Hugo
I finished this book in just two evenings. Thumbs up. There's a reason it won a Hugo and a Nebula.

Science fiction
The first section of the book is fantastic. A meteor strike wipes out D.C. and devastates the whole Eastern seaboard. Elma York and her husband Nathaniel (who both work for the government's space program) struggle to survive the immediate aftermath of the catastrophe. It's a gripping account. When they get situated at the new capitol in Kansas, Elma and Nathaniel help assess the meteor impact and calculate the long-term effect: it's an extinction event. It will take decades, but the meteor has jumpstarted an unstoppable runaway greenhouse effect that will boil the oceans and leave Earth uninhabitable.

Socially conscious alternate-history space-race fan fiction
The only solution is to become a multi-planet species. And so, enter the second and longer portion of the book: socially conscious alternate-history space-race fan fiction. Which is not terrible. It gives strong For All Mankind vibes, and it's fun. Really. But the actual science fiction (the meteor strike and the coming extinction event) takes a back seat to the fan fiction. A great deal of this part of the book is about Elma's anxieties around public speaking (she throws up a lot), and about the struggle for women's rights and for black civil rights, all in the context of a space race set a decade earlier than the one in our real-life timeline. That's not a bad story, but I liked the science fiction portion of the book far more.