I gave it 3 stars because the ideas are top-notch. Kay Kenyon created a very imaginative scenario and I read this book quickly because I was so eager to see how she pulled things together and answered the questions built up from the beginning. The problem I have with this book, and every other novel I've read from this author, is characterization and motivation.
Too many characters' actions don't make any sense. For example, the crew of Road Star turn on the man that brought them children when they found out the kids were purchased from desperately poor families. Why were they so angry? They wanted children, and the children they planned to adopt needed help to escape the desperate poverty and hopeless future of thier own lives.
Why would they decide not to keep the children and turn on the man who arranged it? I can't imagine such a reaction in reality. They loved and treasured those children before the crew had actually met most of their adopted offspring. It's too unrealistic to think they'd just say 'oh no, this is awful. We won't adopt them now, send them back to their miserable lives and parents that sold them off at bargain basement prices." That's ridiculous. Learning the children's true origin would have increased the crew's desire to keep the children safe with adopted families that loved them.
The origin story of Star Road is a bit shaky too. A population with genetic immunity to a deadly illness is definitely realistic. There are many examples in reality. Hatred towards the small group with immunity is also very realistic, although I have trouble with some of their actions too. A civilization advanced enough to create Star Road must have brilliant scientists, and quite a lot of them. I can't imagine world governments with scientific advisers would have decided 'let's kill the gypsies' instead of protecting them and using their genetic immunity in research for a potential cure or vaccine. Anyone capable of rational thought with a solid scientific education would have advocated to save every single gypsy, if for no other reason than reproduction if everything else failed. There's absolutely no way that the immune population would receive the ship meant to leave earth. That's completely unrealistic.
The surviving earth populations were interesting, but I was disappointed after finishing the book. Kenyon never went beyond very superficial explanations, and many characters' actions didn't make much sense. I would enjoy this author's work if she had more solid characters and reasonable motivating factors. Even the dispute between the captain and his first mate didn't seem plausible. The supposed reasons for split loyalties in the crew and rebellion against the captain were very weak. No one had any solid reason for the hostilities that developed. It was obviously a necessary part of the story, but the author didn't really bother to make those hostilities believable. I would have preferred to know more about the ecological recovery plans as well. That was the most important factor for every human involved, but it got very little attention.