As the story opens the moon explodes into a number of big chunks for reasons that are unclear, though most likely it was hit by some space object. The shock and awe among Earth's human population is soon exacerbated when scientists announce that the moon chunks will inevitably collide with each other, break up into smaller and smaller pieces, and - in two years time - begin to rain down on the Earth. This 'hard rain' will last five thousand years and destroy the entire surface of the planet.
In an effort to preserve the human race, world leaders and scientists plan to construct a space habitat for a couple of thousand people - with the International Space Station (ISS) as the hub. The book is almost 900 pages long and approximately the first three-quarters describes, in great detail, the construction of this habitat. This part of the book is very technical and (for me) hard to picture.
My overall impression is that people would live in roundish space pods, each about the size of a trailer home, that can attach to and detach from the ISS and each other. This would allow the pods to move about to avoid being hit by space debris. It would also enable them to connect to each other and rotate, to generate a gravitational field.
The space habitat would need to generate food, oxygen, and energy. It would also need to house a huge amount of tools, medicine, scientific equipment, technology, and so on. The habitat would also include a chromosome bank to preserve the genomes of plants, animals, and humans left behind on Earth. The long range plan is, when the Earth becomes habitable in five thousand years, it will be repopulated by the descendants of the space people as well as other living things generated from the genome bank.
Naturally there's some drama attached to the selection of the tiny percentage of people who will be sent aloft, since everyone else would die. The author doesn't really address what would happen among those destined to be left behind. He seems, in fact, to suggest they would (for the most part) benignly accept their fate. This seems completely unrealistic. Then again, covering this issue would probably be another book.
Meanwhile there's plenty going on in the space habitat. As always when there are more than five people involved in an undertaking, politics rears it ugly head. Thus things don't proceed according to plan and lots of unexpected things happen. I don't want to give away spoilers so I'll just say that - after many many pages - the hard rain starts and life in space commences.
Jump ahead five thousand years and the descendants of the original humans in the space habitat - now numbering several billion people divided into seven races - start to terraform and return to Earth. Inevitably, given human nature, the races are divided into the equivalent of two "countries" called Red and Blue. These are somewhat reminiscent of the old Soviet Union and the United States. There's tension between Red and Blue, and wars and peace treaties result - much as occurred on Earth before time zero (when the moon exploded).
In the last part of the book a group of seven people, including an individual from each race, are sent down to Earth on a scouting expedition. Their mission is to investigate some odd sightings and to see what's what down there. This leads to the climax of the book and perhaps presents some hope for the future of humankind.
I'm not quite sure how I feel about this book. The premise is fascinating, and the author - who clearly did a phenomenal amount of research - seems to cover every aspect of what's required to make a successful space habitat. The details of constructing the habitat, however, are overly long, detailed, and tedious. For me, the human interactions in the habitat are more interesting. I also feel that the story becomes more compelling toward the end, when the new human races are preparing to go back to Earth. There are a few surprises in the story, which some readers might anticipate.
I think a mini-series based on this book might be quite successful. It would provide visual images of technical details that are hard to follow when described in scientific jargon. It would also provide a picture of the new races - which seem to be signifcantly different from modern humans. Just a thought.
I'm not sure the average reader would enjoy this book but I'd certainly recommend it to fans of 'hard' science fiction. In any case, this book could definitely generate lots of intelligent, engaging conversations at book clubs, dinner tables, and parties.