Split-merge craft are spacecraft that are composed of many separable parts, each capable of travelling on its own. They can also be merged with other such craft. They can also change their route, either by reactionless drive, very efficient fuel or an ability to refuel off the land on other worlds. To allow them to merge, their rendezvous hatches are made to be adjustable in size and shape, not made in one form and stay that way. With such spacecraft around, ethical or psychological objections on the lines of "the crew would have to stay in the same group through the whole mission" or "children born aboard would be stuck" would no longer be an issue as long as the ship is within reach of Earth within the child's lifespan.