Space Shuttle Fault Haunts Nasa

Nasa's beleaguered engineers were still trying to find out how to get the shuttle Discovery back into space last night as the questions continued to be raised over its aborted launch.

The agency's first manned mission since the Columbia disaster in 2003 was scrubbed 140 minutes before its scheduled liftoff on Wednesday owing to a faulty fuel tank sensor, with the seven astronauts already on the flight deck.

But some experts believe Nasa should have conducted further tests after the shuttle was rebuilt from scratch following an identical problem in April.

"Any engineer will tell you that you can't beat a good test," said Randy Avera, a former Nasa engineer who helped build Columbia. "Nasa should have tested that."

The shuttle's fuel system electronics were built from scratch after the sensors failed in April, and Nasa was confident that the problem was resolved.

But managers have now admitted they are baffled about the problem's recurrence. "It's an unexplained anomaly," said the Nasa administrator Mike Griffin, who has acknowledged that the fleet, due to be retired in 2010, is beginning to show its age. "If you asked me to try to find a single electronics box in my house that's 25 years old and still working, I don't think I could."

Nasa says the shuttle will not be launched until Sunday at the earliest.