[Turing_Machine]

<I>The earth is where we have to make our stand</i>

Why? We have a whole solar system right on on our front porch.

<I>but the total possible water supply we can access (and this includes the glaciers)</I>

Water doesn't just get used once and then disappear forever. Now, one could certainly make an argument against (say) sucking water out of the Oglalla Aquifer faster than it gets replenished, but that's a localized problem. We aren't going to "run out of water". Really.

[GeneTraylor]

>>> Why? We have a whole solar system right on on our front porch <<<

This is an honest question, not a rebuttal. How would you get those resources here, or us over there?

>>> Water doesn't just get used once and then disappear forever. Now, one could certainly make an argument against (say) sucking water out of the Oglalla Aquifer faster than it gets replenished, but that's a localized problem. We aren't going to "run out of water". Really. <<<

Well it would have been labored to add that although it would take a large amount of energy to purify water, or perhaps desalinate our oceans, the loop can be closed. The problem really is that it takes resources to keep that loop closed.

I think that all of our issues today boil down to how we convert energy to extract work, if we do manage to make something like fusion work then all of these concerns could be undermined with technology, but how do we get from here to there?

[randallsquared]

How would you get those resources here, or us over there?

There are a number of high-startup-cost, low-running-cost schemes for putting stuff in LEO, and as someone once said, LEO is halfway to anywhere in the solar system, energy-wise. Some possibilities are laser launching, space elevators, launch loops, and electromagnetic guns.

Some of these have much higher startup costs (to be effective for human launching, a gun would have to be very, very long and possibly cost the most of any of these, but shorter ones could launch material at very high acceleration with much less up-front cost), and some of them have awful failure modes (the space elevator could spread destruction in a narrow path wrapped all the way around the earth if it failed in the wrong way, via separation from the counterweight), but they're feasible.

The cheapest ways of getting to orbit, therefore, approach the cost in electricity of the task, which is less than 10 KwH per kilogram, or about $2USD per kilo. A recent flight from coast to coast cost more than that for me.

[GeneTraylor]

I've actually thought about this a bit, and I think that a really interesting design for a launch gun would be to encase the payload in a cover composed of diamagnetic material and then pump liquid nitrogen through channels made inside the material. The material would become perfectly diamagnetic and it will repel all fields, so you can easily levitate it and use it's tendency to go from a stronger to a weaker field to accelerate it.

I don't know for sure, but I think that the main advantage of doing something like this versus just putting a magnet over there, would be the fact that an accelerating magnet in such an assembly would generate huge amounts of back-emf and you would have to a) sink that and b) compensate for that.

However I don't know much about what other people's designs are, but I would like to change that. Do you know any resource where I can learn more from the scratch on how to build something like this?

Also where can I read more in detail about launch loops, laser launching, and space elevators?

[randallsquared]

I haven't paid much attention to these areas in a while, but I would start here: http://en.wikipedia.org/wiki/Non-rocket_spacelaunch