The article is well done and it was a pleasure to read. It is certainly the most detailed consideration of the realities of long distance space travel, as we currently understand them, that I have seen.

My statements were primarily directed at making two basic points. The first point is that even if they are not impossible ‘in theory’, whatever that means, it is apparent that the technological requirements of even the most plausible of the cases considered in this article render long distance space travel, effectively, impossible. This will be true for quite some time and, in fact, I would expect that we must have a civilization of current or better technological prowess in place for at least a few hundred years more before such a thing were even seriously attempted. This leads into the next point; that we don’t really have a great track record as a species for making such long term plans. I dispute the claim that our high-technology society has been demonstrated to be sustainable for such a long period of time. We have many challenges; technological, socio-political, and economic that no good answers have been found for. The direst consequences of this lack of long-term planning have been held at bay with relatively cheap and abundant resources, not least importantly, energy. The assertion that global ecological concerns are not pertinent to this discussion is fallacious. It is precisely those skills of long term resource management, and due consideration of the ecological impacts of technological systems for life support and resource manufacture, that would be most relevant to a succesful expedition. The belief that the problem is somehow easier because you are dealing with a closed system which, technically speaking, you aren’t, is mistaken. If anything, the components of such an ‘ecosystem’ would be more strongly coupled together, not less.

Furthermore, the example of imperial Egypt as a model for stability seems rather beside the point. Firstly, I am sure the ancient Egyptians exploited external human populations from time to time. Most empires did, that might even be part of the definition of empire. I am looking for examples of human society that don’t do this as there will be no external human resources, or hardly any others, for a deep-space expedition to exploit. The socio-political aspect of the expedition is thus highly coupled to the technological nature of the endeavour and not to be considered in some isolated fashion.

Finally, while it is likely true that humans have existed on this planet for hundreds of thousands of years only the last hundred have been in a society which comes close to having the capability to generate, utilize, organize, and manage the resources necessary to succeed at this endeavour. That is the society that would have to be sustained on a 700 year trip. Not some pre-historic hunter-gatherer society.

The Ark is certainly a Himalaya to go, an incredibly ambitious project. While obviously I have for this project the tenderness of a mother, my purpose is not to say arrogantly: “We will do it! Of course it is possible to do it!, Etc”. What’s would be the point? It will happen in the future, and if it will happen it will be without me.

My point is purely scientific, I hope (though emotionally I put a personal price on it), the question being: if we’d go, which would be the required resources, both human and technical?

From a technical point of view I am the first scared by the big numbers. That point by itself requires already a philosophical reflection.

Initially, the Ark is the compromise between two requirements. We need to collect both large mass or high energy. Where is the optimum? Which is the energy-mass bottleneck to go live out of Earth?

* ENERGY MANAGEMENT: we must master thoroughly and massively the thermonuclear fusion of all types of fusible isotopes in the solar system. And not just to boil hot water in boilers. Mastery of thermonuclear fusion should be up to the point of ordered ejection of 4 MeV ions through magnetic nozzles. Deuterium mostly, then Helium-3 maybe. This is the only possible source for the wild power of the Arch. Humanity is currently spending power in amounts of about 16 terawatt (10^12W) in 2010. For the Ark, it must reach 4 ExaWatts (10^18W). Just to send a human civilization of 10^4-10^5 individuals at 10 light-years from earth in less than 1000 years travel. Faster solutions require more energy, if we want to send at least as many people, and also considering that worlds at less than 10 ly does not certainly allow “bare head” colonization.

* MASS GATHERING: 20 Gt, of which 8 Gt of water. About 12 Gt of walls and building structure in space. Here, faster solutions are less massive by a factor of at least 1000. But again, they require that the target planet is inhabitable, that I think unlikely in less than 100 light-years radius, and very unlikely in less than 10 light-years.

So finally I came to the conclusion that there is not much choice, and that the solution is either an arch or again the same arch. Maybe smaller or larger, but likely measured in gigatons.

From a human point of view:

* We need a strong human civilization to undertake this.
* A new independent mini-civilization shall be founded, which is viable in long-term and independent from Earth. Then we need only to describe what such a civilization need to be at its core (small size, strong historical records, policy defined by its future targets, etc.).

I just want to detail this scope, at least roughly, of what is needed to leave the Earth.

Of course whether it will be done or not is another story. I clearly hope personally that it will be done. But I make no predictions.