New Study Says Asteroid Retrieval and Mining Feasible With Existing and Near-Term Technologies

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Illustration of an asteroid retrieval spacecraft in the process of capturing a 7-m, 500-ton asteroid. (Image Credit: Rick Sternbach / KISS)

By Douglas Messier
Parabolic Arc Managing Editor

A new study sponsored by the Keck Institute for Space Studies (KISS) has concluded that it would be possible to return an asteroid weighing approximately 500 metric tons to high lunar orbit where it would be mined for resources by 2025.

The Asteroid Retrieval Feasibility Study, published on April 2, was prepared for KISS, NASA Jet Propulsion Laboratory, and the California Institute of Technology (Caltech). Co-leaders of the study included John Brophy of NASA JPL/Caltech, Fred Culick of Caltech, and Louis Friedman of The Planetary Society and participants included representatives of other NASA centers, various universities, institutes and private companies.

The report may provide a preview of what a new company named Planetary Resources spearheaded by the X PRIZE Foundation’s Peter Diamandis will unveil during a press conference in Seattle next Tuesday. Two of the 34 study participants were Planetary Resources President and Chief Engineer Chris Lewicki and former astronaut Tom Jones, who is an adviser to the company. The start-up – which is backed by Google billionaires Larry Page and Eric Schmidt, Microsoft mogul Charles Simonyi, filmmaker James Cameron, and Ross Perot, Jr. – says it will “overlay two critical sectors – space exploration and natural resources – to add trillions of dollars to the global GDP. This innovative start-up will create a new industry and a new definition of ‘natural resources’.”

Asteroid return mission concept. Return flight time of 2 to 6 years depending on the asteroid mass. (Source: Asteroid Retrieval Feasibility Study, KISS)

The study examined the possibility of launching an asteroid retrieval spacecraft aboard an Atlas V launch vehicle, which is already operational. The study calculated the “full life-cycle cost of an asteroid capture and return mission at ~$2.6B[illion].” That cost is likely low enough to the mission could be attractive to private companies like Planetary Resources, providing they can locate an asteroid with sufficient resources to make a profit.

Conceptual spacecraft in the cruise configuration with the capture mechanism deployed. (Source: Asteroid Retrieval Feasibility Study, KISS)

Study participants said the mission is feasible and would fit in well with NASA’s long-term objectives for exploring beyond low Earth orbit.

The two major conclusions from the KISS study are: 1) that it appears feasible to identify, capture and return an entire ~7-m diameter, ~500,000-kg near-Earth asteroid to a high lunar orbit using technology that is or could be available in this decade, and 2) that such an endeavor may be essential technically and programmatically for the success of both near-term and long-term human exploration beyond low-Earth orbit….

“The proposed Asteroid Capture and Return mission would impact an impressive range of NASA interests including: the establishment of an accessible, high-value target in cislunar space; near-term operational experience with astronaut crews in the vicinity of an asteroid; a new synergy between robotic and human missions in which robotic spacecraft return resources for human exploitation and use in space; the potential to jump-start an entire industry based on in situ resource utilization; expansion of international cooperation in space; and planetary defense. It has the potential for cost effectively providing sufficient radiation shielding to protect astronauts from galactic cosmic rays and to provide the propellant necessary to transport the resulting shielded habitats. It would endow NASA and its partners with a new capability in deep space that hasn’t been seen since Apollo. Ever since the completion of the cold-war-based Apollo program there has been no over-arching geo-political rationale for the nation’s space ventures. Retrieving an asteroid for human exploration and exploitation would provide a new rationale for global achievement and inspiration. For the first time humanity would begin modification of the heavens for its benefit.

Below are the report’s Executive Summary and Conclusions sections, which provides a good overview of the study.  You can also read the full report.

Conceptual ACR spacecraft in the stowed configuration. (Source: Asteroid Retrieval Feasibility Study, KISS)

EXECUTIVE SUMMARY

This report describes the results of a study sponsored by the Keck Institute for Space Studies (KISS) to investigate the feasibility of identifying, robotically capturing, and returning an entire Near-Earth Asteroid (NEA) to the vicinity of the Earth by the middle of the next decade. The KISS study was performed by people from Ames Research Center, Glenn Research Center, Goddard Space Flight Center, Jet Propulsion Laboratory, Johnson Space Center, Langley Research Center, the California Institute of Technology, Carnegie Mellon, Harvard University, the Naval Postgraduate School, University of California at Los Angeles, University of California at Santa Cruz, University of Southern California, Arkyd Astronautics, Inc., The Planetary Society, the B612 Foundation, and the Florida Institute for Human and Machine Cognition. The feasibility of an asteroid retrieval mission hinges on finding an overlap between the smallest NEAs that could be reasonably discovered and characterized and the largest NEAs that could be captured and transported in a reasonable flight time. This overlap appears to be centered on NEAs roughly 7 m in diameter corresponding to masses in the range of 250,000 kg to 1,000,000 kg. To put this in perspective, the Apollo program returned 382 kg of Moon rocks in six missions and the OSIRIS-REx mission proposes to return at least 60 grams of surface material from a NEA by 2023. The present study indicates that it would be possible to return a ~500,000-kg NEA to high lunar orbit by around 2025.

Bottom view of the conceptual ACR spacecraft showing the five 10-kW Hall thrusters and the RCS thruster clusters. (Source: Asteroid Retrieval Feasibility Study, KISS)

The idea of exploiting the natural resources of asteroids dates back over a hundred years, but only now has the technology become available to make this idea a reality. The feasibility is enabled by three key developments: the ability to discover and characterize an adequate number of sufficiently small near-Earth asteroids for capture and return; the ability to implement sufficiently powerful solar electric propulsion systems to enable transportation of the captured NEA; and the proposed human presence in cislunar space in the 2020s enabling exploration and exploitation of the returned NEA.

Top view of the conceptual ACR spacecraft showing the instrument suite and capture mechanism prior to being deployed. (Source: Asteroid Retrieval Feasibility Study, KISS)

Placing a 500-t asteroid in high lunar orbit would provide a unique, meaningful, and affordable destination for astronaut crews in the next decade. This disruptive capability would have a positive impact on a wide range of the nation’s human space exploration interests. It would provide a high-value target in cislunar space that would require a human presence to take full advantage of this new resource. It would offer an affordable path to providing operational experience with astronauts working around and with a NEA that could feed forward to much longer duration human missions to larger NEAs in deep space. It would provide an affordable path to meeting the nation’s goal of sending astronauts to a near-Earth object by 2025. It represents a new synergy between robotic and human missions in which robotic spacecraft retrieve significant quantities of valuable resources for exploitation by astronaut crews to enable human exploration farther out into the solar system. A key example of this is that water or other material extracted from a returned, volatile-rich NEA could be used to provide affordable shielding against galactic cosmic rays. The extracted water could also be used for propellant to transport the shielded habitat. These activities could jump-start an entire in situ resource utilization (ISRU) industry. The availability of a multi-hundred-ton asteroid in lunar orbit could also stimulate the expansion of international cooperation in space as agencies work together to determine how to sample and process this raw material. The capture, transportation, examination, and dissection of an entire NEA would provide valuable information for planetary defense activities that may someday have to deflect a much larger near-Earth object. Finally, placing a NEA in lunar orbit would provide a new capability for human exploration not seen since Apollo. Such an achievement has the potential to inspire a nation. It would be mankind’s first attempt at modifying the heavens to enable the permanent settlement of humans in space.

Conceptual spacecraft with solar arrays folded back to facilitate matching the asteroid’s spin state during the capture process. (Source: Asteroid Retrieval Feasibility Study, KISS)

The report that follows outlines the observation campaign necessary to discover and characterize NEAs with the right combination of physical and orbital characteristics that make them attractive targets for return. It suggests that with the right ground-based observation campaign approximately five attractive targets per year could be discovered and adequately characterized. The report also provides a conceptual design of a flight system with the capability to rendezvous with a NEA in deep space, perform in situ characterization of the object and subsequently capture it, de-spin it, and transport it to lunar orbit in a total flight time of 6 to 10 years. The transportation capability would be enabled by a ~40-kW solar electric propulsion system with a specific impulse of 3,000 s. Significantly, the entire flight system could be launched to low-Earth orbit on a single Atlas V-class launch vehicle. With an initial mass to low-Earth orbit (IMLEO) of 18,000 kg, the subsequent delivery of a 500-t asteroid to lunar orbit represents a mass amplification factor of about 28-to-1. That is, 28 times the mass launched to LEO would be delivered to high lunar orbit, where it would be energetically in a favorable location to support human exploration beyond cislunar space. Longer flight times, higher power SEP systems, or a target asteroid in a particularly favorable orbit could increase the mass amplification factor from 28-to-1 to 70-to-1 or greater. The NASA GRC COMPASS team estimated the full life-cycle cost of an asteroid capture and return mission at ~$2.6B.

Conceptual flight system configuration before deployment of the capture mechanism showing the locations of the cameras on the solar array yokes used to verify proper deployment and subsequently to aid in the asteroid capture. (Source: Asteroid Retrieval Feasibility Study, KISS)

CONCLUSIONS

The two major conclusions from the KISS study are: 1) that it appears feasible to identify, capture and return an entire ~7-m diameter, ~500,000-kg near-Earth asteroid to a high lunar orbit using technology that is or could be available in this decade, and 2) that such an endeavor may be essential technically and programmatically for the success of both near-term and long-term human exploration beyond low-Earth orbit. One of the key challenges – the discovery and characterization of a sufficiently large number of small asteroids of the right type, size, spin state and orbital characteristics – could be addressed by a low-cost, ground-based observation campaign identified in the study. To be an attractive target for return the asteroid must be a C-type approximately 7 m in diameter, have a synodic period of approximately 10 years, and require a ∆V for return of less than ~200 m/s. Implementation of the observation campaign could enable the discovery of a few thousand small asteroids per year and the characterization of a fraction of these resulting in a likelihood of finding about five good targets per year that meet the criteria for return.

Notional NEA Human Mission Concept of Operations with Pre-deploy. (Sources: Asteroid Retrieval Feasibility Study, KISS)

Proof-of-concept trajectory analysis based on asteroid 2008 HU4 (which is approximately the right size, but of an unknown spectral type) suggest that a robotic spacecraft with a 40-kW solar electric propulsion system could return this asteroid to a high-lunar orbit in a total flight time of 6 to 10 years assuming the asteroid has a mass in the range of 250,000 to 1,000,000 kg (with the shorter flight times corresponding to the lower asteroid mass). Significantly, these proof-of-concept trajectories baseline a single Atlas V-class launch to low-Earth orbit.

The study also considered an alternative concept in which the spacecraft picks up a ~7-m diameter rock from the surface of a much larger asteroid (> 100-m diameter). The advantage of this approach is that asteroids 100-m in diameter or greater are much easier to discover and characterize. This advantage is somewhat offset by the added complexity of trying to pick up a large 7-m diameter rock from the surface, and the fact that there are far fewer 100-m class NEAs than smaller ones making it more difficult to find ones with the desired orbital characteristics. This mission approach would seek to return approximately the same mass of asteroid material – of order 500,000 kg – as the approach that returns an entire small NEA.

Conceptual Human NEA Mission Excursion Vehicle Using SEP System (Image Credit/Source: NASA / AMA, Inc.)

The proposed Asteroid Capture and Return mission would impact an impressive range of NASA interests including: the establishment of an accessible, high-value target in cislunar space; near-term operational experience with astronaut crews in the vicinity of an asteroid; a new synergy between robotic and human missions in which robotic spacecraft return resources for human exploitation and use in space; the potential to jump-start an entire industry based on in situ resource utilization; expansion of international cooperation in space; and planetary defense. It has the potential for cost effectively providing sufficient radiation shielding to protect astronauts from galactic cosmic rays and to provide the propellant necessary to transport the resulting shielded habitats. It would endow NASA and its partners with a new capability in deep space that hasn’t been seen since Apollo. Ever since the completion of the cold-war-based Apollo program there has been no over-arching geo-political rationale for the nation’s space ventures. Retrieving an asteroid for human exploration and exploitation would provide a new rationale for global achievement and inspiration. For the first time humanity would begin modification of the heavens for its benefit.

  • Geoff T

    Would a 500-1000 tonne asteroid be a suitable counterweight for a space elevator system? If so, that’ll really be something.

  • Burke Burnett

    Great sleuthing. Kudos.

    “To be an attractive target for return the asteroid must be a C-type approximately 7 m in diameter…”

    Given PR’s apparent premise of profits from metals (which are likely found only [?] in X-class asteroids), what would one do with a C-class (carbonaceous) asteroid?

    This thing is going to get everyone racing back to study John Lewis’ book.

  • warshawski

    At 2.6B for 500,000 kg I do not see anyone making a profit from selling the refined product evin in orbit as you could get 20 Falcon Heavy flights for that price and put 1000,000 kg in orbit.
    Although you would not make a profit on this mission it is a great goal as the amount you would learn would be amazing and follow on missions could be bigger and cheaper eventually making profits. This type of mission should be done to kick start exploitation of space resources.

  • Michael Turner

    The buzz: this Planetary Resources startup will be about asteroid mining. But everyone seems to forget that James Cameron just came back from the bottom of the ocean, and that asteroids aren’t planets.

    I’m betting that the company will have more to do with the intersection between exploiting Earth seabed “planetary resources” and telerobotic prospecting off-Earth. The synergies between the kinds of analysis done for “oceaneering” projects and off-Earth construction have already been pretty well developed: http://spirit.as.utexas.edu/~fiso/telecon/Cadogan_4-25-07/Robot_optimization%20-%209-21-00.pdf

  • Michael Turner

    “… what would one do with a C-class (carbonaceous) asteroid?”

    Bulk biomass elements, specifically H and C.

    Carbon is in short supply on the moon, but not in C-2 asteroids. Carbonaceous chondrites are also H2O-rich (as high as 22%). And it’s long been understood that long-duration human space presence will require growing food in CELSS (Closed Ecological Life Support Systems.) That means: hydrocarbons and water. And water might be more easily gotten from C-2 asteroids than from the moon.

    Apart from the logistical and psychological advantages of vegetable gardening on spacecraft and in bases, a sufficiently rich “biosphere” around the crew quarters would offer some added solar-storm shielding value. The crew could always keep eating from food storage while the CELSS “reboots” from seeds and better-shielded saplings, after a solar storm kills the garden. Any water from the C-2 asteroid in excess of what’s needed for plants could be used as additional solar-storm shielding, frozen between an IR-reflective coating facing vacuum and an insulating lining between the ice shield and the internal biosphere. This shield could also double as a food freezer, since frozen food is, to a good first approximation, frozen H2O. Solar storms aren’t very long, so there’s no danger that the crew would starve while waiting one out. The bigger starvation danger would be in the wait while the garden regrew. But if your food store is also part of the outer shield, and your biosphere is part of your inner shield, and if the outer shield can be easily recycled at the destination for other purposes (it’s a lot easier to melt ice than aluminum) ….

    Space might be the best place, in the *very* long run, for metallurgical and (inorganic) chemical industries. But those industries will require lots of expensive, heavy equipment. Within the lifetimes of these Planetary Resource startup people, it’s going to be mostly about enabling government-funded manned missions and privately-funded space tourism. And that means growing food up there. ISRU-based food is probably the best deal, logistically. And C-2 asteroids are probably the best (known) ISRU base for CELSS.

  • Michael Turner

    “At 2.6B for 500,000 kg I do not see anyone making a profit from selling the refined product evin in orbit….”

    That’s the estimated cost of the proposed *mission*. Much of that expenditure would go toward R&D to produce a design for an asteroid-capture-and-return craft. That craft should be reproducible for a fraction of the mission cost. If each of our computers cost the cumulative R&D cost to make them, a laptop would cost a lot more than $2.6B.

    The bigger problem here might be the time value of money: 10 years to wait for a highly uncertain return on sunk costs is a lot of time, by the standards of financial markets, not to mention a lot of risk. IIRC there was an International Space University student paper that worked out just how bad an investment it would be ….

    Still, it’s a great idea just for its scientific value, and at $2.6B, well, that’s like the cost of 2 or 3 Shuttle flights. Nor need it be a one-off. If they can pick a chunk of rubble off a C-2 asteroid, they might be able to pick one off Phobos, which would give people a head start on figuring out how to put a manned base there. They might be able to fund it just from selling samples …. and, as I mentioned earlier, it would provide a huge base for biomass for CELSS in Earth orbit, on the moon, and for interplanetary missions.

  • W. D. Kelly

    Interesting. In 2009 and 2010 my colleagues and I submitted SBIRs to NASA regarding transport of volatile materials from the asteroid belt to the surface of the moon. We based our study on repeated Mars flybys, solution of the Lambert time of flight problem, targeting Lagrangian points, solar electric propulsion and tether applications. Our comparison point was previous studies based on launching from Earth or exploiting Phobos for similar supplies: the velocity deltas and attendant risks. Our main objectives were main belt asteroids suspected of hydration, low inclination and eccentricity. We did not intend to deliver entire asteroids, but obtain water, carbon, hydrogen and nitrogen compounds that were not present by any measure in ABUNDANCE on the surface of the moon.

    We did not get a go-ahead either time. In one instance, based on pre-submittal inquiries, we were informed that supplying H,C & N from off the surface of the moon was not in situ resource utilization and therefore not worthy of any evaluation. Perhaps we were contacting the wrong organization for research sponsorship of such nature?

  • Michael Turner

    KISS is perhaps halfway through its $24M, so maybe you still have a chance. NASA has to know what it’s doing, but politics cause the goalposts to crawl all over the field. They’ve given up on the moon (as Obama put it, “We’ve been there,” and can’t afford Mars. They probably can’t afford a manned mission to an asteroid either. One difference between your proposal and the KISS proposal is that theirs proposes that the captured asteroid become an astronaut destination in cislunar space. (L1 would probably be a good destination for a variety of reasons.)

    “Five general categories of benefits from the return of an entire NEA were identified:
    1) Synergy with near-term human exploration;
    2) Expansion of international cooperation in space;
    3) Synergy with planetary defense;
    4) Exploitation of asteroid resources to the benefit of human exploration beyond the Earth-moon system; and
    5) Public engagement.”

    Nothing there about actually going to the moon, though the possibility is implied in (1).

  • Michael Turner

    Mining very high grade ores for very expensive metals are still not very attractive when the capital investment is high, the return is far in the future, and the market is terrestrial. http://www.kemcom.net/EconAnal.pdf

  • Anonymous

    warshawski, the comparison is incorrect. The Falcon Heavy can put 53k kg to LEO, which you used to calculate 20 flights for 1M kg, or 16k kg to translunar trajectory. In this case, the asteroid will be brought to a lunar orbit, so the 16k figure needs to be used; 62.5 flights. The cost calculation is curious; 2.6B for 20 flights means one Falcon Heavy costs $130M?! Given that EELV costs more than that for about half the lift, I just do not believe it, even with a block buy. Remember what has happened to the “cheap” Falcon 1?

    GeoffT, the counterweight size is dependent on where it is placed above the stationary/synchronous point (roughly but not GEO), the weight of the cable beneath that point, and how much we want to lift. Keep in mind, though, that an asteroid is probably not entirely solid and structurally sound. So given the value of the counterweight position as a station and the equipment that will need to be there, I highly doubt an asteroid will be used _purely_ for a space elevator counterweight.

  • http://www.apse-inc.com(underdevelopment) APSE Inc.

    Just received a copy of the KISS study Sunday evening. APSE Inc. initial opinion is that the KISS approach is incorrect. For what they are doing, it would be more advantageous at this time to do small scale prospecting/sample return missions. Two points of note: 1.) Moving an asteroid – even a small one, any where near Earth is going to get a lot of people upset, and 2.) Given the mass of a ‘large’ asteroid and the mass of the eventual required mining equipment, it would be easier to move the mining equipment and do the benefication activity ‘in situ’ and ship out concentrate. Yes, the initial capital cost will be very high, but the equipment can be re-used on other asteroids and the energy ‘moving cost’ will be minimal. Also, regarding the capital cost for mining an asteroid, compared to what? Large scale terrestrial mines now cost in the order of US$ 500 Million to 1.0 billion to fully develop.

  • Marcus Zottl

    @ Moving an asteroid – even a small one, any where near Earth is going to get a lot of people upset.

    I disagree. It will upset a few lunatics (probably the same kind that demonstrated in advance of Cassini’s flyby of Earth out of fear for radioactive material onboard…). For the large public it will be like anything space related: they won’t care at all.

  • drs

    Never mind Falcon; Russian launches are $4000/kg. So 500,000 kg to LEO would be $2 billion. GEO, $8 billion I think.

  • Chuck Pell

    At current launch cost ($25,000/kg), the 500,000kg asteroid is worth $12.5B USD irrespective of the value of the materials which might be extracted, *as long as it remains off-planet.* Other than a few high-value items (gold is ~$50,000/kg, etc.) I see no need to return any of the captured material to Earth. The greatest value appears to lay with off-planet processing for those things useful in the immediate sense (e.g., ISS & the like), for example water, breathable gasses and bulk shielding against radiation and micrometeorites. Otherwise, just tunneling into a small body provides the foundation for useable habitat, minimal (flexible, composite, nonmetal) pressure vessels, perhaps fibers spun up from melt glass in situ. I’ve no issue with moving a small asteroid into LEO – no one is fighting having the ISS Up There.

  • Paul451

    Michael Turner,
    “For the large public it will be like anything space related: they won’t care at all.”

    Some mainstream media have already been covering Planetary Resources’ announcement. And that’s without any hardware. I think they would cover a several stages of an actual asteroid capture and return. And if there are idiot-protests and lawsuits, that will obviously be covered as well. So I’ll think there’ll be a fair bit of public interest. (Well, for the first, then it becomes routine and boring.)

    Remember, the closer we get to arrival of a captured asteroid, the more you’ll see media-friendly science-commentators (Neil deGrasse Tyson, for example) hyping the potential. And media loves hype.

    Chuck,
    It’s clear they are hunting for water first. That means fuel. Everything else follows from that. Once you have free fuel, each subsequent asteroid capture is also free.

  • Toreus

    I wonder if the capture craft discussed in this study could be reused- it doesn’t look like anything other than the fuel would be used up.

    On a related front, can Hall thrusters operate on gases other than Xenon (ie, water vapour?)

  • http://www.congrex.nl/11C03/papers.zip Zoran M. Ilitz

    The KISS study was based on my papers presented on the last planetary defense conference (follow the supplied link to get the papers (posters 35 and 44), or google ’2006 WB Ilitz’, follow the first link and read the comments below). Half a dozen of KISS study participants were also present in the mentioned conference and I explained them in detail what it is all about. The text above mentions that they considered fetching a boulder from an asteroid, which was my original idea, together with retrieval of an asteroid to cislunar space. Before I came up with this proposal, the best plan that NASA had was OSIRIS Rex mission to retrieve a tea spoon of sample material…

    Since none of my papers is mentioned in references, I write this comment to clarify where this all came from.
    Thank you for the story.

  • W. D. Kelly

    Mike Turner,

    Discussing the news with co-workers on our proposal for moving asteroid materials, it was noted that a big distinction between what the Keck Institute study looked at and what we were proposing was this: We concentrated more on main asteroid belt objects with low eccentricity and inclination and likelihood of aqueous composition, though NEOs were not excluded. The initial reports about Keck study and Planetary are talking about Near Earth Objects. Secondly, if the candidate was a bonanza, we would extract in situ and haul back.

    For lunar applications, H20, H, C and N compounds would be the targets. They could be used for propellants and perhaps even for tether or polymer packaging. If lunar surface settlements were to be established, the size of a “colony” of 10, 100, or 1000, whether now or 500 years from now would be constrained by how much H2O, C, H and N compounds (volatiles) you had access to. You could drill for them on the moon, you could haul them up from Earth, search and retrieve them from NEOs or the main belt or even Phobos or Deimos, but there would be in each case trade parameters, risks and costs.

    My own pre-analysis intuition is that ice concentrations increase with solar distance. You don’t want to go to Saturn’s moons which are practically orbiting glaciers, but NEOs are going to be dessicated. Even with Vesta data coming in, the volatile content is not really leaping out at us. Maybe Ceres will be a different story. But it is quite possible at this stage to return a NEO property to the lunar or terrestrial vicinity that is not worth the $/kg invesment.

  • Charles R. Nichols

    I read the KISS study. Frankly, I don’t get it. Even with the surprisingly low $2.6B price tag, there doesn’t seem to be much value added by dragging an asteroid back to HLO. If the goal is science, it would be cheaper to do the science in situ (in the asteroid’s original orbit). If the goal is resource utilization, it makes more sense to beneficiate in situ, and just move the valuables.

    I think Planetary Resources is doing the right thing, focusing on mining water first, as an enabler for all future space operations.

    Douglas Messier (above) speculated that Planetary Resources may plan to execute the KISS plan. But that was five days before PR’s website went live. There’s no support for that theory either on PR’s website or in the 4/24 press conference videos. PR’s website is actually pretty clear about mining water first, then metals. I don’t recall seeing anything about hauling entire asteroids around.

    —–

    Burke Burnett wondered, “… what would one do with a C-class (carbonaceous) asteroid? This thing is going to get everyone racing back to study John Lewis’ book.”

    Google “volatile products from carbonaceous asteroids”. It will show you a chapter from John Lewis’ other book, “Resources Of Near-Earth Space”. It turns out you can make quite a lot of industrial commodities from carbonaceous ore.