"Columbia" accident
This usually remains on the Shuttle for launch and landing, however as the next mission, STS-124, is carrying a very large payload - the JEM pressurised experiment module - the OBSS cannot be accommodated in its payload bay for launch. Because it will be needed once the Shuttle reaches orbit, "Discovery" will collect the OBSS from the ISS, and return it to Earth at the end of its mission. The OBSS is used to conduct photography of the orbiter's heat shield, in order to check for damage, similar to that which caused the 
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Space Shuttle Endeavour launches for mission STS-123