Research Papers I Wish I Could Con Someone Into Writing Part I: Lunar ISRU in the Age of RLVs

One of the things I’d love to do if I were successful enough at Altius to afford it would be to sponsor graduate-level research into space technology, business, economics, and policy topics that I’m interested in. Not just because I don’t have time to dig into these topics as deeply myself as I would like, but also because frankly there are lots of graduate students out there who have better analytical tools they could bring to bear than the crude ones I could come up with informally. I decided to share some of these ideas via a blog post in the hope that maybe I could either inspire someone in grad school who is looking for a research topic, or if not I could at least plant the seed for conversation on this blog. If someone is interested in doing one of these research topics, I’d love to do a review of the final paper when it comes out.

Topic One: Lunar ISRU Economics In The Age of RLVs
This is one that I often discuss with coblogger Chris Stelter on Twitter. There have been a lot of papers over the years looking at ISRU economics, but the vast majority, if not all of them, have made the assumption that launch costs are more or less static. I think I understand the usual reasons for doing so–either a) these papers are trying to recommend a policy change, and therefore are being compared against the status quo approaches of say government exploration missions using entirely earth-launched propellants, or b) at the time of the papers, RLVs weren’t taken very seriously, and the last thing they wanted to do was to make ISRU look less respectable by making it look like it depended on RLVs.

But now is probably a good time to start looking into what lunar ISRU economics look like if you assume RLVs can be successful in driving down launch costs in the foreseeable future. I’ve seen a lot of SpaceX fans recently who have made the argument that lunar or NEO ISRU is totally irrelevant because BFR costs are guaranteed to be so cheap that there’s no way lunar ISRU could possibly compete with it. I think this is… mildly overoptimistic, but one result of lunar ISRU studies that assume status quo earth-to-orbit launch costs (both for launching ISRU infrastructure, and as competition) is that the lunar ISRU price points they quote really do seem kind of high compared to potential RLV price points. I personally don’t think lunar ISRU is in as much trouble as all that, but I do think that since it is more likely that we’re at the dawn of the age of the RLV, that those interested in lunar ISRU economics should at least start looking as RLVs become available.

Some thoughts on approaches:

  • Launch costs are going to vary over time–even if gas and go RLVs happen in the foreseeable future, it’ll still take time to get there. So instead of treating launch costs as static, make a few scenarios where you make different assumptions about the shape of the launch cost vs. time “S-Curve”. How long does it take for significant reductions in $/kg to start appearing? How low can they realistically get before hitting diminishing returns? How steep is the slope of $/kg over time once that initial decrease starts creating new demand that creates virtuous cycles? The nice thing is that you can probably characterize these S-Curves with only a few parameters, and then you can come up with say at least three scenarios–a pessimistic one where RLVs are only mildly successful, and launch costs decrease slowly, hitting diminishing returns at a moderate price point, an optimistic one, where RLVs are very successful, and the transition is fast, with the point of diminishing returns being dramatically lower than current prices, and then a middle of the road S-Curve shape.
  • Assume that lunar ISRU developers are smart enough to leverage RLVs as they become available, so that launch of ISRU hardware can take advantage of the decreasing costs over time. For example–George Sowers was mentioning a recent CO School of Mines analysis that showed it was possible to extract water from the lunar poles for $500/kg of extracted water on the lunar surface. But he was assume a $35,000/kg delivery cost to the lunar surface for all the infrastructure.
  • It would be interesting to see analyses that reflect the idea that lunar ISRU developers might be able to leverage decreasing launch costs to also lower the exploration and development costs of their lunar ISRU capabilities.
  • It would be good to include scenarios for how hard lunar ISRU ends up being, ranging from scenarios where trying to crack oxygen out of the regolith is the best we can do, through lunar polar ice being legit, all the way through Warren Platts’ lunar aquifers scenario. My guess is that this could also be modeled by some sort of S-Curve as well, as there’s going to be a learning curve for developing lunar mining, that eventually snowballs, but then hits diminishing returns, but the timing, depth, and steepness of the curve could vary.
  • It would be cool to see analyses that assume different cislunar transportation architectures for getting lunar ISRU propellants back to LEO. Not just rocket only, but also architectures that use propellantless launch options (see my unfinished “Slings and Arrows” series), aerocapture, SEP transfer, nodes at different cislunar orbital locations (LEO, EML1/2, LLO, etc).
  • It would be interesting to see with these analyses where the equillibrium point ends up being for lunar ISRU vs RLV-earth-launched propellants under different assumptions. I could see some cases (optimistic RLVs, pessimistic lunar resource difficulty, lame approaches to cislunar transportation) where lunar ISRU isn’t even competitive on the lunar surface, while there may be other scenarios, where lunar ISRU wins hands down even in LEO. But it would be interesting to see patterns and what assumptions lead to which outcomes.

Anyhow, I just wanted to seed the thought. I’ll probably turn this into a series for other research topics I’d like to see others write, but I wanted to throw this one out there.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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21 Responses to Research Papers I Wish I Could Con Someone Into Writing Part I: Lunar ISRU in the Age of RLVs

  1. gbaikie says:

    Are we in the space age, or has come and went, or is it about to happen.

    I think we are in the space age because the global satellite market is 200 billion a year
    industry. Because one can’t afford not using satellites and satellites are critical to national security. So satellite market marks beginning of space age and it will continue
    and it will peak with harvesting solar energy in space for earth surface use.

    In terms of lunar water mining, it will begin with 5 to 10 billion dollar per year market
    and when the lunar market is 100 billion dollar per year there will be no doubt we are in the Space Age. And from 5 to 100 billion could take less than 10 years, though it could also take more than 50 years.
    It seems cheaper earth launch cost main affect will be to possibly reduce that time- have be more likely to be 10 years or less as compared to 50 years or more.

    The most important factor related to getting the lunar market to 100 billion dollar is
    is lunar exploration to determine whether the lunar poles have minable water.
    And exploration of Mars to determine if and where Mars settlements are viable is also
    related to when the lunar market reaches 100 billion per year.

  2. DougSpace says:

    That would be a wonderful study. I would assume that the most expensive propellant to deliver from Earth would be that propellant used for ascent from LEO to LLO. For a reusable lunar lander – ferry this is necessary and a fair amount of mass. So one could start there to look at which is more cost effective.

    I think that someone needs to write an article skeptically analyzing the likelihood of the BFR-BFS. There’s a lot of space advocates assuming that it will become reality. It might and it certainly would be great, but there’s a whole lot between here and there. Elon is talking about starting with the BFS and it being SSTO. That would definitely be a first and I can’t imagine very easy. The Grasshopper exploded. How much setback would there be if a BFS exploded during development. Even if BFS made it to orbit, could it have enough propellant to land. Would we have another series of crashes requiring the construction of a new BFS each time? And one cannot fully test the BFS at Moonor Mars re-entry speeds without also developing the BFR. So, we could be waiting many years before we know for sure that the BFR-BFS is something we can confidently plan around. Meanwhile we’ve got a proven FH that could do much to establish a permanent lunar base.

  3. DougSpace says:

    I’m concerned with how some people place lunar-derived propellant on the critical path to either the Moon or Mars. It makes lunar return harder to sell because one has to overcome the uncertainties about the nature of the polar water and about the challenge of harvesting ice from those environmental conditions. Rather, I advocate that lunar polar ice should be a “value-added” thing and that any plan should include a Plan B in case harvesting water ice for propellant never becomes economically viable.
    Launching two FHs per mission is a straightforward solution.

  4. Paul D. says:

    One thing lower launch costs would do is reduce the cost of exploring the lunar surface for high quality deposits. So it would shift scenarios away from “what can we do with average regolith” to “what can we do with rare ore bodies”. Of course this requires making assumptions about those ore bodies, but the standard scenarios already make the assumption they aren’t there.

  5. Paul451 says:

    “That would definitely be a first and I can’t imagine very easy. The Grasshopper exploded. How much setback would there be if a BFS exploded during development.”

    It depends on whether they are building them on an assembly line, like Falcons, or as bespoke one-offs.

    Paul D.,

    lower launch costs would do is reduce the cost of exploring the lunar surface for high quality deposits

    That was, in effect, Jon’s third point. Lower launch costs also lowers the price of surveying and development, making it easier to fund, making it more likely that someone actually does that.

    For example, lower launch costs mean that you can fly equipment at an earlier level of development. That lets you test early and develop iteratively. Lower first-iteration development costs (combined with lower launch costs) means that smaller companies (and smaller projects in larger companies) can afford to reach at least that first step. That increasing the number of ideas being explored and (hopefully) increases the potential pool of investors willing to fund at that level.

    High launch costs mean you pretty much have to launch your final production equipment, that means you have to be much more certain that it will work out of the box, and that drastically increases the development costs. That means you need a much larger potential market in order to have a sufficient return to pay for that larger investment, which means a larger-scale of production to supply at that level, which increases the cost of development (and launch) yet further. All of which reduces the number of companies capable of doing that work, and the number of investors will to throw that much money into such a high-risk venture.

    IMO, the difference in costs could be orders of magnitude.

    So while lower launch costs lowers the price of your “rival product”, the minimum size of the potential market necessary to justify the scale of investment can be much less. The question is which effect wins. (Which is where Jon’s hypothetical pet grad student would come in handy.)

  6. gbaikie says:

    If you don’t count the potential of mining water on the moon, how does 2016 HO3 compared to the Moon?
    2016 HO3 is probably 40 meter or more in diameter
    40 meter diameter sphere has volume of 33510 cubic meter and if each cubic meter
    has density of 2 (twice water density) it has mass of 67020 tons or 67,020,000 kg.

    Rather then cost to get to it, what cost to get mass off 2016 HO3. Moving 50,000 kg from
    it to Earth surface?
    And how much cheaper if instead earth surface move it to a high earth orbit and/or Earth/ moon L point?
    A 40 diameter sphere has surface area of 5026 square meter. Or 50 meter by 100 meter
    field is 5000 square meters.
    There could be one rock on the surface which was 50,000 kg, there could 10 rocks equalling 50,000 kg. Or could get 50,000 kg of gravel/sand. Or could mine 50,000 kg of something say, iron or anything picked up with a magnet.
    And 1 rock which is 50,000 kg is about 3 to 5 meter across (or in diameter).

    Say whomever going to mine 2016 HO3 is going send at least 50,000 kg towards Earth but if someone wants to bring at to earth surface or use it space, they have to pick it up
    in some Earth orbit. So miner going have the 50,000 kg hit Earth’s atmosphere have one
    pass aerobrake and it. Might be orbit with apogee of less than 5 million km from Earth.
    And question where is it wanted and how much would someone paid per kg?

  7. Jeff Greason says:

    There is SO much work to do. I’ve got a backlog of papers that need a grad student co-author that could keep me busy for years. I work something out to the point where *I* know the answer, then I just never have the time to typeset the formulas and do the analysis at the one more level of detail that makes it publishable. I, too, have toyed with the idea of doing that, but there’s always something more urgent.

  8. Jim Davis says:

    It’s worth pointing out that ISRU, like all infrastructure projects, will follow demand, not proceed it. Bridges aren’t built so people can cross rivers; they’re built because people are already crossing rivers. Roads aren’t built so people can so somewhere; roads are built because people are already going somewhere. The Panama canal wasn’t built so ships could get to the Pacific; it was built because ships were already going to the Pacific.

    If the advent of RLVs leads to a lot of lunar traffic, then ISRU will follow naturally. If it doesn’t, then ISRU won’t happen. Big airports don’t get built hoping that they will turn small towns into big cities.

  9. gbaikie says:

    “If the advent of RLV leads to a lot of lunar of lunar traffic,…”
    A round trip ticket to Lunar surface is about 1/2 billion dollar per seat and flyby is
    about 1/4 billion. And flyby is about only thing “discussed” in terms of being offered.
    When SpaceX is flying crew to ISS, it will have something with 7 seats and the Heavy Falcon at 150 million per launch and reusable dragon, so a flyby of Moon could be around 100 million per seat.
    Will people buy flyby seats of Moon if fairly safe and at 100 million?
    If had a lunar lander would people buy seat at 250 million.
    It seems government space agencies would buy a seat to lunar surface because it’s cheaper and better than a robotic lunar lander.

  10. gbaikie says:

    It seems if had 10 people who paid total of 5 billion to land on Moon and return safely to
    Earth someone would provide that service at that price.

    I think NASA should have Lunar exploration program which had total cost of 40 billion
    dollars. 20 billion or half total program cost is robotic and that begins, now (and apparently it is beginning now) and then after few years of robotic exploration, NASA
    should 2 or 3 years of lunar crew exploration, that ends while starting a NASA Mars
    exploration program with about 1/3 of it’s total budget robotic or 2/3 manned exploration.
    And the lunar manned sends about dozen crew in total to lunar surface.
    And it seems after NASA lunar manned, seats to lunar surface could be about 1/4 billion. Or if 10 people willing to pay 2.5 billion dollar, someone could deliver that service.

  11. gbaikie says:

    NASA goes to Congress says going finish lunar exploration within 10 years and then at cost of 50 billion per decade will explore Mars.
    Lunar robotic and manned program has total taxpayer cost of 40 billion or about 1/5th
    of ISS program. In terms of added budget (congress costs) about 5 billion.
    Or “congress cost” is 5 billion and that’s for both lunar and Mars exploration, and part
    of this is reorganization costs, and this reorganization is going about focused upon delivering Lunar and Mars exploration at low cost to tax payer, which will involve not spending as much money on things not having to do with lunar and Mars exploration and involves dealing ISS so NASA not continuing (the cost) ISS when Mars program begins.
    So continuation of NASA plus 5 billion added in near term, or 1 billion per year for next
    5 years and then returning to “baseline” (whatever that is).
    And I think preserving ISS for international use and depot in 28 inclination should be
    finished/finalized within 5 year.

  12. DougSpace says:

    > The Panama canal

    One of the problems with historic analogies is that they don’t allow for the exceptions including doing new things that haven’t been done before.

    ISRU might be the exception. By your reasoning, the ISRU production of propellant on Mars won’t happen until there are already spacecraft that send all of their return propellant to the surface of Mars first. Only when this is happening will the people already accessing the surface of Mars demand that their return propellant be produced by ISRU. And so ISRU will naturally result from the demand of those already spending a whole lot of money accessing the surface of Mars in an inefficient way.

    The reason why this ISRU might be employed from the get-go rather than after an inefficient crewed transportation system to Mars is already running is that sending large craft to the Martian surface without ISRU is a known, very expensive approach which space architects have long been planning to avoid by incorporating propellant ISRU from their first large mission. So, I prefer to reason from the relevant factors (e.g. first principles) rather than only by historic analogies. This is the way that Elon says that he reasons and it seems to be working pretty well for him including innovating a lot of new things.

  13. gbaikie says:

    I don’t NASA should mine lunar water (and/or make lunar rocket fuel) for a number of reasons. But one reason is it would be more expensive.

    In terms of NASA exploration of Mars, I think NASA should import H2 and make rocket
    fuel using Mars air. So with NASA mars exploration, also not have NASA mine water.

    A Mars settlement needs to mine water and make rocket fuel on Mars and a Mars settlement would have enough demand for Martian water, to make cheaper than importing water.

    With lunar water mining one needs to get to point of mining about 1000 tons of water per yea for it to be viable.
    And NASA mining 1000 ton per year is crazy and undesirable.
    With Mars, NASA might mine mars water because looking for water by drilling
    for water is the same as exploring Mars in terms of finding cheap water. And mars settlement site could be wherever there is water well which provide enough water for a future settlement.
    But I don’t think one starts Mars exploration by looking for site to drill for water, such a search might occur several years after establishing a Mars base.

  14. Jim Davis says:

    The reason why this ISRU might be employed from the get-go rather than after an inefficient crewed transportation system to Mars is already running is that sending large craft to the Martian surface without ISRU is a known, very expensive approach which space architects have long been planning to avoid by incorporating propellant ISRU from their first large mission.

    If propellant production on Mars turns out to be as easy as these plans require we will certainly see it at a very early stage. If it turns out to be as complicated as it is on earth it might be awhile.

    So, I prefer to reason from the relevant factors (e.g. first principles) rather than only by historic analogies. This is the way that Elon says that he reasons and it seems to be working pretty well for him including innovating a lot of new things.

    But note that Musk is very vague about what happens after his ships land on Mars with empty tanks. Propellant production seems to be at the black box level; CO2, H2O, and solar power in, LCH4 and LO2 out. He needs it for his Mars architecture to work but so far we have been given few details. While I’ve been surprised by Musk’s successes in the past (the first stage recoveries still amaze me) I do not automatically assume that what Musk needs will appear when desired. I hope a prototype (analogous to the Grasshopper tests ) of his propellant plant are revealed in due time.

  15. Paul D. says:

    Perhaps it would be useful to consider the simplest possible ISRU scenario you can think of.

    My take on that: mining the moon for shielding mass to be used for high orbital crewed stations. The latter would be for satellite maintenance and construction. I’m imagining very large satellites being created in GSO that would benefit from crew on hand to assemble and maintain them.

    The reason this is simple is that all it would require is landing a vehicle, loading up on regolith, and launching again. The mass payback might be positive if LH2/LOX propellants were used.

  16. gbaikie says:

    Well if you want shielding, mine our other moons. But NASA shouldn’t mine nor make
    rockets. NASA should explore space in order to establish/start different markets in space
    which private investment can fund.
    The most important activity in space is the global satellite market which is a 200 billion
    market. In
    NASA should explore the lunar poles to get more information about lunar water,
    armed with information, people can decide if they want to invest in lunar mining.
    If that happens, one gets a lunar market which could be a tens of billion market within a few years.
    After exploring lunar poles, NASA explore Mars (to possibly start another market) and NASA should look for viable places for towns and cities on Mars.

  17. Warren Platts says:

    The Colorado School of Mines has recently started a master’s program in space mining, so getting some ideas for possible projects out there might actually get them done!

    The thing is, if you’re going for lunar rocket propellant (water), the lower the launch costs from Earth, the lower your profit margin. Thus, under an optimistic scenario, you would almost have to find some relatively shallow sublunarian aquifers!

    The price of gold, on the other hand, does not depend on Earthly launch costs….

  18. gbaikie says:

    The price of gold, on the other hand, does not depend on Earthly launch costs….

    If you don’t have lunar rocket fuel, good brick on lunar surface are not worth bringing back to Earth.
    Or lunar regolith brought back to Earth is worth more than same weight in gold.

    The main problem of making lunar rocket, is getting enough demand for lunar rocket fuel. That is why NASA shouldn’t mine lunar water, as NASA is incapable of generating market demand for lunar water or lunar rocket fuel.

    A company which mines lunar water, could also mine lunar iron. Or if extracting water from regolith, you can at same time extract iron ore.
    Also one could extract gases from regolith, Co, Co2, h2, He, etc.
    One also has the basic equipment to mine gold, and make landing zones, roads, and stuff like burying habs. So the business is “earth moving” and mining, and you start with mining water. And by selling water (to company which needs water to make rocket fuel) you will be making it cheaper to go to the moon, and thereby creating a larger local market for iron (and mining company and other companies could need stuff made out iron- if the lunar manufactured iron products are cheaper than bringing something of similar function from Earth).

    If you are drilling for water, you are exploring for underground water.
    If NASA is exploring for lunar water by drilling for it, that could be expensive and if NASA finds water, it is then “mining water”.
    I think NASA should just look at surface of the small region of lunar poles and bring back lunar samples, and then NASA should explore Mars. And after a few years on Mars, I think NASA should explore for Mars water by drilling for water.
    If NASA could find underground water which could provide enough water, that could be location for a Martian town.
    If NASA finds movable lunar water at the surface, then investors could invest money to mine water and other activities on the Moon. And if lunar water is being produced at 1000 tons per year, one could get private exploration, looking water one could drill, or look for gold and PGMs or whatever.
    If you get to point of Mars settlement, then you could have a much larger market for lunar rocket fuel, and if can find large supply lunar well water when there is large demand of water, then that could be worth a lot more than cost of finding it.

  19. gbaikie says:

    Let do goldilocks.
    I think most important aspect of minable water in the space environment is the demand of water. Now much water would be bought per year and the amount bought depends on supply and price of water. And we assume everyone can buy water or sold as any other commodity and we will assume the market is not crazy but rather finds the correct price which reflects what is knowable.
    Or I think lunar water is worth about $500 per kg and one needs to mine about 1000 tonnes per year (1 million kg per year).
    A problem with 1000 tonnes year is having enough demand and it requires a lot electrical power to split 1000 tons of water in one year.
    What would be knowable is how much water, a company plans on mining and could know how much electrical capacity is being planned on being installed.
    And if there is a perceived shortage of what is planned to mined, and/,or amount electrical power installed, that could be seem as opening for other business to compete by meeting those perceived demand needs.

    So if lunar water mining company plan is to mine 100 tons, and roughly double production per year: 100, 200, 400, 800 tons by year four of operations.

    Such a lunar water mining company could get other companies who also mine lunar water. They could mine within say 50 km of first company, or could mine on the other lunar pole. And part of deciding to have new lunar water companies is the perception of higher future demand for lunar water. Though being able to “own” different areas of Moon is probably as a great a driver related starting other lunar water mining companies
    on the moon.
    But let us go back the one beginning company and have 3 choices:
    1) 10 tons per year and doubling
    2) 100 tons per year and doubling
    3) 1000 ton per year and doubling.
    Would any of these choices alter the price of lunar water?
    If they didn’t, then probably one should pick 3). And perhaps
    consider starting with 5000 or 10,000 tons and doubling.
    There are some factors other than the price of water.
    It will cost more to start with higher production, and your cost of money
    (should have higher risk) would be higher. And the other aspect of having
    enough electrical power to split the water. One advantage is it would tend to
    discourage other lunar water companies from starting near you (could encourage
    lunar mining at other pole).
    Another plan, could be to mine a lot of water in beginning, and shift mining operations to other types of mining, like mining lunar iron. And a portion of mining equipment can
    used for things other than mining for any kind of ore (building roads, burying living space, etc).

  20. Jim Davis says:

    Or I think lunar water is worth about $500 per kg…

    Then lunar settlement is pretty much out of question, wouldn’t you think?

  21. gbaikie says:

    “Jim Davis says:
    April 30, 2018 at 10:46 am
    Or I think lunar water is worth about $500 per kg…

    Then lunar settlement is pretty much out of question, wouldn’t you think? ”

    Yes, at that price.
    Same goes for Mars.
    With Mars one should be able to have much cheaper water, though
    with Mars you would dealing with quite different situation.
    A location to mine lunar water could have as little as 10,000 tons of minable water.
    A site for Mars town requires millions of tonnes of easily accessible water, and not mining dirt or even ice, but drilling water wells.

    So, 10 million kg at $500 is 5 billion dollars gross value of water at a site.
    Whereas one billion kg of water at $10 is a location for a town which will need at least that much water for the life of the town in coming decades. Though 1 trillion kg of water is lot better than 1 billion kg for a mars settlement, but a million tonnes would minimum for Mars vs 10,000 tons as minimum for lunar mining site.
    With the moon, after a decade, one could move the operation to another site within the polar region.

    With the Moon you want to start with mining area of about 1 square km or less.
    If there is 100 kg of recoverable water per square/cubic meters of regolith then 1000 by
    1000 is million times 100 kg, or 100 million kg (100,000 tonnes) and would require decades to mine. You don’t need that much. One looking for best football field area to mine.
    Whereas with Mars settlement the site could be the size of the entire lunar polar region – hundreds of square km. And with Mars site, you don’t want to have a ghost town in the future.
    The Moon is where one mines, and Mars is where one grows crops.
    And place with food and water is where you live. And Mars is months away, and moon is a days away from Earth (and with teleoperation, most people working on Moon, can live on Earth).
    50 years after lunar water begins, water could be imported to the moon. The moon importance is a possible location of where water mining in space, begins.
    Or where the market of water from solar system, starts.
    Moon is gateway to the solar system, which has more water than the oceans of Earth.
    So 50 years after, lunar water mining begins to would be cheaper to mine water on the moon, but getting water from space rocks could be even cheaper to ship it to the moon than mining lunar water.
    And lunar settlements could be in lunar orbits. Which can import food from Mars.
    Or if can get cheap water and cheap CO2 one could also grow food in Earth/Lunar orbits.
    Less than a million tonnes of lunar rocket fuel within a century will transform us into spacefaring civilization- and Earth will get unlimited energy from space. And the SPS might be made from lunar material or from space rocks or quite locally, from the few quasi moons of Earth.

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