About four and one half years ago one of my first blog entries was “Amazon Women on the Moon.” At that time I inferred a new military service, a “Space Navy” would best be composed of young women in the interest of effecting Survival Colonies on the Moon. I have not changed my mind on this. With an embryo bank of fertilized ovum available a wild guess at the number of females required to rebuild the human race would be a few hundred. With a sperm bank the number of females required goes up. With males and females in pairs this would likely increase to a couple thousand if interbreeding problems are to be avoided. The number of females necessary and how to sustain them off-world is an interesting problem. Our newest graduating class of astronauts, nicknamed “the turtles”, has six females and seven males.
Image by Mark Felix
This is the extremely important question: if Earth is suddenly uninhabitable could the human race survive? If an asteroid or comet several times the size of the previous dinosaur killer were to hit Earth in just the right place tomorrow only microscopic life would survive. That some underground facility or nuclear submarine with a band of courageous survivors might repopulate the Earth is a great science fiction story but not plausible if the surface and oceans were nearly sterilized. The best place to survive such an extinction event would be off-world.
A planet killer was, by the way, the plot of the movie “Armageddon”; from https://grantland.com/hollywood-prospectus/bragging-rights-armageddon/ “Over the years, the big-budget story of an asteroid on a collision course with Earth and the lovable oil-rig roughnecks tapped by NASA to do something about it has become its own shorthand. Armageddon means explosions, it means ’90s Hollywood, it means emotional manipulation — “That man’s not a salesman. That’s your daddy” — and it means that unholy union of Jerry Bruckheimer and Michael Bay. Armageddon makes people roll their eyes. “Movie isn’t actually the best word to describe ‘Armageddon,’” wrote Janet Maslin in the New York Times in 1998. “More accurately it’s a product, a feat of salesmanship.”
The movie “Deep Impact” was much better than, but unlike “Armageddon”, had the caveat humankind might survive the worst case. Besides a planet killing impact another worst case scenario is the engineered pathogen. I have argued on a few forums, before I was banned from them for being a critic of NewSpace, that a 100 percent lethal pathogen is one valid reason to establish Survival Colonies. There was always some rabid neoliberal Ayn-Rand-in-Space type who would reply some humans would always have some “natural resistance” to the pathogen so life would go on. Not true of course.
After a planet killer and engineered pathogen comes the possibility of a super-volcano epic coating the surface of the planet in ash, contaminating the atmosphere and poisoning the oceans, and freezing the surface of the planet by blocking sunlight. If the epic was intense enough it would have the same effect as the planet killer. Of the three I would say the engineered pathogen is the biggest danger. I listed it second only because an impact is far more likely to depopulate the planet and can be most easily guarded against. While a large impact might leave a small remnant alive it would make those few far more vulnerable to a secondary plague or other event ending us. As I have stated several times, any extra-terrestrials observing our extinction by way of an impact or volcanic epic would remark we were just too stupid to survive. Natural selection.
Unnatural selection by way of purposefully murdering each other with an engineered pathogen is, in my view, the biggest danger. A lesser impact or single super-volcano that destabilizes civilization and leads to nuclear war could end with a last ditch biological agent being released. Even a Carrington Solar Event could be the catalyst of extinction. Climate Change will doubtless have unintended consequences we cannot foresee now that could likewise set humankind up for a catastrophe leaving us vulnerable to a secondary event extinguishing our species. Our optimism bias conditions us to ignore such possibilities. We believe we will somehow be lucky enough to survive whatever comes our way. This could end us.
The views I have expressed on this blog over time have not changed much and the original concept of women in space as insurance for the human race has not changed at all. We know eggs are viable after being frozen for a quarter century and woman in their 70’s have given birth so it is not unreasonable to expect women tasked with restoring the human race might produce around ten more females each. The question is where these children would be born. If there are giant lava tubes under the Moon this could be the answer. 
Graphic from Purdue University
A lava tube with an interior over 3000 feet in diameter would allow for a circular train generating artificial gravity to be constructed, what I call a “sleeper train.” Such trains, perhaps stacked on top of each other to utilize available space, might allow for a small population to survive and even thrive on the Moon. On icy bodies such as Ceres and the ocean moons of the gas and ice giants it might be even easier to effect these constructs. Such distant outposts are unlikely unless Survival Colonies under the Moon are in need of water or volatiles. It is possible that resources on the Moon could be limited in which case expanding outward would eventually be necessary for sustaining the lunar population.
If the Earth was to suddenly die, this kind of existence in small subsurface communities might be the only viable path since the massive resources we now enjoy to support building true space habitats would not be available. Slowly rebuilding the population by expanding these kinds of communities into the solar system until a workforce and resources were available for building habitats might take centuries. In the event of a catastrophe on Earth it would be far better to already have or be in the process of setting up lunar factories enabling miles-in-diameter-artificial-spinning-hollow-moons. Gerard K. O’Neill envisioned the Earth eventually becoming a place people went visit like a national park with almost the entire human race living in space.
Ice on the Moon 