Maybe a possible spoiler right at the end, though probably not. Still, be warned! 
 The first two sections were surprisingly good; the last section, set 5,000 years after the events in the first two, was a real slog. Be aware, though, that there was a lot of scientific and engineering jargon in all three sections. I read most of it in the first two sections but found myself skimming it in the third. 
 On an otherwise unremarkable summer day, something slammed into the Moon with such force that the Moon broke into seven big pieces. At first, nothing else happened - people around the world were stunned to look into the night sky and see the pieces hovering in the same relative position they had been in when they were part of the whole. But before long they started bumping into each other, sending fragments flying off into space that then fell to Earth as large meteors. One media-savvy astrophysicist - who seems to have been modeled on Neil deGrasse Tyson - posited that the pieces of the Moon would soon destroy each other and bolide hits would increase until they became a Hard Rain, spelling the end of life on Earth in a huge conflagration. Simultaneously, scientists from around the world came to the same conclusion. 
 I have to say, it was so well told that I cried more than once as the people on Earth and in the Space Station started to work together toward saving at least en0ugh people so that when the White Sky happened and the Hard Rain began, the human race would have a chance to survive. 
 As for the third section, much much too wordy, and none of the characters really appealed. Also, a bit of a personal disappointment because I kind of figure out mostly how that part would end, except I thought it would be great if the Martian contingent to come racing back to Earth to save the day - or add to the mix of groups that hated each other, and that was another downer how little human beings had changed in that respect.