Before we get started, let me tell you a story:
Years and years ago, I bought a used copy of Larry Niven's short story collection A Hole In Space because I bought absolutely every Larry Niven short story collection I could find. Niven is probably best known as a novelist, but his short stories are his best work. He has a gift for taking a single, weird idea and turning it into a compelling story. Only Warren Ellis, among contemporary writers, is his match in this regard. Anyway, there's a story in that collection called "Rammer." I won't give away any details, but it's a nifty mystery involving two kinds of (sort of) time travel, Bussard ramjets, and totalitarianism.
What I didn't know at the time was that it was "Rammer" was the first chapter of Niven's novel A World Out Of Time. What I also didn't know was that the first chapter was almost completely unrelated to the rest of the novel. There was a massive time shift and suddenly, instead of being science fiction, it read much more like a fantasy novel in a space setting. It wasn't bad but it was jarring, as though two different stories had been awkwardly spliced together.
If you've read Seveneves, you know why I'm telling you this story. The structure of the book, to me at least, was jarring to the point where it took me out of the story. It's either a novel and a half-novel, or two sets of two short novels, with the second set missing it's final piece.
I'll keep the spoilers to a minimum here. The first two sections of the book concern a disaster which renders the Earth uninhabitable and how humanity tries to preserve the species by taking to space and then what happens once they get there. The story zips along, the science is interesting, and the characters are well-drawn and interesting (if not always sympathetic). These sections would have made a terrific novel on their own.
The final chapter of the second section, however, is a jarring change of pace. There's no subtlety to the way it sets up the next part. It's necessary, I suppose, but it is so very out of place with the rest of the book to that point that it not only takes you out of the narrative, it makes you a little apprehensive about what's coming. And by "you," I mean me. And by "apprehensive," I don't mean it in a good way.
The third section is section skips ahead five thousand years. For perspective, five thousand years before today would be the start of the Mayan calendar, or the beginning of the Kish dynasty in Mesopotamia. We're talking about a span from the Bronze Age to today. I harp on this because the changes at the end of the second section of the book are still very much in evidence and, in fact, the central paradigm of the third section.
Is that reasonable?
It didn't seem reasonable to me. Every time a character behaved a certain way because that's how people of their race behaved, it grated. I understand that Stephenson is writing about societies which were created in a certain image as opposed to just evolving organically, but five thousand years is a very long time for racial traits to remain that distinct in all members of the race.
The story in the final section is fascinating, although the reveals aren't especially surprising. It does end on a note which doesn't feel especially like an "end," so I wouldn't be surprised if Stephenson returned to this story at a later date.
Therein lies my problem with the structure. The second section of the book was extremely similar in tone to the first. They're either a single book, or a short book and a sequel. The third section is wildly different than the first two and it feels incomplete, like there was meant to be a fourth.
I'm probably harping on that more than I should. Seveneves is a page turner, and that's a heck of a trick for hard science fiction. I learned more about orbital mechanics and whips than I ever expected to know, but the lengthy explanations never broke the narrative flow. I felt strongly about the main characters and genuinely wanted to throttle one of them. I strongly recommend it, flaws and all.