This is a LONG novel, too long in my opinion, and I was surprised that

the great circle flight doesn't happen till near the end.

Still I did read to the end because the theme of people disappearing or dying without warning, never to be found again, filters through the novel, warning us gently of what's to come and intriguing me (even if there were parts that stalled these discoveries for far too long). And obviously, the theme of flight in all its forms is intriguing: why people fly, why they want to disappear, what are they fleeing / flying from? And the feistiness and determination, and the loneliness and ostracisation of both the main female characters is portrayed beautifully, believably and heart-breakingly ... you want them to succeed but also, if it's what they decide they want, you want them never to be found again.