types A class of easily recoverable objects (EROs) was identified by a group of researchers in 2013. Twelve asteroids made up the initially identified group, all of which could be potentially mined with present-day rocket technology. Of 9,000 asteroids searched in the NEO database, these twelve could all be brought into an Earth-accessible orbit by changing their velocity by less than . The dozen asteroids range in size from . Asteroid cataloging The B612 Foundation is a private nonprofit foundation with headquarters in the United States, dedicated to protecting Earth from asteroid strikes. As a non-governmental organization it has conducted two lines of related research to help detect asteroids that could one day strike Earth, and find the technological means to divert their path to avoid such collisions. The foundation's 2013 goal was to design and build a privately financed asteroid-finding space telescope, Sentinel, hoping in 2013 to launch it in 2017–2018. The Sentinel's infrared telescope, once parked in an orbit similar to that of Venus, is designed to help identify threatening asteroids by cataloging 90\% of those with diameters larger than , as well as surveying smaller Solar System objects. Data gathered by Sentinel was intended to be provided through an existing scientific data-sharing network that includes NASA and academic institutions such as the Minor Planet Center in Cambridge, Massachusetts. Given the satellite's telescopic accuracy, Sentinel's data may prove valuable for other possible future missions, such as asteroid mining. Broad, William J. Vindication for Entrepreneurs Watching Sky: Yes, It Can Fall, The New York Times website, February 16, 2013 and in print on February 17, 2013, p. A1 of the New York edition. Retrieved June 27, 2014. Mining considerations There are four options for mining: In-space manufacturing (ISM), which may be enabled by biomining. Bring raw asteroidal material to Earth for use. Process it on-site to bring back only processed materials, and perhaps produce propellant for the return trip. Transport the asteroid to a safe orbit around the Moon or Earth or to the ISS. This can hypothetically allow for most materials to be used and not wasted. Processing in situ for the purpose of extracting high-value minerals will reduce the energy requirements for transporting the materials, although the processing facilities must first be transported to the mining site. In situ mining will involve drilling boreholes and injecting hot fluid/gas and allow the useful material to react or melt with the solvent and extract the solute. Due to the weak gravitational fields of asteroids, any activities, like drilling, will cause large disturbances and form dust clouds. These might be confined by some dome or bubble barrier. Or else some means of rapidly dissipating any dust could be provided for. Mining operations require special equipment to handle the extraction and processing of ore in outer space. The machinery will need to be anchored to the body, but once in place, the ore can be moved about more readily due to the lack of gravity. However, no techniques for refining ore in zero gravity currently exist. Docking with an asteroid might be performed using a harpoon-like process, where a projectile would penetrate the surface to serve as an anchor; then an attached cable would be used to winch the vehicle to the surface, if the asteroid is both penetrable and rigid enough for a harpoon to be effective. Due to the distance from Earth to an asteroid selected for mining, the round-trip time for communications will be several minutes or more, except during occasional close approaches to Earth by near-Earth asteroids. Thus any mining equipment will either need to be highly automated, or a human presence will be needed nearby. Humans would also be useful for troubleshooting problems and for maintaining the equipment. On the other hand, multi-minute communications delays have not prevented the success of robotic exploration of Mars, and automated systems would be much less expensive to build and deploy. Asteroid mining could potentially revolutionize space exploration. The C-type asteroids' high abundance of water could be used to produce fuel by splitting water into hydrogen and oxygen. This would make space travel a more feasible option by lowering cost of fuel. While the cost of fuel is a relatively insignificant factor in the overall cost for low earth orbit manned space missions, storing it and the size of the craft become a much bigger factor for interplanetary missions. Typically 1 kg in orbit is equivalent to more than 10 kg on the ground (for a Falcon 9 1.0 it would need 250 tons of fuel to put 5 tons in GEO orbit or 10 tons in LEO). This limitation is a major factor in the difficulty of interplanetary missions as fuel becomes payload. Extraction techniques Surface mining On some types of asteroids, material may be scraped off the surface using a scoop or auger, or for larger pieces, an "active grab." There is strong evidence that many asteroids consist of rubble piles, potentially making this approach impractical. Shaft mining A mine can be dug into the asteroid, and the material extracted through the shaft. This requires precise knowledge to engineer accuracy of astro-location under the surface regolith and a transportation system to carry the desired ore to the processing facility. Magnetic rakes Asteroids with a high metal content may be covered in loose grains that can be gathered by means of a magnet. Heating For asteroids such as carbonaceous chondrites that contain hydrated minerals, water and other volatiles can be extracted simply by heating. A water extraction test in 2016 by Honeybee Robotics used asteroid regolith simulant developed by Deep Space Industries and the University of Central Florida to match the bulk mineralogy of a particular carbonaceous meteorite. Although the simulant was physically dry (i.e., it contained no water molecules adsorbed in the matrix of the rocky material), heating to about 510 °C released hydroxyl, which came out as substantial amounts of water vapor from the molecular structure of phyllosilicate clays and sulphur compounds. The vapor was condensed into liquid water filling the collection containers, demonstrating the feasibility of mining water from certain classes of physically dry asteroids. For volatile materials in extinct comets, heat can be used to melt and vaporize the matrix. Mond process The nickel and iron of an iron rich asteroid could be extracted by the Mond process. This involves passing carbon monoxide over the asteroid at a temperature between 50 and 60 °C for nickel, higher for iron, and with high pressures and enclosed in materials that are resistant to the corrosive carbonyls. This forms the gases nickel tetracarbonyl and iron pentacarbonyl - then nickel and iron can be removed from the gas again at higher temperatures, and platinum, gold etc. left as a residue.Lewis, John S. "Extraction of volatiles and metals from extraterrestrial materials." (1992). Self-replicating machines A 1980 NASA study entitled Advanced Automation for Space Missions proposed a complex automated factory on the Moon that would work over several years to build 80\% of a copy of itself, the other 20\% being imported from Earth since those more complex parts (like computer chips) would require a vastly larger supply chain to produce. Exponential growth of factories over many years could refine large amounts of lunar (or asteroidal) regolith. Since 1980 there has been major progress in miniaturization, nanotechnology, materials science, and additive manufacturing, so it may be possible to achieve 100\% "closure" with a reasonably small mass of hardware, although these technology advancements are themselves enabled on Earth by expansion of the supply chain so it needs further study. A NASA study in 2012 proposed a "bootstrapping" approach to establish an in-space supply chain with 100\% closure, suggesting it could be achieved in only two to four decades with low annual cost. A study in 2016 again claimed it is possible to complete in just a few decades because of ongoing advances in robotics, and it argued it will provide benefits back to the Earth including economic growth, environmental protection, and provision of clean energy while also providing humanity protection against existential threats. Failed mining projects On April 24, 2012 a plan was announced by billionaire entrepreneurs to mine asteroids for their resources. The company was called Planetary Resources and its founders include aerospace entrepreneurs Eric Anderson and Peter Diamandis. Advisers included film director and explorer James Cameron and investors included Google's chief executive Larry Page. Its executive chairman was Eric Schmidt. They planned to create a fuel depot in space by 2020 by using water from asteroids, splitting it to liquid oxygen and liquid hydrogen for rocket fuel. From there, it could be shipped to Earth orbit for refueling commercial satellites or spacecraft. In 2020, the scheme was wound down and all hardware assets were auctioned off. Telescope technology was proposed by Planetary Resources to locate and harvest these asteroids has resulted in the plans for three different types of satellites: Arkyd Series 100 (the Leo Space telescope) is a less expensive instrument that will be used to find, analyze, and see what resources are available on nearby asteroids. Arkyd Series 200 (the Interceptor) Satellite that would actually land on the asteroid to get a closer analysis of the available resources. Arkyd Series 300 (Rendezvous Prospector) Satellite developed for research and finding resources deeper in space. All public plans for The Arkyd space telescope technology have been abandonned, and Planetary Resources assets have been acquired ConsenSys, a blockchain company with no public space facing goals. Proposed mining projects Another similar venture, called Deep Space Industries, was started in 2013 by David Gump, who had founded other space companies. At the time, the company hoped to begin prospecting for asteroids suitable for mining by 2015 and by 2016 return asteroid samples to Earth. Deep Space Industries planned to begin mining asteroids by 2023. At ISDC-San Diego 2013, Kepler Energy and Space Engineering (KESE, llc) also announced it was going to mine asteroids, using a simpler, more straightforward approach: KESE plans to use almost exclusively existing guidance, navigation and anchoring technologies from mostly successful missions like the Rosetta/Philae, Dawn, and Hayabusa, and current NASA Technology Transfer tooling to build and send a 4-module Automated Mining System (AMS) to a small asteroid with a simple digging tool to collect ≈40 tons of asteroid regolith and bring each of the four return modules back to low Earth orbit (LEO) by the end of the decade. Small asteroids are expected to be loose piles of rubble, therefore providing for easy extraction. In September 2012, the NASA Institute for Advanced Concepts (NIAC) announced the Robotic Asteroid Prospector project, which will examine and evaluate the feasibility of asteroid mining in terms of means, methods, and systems.Robotic Asteroid Prospector (RAP) Staged from L-1: Start of the Deep Space Economy nasa.gov, accessed 2012-09-11 As of 2020 (also under NIAC support) technology is being developed by Deep Space Industries. NASA is partly funding planning and development efforts to examine, sample, and harvest asteroids. These plans involve three proposed families of spacecraft: FireFlies are triplets of nearly identical spacecraft in CubeSat form launched to different asteroids to rendezvous and examine them. DragonFlies also are launched in waves of three nearly identical spacecraft to gather small samples (5–10 kg) and return them to Earth for analysis. Harvestors voyage out to asteroids to gather hundreds of tons of material for return to high Earth orbit for processing. Technology is being developed by TransAstra Corporation to locate and harvest asteroids with the Apis family of spacecraft, which comprises three classes of flight systems: Mini Bee is an experimental technology demonstration vehicle designed to showcase the company's patented approach to asteroid mining using concentrated solar energy known as optical mining Honey Bee is a mid-sized spacecraft designed to utilize optical mining technology to harvest asteroids up to 10 meters in average diameter Queen Bee is the largest spacecraft in the Apis family, an evolution of the Honey Bee that is scaled to enable capture and mining of asteroids up to 40 meters in average diameter Potential targets According to the Asterank database, the following asteroids are considered the best targets for mining if maximum cost-effectiveness is to be achieved (last updated December 2018): Asteroid Est. Value (US$ billion) Est.
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