NASA on Wednesday postponed until mid-December the launch of the space shuttle Discovery on its last trip to the International Space Station after cracks were found in its external fuel tank.
"It's a complex problem," space shuttle program manager John Shannon said. "We really need to understand our risk."
The next potential date for launch is December 17, the US space agency said after earlier putting off any launch until December 3.
Discovery was hours from launching on it final voyage to the ISS on November 5 when technicians found a hydrogen leak and later a series of long cracks on the shuttle's external fuel tank.
NASA then put off the launch so repairs could be completed on the aging shuttle.
Discovery's 11-day mission with its all-American crew of six is to deliver a pressurized logistics module called Leonardo to the ISS, which will be permanently attached to the space station to provide more storage space.
The shuttle will also bring Robonaut 2, the first human-like robot in space and a permanent addition to the orbiting space station, as well as spare parts.
Two space walks, for maintenance work and component installation, are scheduled.
The Discovery has launched into space 38 times, and NASA aims to retire the shuttle after its final and 39th voyage.
The launch aims to be the fourth and final shuttle flight of the year, and the last scheduled for Discovery, the oldest in the three-shuttle fleet that is being retired next year.
But the mission has been marred by a series of delays brought on by bad weather and equipment problems.
The three US shuttles -- the other two are Atlantis and Endeavour -- are due to be sent off to become museum pieces after a final shuttle mission to the space station in late February.
That means Russian Soyuz spacecraft, a modernized version of which recently dropped off three fresh crew members to the ISS, doubling the crew to six, will for several years be the only vehicle for transporting humans into space.
However, NASA's recently approved 2011 budget has left the door open to an additional shuttle flight in June.
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