We currently have the technology to build a ship which could reach Alpha Centauri in 88 years.
In the context of this series on outmigration, our primary purpose in doing so is to migrate some small portion of the human gene pool off-world, to enhance the possibility of our continued survival as a species. This seems to be one of DARPA's purposes as well.
In order for Alpha Centauri to actually warrant traveling to it, we would need to know beforehand that a world exists in that system which is significantly more hospitable than Mars or other locations in our solar system, but devoid of life.
We will likely have the ability to determine this with some confidence in the coming decade. The existence or non-existence of an earth-sized world with a significant amount of liquid water on its surface will probably be known within the next few years, assuming that the Exo-Earth Imager telescope is completed.
If no such world is found, then the search would shift to Barnard's Star. If Barnard's Star proved similarly devoid of habitable worlds, Orion will almost certainly not be built. At least, not the interstellar version of Orion. There would simply be no point.
Assuming that a suitable world is found, determining the existence of life will prove more challenging, but high concentrations of atmospheric oxygen or methane on a temperate world would be a pretty good first clue. If we should find conclusive evidence of life on such a world, that would immediately shift the focus away from colonization toward very cautious exploration. Because a war of the worlds, even unintentional and even on a microbial scale, is likely to end badly for everyone involved.
If our hypothetical "goldilocks world" should prove mostly habitable but uninhabited, the first priority of the new colonists will be terraforming it from "mostly habitable" to truly earth-like. By definition, any world worthy of traveling four lightyears to terraform should be pretty quick and simple to terraform. A need for much more than simple oxygenation of the atmosphere (by photosynthetic or chemical means) would tip the balance back in favor of Mars or Europa for permanent colonization. But we cannot begin to estimate what steps would be necessary for terraformation until we have a pretty good understanding of the world as it currently exists. And until we know those steps, and what resources we can hope to find on that world to help accomplish those steps, we have no way of determining how much of the cargo space of the Orion vessel would be committed to carrying either materiel for terraformation, or equipment for surviving on a non-terraformed world.
This brings us to something of an impasse. We have the technology and resources to build the working parts of Orion. But we do not yet have the knowledge to determine what to carry in it, and for a one-way sojourn to the nearest stars, we cannot leave that to guesswork. Until the Exo-Earth Imager is operational, and we have real data about the planetary systems of Alpha Centauri and Barnard's Star, we cannot proceed much further on this tack.
The good news is, if all goes well we may have the answers we need much sooner than we would realistically begin building Orion.
In the mean time, we have left an enormous question unanswered which is critical for human colonization not only of the stars, but of the other worlds within our own solar system. Our quest for a habitable-but-uninhabited world raises the question of how in fact life arises on a world, and how common this process actually is. So I want to take this discussion on a different tack for a bit, and look at how life arose on earth, and how likely it is that life has arisen elsewhere. For the next several posts I'm going to look at abiogenesis, panspermia, and exobiology generally.
Helm's a lee.
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