Unlike space rockets, NASA’s space shuttles, or orbiter vehicles, were designed to be reused. Each has a name, but every mission on which it goes is given a unique number. The acronym STS (Space Transportation System) has been used throughout the shuttle program. The first nine flights were simply numbered STS-1 to STS-9. A more complicated system was then used, but the original system of STS + number has been revived. The do not always follow numerical order, as a mission may be delayed and a later-numbered mission may take its place before it can be rescheduled.