Distress calls usually work good...
You could also use an unexplained radio silence, perhaps a new object has appeared and needs surveying, maybe a ship disaster beacon has been detected, could be participating in a technology test (perhaps an upgraded jump drive, this could in turn not work properly and prevent them from returning home for a few missions), responding to report of an illegal fissionables mining op, doing a race (dunno why enlisted personnel would do this), going out on a dare, or if you want to get all mystical throw a dream or some other turn of events that just inspire the characters to go.
