NASA space shuttle managers formally decided Thursday to push back the launches of three orbiter missions in 2007 to allow extra time for spacecraft processing.

"It's a combination of processing and orbiter turnaround," Kyle Herring, a NASA spokesperson at the Johnson Space Center, told SPACE.com. Herring added that shuttle external fuel tank preparations are a priority: "We know that the tank processing is one of the critical path items and has been since return to flight."

NASA's first shuttle flight of 2007--STS-117 aboard Atlantis--is expected to carry new fuel tank modifications when it launches no earlier than March 16 on a mission to deliver new solar arrays and truss segments to the International Space Station (ISS).

The shuttle Endeavour will follow no earlier than June 28, on NASA's STS-118 mission, to add another truss segment and a spare parts storage platform to the ISS. Those missions pave the way for the planned Sept. 7 launch of Atlantis' STS-120 mission to install a U.S.-built hub [image] that will link future laboratories to the ISS.

The three shuttle missions were originally slated to launch on Feb. 22, June 11 and Aug. 9, respectively, but were pushed back to accommodate shuttle processing needs, as well as plans for upcoming ISS astronaut spacewalks, crew rotations and automated cargo shipments.

NASA has scheduled five shuttle flights in 2007 to continue space station assembly.

"The other two flights are set for October and December in 2007," Herring said.

October's flight, STS-122 aboard Discovery, will deliver the European Space Agency's Columbus laboratory to the ISS. Endeavour is expected to launch its STS-123 mission in December carrying the logistics module for Japan's Kibo laboratory [image] and Canada's Dextre robotic arm attachment.

NASA's next shuttle mission of 2006--STS-116 aboard Discovery--is scheduled to launch on Dec. 7 to add a new truss segment to the ISS and rewire the outpost's electrical grid.

At least 14 shuttle flights, including STS-116, are expected to complete the ISS by NASA's September 2010 deadline, when the agency plans to retire its three-orbiter fleet.

The flight schedule also includes room for a pair of extra ISS-bound flights to haul spare parts and other items to the orbital laboratory, as well as a 2008 flight to service the Hubble Space Telescope.

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