STS-400 was the Space Shuttle contingency support (Launch On Need) flight that would have been launched using Space Shuttle Endeavour if a major problem occurred on Space Shuttle Atlantis during STS-125, the final Hubble Space Telescope servicing mission (HST SM-4). The mission was first rolled out in September 2008 to Launch Complex 39B two weeks after the STS-125 shuttle was rolled out to Launch Complex 39A, creating a rare scenario in which two shuttles were on launch pads at the same time. In October 2008, however, STS-125 was delayed and rolled back to the VAB.