With NASA engineers still unable to determine the cause of cracks on space shuttle Discovery's external fuel tank, the launch has been pushed back to 3 February at the earliest, the US space agency says.

Engineers need more time to conduct tests to get to the root cause of the cracks on two 21ft-long (67.4m) aluminium stringers on the shuttle's external tank. 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 will attempt to replicate the cracks that occurred when loading the tanks to obtain higher-fidelity information about what goes on under the foam during cryoloading, says Bill Gerstenmaier, NASA's associate administrator for space operations. After removing the protective foam and instrumenting the stringers, the foam will be reapplied before filling the tank.

"Between those two tests, we think we'll get enough information that we can move forward," Gerstenmaier says. The testing also will confirm the integrity of repairs to the ground umbilical carrier plate, which leaked an unsafe amount of gaseous hydrogen during Discovery's 5 November launch attempt.

The date of the test is under evaluation, but NASA says it is working on a timeline and hopes to complete the work before the end of 2010. Even if testing and evaluation are complete by year-end, NASA has other space traffic it must work around to reschedule Discovery's launch.

Russian-launched resupply and crew swap missions are continuing as scheduled, keeping the impact of Discovery's delays to a minimum on the International Space Station (ISS), NASA says. But Discovery's delays mean that the earliest opportunity for the liftoff of the final scheduled shuttle mission -- STS-134 with Endeavour - is now 1 April.

Discovery has faced a string of problems trying to get off the ground for its final flight. The original September launch was initially pushed back when payloads to be delivered to the International Space Station (ISS) were not ready on time.

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.

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.

Source: Flight International