STS-400

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).

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. This created a rare scenario of two shuttles being on the launch pads at the same time. In October 2008, however, STS-125 was subsequently delayed and rolled back to the VAB.

Initially, STS-125 was retargeted for No Earlier Than February 2009. This changed the STS-400 vehicle from Endeavour to Discovery. The mission was redesignated STS-401 due to the swap from Endeavour to Discovery. However, STS-125 was then delayed further, allowing Discovery mission STS-119 to fly beforehand. This resulted in the rescue mission reverting to Endeavour and the designation STS-400 being reinstated. In January, 2009, it was announced that NASA was evaluating conducting both launches from Complex 39A in order to avoid further delays to Ares I-X, which, at the time, was scheduled for launch from LC-39B in the September 2009 timeframe. It was planned that after the STS-125 mission in October 2008, Launch Complex 39B would undergo the conversion for use in Project Constellation for the Ares I-X rocket. Several of the members on the NASA mission management team said at the time (2009) that single pad operations were possible, but the decision was made to use both pads.