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Some thoughts:
* What about not bothering with separating N from CO2, and just shipping the whole 96:3 mixture as-is to Mars from Venus? Maybe deal with the CO2 on the Mars end, binding it to the biosphere there? Could they use any CO2 or do they already have more than enough?
* The Landis aerostat theory seems viable for Venus, and gas processing is going to be a crucial tech for the aerostats. If we use unmanned aerostats, gas processing can even be their whole singular purpose. Assuming minor investment in gas processing tech, would it be reasonable to assume that gas processing the N from Venus and shipping to Mars would take less energy than mining from the easier Titan and shipping to Mars? If we have manned aerostats, they're going to want to have surplus nitrogen production anyway, to have storages of nitrogen for emergency purposes.
Wouldn't nitrogen from Venus require less energy to transport?
I don't believe that governments will be willing to spend the kind of money that is needed to colonize, not for 50-100 years still, maybe more.
If the first steps are to be taken in 10-50 years, they will be made by private enterprise, and led by people like Branson, Bezos and Rutan.
I do think that with people like that in the helms of big business, we will see colonization coming from commercial space tourism, rather than governmental space exploration. Governments are too slow.
The floating cities idea is brilliant. I never thought something like that could be done without something as sci-fi as antigravity generators, but the lifting gas phenomenon solves the problem elegantly and simply. In effect, living inside massive zeppelins filled not with helium but breathable air.
This concept, to me at least, seems to make colonizing Venus easier than Mars, even. However, there are a few questions about practical details.
Transport between orbit and the cities? Space shuttle type solution? We still don't have sufficient tensile strength materials to build a space elevator. However, I'm having a hard time visualizing a floating city that can take the kind of punishment that Cape Canaveral does. Wouldn't the backfire of a shuttle taking off pretty much destroy the city?
Food. In order to really be able to colonize anything, we're going to need a completely closed bio-cycle; shipping food from Earth for hundreds, thousands, millions is unfeasible for a permanent colony. How much agricultural space needs to be built per inhabitant? Would the soil have to be shipped from Earth? Are there plants that could be grown without soil? Aquatic plants perhaps? Sprouts et al? Would fish pools be more practical than shipping soil over?
Sulfur. Pretty much all of the atmosphere can easily be transformed into other useful gases and water, but what use is sulfur? What can we use it for? Are we gonna end up with huge sulfur mountains leftover from chemical reactions that we won't know what to do with?
Any thoughts?
I don't really see how transporting the nitrogen from Venus to Mars in gasbags is feasible. Gasbags are too fragile; a single micrometeorite (and there are MANY between Venus and Mars) would puncture the gasbag, and suddenly the whole shebang would empty like a runaway balloon. No gasbag would make it to Mars.
Another possibility is freezing the nitrogen in Venus orbit into solid chunks -- 100% nitrogen asteroids -- and then push them onto a trajectory for Mars. Put a homing beacon on them to track them for extra logistic control.
If necessary, they can be broken into smaller chunks with weaponry when they're on final approach towards Mars. Then just plunge to the planet, aerobrake -- ideally, chunks sized so that they never hit the surface, but are vaporized into the atmosphere by the friction of aerobraking.