Probably my second favorite Robinson novel (after The Years of Rice and Salt), this one is gritty and realistic, but also intellectually and philosophically challenging. It's much better than his cli-fi and terraforming trilogies, because it's less optimistic and doesn't rely on simplistic solutions. I would have given this book five stars, but the last part let me down. It's like Robinson can't bear to finish his story without resorting to a rainbows and butterflies finale. Oh well, the rest of it is quite good.