Ok, this is pretty vague, but I read a sci fi book a while ago and can't remember the name. Basically, aliens for some unknown reason have started sending robot spacecraft towards earth every five years or so. These craft are bent on destroying earth, and each is more deadly than the last. However they are not total annihilation craft; they are actually clever puzzles that humans must solve or the planet will be destroyed. Every time, through ingenuity, cooperation, a global prediction market, and bravery of a small team of humans, they are able to defeat this alien craft. Of course people die, but you can see from reading the book that this whole process makes us stronger every time, so maybe the aliens are doing us a strange kind of favor. The human team that goes off to defeat this alien craft never comes back alive except one of the guys from the first team, and a woman from the last team. Both cases they are missing an arm, and they become the overall leaders next time. I can't remember the name. Anyone remember what this is? Thank you!