Space shuttle Discovery has returned to the Vehicle Assembly Building (VAB) from the launch pad at NASA's Kennedy Space Center in Florida to finish external fuel tank testing, the US space agency says.
The manoeuvre, known as a "rollback," began on the night of 21 December, taking six hours to make it down the 5.47km (3.4mi) track from launch pad 39A back to the VAB. The move was delayed by a day due to problems with the levelling system on the crawler-transporter that moves the shuttle to and from the launch pad.
In the VAB, technicians are using x-ray scans to see beneath the foam insulation on all 108 support beams, called stringers, on Discovery's external tank. The team will also remove the 89 strain gauges and temperature sensors still attached to the tank from the 17 December tanking test and reapply foam to those areas.
Shuttle programme managers are keeping options open to modify some of the stringers, depending on the results from the additional testing. Managers are expected to decide on 30 December whether modifications are needed.
If no further modifications are needed Discovery is expected to be back on the launch pad in January ahead of its next launch window, which begins 3 February.
Abnormally frigid temperatures and high winds have slowed the testing process as the US space agency's engineers continue to attempt to determine the root cause of cracks in the tops of two 21-foot-long aluminium stringers during the shuttle's 5 November launch preparations.
Engineers have been trying to determine the root cause of the cracks on two 21ft-long (67.4m) aluminium stringers on the shuttle's external tank for more than a month. Though the cracks have been repaired and thermal protective foam reapplied, NASA says more tests are required to determine if Discovery can launch safely.
NASA is attempting to replicate the cracks that occurred when loading the tanks to obtain higher-fidelity information about what goes on under the foam during cryoloading. The tank holds super-cooled liquid oxygen and liquid hydrogen, which cause the tank to contract by about half an inch as it is filled.
Then the planned 1 November take-off was repeatedly scrubbed, first to repair leaking helium and nitrogen gas lines on the Shuttle's orbital manoeuvring system pod, which took longer than expected. Electrical problems discovered during a routine engine power-up and check-out and weather added further delays.
When it finally launches, the mission will deliver a pressurised logistics module called Leonardo to the 10-year-old ISS. Leonardo, built by the Italian space agency, will be permanently attached to the space station and used to transfer cargo to and from the station. The Shuttle will also carry Robonaut 2, a 136kg (300lb) humanoid robot developed by NASA, making its first trip into space for testing