On the Habitability of the Moon and Mars

Moon Mars cities

Humanity’s future homes? Understand the problem first. (Artwork by R. A. Smith)

After reading the comments to a question recently posed at Reddit, I’m once again struck by how quickly a serious discussion about space can fly off the rails without knowledge of basic facts and their implications. The question that was raised is “Why is everyone so eager to colonize Mars, while the Moon, with its proximity and low gravity, sits empty?” As you might expect, the comments on this question vary widely in their relevance and cognizance. I thought it might be useful to collate some of the relevant facts that must be considered in determining which body is most useful for learning the life skills of an off-planet species.

Of course, the word “colonize” is loaded with different interpretations, but in this case, I take it to mean the establishment of permanent human settlements on either world. As is so often the case, the discussion at Reddit quickly turns toward comparing the two objects in terms of their resources and surface environments. While some of the comments are well informed, many misconceptions about the properties of both objects are readily evident – both confusing the casual reader and inhibiting the discussion.

Humans need raw materials, wherever they live, including light elements (e.g., oxygen, hydrogen and carbon (see page 4 of this paper), usually associated with the needs of life support, such as air, water and food) and heavier elements (needed to make things, including structures and machines). Energy is required to process this material into whatever form is required. Fortunately, all of the objects of the inner Solar System are rich in materials, although their concentrations vary from place to place. The critical controlling factor on whether a place can be inhabited is the availability of a reliable and continuous source of energy.

There is no “Second Eden” in our Solar System. Wherever people travel in the space around our Sun, they will have to create a protective environment to shield their bodies from the harsh conditions that they encounter. Because we are talking about not merely exploring, but rather living off the Earth, we need to be able to make what we need to survive from locally available materials. Naturally, some places are easier to settle than others, but when deciding which locations have more merit, it is important to fully understand all the requirements for habitation, not just the most obvious (albeit critical ones), such as the availability of water or the depth of the local gravity well. The key light element materials needed to support life are the so-called CHON elements (carbon, hydrogen, oxygen and nitrogen). Water supplies the middle two, but sources of both carbon and nitrogen must be found and available for harvest. After collection, we must be able to find or synthesize the substances needed, which involves a lot of chemical processing, time, and energy.

The Moon is depleted in light elements (although large quantities are present near the poles) but is well endowed in the heavier rock-forming elements (e.g., iron and aluminum). Over billions of years of micrometeorite bombardment, the lunar surface has been ground into a fine, grain-sized dust of jagged, angular fragments of minerals and glass. Moving parts quickly become immobile when coated with this talcum powder-like, abrasive dust. Future lunar inhabitants will need to mitigate these effects, as well as protect themselves from the transfer and inhalation of the local surface dust. The Moon has no appreciable atmosphere (its exosphere has a surface pressure of 10-15 bar, or about one-thousandth of a trillionth of the atmospheric surface pressure of Earth). The lack of a global magnetic field means that the lunar surface is a hard radiation environment. Both solar particles (including coronal mass ejections) and galactic cosmic rays bombard its surface. Over the course of a single lunar day (28 Earth days) at the equator, the Moon experiences thermal extremes ranging from 100° C to -150° C, while at the near-permanently lit areas near the lunar poles, the temperature is a constant -50° C. Compared to the planets, the Moon’s low gravity (about 1/6 that of the Earth) makes it a relatively easy object to access and leave (something we did successfully on six occasions, 45 years ago).

Mars appears to be richer in light elements than the Moon. We know very little about the nature and abundance of the heavier elements on Mars, but meteorites (that we believe come from Mars) suggest that its crust is made of rocks quite similar to those that make up both Earth and Moon. Thus, it is likely that iron is very abundant, and it is probable that aluminum and other metals can also be found in quantity. Like the Moon, Mars also has very fine dust, but it appears to be composed of clay minerals and thus, it is likely to be both softer and less abrasive than lunar dust. However, analysis of data from landed probes suggests that Mars dust may be highly reactive chemically (including the presence of toxic substances, like peroxides). Future Mars inhabitants will need to protect themselves from these substances.

Mars has an atmosphere but it is extremely thin (surface pressure is about 6 millibars, or six thousandths of an Earth atmosphere) and is composed almost completely of carbon dioxide. The martian atmosphere can be used to aerobrake (i.e., slow down a spacecraft during landings) but its atmosphere is not thick enough to eliminate the need for significant propulsive braking. This is a problem since the martian gravity is more than twice that of the Moon, or about 3/8 (0.38) the gravity of the Earth. Landing on Mars with heavy (i.e., human-sized) landers remains an important, unsolved issue (called the Entry-Descent-Landing (EDL) problem). The deep gravity well of Mars means that bigger, more energetic spacecraft will be required to get off the planet (and streamlined, as initial passage will be through an atmosphere that, while thin, is still significant). Mars is cold, but warmer periods occur in some areas (temperatures range from about -150° C near the poles, up to almost 20° C during summer at the equator). Although its atmosphere provides some protection, the surface of Mars remains a hard radiation environment, roughly equivalent to what is received by the equipment and crew on board the International Space Station.

On both planets, humans must be protected from the local environment. Pressurized habitats are needed and must include shielding from radiation. Such protection will likely be accomplished through the use of local material as shielding, either water (an excellent radiation protective) or local soil, requiring high-power machinery to excavate and move large amounts of material. Both Moon and Mars contain significant deposits of water. On the Moon, water is found in quantity within the permanently dark floors of polar craters. Hydrogen is also implanted on the grains of the lunar soil in extremely small quantities. Water appears to be more widely distributed on Mars, being found as vapor in the atmosphere, chemically bound in clay minerals everywhere, and in some localities at higher latitudes, near the surface as ground ice.

Energy is the critical pacing item for colonization. Wherever people go in space, they will need energy and lots of it. We must create a special environment to protect ourselves, something we get naturally here on Earth. The principal sources of electrical energy in space travel are solar and nuclear. The closer you are to the Sun, the more solar energy is available. Because Mars is about 1.5 times as far from the Sun as the Earth-Moon system, solar energy is less than twice as intense there (inverse-square law). This allows small robotic spacecraft to operate on Mars with solar panels, but solar electric, as the sole source of energy for larger vehicles and facilities (such as human habitats), is not practical. It is certainly inadequate for the amounts of energy needed for resource processing necessary to support a human colony. For this reason, credible plans for the colonization of Mars rely on the continuous operation of nuclear reactors.

On the Moon, a day/night cycle of two weeks duration (at the equator) means lunar inhabitants must survive a very long, cold night without solar power. In the past, ideas about lunar habitation have always collided with this reality, leading to a requirement of a nuclear reactor. Recently, however, mapping of the Moon’s surface found areas near the poles of the Moon that remain in sunlight almost continuously. This is possible because the Moon’s spin axis is nearly perpendicular to its plane of orbit around the Sun. This discovery makes lunar habitation much more likely. We can now envision an initial human presence off-planet without the need for the near-term development of a practical space nuclear reactor (an item that does not currently exist and will require several billions of dollars for development).

One last consideration is the distance from Earth. The Moon has the advantage of being relatively close – about three days away on typical trajectories. Moreover, as it is in orbit around the Earth, the Moon is constantly available for both arrival and departure, so a quick bug-out is always an option. In contrast, launch windows to Mars occur infrequently, on the order of every two years with current technology. Transit times (one-way) are several months in duration and do not offer easy abort options. The proximity of the Moon results in instantaneous RF communications (3 seconds round trip) while the distance of Mars means that communications between Earth and Mars have time-lags of tens of minutes.  Thus, habitation requires much more local autonomy at Mars than the Moon.  Unless the first colonists have a death wish, these issues of proximity and access must be addressed.

We know about de-conditioning of the human body in zero gravity, but we are completely ignorant of such effects in the fractional gravity the Moon and Mars. We think that problems from radiation can be minimized, but the long-term effects of living in a shielded environment are unknown. Some focus on initial access as the biggest problem, but gravity is only one factor and consideration among many. Any debate about where to “settle” in space must be cognizant of these and many other facts. Both the Moon and Mars have their respective advantages and disadvantages. The decision over where to focus our limited resources in the near term must take into account the relative abundance of materials needed, their locations on the object and our ability to access and process them into a form that we can use.

Debate is good and is to be encouraged but only informed debate is useful and essential.

Related: A comparison of asteroids vs the Moon as a space destination can be found in this 3-part series:

Destination: Moon or Asteroid? Part I: Operational Considerations

Destination: Moon or Asteroid? Part II: Scientific Considerations

Destination: Moon or Asteroid? Part III: Resource Utilization Considerations

This entry was posted in Lunar development, planetary exploration, space policy, space technology, Space transportation. Bookmark the permalink.

53 Responses to On the Habitability of the Moon and Mars

  1. DougSpace says:

    I am frequently struck that, in these debates, the fact that Mars has “oceans” of water is emphasized whereas the Moon is described as dry as a bone and therefore Mars beats the Moon as the destination where we should first establish a base/settlement/colony. That may be relevant for long-term settlement but isn’t such a deciding factor in the near-term. It is not so important the total quantity of water available on the planet that matters but rather the concentration at the best, first location. At one-part-per 18 at Cabeus Crater, that location has significant concentrations of water, albeit several additional factors are relevant for consideration. An analogy could be, if one is living on the shores of Lake Victoria, should one move to the shore of the Indian Ocean simply because there is much more water there? No, Lake Victoria provides far more than enough water for centuries of needs.

    Significant to the point that the Moon is more proximate to the Earth than Mars is that this means that it is within telerobotic reach of relatively low-cost labor on Earth operators and also that it is much closer in terms of transport time than Mars. Secondly, the existing and near-term markets are at and near Earth and so the Moon probably offers a better “exit strategy” for the transition from government financial support to market support.

    It is true that Mars gets about 43% of the solar energy flux compared to Earth. But is the 2.3 times more solar panels needed on Mars so significant so as to require nuclear power?

    • Paul Spudis says:

      But is the 2.3 times more solar panels needed on Mars so significant so as to require nuclear power?

      I think that ultimately, we’ll need nuclear power on both Moon and Mars to support a significant human presence. But initially, we can get a foothold on the Moon easily with solar arrays. Any Mars expedition solely reliant on solar will find itself severely hamstrung.

      • billgamesh says:

        I would be curious if some kind of space mirror could be positioned or a system of space mirrors to provide solar energy to surface arrays during the lunar night. And I would guess that such mirrors positioned on the peaks of eternal light (or whatever the popular term being used) would be more efficient than placing solar arrays there. There is no weather to worry about blowing such huge reflectors down. No Moonquakes powerful enough (?), no atmospheric conditions to inhibit using such mirrors.

        • Paul Spudis says:

          I would be curious if some kind of space mirror could be positioned or a system of space mirrors to provide solar energy to surface arrays during the lunar night.

          In theory, this could be done, but it might be more trouble than its worth. A better approach might be to use rechargable fuel cells to bridge electrical power coverage during the relatively short eclipse periods. Another possibility that we are examining is the use of extended, elevated masts to raise the solar arrays into sunlight; these might need to be only a few meters high to become exposed to full sunlight.

        • Joe says:

          Am not sure of the relative efficiency either, but such a concept was explored in the 1970’s by Krafft Ehricke. It was called a Lunetta. While intended to provide lighting, it would be applicable to power systems as well.

          You can read more about it in paragraph 10.28 at the link below.


          • billgamesh says:

            Thanks Joe, I will take a look. I am not a big fan of Ehricke- he made some statements I don’t think are valid; but then all of the visionaries of that period and earlier were shooting in the dark to a much greater extent than now.

            “The city of Selenopolis would be powered by fusion power plants, because only such an energy-dense energy source could energize a city that had to create its own environment, grow its own food,”

            Solar energy in my view is the only energy resource necessary on the Moon. I consider fusion reactors a scam. Just my opinion. Fusion bombs; completely different matter- they work quite well.

          • billgamesh says:

            His Third Law was right on the money.

            “A perspective for growth, which would necessitate expansion beyond the Earth, would bring about international cooperation, scientific developments, a global industrial revolution, and ultimately, the preservation of civilization and of human growth potential. In contrast, a no-growth, closed-world pathway would lead to irrational anti-science movements, geopolitical power politics, regional chauvinism,-”

            And while I disagree with some of his predictions, his speculation about fusion energy was, on second thought, may not have been too far off the mark. Though I am not a fan of fusion reactors, PACER is one of my favorite concepts for generating power (only in the outer solar system). I hate to give Mars advocates ammunition but it might work quite well there. Some day.


          • Joe says:

            billgamesh says:

            “I am not a big fan of Ehricke- he made some statements I don’t think are valid”

            Ehricke’s confidence in the development of Nuclear Fusion may be misplaced (or not), but his thinking on these subjects generally has at least three distinct things to recommend them:

            (1) Ehricke took the approach that space settlement must begin with space industrialization intended to benefit the population that is not interested in leaving the Earth.

            (2) Ehricke was technically astute and thought through all the details of his proposals in surprising detail.

            (3) Many of Ehricke’s concepts are significantly different than those usually discussed (even in specialized forums). A good example is the Lunetta.

            To quote our host (in the article to which this comment section is attached):

            “Debate is good and is to be encouraged but only informed debate is useful and essential.”

            I am not endorsing Ehricke’s plans uncritically, but they are well thought out, technically astute and (in many cases) significantly different from a lot of things discussed. That makes them worthy of consideration.

      • Vladislaw says:

        Dr. Spudis, are the frequency of dust storms on Mars enough to also include wind power as maybe a backup? Or is wind just to infrequent on Mars?

    • I think it would be excessively dangerous to deploy astronauts on the surface of Mars for several months or longer without at least some nuclear power supply especially with the potential dangers of dust storms making solar power production almost impossible for a few months.

      I would agree with Dr. Spudis that using fuel cells to supply back up energy on the Moon would probably be the cheapest and most convenient way to sustain early outpost.

      Lunar water could be electrolyzed into hydrogen and oxygen and stored in pressurized tanks either as a pressurized gas or a liquid. Hydrogen and oxygen gasses could also be simply stored in inflatable gas bags.

      Human waste and other hydrocarbon waste could also be converted into methanol through plasma arc pyrolysis. Methanol can be used with oxygen in fuel cells to produce electricity. The resulting carbon dioxide and water from the fuel cell could be recycled to manufacture methanol and oxygen again.


  2. billgamesh says:

    The distance is the overwhelming factor due to radiation exposure. In fact, I strongly believe this places any voyage to Mars in the realm of fantasy until a lunar infrastructure is established. The type of nuclear propelled, massively shielded, rotating spaceship required to transport human beings on multi-year missions to Mars can only come from the Moon. And that makes the whole question of where to go first not even worth discussing.

    Unfortunately, because of the legions of New Space proponents who buy into the infomercial-like schemes for retiring on Mars, basic facts must be explained and even then needlessly argued. Not only this, but any forum where this issue is aired risks being flooded with vehement denials from outraged New Space sycophants. It is very frustrating and even more so when periodically dozens of articles concerning bizarre make-believe missions appear on the internet. The recent spate claiming NASA will colonize Venus with cloud cities is just one example.

    The public has been and continues to be deluded with misinformation and false advertising that really amounts to nothing more than infomercials. As long as the public only has this kind of false advertising to consider then the situation can only get worse. Over the last several years it has become disturbingly evident the citizenry has very little basic knowledge about space. Almost all of what the average American knows about human spaceflight comes from movies and New Space “press releases” which are actually advertisements disguised as news.

    The political decisions concerning funding for space exploration are heavily influenced by public opinion. The infomercial is thus playing an important role in the U.S. space program and that is…pathetic.

    • Just how deadly is space radiation to Mars? I understand that NASA cannot morally create a situation that is detrimental to the astronaut’s health, and therefore cannot propose a space mission that leads to increased cancer risk. But if the astronauts decide to take these risks knowingly, are they reasonable as compared to mountain climbing, or some other dangerous activity (smoking?).

  3. Joe says:

    “but sources of both carbon and nitrogen must be found and available for harvest”

    Dr. Spudis,

    I know we have discussed this before, but could you say a little about the availability of carbon and nitrogen at the lunar poles?

  4. billgamesh says:

    The ice on the Moon is at the poles and this leads me to ask the question of how does this affect launching payloads from Earth? Do we have to launch lunar polar payloads using an Earth-polar-orbit-type launch? If so what is the penalty for not using the Earth-rotation-assist launch practices?

    One reason I ask this is to clarify how this effects all the New Space claims that everything a Super Heavy Lift Vehicle like the SLS can do can be done from LEO using depots and orbital assembly techniques. I am highly critical that any of these schemes would be a successful substitute for a larger direct-to-the-Moon launcher with hydrogen upper stages.

    • Paul Spudis says:

      Ultimately, the harvesting of lunar water makes a heavy lift vehicle unnecessary. The depot idea — supplied by water from a source in space with low gravity (e.g., Moon or asteroid) can supply all needs to move payloads throughout the Solar System.

      The issue with depots v. heavy lift today is different. The New Space fetishism with fuel depots is based on their idea that they can launch payloads from Earth at costs so low that it beats the cost of developing and operating a heavy lift vehicle.

      • billgamesh says:

        I apologize for not phrasing my question properly. I look at it now and it is pretty confusing.
        Presently, (not ultimately) as far as I know, we launch everything leaving Earth orbit on a roughly eastern flight path to get an assist from the Earth’s rotation. To change that direction and enter a lunar polar orbit for landing on the lunar poles requires more energy than when landing near the lunar equator. Correct me if I am wrong. My question is partly how much (if any) mass penalty this incurs in terms of the approximate payload to be soft-landed. Would it require less energy (and less mass penalty) to launch directly into an Earth polar orbit and then on to the lunar polar orbit and landing?

        The New Space fetish of building everything in LEO means they do not have the option of launching from the Earths surface directly into an Earth polar orbit and on to a lunar polar orbit. Does this Earth to LEO and then to Lunar polar mission require more total energy or less than the direct polar launch? Or am I completely clueless about how all this works (that is a possibility:)

        • Paul Spudis says:

          There is a small penalty for going to lunar polar vs. equatorial orbit (a few tens of meter per second). but the Moon is so distant from the Earth that a polar orbit is attainable from an Earth equatorial departure orbit without any significant difficulty. If you send cargo to the Moon via a “slow boat” route (e.g., weak stability boundary trajectory), there is essentially no delta-v penalty at all.

    • I’m not sure what the argument is between fuel depots and heavy lift vehicles since both are mutually beneficial to each other. You’re not going to be able to use propellant depots to transport large payloads to the lunar surface from LEO for an outpost without first being able to transport relatively large spacecraft to LEO plus large amounts of fuel to LEO. And the SLS will be able to do this much more cheaply and efficiently than the Delta-IV heavy or the Falcon Heavy.

      The principal advantage of a heavy lift vehicle is that it makes it a lot simpler and less expensive to deploy large heavy objects within cis-lunar space and onto the lunar surface.

      With an Altair-sized cargo landing vehicle, that could only be fitted withing the payload fairing of a heavy lift vehicle, you could place a large lightweight SLS fuel tank derived multilevel outpost on the lunar surface– with a single SLS launch (an instant large and spacious lunar outpost that could be easily regolith shielded from radiation).


      • billgamesh says:

        “I’m not sure what the argument is between fuel depots and heavy lift vehicles-”

        The argument concerns an HLV going directly to the Moon with a worthwhile payload and the New Space schemes of using much smaller vehicles and inferior propellents to supposedly accomplish the same mission for far less money. I do not consider such schemes to be either possible or “mutually beneficial.” I consider them a scam.

  5. Will there be a follow-up on the habitability of O’Neill colonies?
    Take it smaller, and it’s a “Mars Direct” tuna can covered with sandbags or artificial rock aggregate for shielding, spun up for full Earth G on a truss made of locally sourced rock and metal. the peurpose of it is to allow to build up habability so crews don’t need to rotate back to Earth until they’re ready to retire, and to produce ices and maybe even metals for shipment back to LEO. Or to support Mars exploration with zero time-delay telerobotics (while mining water ices to send back)

    • Paul Spudis says:

      Will there be a follow-up on the habitability of O’Neill colonies?

      Not from me.

    • billgamesh says:

      O’Neill envisioned only Earth Lagrange colonies constructed from lunar resources as far as I know. I just took a quick look at my copy of “The High Frontier” and I don’t see anything about Mars. Though he did address eventually mining asteroids for water (there was no evidence of ice on the Moon at that time) he did not think that Mars, or any of the planets or their satellites, were good candidates for human habitation.

  6. Protecting humans from radiation on both the Moon and Mars is probably pretty easy to do, IMO, since it only requires a few meters of regolith to reduce the radiation exposure of a habitat below radiation levels of exposure legally allowed for radiation workers on Earth.

    But the long term effects of exposure to a continuous hypogravity environment (lower than the Earth’s gravity) on the human body is the big unknown!

    We already know that a– microgravity– environment is inherently deleterious to human health over several weeks or months of exposure– and may even cause blindness and even sterility in some individuals.

    But we simply don’t know if the low gravities of the Moon and Mars are deleterious to human health and reproduction. And if such environments do turn out to be harmful, we don’t known whether simply wearing a weighted vest or temporary exposure to hypergravity machines could resolve this potential long term health issue.

    Of course, we could quickly find out if a low gravity environment is significantly harmful to the human body by simply establishing an outpost on the lunar surface where astronauts could stay for a year or more. Why even talk about going to Mars (a multi-year mission under microgravity and low gravity conditions) if we can’t do that?


    Living and Reproducing on Low Gravity Worlds


    What if You Were Born in Space?


    SLS Fuel Tank Derived Artificial Gravity Habitats, Interplanetary Vehicles, & Fuel Depots


    • We should be able to build a small centrifuge on the ISS today and test a few generations of mice at various gravities. I still can’t figure out why this hasn’t been done yet.
      There have been many proposals, but no action.

      • billgamesh says:


        Artificial gravity was generated successfully with a tether system in 1966. And if you are going to spin equal masses at the end of a tether you might as well generate a full Earth gravity. Dancing around this solution in the interest of going cheap is no solution.

        • The question I wanted to answer is not how to have gravity for a ship, but can we survive and reproduce in low gravity environments, such as the Moon and Mars. Animal experiments on the ISS would certainly help to answer the question, even if it wasn’t a definitive answer.

          As far as tethers go, I have absolutely no problem with them or any other method that works. I do know that large rotating dynamic systems with only tension elements are probably quite complex and may have induced harmonics and other problems that are not evident. It’s hard to dissipate energy in unwanted systems oscillations with only tension elements and nothing to work against.

    • Dear Marcel,

      As I would hope to be able to say regarding my own contribution to this discussion (see comment below), I think the materials you’ve linked to represent some quite enlightening background reading for this post by Dr. Spudis. And who did the wonderful illustrations for your article on the SLS-derived habitat?!?

      Still on my wish list: a brief refresher course on the physics of centrifugally-induced gravity.

      G. W. (Glenn) Smith

  7. LocalFluff says:

    I do not buy the argument that going to the Moon will be a stepping stone to Mars. If you want to go to Mars, go there directly, Dr. Zubrin is right in that respect although he greatly underestimates that challenge.

    My point is that since the Moon will be explored first, every new investment in the Moon will make another investment in the Moon cheaper and more beneficial. The relative gap between another mission to the Moon and the first mission to Mars will become bigger and bigger.

    While there will be more general Lunar lessons learned which would be useful on Mars, almost all of the equipment would have to be redisgned from scratch because of the differences. Like basing an expedition to Antarctis on experiences from an expedition to Sahara. Transfer ship, EDL, surface habitat, power, communication, ISRU will all be different. Even the most trivial items would need at least adaptations to dust, gravity, temperature, degree of autonomy. That very different Mars arcitecture might be constructed on the Moon, but would just be another very big difference between the two!

    • billgamesh says:

      “If you want to go to Mars, go there directly,-”

      The first solar flare will irradiate you beyond what the already hostile deep space radiation environment would and combined with zero gravity debilitation and the tiny cabin would leave you irradiated, debilitated, and probably psychotic. Good luck with that.

      The Moon is the only place to acquire space radiation water shielding, assemble, test, and launch the nuclear propelled, artificial gravity equipped spaceship required for Human Space Flight Beyond Earth and Lunar Orbit (HSF-BELO). Denial of this requirement is not a solution.

    • Trying to go to Mars directly from LEO and back and without using lunar resources for propellant, air, water, and radiation shielding would dramatically increase the delta-v requirements.

      Trying to go from LEO to a Mars Transfer Orbit would require a delta-v of 4.3 km/s.

      But departing from an Earth-Moon Lagrange point to Mars Transfer Orbit would only require a delta-v of less than 1.1 km/s.

      Trying to supply your interplanetary vehicle at LEO from Earth with the propellant, air, water, and radiation shielding need for an interplanetary trip to Mars would require a delta-v of more than 9.2 km/s.

      But supplying those resources from the Moon to an Earth-Moon Lagrange point interplanetary departure site would only require a delta-v of less than 2.6 km/s.


  8. Dear Dr. Spudis,

    A terrific post — clear-eyed but also forward-looking — regarding the relative habitability of the Moon and Mars.

    For me a key point is your early reference to the question of “which body is most useful for learning the life skills of an off-planet species”. Yes, given the proximity of the Moon, it may well be a better destination than Mars for our initial camping trips; but given also its low gravity and direct exposure to the cosmic microwave, the critical long-term value of the Moon — and this, I hope, not out of line with your many previous posts on the subject — is more likely to be as a site for the autonomous factories which will create the Cadillacs of space exploration.

    Speaking of which — autonomous lunar manufacturing, that is — I am wondering if it would be out of place for me to introduce your readers to my own contribution to the subject, with the hope that there will be other informed “amateurs” who will not be afraid to “put up or shut up”?


    G. W. (Glenn) Smith
    New Orleans, LA

  9. William Mellberg says:

    “Unless the first colonists have a death wish, these issues of proximity and access must be addressed.”

    Dr. Spudis, you have written another outstanding analysis that deserves widespread readership. In particular, you have provided a very large dose of reality to those who embrace fantasy, and your “death wish” comment reminds me of Elon Musk’s claims that he plans to “retire on Mars.”

    Given your partial description of the martian environment, a logical question to Mr. Musk would be, “Why would anyone in their right mind WANT to ‘retire’ on Mars?”

    Musk’s cadres of faithful NewSpace followers have all bought into the notion that Mars colonies are just around the corner. “Thousands” of settlers will be traveling to the Red Planet within the next twenty years. But who on Earth would want to live on Mars?

    The thin atmosphere on Mars gives it an Earth-like appearance in the images returned by our various robot landers. But Mars is a very hostile place. You could not walk outside without a spacesuit. There are no plants or trees to be found. There are no fields, grasslands or dairy farms. You won’t find a supermarket or even a convenience store. Entertainment? Forget about it. No sports arenas, no concert halls, no movie theaters, And you would never be able to have a two-way conversation with friends and relatives back on Earth because of the communications delay. Which, I suspect, would be very frustrating for a generation that is so keen on instant communication (and gratification).

    So I repeat … “Why would anyone in their right mind WANT to ‘retire’ on Mars?”

    The Moon has a harsh environment, as well. But, as you point out, “proximity and access” are in favor of permanent facilities on the Moon, much like we have permanent facilities in Antarctica.

    Apollo 17 Astronaut-Scientist Harrison Schmitt has called the Moon “a natural space station.” But unlike the International Space Station, where every drop of water and every molecule of air has to be transported from Earth, lunar resources will make it possible to achieve some measure of autonomy on the Moon. Food can be grown on the Moon. And communications with Earth, as you point out, is near instantaneous. The Near Side of the Moon would be a great place for Earth observations. The Far Side of the Moon would be an excellent place for astronomical observations (both optical and radio). And the lunar surface provides a vast area for exploration of the sort that will build the experience necessary to settle worlds beyond cislunar space, and to better understand how those worlds formed and what they might hold in terms of resources.

    It is hard for me to understand why it is so difficult for most people (and some politicians) to comprehend that the Moon, not Mars, should be the focus of human presence beyond Low Earth Orbit. But, again, you have identified some of the problems: an ill-informed public, an uninformed press corps and the misinformation generated by special interest groups.

    Given the reality of the martian environment, perhaps we should be a little more aggressive in asking the ‘space cadets’ and ‘musketeers’ …

    “Why would anyone in their right mind WANT to ‘retire’ on Mars?”

    Given the limited resources here on Earth for space exploration (i.e., money), Mars represents a diversion from the more logical next step of creating permanent facilities on the Moon to develop the capability to “live off the land” on other worlds. That’s the reality of the situation.

    In short … I agree with you 100%!

    Bill Mellberg

    • billgamesh says:

      In my view the New Space end game is LEO tourism and the promises of space travel are a facade to hide this scam. Preying on the gullible to finance something the unwashed masses would never support is a time honored-tradition among “entrepreneurs.”

      Going to the Moon would dump the New Space manifesto in the trash can, which is why they are fanatically opposed to the SLS. Going straight to the Moon with a HLV and bypassing the whole DIY hobby rocket fantasy would expose LEO as a dead end.

      Nothing gets a reaction out of a New Space fanatic like praising HLV’s; it drives them nuts. The only thing worse is to express words to the effect that “there is no cheap.” The reality that only massive governmental resources can get humankind into space is anathema to the whole New Space scam. That want to “hand it over to Musk.” It has more in common with a cult than actual spaceflight advocacy.

  10. Thanks Marcel, the link leads to a very clear and interesting explanation of the requirements and risks regarding radiation shielding.

    So 240 tons in the form of 50 cm of water would be adequate for a risk level that makes sense, and is far from trivial. 500 kg per m2 of surface area= 480 m2, for an internal volume of about 1500 m3, or twice the ISS? I expect careful choice of travel period might reduce this a bit, but is not logical for the design of a system that aims at colonization, or frequent use.
    I can also guess that in the Mars one concept, the single dose radiation of the single trip is part of the reason you don’t want to come back the same way. And that the modules we usually see illustrated for that concept seem inadequate for long term, or even mid term, habitation.
    It’s interesting to see that the obstacles are not technical, but financial. I wonder where the break even point is between sending shielding mass up from the Earth and producing it on the Moon? I guess it’s even more a question of the fuel required to move that extra shielding mass to Mars as well, that favors the Moon production of fuel, and solves the question of shielding at the same time.

    Michel Lamontagne

    • billgamesh says:

      According to Eugene Parker in his 2006 article “Shielding Space Travelers”, the minimum shielding for a capsule would be 14 feet of water at about 400 tons. The reason there is no way around such massive shielding is secondary radiation. I believe Parker is correct and this means chemical propulsion is pretty much useless. Nuclear energy is required. It is the elephant in the room nobody wants to talk about but it is what it is.

      • billgamesh says:

        I would add that the shock this 14 feet-of-water requirement has on spaceflight enthusiasts is mind-numbing and usually leads to the person going into automatic denial. It is hard to deal with but…..it is not a show-stopper. I am sure when plans for the F-1 Saturn V first stage engine were first displayed many people just shut down and could not accept it. The mass changes a long held worldview that spaceships must be in some sense like an airliner (like the space shuttle). The spaceship will ironically be more like an actual ship in the ocean wrapped in several thousand tons of water. Nuclear energy is certainly powerful enough to push this mass through space- the public simply has to acclimate to the idea. And any such true spaceships will have to operate from the Moon.

      • Well, who is right here? 2 feet or 14 feet? that’s 3000 kg/m2. How did mr Parker define the minimum shielding required? From Marcel’s text, the 14 feet thickness corresponds to a dose of 5 rem, which is the maximum yearly dose for radiation workers on Earth. 2 feet is the 50 rem dose, or an increase in cancer risk of 3%, which is of course unacceptable for a worker in a radiation plant, but perhaps an acceptable risk for an astronaut that does one trip per decade?

        • billgamesh says:

          We used to send hazmat workers into toxic waste sites in jeans and T-shirts. That changed. In my early years in the aviation industry I was exposed to all kinds of chemicals and heavy metals and I watched that completely change. The entire philosophy of “managing” radiation exposure is a similar situation in my view. And zero gravity debilitation is also the same game. The fact is that human beings evolved in Earth gravity and Earth near sea level radiation and to send people into space for a large part of their lives means providing that environment if they are to not suffer permanent damage. Trying to go cheap always ends up costing more in the end.

          • billgamesh says:

            I would add I am sorry I did not directly answer your question of “who is right”; the answer is that there is not enough data to know for certain. This is always used by the New Space crowd to pronounce radiation mitigation as trivial and a non-issue. When this happens I always think about the Apollo astronauts having trouble sleeping because of the flashes they saw even with their eyes closed. Those flashes were cosmic rays blowing holes through their brains. I think Parker is right.

      • DougSpace says:

        What do you make of this NASA graph?


        I read it to mean that about 30 cm of water could cut the GCR exposure in half. For the initial missions to Mars (e.g. flyby, Phobos-Deimos, go-and-return) cutting total radiation exposure (GCR, SPEs, solar wind) to 20-50% could mean keeping older male astronauts well within their career limits.

    • DougSpace says:

      > I wonder where the break even point is between sending shielding mass up from the Earth and producing it on the Moon?

      Hi Michel, It is a fairly complex question depending upon the specific context. I think that for early missions to Mars, water for shielding from lunar sources would be dependent upon the ease and quickness by which ice harvesting operations are established. If all goes well, even after the first landing of ice harvesting equipment sized for a largish lander, enough water and hence propellant could be delivered from the Moon to orbit and pushed towards Mars. However, if ice harvesting operations are harder than hoped for then an alternative is to launch water (or polyethylene) from Earth to LEO and then have ion propulsion slowly spiral it to Mars. Either way, I think it best to send the shielding into a double synodic Earth-Mars cycler orbit so that the cost of one launch can be spread out over multiple uses. The shielding of the shielded habitat could be increased with either additional launches or as lunar (or asteroidal) resources come on-line.

      My opinion is that the harvesting of lunar water ice will be easier than some presume. The ice concentration is likely good, the ice harvester could be large, equipment could be designed with quick-release mechanisms for swapping spare parts, the equipment could warm itself (the vacuum prevents heat loss through convection), and equipment could be developed in realistic environments (e.g. the Space Power Facility with vacuum, cryogenic, and 5/6ths weight suspension).

      • billgamesh says:

        If the ice is readily accessible then landing humans may not be initially necessary- a robot lander might be able to land on the ice, harvest a load and convert it into fuel, and take off back into lunar orbit to transfer the water to an empty upper rocket stage. Then go back down and do it again. If this could be made to work then Astronauts could arrive in lunar orbit with a fully shielded radiation sanctuary waiting for them- no worries about a solar storm suddenly turning their mission into a disaster. These water filled space stations in lunar orbit could be- I am of course just speculating- could be the best precursor to actually landing and initiating construction of a Moon base. Since we have an HLV almost ready but no human lander on the horizon the semi-expendable robot lander could be the way to go.

  11. gbaikie says:

    **”The question that was raised is “Why is everyone so eager to colonize Mars, while the Moon, with its proximity and low gravity, sits empty?”

    Of course, the word “colonize” is loaded with different interpretations, but in this case, I take it to mean the establishment of permanent human settlements on either world. As is so often the case, the discussion at Reddit quickly turns toward comparing the two objects in terms of their resources and surface environments. While some of the comments are well informed, many misconceptions about the properties of both objects are readily evident – both confusing the casual reader and inhibiting the discussion. **

    Venus is the most Earth like planet. Venus lacks water- though is has Hydrogen and Oxygen in chemical compounds- the acid clouds have hydrogen and the air is CO2 from which one can get oxygen. So it has a lot of water which can made, but doesn’t have much water which relatively easy to obtain. Though general idea with settlement on Venus is one cities in it’s sky.

    But there is no shortage of places to live on Earth- 70% of earth is ocean, and it would easier to have settlement on Earth oceans than Venus or anywhere is space. And Venus has same “problem” as Earth- it’s difficult to get out of it’s gravity well.

    One thing we would want from Space is cheap electrical power. The US and the world has spent hundreds of billions dollar “trying” to get affordable electrical energy from solar energy. It has been failure due to the nature of Earth itself. Getting 12 hours of solar energy per day is not very
    useful for our needs and we don’t even get 12 hours of solar energy on Earth. Germany is one of worst places to get solar energy. One could say if you make solar energy work in Germany, one could make solar energy work in the rest of the world [because few places where people are living are worst locations]. And Germany’s effort to be the solar capital of the world is driving the country toward poverty. Space exploration [though some may think is wasteful use of money] has never driven any country towards poverty- one argue that’s quite the opposite. No one can argue that Germany’s obsession with it’s solar energy governmental program, has been helpful to it’s society- unless driving up the price of electricity conforms with one confused ideas of being “helpful”.Nor has it lowered Germany’s CO2 emission and causing more coal and wood use, which per unit of energy make the most CO2.
    The region of Germany gets yearly average per day of about 2000 watt hour of useable solar flux- if got 20% as electrical power that is 400 watt hours per square meter per day. The rovers on Mars are getting more electrical power than this. Anywhere on Mars is better location than Germany. And with Mars if you do something to inhibit global dust storm, it would be better place to harvest solar energy than compare to where most people live on Earth [not many people live in deserts and nor vast ocean area which less cloudy].
    One could argue that like Earth one would need nuclear energy on Mars.

    Anyhow the best place to harvest solar energy for people living on Earth is from Earth high orbit- where one can get a constant source of solar energy.
    And the main problem with getting solar energy from high earth orbit is the cost of getting to high earth orbit.
    People commonly are concerned about to issue of transmitting the power from Space. And would say that this involves another benefit from harvesting solar power from space- it’s “giving you” a global network that provide electrical power anywhere and at any time on Earth.
    Or it’s providing way that people could live in the open ocean or anywhere not near current electrical power grids. Or it’s cheaper and better electrical power grid.
    Or if one could get solar energy in space, one could want a better power grid. Or we already have a communication grid which space based. If it was cheap to get to high orbit, we would establish a
    electrical power grid simply to move electrical power generated on Earth to other locations on Earth. And in addition to that you get abundant, clean, and cheap electrical power to entire earth for as long as the Sun burns.
    So harvesting solar power from Space is dependent on much cheaper access to high earth orbit.

    But we can’t go directly to getting cheap solar energy from space, in same way we can’t get automobile made 2000 years ago. Or factor in having airplanes was having powerful enough and lighter enough engine. So we could not have gotten airplane [regardless of how smart people were] 200 years ago- as engine had not technological advanced enough. But there was also other factor involved, such as economic development of wealth- lots people who could buy airplane tickets [and wanted to].

    The Moon is the first destination because it has low gravity well and because it might have minable water. It’s minable because it’s in a low gravity well and it’s in high earth orbit, and it’s too expensive to get things to high earth orbit.
    For lunar water to be directly related to getting solar energy from space to earth requires water and or electrical energy to be very cheap and available on lunar surface. And cheap is somewhere around $1 per lb of water and $1 per kw hour of electrical power.
    If one put blinders on and only think of the Moon- such a cheap price is not really possible- not in the “foreseeable” future [or not within a century]. Or in order to get this in foreseeable future one has see the moon for what it is, which is the gateway to the solar system.
    Or if focus on colonization the moon, it’s possible it work against getting to such low price.
    It depends upon what the people do, but just saying it’s possible people could prevent it.
    Because their is shortage of water on the moon in terms of human consumption of water- and people going to want swimming pools and farm things- the Moon doesn’t have enough water for this purpose. The Moon has endless supply of water for the purpose of rocket fuel. The Moon’s water could help send more than million people to Mars.

    So the Moon needs to be exporter of stuff rather domestic consumer of stuff. And if it does this fast it would soon be able to import all the water it needs in the future [sooner]- because this solar system has earth oceans of water. It’s possible that water is dumped on the Moon, simply for the purpose of generating “hydro power”. Though long before one gets to point importing massive amount of water on the moon, you have earthling getting electrical power harvested from Space.

    So one should think of how to get the moon in the export business. And starts with commercial mining of water.
    Now Mars also has relative to the Moon, cheap water. Mars could even have cheaper water compared to Earth. Mars is not a good location to export rocket fuel, but could be a good location for the consumption of water for farming [and human use in general- Martians could “afford” swimming pools]. So what NASA should do is look for cheap water on mars- and cheap water will drive human settlements on Mars.
    So obviously such cheap water would be below the mars surface, and probably mean water in liquid form which can be pumped to the surface. Mining permafrost is not I what mean by cheap water. So we know Mars has trillion of tonnes of water in polar cap and frozen ground, I think there could trillions tonnes of water available below the surface as liquid. So Mars could only have
    a few spots there there is water which easy to pump from the ground, or it could have many locations- or none. And that one thing NASA should explore for.

  12. That’s what i see as well, and it agrees with the reference given above. The question seems to be more is 50, 25 or 5 rem acceptable? Parker says 5, the reference from Marcel says 25 but with some risk, we could go with 50 and even more risk, mostly for men. I would personally accept 25 for a 6 month trip follower by 5 for the rest of my life. But that’s hardly a design basis! I think that 500 kg/m2 would certainly reduce the flashes from what little shielding the Appolo guys had

    • NASA has set the maximum career ionizing radiation exposure limit for a 25 year old woman (the most vulnerable interplanetary passenger to radiation) at approximately 100 Rem for a 3% increase in the cancer rate.

      The briefest round trip scenario during the 2030’s (more than 500 days) would probably just barely exceed 100 Rem of radiation exposure during solar minimum conditions with no radiation shielding. Of course, you’d still have to protect her and the other passengers from the long term of effects of potentially brain damaging heavy nuclei and major solar events. Just 20 centimeters of water shielding would appear to be enough to protect astronauts from heavy nuclei and major solar events.

      But, philosophically, I don’t believe that any– single interplanetary mission–should be a career-ender for an astronaut because of excessive radiation exposure, especially for a person still in their 20’s. (Could you imagine traveling to Mars in your 20s and then being banned from traveling into space again for the rest of your life!)

      So I would recommend that crewed interplanetary vehicles have trans-habs with at least 50 centimeters of water shielding to reduce annual cosmic radiation exposure to approximately 25 Rem per year during solar minimum conditions.

      Permanent rotating artificial gravity habitats in orbit within the solar system, however, should be much more heavily shielded, IMO, probably with at least 50 cm of dense iron ore. Fortunately, there are plenty of iron ore resources on the Moon and in the asteroids and meteoroids.

      Happy New Year!


      • billgamesh says:

        From Shielding Space Travelers:

        “Above every square centimeter of surface is a kilogram of air. It takes a vertical column of about 70 grams about 1/14 the distance through the atmosphere, achieved at an altitude of 20 to 25 kilometers (60,000 to 80,000 feet) before the average incoming proton hits the nucleus of an atom in the air. The rest of the atmosphere serves to absorb the shrapnel of this initial collision.”

        “Mars astronauts would receive a dose of more than 80 rems a year. By
        comparison, the legal dose limit for nuclear power plant workers in the U.S. is five rems a year. One in 10 male astronauts would eventually die from cancer, and one in six women (because of their greater vulnerability to breast cancer). What is more, the heavy nuclei could cause cataracts and brain damage.”

        “To match the protection offered by Earth’s atmosphere takes the same one kilogram of shielding material per square centimeter, although astronauts could comfortably make do with 500 grams, which is equivalent to the air mass above an altitude of 5,500 meters. Any less would begin to be counterproductive, because the shielding material would fail to absorb the shrapnel. If the material is water, it has to be five meters deep. So a spherical water tank encasing a small capsule would have a mass of about 500 tons.”

        My mistake, I thought it was 400 tons. This is obviously different than the information Marcel is using.

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