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Interstellar Expansion: Colonizing Ice Dwarfs

Are habitable planets the best places to look for life? The question seems odd, because we’re assuming life has to have clement conditions to emerge and survive. But step beyond the question of life’s formation and the issue can be framed differently. Where beyond its birthplace might life migrate? In SETI terms, where might we look for the signature of a civilization advanced enough to move beyond its home world and expand between the stars?

A lot of ideas seem to be converging here. In Huntsville, Ken Roy (whose description at the recent interstellar conference was ‘an engineer living and working amidst the relics of the Manhattan Project in Oak Ridge, Tennessee’) described potential habitats stretching far out into the Solar System and beyond. Roy has been working for some time with Robert Kennedy and David Fields on colonization scenarios.

My own talk covered the kind of places where we might extract resources, ranging from icy dwarfs like Pluto to cometary objects and ‘rogue’ planets without any star. And science fiction author Karl Schroeder, in a recent blog post called A Tale of Two Worlds, also brought the topic up. Let me quote Schroeder, because I want to return to his post in a day or so:

…it’s important to bear in mind that habitability and colonizability are not the same thing. Nobody seems to be doing this; I can’t find any term but habitability used to describe the exoplanets we’re finding. Whether a planet is habitable according to the current definition of the term has nothing to do with whether humans could settle there. So, the term applies to places that are vitally important for study; but it doesn’t necessarily apply to places we might want to go.

Both Schroeder and Roy are assuming not near-term projects but the kind of settlement and terraforming that draw on huge resources of energy. The premise, in other words, is that we’re talking about a culture that ranges freely through its own system, having mastered fusion or other technologies and being capable of large-scale building projects in space and on planetary or other surfaces. Grant that premise and then think about what kind of structures it might make sense to build when exploiting local resources and looking out toward the stars.

Pluto and the Ice Dwarfs

Pluto is a case in point. Here we have a surface that appears to be a shell of nitrogen ice covering water ice. When New Horizons gets to the Pluto/Charon binary in 2015, one thing to look for is an equatorial bulge that could have been left over from the early days of Pluto’s formation. No bulge makes the case for stretching of the ice shell over Pluto’s lifetime, strengthening the possibility some are noting that the ice dwarf could contain an ocean beneath about 165 kilometers of crust, an ocean that may be just as deep as the crust is thick (see The Case for Pluto’s Ocean for more).

As Roy told the crowd in Huntsville, icy worlds like Pluto are rich in volatiles, and of the tens, if not hundreds of thousands of Kuiper Belt objects out there between 30 and 50 AU, several hundred may be Pluto-size. Such worlds are doubtless common not just here in our own system but as rogue planets in interstellar space and perhaps circling brown dwarfs, those dim objects that blur the distinction between gas giants like Jupiter and true stars like Proxima Centauri.


Image: This artist’s conception of the ‘scattered disk’ object Sedna reminds us that even beyond the Kuiper Belt and as we move into the Oort Cloud, vast numbers of icy objects are thought to exist. Can we exploit these as we move outward toward another star?

Build a settlement on an ice dwarf in the outer system and you are not only creating space for living and doing science, but also building the technologies that will one day be used in interstellar colonization missions. But Roy noted that the science fictional image of a domed city in a harsh landscape just won’t work here. Induce Earth-class atmospheric pressure inside such a dome and even a small one (1000 feet in radius) would require a four-inch thick layer of steel to keep the dome intact. Moreover, ice dwarfs have but feeble gravity, creating medical issues from muscle atrophy to bone problems, loss of body mass, sleep disturbance and more. A better choice, then, is to move inward, creating the colony deep within the ice dwarf itself.

At 160 meters, the ceiling of a colony hollowed out within Pluto would be fully supported by the air pressure inside. Artificial light would be essential, of course, and we still have a gravity problem, for Pluto’s gravity is only 6.7 percent that of the Earth — a 200 pound person on Earth weighs but 14 pounds on Pluto. Roy suggests a rotating torus in this setting could provide living and working spaces at 1 Earth gravity. At 1 revolution per minute, a 1790-meter torus supported by maglev rails could accommodate, by Roy’s estimation, 10,000 people living in conditions that would more or less resemble the worldships so often imagined by science fiction writers.

We’re assuming technologies that can create large rotating structures in low-gravity environments, with the ability to move spacecraft at velocities of 0.001 c to build and supply the colony. We’re also assuming proven fusion power plants and considerable expertise in mining and construction. We would put these tools to work to extract local silicates and metals from the surface and, perhaps, rock from buried impactors. We would be working in an environment rich in H2O, but also in methanol, hydrogen cyanide, formaldehyde, ethanol, ethane and long-chain hydrocarbons, all within a salty ice mantle.

Interstellar Migration

Here’s long-haul migration to the stars presented as a series of steps at 0.001 c. Moving roughly 400 AU at a time between various objects in the outer system and, eventually, interstellar space, we spend 50 years at each to establish a colony and then build and crew another ship. The 4.2 light years to Proxima Centauri in this scenario demands 664 such jumps and reaches the star in 38,000 years, leaving a chain of colony worlds behind that are self-sustaining.

The technologies needed for this kind of expansion are well beyond us, but it is not inconceivable that more advanced cultures as they move up the Kardashev scale may have accomplished such things. Places that are habitable, as Karl Schroeder says, are not the same thing as places that are colonizable, and it’s also true that we have to be wary of imputing human motivation to hypothetical extraterrestrial civilizations. Their detectable artifacts, in other words, might extend between the stars far into the interstellar deep and so, in some remote futurity, might ours.


Comments on this entry are closed.

  • GaryChurch February 21, 2013, 22:15

    “SpaceX commercial trip to Mars”

    It is a scam; no way to get to Mars without nuclear propulsion. Radiation shielding masses hundreds of tons and chemical propulsion is not going to be enough.
    Mars is a rock anyway- it seems like it is “just close enough” but all things considered it is not. A nuclear mission would have to be launched from a Moonbase; and there is water on the Moon to fill up a radiation shield.

    The hobby rocket is not going to be around for very long; it has so many cheap and nasty features I doubt it will ever be qualified to carry humans.

    There is no substitute for a Heavy Lift Vehicle with hydrogen upper stages. That is how we went to the Moon last time and the laws of physics have not changed.

    The money we have spent on the ISS and commercial crew has been a complete waste.

  • Ronald February 22, 2013, 9:53


    thanks, your argumentation is completely reasonable and I can only agree with most of what you say.
    Funny enough though, the exact same arguments brought me to an opposite conclusion! I.e. it made my ‘ case for Mars’ even stronger.
    Let me explain a bit: I fully agree that terraforming Mars is going to be a very tedious, long-term heck of a job, if feasible at all. And yes, particularly the necessary atmospheric N2 is going to be a real bitch (although the Martian soil does seem to contain some nitrates).
    In fact, terraforming guru Fogg does not even consider Mars as a true terraformable planet (or at leat not what he calls ‘easily-terraformable’, let alone biocompatible, let’s not even mention habitable).
    However, it is this very fact of relative ease of incremental habitation (by means of domes etc.) that, I think makes Mars such a strong case for colonization in comparison with space colonies. The initial costs for transport and settling may be high, but once you are there and settled you can largely ‘live of the land’ (Zubrin term), most required resources are available and you can just keep pumping out more habitats etc.
    I can imagine a future in which more and more people will be settling on Mars, gradually expanding the habitats as needed, and at the same time working (or at least attempting) toward that bigger goal of terraforming. A very cautious two-pronged approach. And I think this is more or less how many SF writers and Mars visionaries visualize it.

    Having said that, I also realize though that this would then have to be compared with the alternative of asteroid resource exploitation for space colonies.

  • Dmitri February 22, 2013, 10:37

    GaryChurch – Listne to Elon Musk 20 min. interview to BBC in March 2012. He quite openly disusses the matter and the host is a prominent in the field. So far Elon has delivered. His main point is that energy wise trip to Mars is the same as to Moon.


  • Dmitri February 22, 2013, 10:44

    Is NASA working on nuclear rocket propulsion?

    Russians have and they set the course on this in 2009 w/ the goal to reach it around 2024.

    Thing that SpaceX achieved is space vechicles on which Russians (http://ru.wikipedia.org/wiki/%D0%9F%D0%B5%D1%80%D1%81%D0%BF%D0%B5%D0%BA%D1%82%D0%B8%D0%B2%D0%BD%D0%B0%D1%8F_%D0%BF%D0%B8%D0%BB%D0%BE%D1%82%D0%B8%D1%80%D1%83%D0%B5%D0%BC%D0%B0%D1%8F_%D1%82%D1%80%D0%B0%D0%BD%D1%81%D0%BF%D0%BE%D1%80%D1%82%D0%BD%D0%B0%D1%8F_%D1%81%D0%B8%D1%81%D1%82%D0%B5%D0%BC%D0%B0)

    and Americans (http://en.wikipedia.org/wiki/Orion_(spacecraft)) are just working on w/ deadline around 2018-2024. This gives to SpaceX 8-10 years headstart.

  • GaryChurch February 24, 2013, 23:54

    “GaryChurch – Listen to Elon Musk 20 min. interview to BBC in March 2012.”

    Thank you for the link but I want nothing to do with him. I have had some bad experiences with his “followers” and been banned from two sites that promote his company. Their vicious online behavior taught me a great deal about private space claims since I would research many private space talking points- such as fuel depots- and found them too good to be true. Any criticism of the private space agenda resulted in insults and a dogpile of commenters calling me a liar.

    Unfortunately this has left me quite unhappy with people who promote SpaceX and I usually lash out at them.

    Fortunately there are some forgiving moderators that have calmed me down and are teaching me to be civil again.

  • Wolfram Lunscher February 25, 2013, 12:48

    I am of the same opinion as this article. The frequent emphasis on habitable/terraformable worlds is short-sighted. Cold worlds without an atmosphere hold many advantages for a technologically advanced civilization.

    Heat rejection is a big problem with any civilization that consumes a lot of energy. Furthermore, the efficiency of heat engines, at the core of any deep-space power plant, increases as the temperature of the heat sink decreases. These ice dwarves are massive, very cold heat sinks.

    Living deep beneath the surface is the place to be, shielding organisms and equipment from cosmic rays. Being far from the sun, ice dwarves will be less susceptible to solar weather. The airless surface and low gravity would mean easy access to space for interplanetary travel and commerce.

    The concerns about life there, I believe, are misplaced. Humanity will evolve, or be evolved through genetic engineering, to adapt to living in the low gravity; and probably have little desire to return to the deep gravity well here on Earth. Even so, alternate bodies, genetically engineered or artificial, would be available for that purpose. Building the habitats on the dwarves, and elsewhere in deep space, will become the purview of robots, not human labor. In fact, robotic seed factories could be sent off to set to work on any or all potentially interesting destinations so that when we arrive all the necessary infrastructure would already be in place. Not that different from how Earth was colonized where nature took care of the infrastructure.

  • Mike Prather February 27, 2013, 21:26

    The last few articles here about habitability got me thinking about binary systems with stars of similar mass. Someone please correct me if I’m wrong, but wouldn’t the barycenter in such a system be pretty much a sweet spot for planetary accretion? A stationary planet with two stars orbiting it would be Copernicus’ dream… And with two stars warming such a planet, I would assume the average distance from the planet to each star could be much greater than the normal habitable distance for a single star. Detection of such a planet would certainly be a trick, except maybe each star transiting behind it might leave a noticeably similar signature. I’m sure it would be rare but what a find that would be.