STS-400

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).

Due to the much lower orbital inclination of the HST compared to the ISS, the shuttle crew would have been unable to use the International Space Station as a "safe haven," and NASA would not have been able to follow the usual plan of recovering the crew with another shuttle at a later date. Instead, NASA developed a plan to conduct a shuttle-to-shuttle rescue mission, similar to proposed rescue missions for pre-ISS flights. The rescue mission would have been launched only three days after call-up and as early as seven days after the launch of STS-125, since the crew of Atlantis would only have about three weeks of consumables after launch.

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.