The Outer Edge

For the planetary chauvinists in the space movement, the prospects are not bright. In our Solar System only two planets (Mars and Venus) are suitable for human colonization, though colonizing Venus would require terraforming that planet (which would take at least some 200 years). On the other hand suitable planets in other planetary systems are minimally several light years away from us, and any journey to those exo-planets will take a period comparable to terraforming Venus. And in case we arrive at a yet uninhabitable planet, we would need to terraform it anyway.

Well, this analysis assumes that our understanding of our Solar System is complete. Unfortunately, our understanding Solar System is far from complete. As far as we know, there are four terrestrial and four giant planets in our Solar System. (Regardless whether you consider Pluto as a planet or not – we do not chose a side in that controversy -, Pluto is unsuitable for colonization.) The giant planets are unsuitable for colonization, and can’t be terraformed and Mercury is too close to the Sun.

Nevertheless there are still people looking for new planets within our Solar System. The question is whether such planets might exist? Depends on what you are looking for. If you consider Pluto and similar objects as planets, then you could expect the discovery of many planets during the next few decades. However, if you are looking for something more substantial, it will be much harder.

The point is that massive objects are exerting gravity upon each other: the orbits of objects in our Solar System are the result of complex interaction between the several planets and the Sun. Scientists consider the Solar System as a chaotic system, i.e. a small change in the configuration of the Solar System, will have enormous consequences for the orbits of all Solar System objects.

At this moment planetary scientists are able to explain the orbits of the planets at the hand of all known planets. (The so-called anomaly of the orbit of Uranus has been solved, after the mass of Neptune was determined more accurately.) Consequently the gravitational effect of any trans-Neptunian planet on the known planets should be negligible. Either because such planet has a small mass (as in case of Pluto) or because such planet is located at large distance from the known planets.

But if you are a planetary chauvinist looking for planet suitable for colonization, you want this distance to be as small as possible. Hence what is the minimal distance at which a planet might be located?

Many astronomers have proposed trans-Neptunian planets in the last two decades. Computer simulations performed by Rodney Gomes showed that a Mars-sized planet could exist at 53 AU, and a Neptune-sized planet at 1500 AU. An Earth sized planet could exist in between. Calculations done by Lorenzo Iorio show that a planet twice the mass of Earth, should have a minimal distance from the Sun of 496 to 570 AU. An Earth-sized planet might consequently exist somewhere between 53 and 500 AU from the Sun.

(Recent empirical evidence seems to rule out Saturn-sized planets up to 10,000 AU from the Sun, and Jupiter-sized ones up to 26,000 AU.)

Even in the optimistic case that we find a Earth-sized planet within 500 AU from the Sun, the planetary chauvinist has to face an important issue. At such a distance away from the Sun, the planet is most likely to be covered with ice of frozen water, nitrogen and similar substances. In other to terraform such planet, one would need to heat it up.

At first glans Solar power seems to be unsuitable for this task. However, by using a vast amount of space mirrors it might be possible to concentrate enough Solar power to heat up a planet. The important question is, of course, how much we want to spend on Solar mirrors to make a far-away planet inhabitable for humans.

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12 thoughts on “The Outer Edge”

  1. I guess I’m a planetary chauvinist. There’s something humanly reassuring about being gravitationally tethered to big natural space rock in orbit around a life-giving star.

    Aside from Mars, there are other bodies in the solar system much more suitable to human colonization than Venus such as the Galilean Moons (Jupiter), Titan (Saturn), and Triton (Neptune).

    1. The Galilean moons are located deep in Jupiter’s magnetosphere, and hence are subject to immense radiation. It’s therefore highly unlikely that those moon will be colonized.

      The gravity on both Titan and Triton is quite low, and poses a serious health risk to colonists.Some of these health issues (bone and muscle degradation) can be overcome if colonists will be spend several hours a day in the gym.

      Rotating space habitats in free space are a better alternative to colonies on planetary or lunar surfaces. The centrifugal force generated by the rotation of the habitat, will substitute gravity and hence prevent the health issues related to low gravity. Further the Solar System contains enough resources to build such habitats.

      1. Building underground colonies would provide additional protection against dangerous radiation. Since the interplanetary medium is also subject to intense radiation, all space habitats would require heavy shielding unless they were protected through other means (planetary atmospheres, magnetic fields, etc.).

        Planetary sites would also have the advantage of close proximity to natural resources (water, minerals, etc.). Their lower gravities (compared to Earth) are a potential problem, but not an insurmountable one in my opinion.

        On the other hand, terraforming Venus seems highly implausible. Sequestering the vast quantities of atmospheric carbon dioxide necessary to decrease surface temperatures would be a monumental undertaking considering that planet’s corrosive effects on metals as construction materials. Furthermore, there is little to no water on Venus.

        I’m not opposed to torus-based space habitats at all (they’re in my novel). It would be prudent for space colonization to utilize diverse technologies. Putting all our eggs into one basket could be problematic. And, I’ll reiterate my point about human psychology. People may have trouble adapting to living in space. Would habitation on a planetary body lessen the effects? I don’t know.

        Great topic for discussion, Mord!

        1. >>all space habitats would require heavy shielding unless they were protected through other means (planetary atmospheres, magnetic fields, etc.).

          Of course, radiation is problem the most important issue to deal with in space. See “The Dangers of Space Colonization” for a more detailed discussion of this issue on this site.

          >>Planetary sites would also have the advantage of close proximity to natural resources (water, minerals, etc.).

          Asteroids and comets will provide natural resources to space habitats, which could be located in proximity of those bodies without the need of having a colony upon them.

          >>Their lower gravities (compared to Earth) are a potential problem, but not an insurmountable one in my opinion.

          Depends on the actual strength of gravity and the amount of time you want to spend on gravity-replacing exercises.

          >>On the other hand, terraforming Venus seems highly implausible.

          I agree with you on this, which would not be a surprise, of course. However, terraforming Venus would technically possible, but it would require the importation of a tremendous amount of hydrogen (used in Bosch reaction to transform CO2 into carbon and water). Nevertheless the costs associated with such enterprise will be prohibitive.

          >>I’m not opposed to torus-based space habitats at all (they’re in my novel).

          Good to hear that you support rotating space habitats. Besides torodial habitats, there are cylindrical and spherical habitat designs (just to bring the variety to attention). [On a side note, some of my stories are set on (or more accurately under) planetary or lunar surfaces.]

          >>And, I’ll reiterate my point about human psychology. People may have trouble adapting to living in space. Would habitation on a planetary body lessen the effects? I don’t know.

          A interesting point, definitely worth discussion. One of the ideas behind space habitats is that they large enough for the creation of an imitation terrestrial landscape, which would lessen the adaptation problem. On the other hand living in a underground habitat with lower gravity might be more different from Earth than live in a space habitat.

    1. Nemesis belongs to the same category as Lemuria, hypotheses proposed by serious scientists but disproved by further scientific inquiry. However, in both cases there is still a lot of people, who believe in them.

        1. >>It’s another example of our species inclination to Doomsday thinking.

          In the case of the nemesis hypothesis, this is quite clear. The Lemuria case is less clearly linked to doomsday thinking, except that the continent was “lost”. However, this is quite funny, as there are real lost continents such as Zealandia, but almost no one talks about them.

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