STS-400 was the
Space Shuttle contingency support (Launch On Need) flight which 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).
[1][2][3][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.
[3] Instead, NASA developed a plan to conduct a shuttle-to-shuttle rescue mission, similar to
proposed rescue missions for pre-ISS flights.
[3][5][6] 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.
[2]
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. This created a rare scenario of two shuttles being on the launch pads at the same time.
[3] In October 2008, however, STS-125 was subsequently delayed and rolled back to the
VAB.