There's actually three acts: Earth, Space, Moon. It's really a trilogy smushed up into one book.
I agree, though. The first two acts were great, and the third fell flatter than the earlier two. But I enjoyed it nonetheless. It's nice to imagine what would happen, and it's pretty hard scifi throughout, albeit greatly accelerated.
Yeah, though I tend to like breaking it into 'the end of Earth and our efforts to survive that end' and 'what happens to a society that's been through that trauma and their return home' split. The time jump really makes the jump feel more solid between book/acts 2 and 3 to me.
I agree, though. The first two acts were great, and the third fell flatter than the earlier two. But I enjoyed it nonetheless. It's nice to imagine what would happen, and it's pretty hard scifi throughout, albeit greatly accelerated.