Shell worlds

This idea is currently making the rounds:
http://www.space.com/23063-terraforming-planets-shell-worlds.html

I know I've talked about this before, but proposals like this really impress on me what a wrench it is for most people to consider living on something other than a planet.

I've been up on this soapbox before. The last time was when someone speculated that if only we had a magic wand to compress the moon to a higher density, we could bring the surface gravity up to 1 G and then lay down an atmosphere.

Now here we might have a slightly-more-realistic proposal, but still one that assumes an engineering capability far beyond what's needed to make O'Neill-style orbital habitats.

For starters, let's look at their bulleted list of advantages:

* Industry and facilities that could benefit from access to a vacuum could use a port that connects to the outside of the shell.
* Ultraviolet radiation from a star would not be a problem since the world would be fully encased in the shell.
* The heating, cooling and the length of a day on the world would not be dependent upon the orbit of the planet around a star.
* The shell would provide radiation protection.
* The world would provide an almost limitless playground for design. For example, cities could hang down from the interior of the shell.

Please note that all of these advantages would be equally-true for O'Neill habitats, which can be realized with a far more modest technical capability in a much nearer-term future. (OK, maybe not that last one, but I'm still not sure what the benefits would be of a "hanging city". I've always assumed orbital habitats will have lots of structures up near the spin axis; I suppose those would be a place to go to and look down on the cloud-tops.)

Of course the great advantage of the High Frontier proposal over this (or any of the other terraforming proposals for that matter) is that one needn't start out working at the scale of an entire world. You could begin much more modestly, only trying to arrange living areas for a few thousand people. Maybe later, larger designs accommodate a few million. But on the other hand, you can just keep building more, and don't need to stop at merely-planetary amounts of total living area.

I almost have more respect for those thinkers who talk about domed communities on the moon or Mars, and envision the domes proliferating to the point that the entire surface is covered with an interconnected set of domes appearing like a foam. They then say that at that point you have essentially "roofed over" the planet. At least this scenario assumes starting out with small communities rather than planetary-scale engineering endeavors.

With an opaque roof, this proposal assumes artificial suns. This is another thing which might be easier said than done. I've heard some people blithely talk about "fusion generator mini-suns". But such assumptions take us beyond the limits of near-term-extrapolated technology (something O'Neill never allowed himself to do). To me, assuming mirrors, windows, and use of natural sunlight is a much more conservative assumption.

I also note that with this idea we're still left with the gravity of the celestial body being used. To me, one of the chief advantages of orbital habitats is that we can arrange a full 1-G if it turns out nothing less than this is healthy. The promoter of this concept mentions that the lower gravity enables human flight, but as we all know such is also possible in orbiting habitats nearer to the spin axis. Having 1-G where you live and work but still having access to areas of reduced gravity is to enjoy the best of both worlds.

Again, I think the only reason proposals like this ever even get brought up is because, for most people, living on something other than a planet is such a foreign concept that they refuse to even give the idea fair consideration. I never cease to be amazed at how strong this tendency is, even among people willing to consider vast extraterrestrial technical capabilities.

How do the rest of you feel about this topic?

Regards,
Mike Combs