You could cleanly think about it in three parts, but the time jump in the middle which involves a whole new set of characters is another way to split it.
Yes the book itself is broken into 3 parts but if you're looking at it from a scientific plausibility scale parts 1 & 2 are in the same vicinity of that scale where part 3 jumps a good ways into more traditional scifi levels of plausibility.
Heh, true. My brain went into "pre space" and "post space" as halves :)