The question might have been asked: Why just a piece of it? Why not bring the whole comet core back, if water was so desirable? What was the point of sending a large nuclear reactor into space if you weren’t going to use it? And the answer lay in the fact that even a large nuclear reactor did not even come close to having enough power to move such a big piece of ice. The mission would have lasted more than a century, assuming the existence of some kind of a miracle reactor that could operate at full power for that long. In order to get this done in any reasonable amount of time, they could only bring back the bare minimum of ice needed to rendezvous with Izzy and accomplish the Big Ride.