The sequence of Space Shuttle flights for the next three missions will be STS 121/Discovery, STS 115/Atlantis and then Discovery for STS 116, NASA’s associate administrator for space operations William Gerstenmaier and its administrator Michael Griffin announced on 18 August. The next flight was to have been STS 121/Atlantis, but Atlantis is needed for STS 115 because it is a lighter orbiter than Discovery and better able to lift the International Space Station’s second port truss. Atlantis would otherwise have had to fly missions 121 and 115 back-to-back. Endeavour is not available because it is held up in its maintenance period due to Discovery’s and Atlantis’s return to flight activities.

Source: Flight International