Robert Heinlein sold me the moon; Ray Bradbury sold me Mars. Because of this, I love the idea of space colonization. This is a trait I share with Elon Musk. But unlike Musk I am willing to look critically at the arguments for (and against) investing in such colonization.
One good argument for colonizing Mars is the survival argument. As many others have pointed out, humanity is a one-world species: we have all our eggs in this one basket. So, if all the humans on earth are killed, then that is it for humans. But, so the argument goes, if we colonize (for example) Mars, then the odds of humanity surviving increase significantly. While this argument has some appeal, it needs to be considered in a bit more depth.
For a space colony to provide that second basket for survival, it would need to meet (at least) two conditions. First, it would need to be isolated from earth extinction events. Second, it would need to be self-sufficient (or not reliant on earth).
In terms of the first condition, the reason for this is obvious: if the colony would be extinguished by the same extermination event that ends humanity on earth, then the colony would not allow humans to survive. There are, of course, extinction events that a space colony would be isolated from. One example is the killer asteroid: unless a second asteroid is on its way to the colony, a space colony would be unaffected by the earth getting hit by a big space rock. One concern about spending resources on a colony for this purpose is that one could argue that spending the same resources on survival on earth would save more people. The clear exception would be a strike that would kill everyone on earth, no matter what we did in terms of such things as survival bunkers and other preparation.
Another example is environmental collapse on earth. This would not directly impact the colony since it would be on another planet (or elsewhere). A concern with this example is that almost any plausible environmental disaster would still leave earth vastly more habitable than places like the moon or Mars. So, if the goal is to survive an environmental collapse, then investing on earth would be a more cost-effective approach. What would be a vastly superior approach would be to use these resources to prevent this collapse. But if the collapse was unavoidable and would render the earth worse than (for example) Mars, then a colony would make sense.
Unfortunately for the idea of using a space colony as insurance against extinction, there are many extinction events that would also destroy the colony because it could not be isolated from them. One obvious example is war: if a war on earth results in extinction there, this war would almost certainly result in the destruction of any space colonies as well. It is possible they might survive, but unlikely in such a war of extinction. Another extinction event that would easily spread to a colony would be disease. In many scenarios, the disease would not be recognized as an extinction level threat until it already reached the colony. Even if the threat of the disease were recognized in time, the COVID pandemic has shown that “Space Republicans” are a very real possibility and that they would undermine efforts to isolate the colony. Humanity could thus go extinct so that a few politicians could score some last political points and own the “Space Liberals.”
One can also make the argument that an isolated space colony could help humanity rebuild from a planet-wide disaster. This is not a bad argument, aside from the weakness that having disaster recovery in place on earth would almost certainly be more cost effective and more practical. In terms of surviving an extinction event, if it is something that the colony can be isolated from, then it could provide that second basket that would allow humanity to survive. This would require that the colony be self-sufficient.
In terms of self-sufficiency, this is an obvious requirement. Even if a colony survived the extinction event that destroyed humanity on earth, the death of the colony would only be postponed if it depended on earth. As such, a colony justified by the survival argument would need to be able to survive without support from earth. Naturally, if there are multiple colonies, then they could depend on each other as long as they were collectively self-sufficient.
Since the other planets and moons in the solar system are incredibly hostile environments, creating a self-sufficient colony with a large enough population to keep the human race going would be incredibly challenging. Colonizing Death Valley, Everest, or the ocean would be simple and easy by comparison. This is not to claim that a self-sufficient colony is impossible, but this would require immense investment and probably centuries. Colonization efforts here on earth have failed on numerous occasions, even with support. As such, counting on a space colony as a second basket for humanity’s eggs is unreasonable for the foreseeable future. As many others have argued, investing in making life on earth better would provide a far greater return. This, I admit, pains me to say—for I have long dreamed of Mars and worlds beyond.