[jcranmer]

You're ignoring what I believe to be the most important point: how do we know that is actually possible to build an interstellar colonial vessel?

Or, to bring a concrete element to the question: all known rocket fuels require some nonrenewable Earth resource. How much fuel does it take to launch a vessel built with currently-known technology that would be capable of landing a living human being on a hypothetical planet around Alpha Centauri? And what does that fuel number look like, when expressed in terms of "years of current resource extraction" (or, somewhat more audaciously, percentage of known proven reserves)?

To my knowledge, interstellar colonization is flat-out impossible with current technology. (I'd argue that even intrastellar colonization is impossible, rather than impractical). Arguing that we should discount this non-knowledge of how to do it when explaining why it hasn't happened to our observation is irresponsible to me.

[m4rtink]

Well, the ideal chemical rocket fuel is hydrogen + oxygen - eq. the two things that make up water. Thats perfectly renewable once you pump some energy into it to split water into the two.

Methane + oxygen popular in the new generation of rockets is not much different, you basically add some carbon to the process & Methane can be made on Mars from local resources.

Best fuel for nuclear thermal rockets is again hydrogen, abundant everywhere.

Current Ion Engines run on noble gases, but supply does not seem to be a problem so far.

Sure, the uranium needed for Orion and Nuclear Salt Water rocket indeed is not really renewable but given the performance &opening the way to grab more of it from space make it a non-issue as well.