Choosing A New Home For Humanity

There five important factors for long term human space colonization:

Resources - humans need certain kinds of resources to survive and continue a technological civilization: carbon, nitrogen, oxygen, hydrogen, silicon, metals. We also need energy: solar, nuclear, or geothermal.

Accessibility - this is what people often mean when you say "gravity well". How much energy does it cost to get there or leave there? Imagine you live on your own personal island. Sure, it's quiet and there's no neighbors to annoy you, but if you don't have fuel for your helicopter or a nice speedboat your economic sphere of influence will be significantly constrained.. much as we are down here on Earth.

Radiation Protection - also, down here on Earth, we have an atmosphere that protects us from the harsh solar and galactic cosmic radiation, but just about everywhere else in the solar system we're going to have to make do with a more substantial physical barrier - either thick metal plates or, more likely, a couple of meters of dirt.

Gravity - we evolved in a 1g gravity field and all the evidence we have so far indicates that we may indeed need that much gravity - particularly for having offspring. Perhaps we can get by with less, but there's no evidence for that yet.

Technology - this basically comes down to when we go. Our current technological capabilities are insufficient to get beyond cislunar space. Soon we should be able to go anywhere in the inner solar system. Eventually, we'll be able to go anywhere in the solar system and then it'll just be down to delta-v requirements. Some day, the stars.

So let's consider the possibilities. To give an objective analysis of the suitability of each destination I'll adopt a scoring system where 2 points are awarded if something is particularly favorable and comes as an inherit part of the destination. If some application of existing or near-future technology is required then a score of only 1 point will be awarded. I wont score the technological requirements to reach the destination as we will only be considering destinations that are reachable with current or near-future technology.

Score: 1 Res, 0 Acc, 2 Rad, 2 Gra = 5.

Although we're talking about a new home for humanity, I think scoring our current home is a good way to show the motivation to leave it. We certainly had plenty of resources on this planet, but there's also a heck of a lot of us on it now and the availability of those resources is starting to dwindle. The accessibility of Earth is the worst in the inner solar system.. it takes so much delta-v to get off it that we have to use multi-staged rockets. The radiation protection is exquisite, and so is the gravity.

Orbital Habitats
Score: 0 Res, 2 Acc, 0 Rad, 1 Gra +1 Zero-G Bonus = 4.

The traditional O'Neill Colony, Clarke Wheel, or even the ISS imports all its resources. Of course, the accessibility is great but as a result of the high cost of import there's few materials to use for effective radiation shielding. Artificial gravity can be provided, and the easy access to zero-g is a bonus to scientific work.

Score: 1 Res, 1 Acc, 1 Rad, 0 Gra = 3.

The Earth's Moon is often considered a barren destination with little in the way of resources. This picture has been disputed over the years with many people pointing out the wide availability of metals, silicon and oxygen in the lunar regolith. Recently, the discovery of a water cycle on the Moon and the expectation that there may be as much carbon as there is hydrogen firmly busts the "desolation" myth. That said, it certainly will be a lot of hard work to live there. The accessibility is poor, but not nearly as bad as Earth. Radiation protection would be of the underground variety. The gravity is terrible for human reproduction and there's nothing we can do about it.

Score: 1 Res, 0 Acc, 2 Rad, 2 Gra = 5.

Colonizing the surface of Venus is mostly out of the question due to the surface temperature and pressure (460C and 93 atm). Although it may be possible to colonize Venus with platforms floating in the upper atmosphere, the only resources available would be the toxic atmosphere and anything you can collect, with great expense, from the surface. And the gravity well is almost as bad as the Earth. Radiation protection from the atmosphere alone would be same as Earth, but the magnetic field is much weaker. Venus also has another thing going for it: it's the only body other than Earth in the inner solar system which has a near-one gravity.

Score: 2 Res, 1 Acc, 1 Rad, 0 Gra = 4.

Mars is almost universally considered the premier destination for human expansion into the solar system. The reasoning is that it has a sidereal and geological similarity to Earth. So we can expect the resources to be abundant, and getting at them will be a similar experience to getting at them on Earth. The gravity is under half of that of Earth which means the accessibility is pretty good. The radiation protection requirements places all infrastructure for living and industry underground. The weak gravity also means that having offspring there may be impossible.

Asteroids etc
Score: 2 Res, 2 Acc, 1 Rad, 1 Gra +1 Zero-G Bonus = 7.

The near Earth asteroids, the moons of Mars and the asteroid belt are the most abundant resources in the inner solar system. Getting to them and moving between them is the easiest, in terms of energy, of all the alternatives. Radiation protection is available simply by digging into them which, considering the average density of most the asteroids we know about, could be done by robotic precursor missions or even by hand if necessary. Using rotating habitat structures, or even just a simple train driving around an endless loop of track, a full artificial gravity can be provided to the colonists. The easy access to zero gravity is also a bonus to industry.


The question of how much gravity humans need to produce healthy offspring must be answered before we can seriously discuss the colonization of the solar system. If it turns out that we need a full Earth gravity then it really does rule out most destinations until some future technology is developed to overcome it. To my mind, this question should be the primary focus of human adaptability experiments on the ISS and elsewhere in space. For example, a new short arm centrifuge module should be under production to be used for full gestation small mammal testing. Beyond the ISS, the planned robotic lunar landers should carry microarray experiments to test the affect of reduced gravity on human cells. If the expressed protein sequences are sufficiently similar to terrestrial control results then we can say with some confidence that reduced gravity is worth future study - such as landing a full gestation mammal testing module on the Moon.

Special thanks to Dr Jim Logan for inspiring the scoring system.


  1. Gabriel9:20 PM

    Hello Trent,

    Could be interesting to expand candidates to far bodies (Titan, Europa...).

  2. You should include large in-space habitats as another option. There will be a continuum between non-rotating stations and gigantic rotating O'Neill type of artificial worlds. It's quite possible to consider a sizable but fairly near-term habitat that uses a tether and counter-mass to provide significant spin-gravity if not 1 g.

    WRT rad protection, I agree that on the near term, simply piling up bulk mass will suffice for surface habitats. Perhaps there will be caves available as well. (In-space habitats can also build up mass over time. Each vehicle that comes to the habitat will leave behind additional material. Waste materials should be used for rad protection rather than returned to earth.)

    On the longer term, though, there could be more imaginative approaches.

    For example, water is an excellent rad protection material. (As is any hydrogen rich material. Much better than metals, which produce nasty secondaries.) It also has the nice feature of being transparent. Since water needs to be stored in large tanks anyway, they could be used as walls for habitats. Add windows on the sides and you have sunlight streaming into your living area.

  3. > The question of how much gravity humans need to
    > produce healthy offspring must be answered
    > before we can seriously discuss the
    > colonization of the solar system.

    Really the answer is you need full G to stay healthy. Space advocate folks don't like it, but folks on the ground don't like needing exercise. Given we can pretty well duplicate zero G bio med effects with bed rest. Its the tidal effect on cardio vascular system acting as exercise. Even doing a couple hours of cardio a day in orbit, is less excessive then spending the day siting up watching TV.

  4. I've also reevaluated Venus with some actual data and linked to a study. It's now on-par with Earth.

  5. Both low-gravity and radiation strike me as somewhat poor / inconsistant metrics to evaluate entire worlds on.

    For one thing, it's fallacious to say that humans need full G to stay healthy. We have essentially no data at all on how human physiology performs in low gravity -- our only two data points are full G and microgravity. We know that microgravity has many negative effects, from calcium loss to suppression of the immune system, but it is erroneous to assume that you can simply make a straight-line interpolation from full G to zero G and extrapolate the results from there. Biological systems don't behave that way. Many of the ill effects of microgravity come from fluids not being able to orient themselves, and it is thought that even a small amount of gravity may be enough to counteract this. So you can't simply say that lunar 1/6th G, for example, will be 1/6th better than zero G. It may be, or it may be 95% better -- we just don't know. But it's a very good bet that the health effects of low gravity will be highly non-linear.

    In the case of radiation, while it's true that most of the surface of Mars and the Moon are bathed in it, it's also true that there are large volcanic caverns on both bodies which form almost perfect pre-made radiation shelters - providing radiation protection on par with Earth's atmosphere - and are likely to be the first colonization sites on both bodies. It's unlikely that asteroids come with such features, so radiation is a more legitimate concern there.

    So, to sum up: radiation should not be considered a problem on the Moon and Mars, and gravity is unlikely to be a problem there either. Radiation probably is a problem on Asteroids, but artificial gravity should take care of any gravity concerns.

    As for resources: those should be measured not in terms of their presence but in terms of their Net Present Value, which would account for both the cost of extraction and the value of time while returning them to earth. Deep gravity wells and long distances both count against NPV... but that's a subject for another rant.

  6. Nathan, the absence of evidence of the reproductive success of mammals in non-standard gravity means we have to assume there will be problems.. not the other way around. More simply, until we know it is safe to have kids on other planets we can't make serious choices about where to build our space colony.

    In regards to radiation, it clearly is a concern as it kills people when they get excessive amounts of it, and slowly kills them even when they get a little of it. Remember, we're not talking about brave astronauts here, we're talking about our children. Mars and the Moon do indeed have plenty of dirt that you could live under, and so do asteroids, and that's why I gave them a score of 1. Earth and Venus, on the other hand, both have *natural* radiation protection and that's why they got a score of 2.

    Both radiation and gravity on Mars, the Moon and asteroids are "a problem". That you think you've got solutions to those problems does not at all change the fact that they are problems.

    I believe the net present value could best be accounted for in the metric I called "accessibility". With that included, you could call it "strategic value". The "resources" metric is really a measurement of the potential for growth and self sufficiency.

    Thanks for the comments.

  7. Rotating habitat structures (like the looping train) are approximately as reasonable on bodies like the Moon and Mars as in asteroids. You just give them a slight slant to add the native gravity to the artificial.

    It's maybe a toss-up as to which is easier to construct; mining inside a loose-aggregate asteroid might be easy, or it might be impossible (the whole body could just fall apart for all we know). We have plenty of experience with terrestrial mining, which will help in a Lunar or Martian context; the material on a planetary surface will vary, but may not be quite so friendly to tunneling as some asteroids.

    However, I'd say the presence of lots of nice round holes on the surface of Luna or Mars could be plenty helpful: build a track around the crater walls (possibly in a cutting) and cover it in stone, regolith concrete, or loose regolith over an imported framework structure.


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