Hull Zero Three by Greg Bear

Sci-Fi thriller that grabs you and doesn't let go until you finish. It has some interesting concepts and it's thoroughly entertaining. The logic doesn't quite hang together at the end, but then we are dealing with people and people are never very good at logic.

We have a giant spaceship that has latched onto a ball of frozen ice (that would otherwise be a comet) and is using it as reaction mass. The ship is turning the ice to water and pumping it to the ship where it is fed to the  motors. The ship accelerated to twenty percent of the speed of light and now the journey is half over and it needs to turn around and start decelerating. The voyage is going to take hundreds of years. There are no people as such. Whenever the ship needs people, like now, when it needs to make a decision on which of several possible star systems to choose for a final destination, it grows them. While they are growing, it programs them with dreams about who they are, what they do, and where they are going.

But something has gone wrong and there are two factions fighting over which course to take. The fighting has gotten ugly and is beginning to look like a war. Our hero is woken from his pleasant dream of an idyllic life on an idyllic planet with an idyllic mate to a run for your life situation. The whole thing is crazy, but it's great fun to read.

My difficulty with the story comes when they start discussing how they are going to deal with whatever life forms they find on their new planet. One faction wants to wipe them out, make a clean slate for humanity to build on. I find that unbelievable. I doubt whether we are going to find any planets that humans could live on, and if we did and there was any life already there, it would be much better adapted to living on that planet than we could ever hope to be. In this story, the ship is busy growing monsters specifically designed to wipe out all life on the target planet. You might get all the big creatures, but you will never get all the microscopic critters, and those are the ones you would need to worry about, should you ever find such a place. I don't see it happening.