NASA said Wednesday the launch of Discovery would be postponed for at least two days to allow for a solution to the problem that a sensor indicating the level of liquid hydrogen fuel in the external fuel tank failed to work properly.

NASA engineers have done a lot of work but what caused the problem at this stage remains unknown and engineers cannot understand the sensor failure normally, Wayne Hale, deputy manager of NASA's space shuttle program, told a news conference about two and a half hours after NASA announcing it has decided to quit the launch that was scheduled for 3:51 PM EDT (19:51 GMT).

NASA has decided that "that's the very best scenario" that NASA would not in any conceivable way perform the launch before Saturday, said Hale.

The launch window runs until July 31. The problem was found during the routine pre-launch check, said Hale, who also leads there turn-to-flight mission management team. It was then around 1:15 p.m., and NASA officials held a long discussion before deciding to quit the launch.

The low fuel warning sensor failed to give back readings to the computer in the check. As a rule, the four sensors for liquid hydrogen installed at the bottom of the tank have to work all properly for the liftoff.

Low run of hydrogen fuel may seriously harm the engines, endangering the flight, according to Hale. A similar problem with one sensor had occurred in the first tanking test in last April, but the problem disappeared in the second test in May. However, Hale noted that Discovery had later replaced with a new tank.

The unexpected event gives a setback in NASA's return-to-flight effort. NASA has made many major safety improvements over the past two and a half years at the recommendation by the Columbia accident investigation board.

NASA Administrator Michael Griffin said Tuesday NASA has done everything it needs to do to ensure a safe flight. He said a safe flight is "utterly crucial" for NASA, for the nation, and for the US space program.

NASA's shuttle fleet, which is currently consisting of Discovery, Atlantis and Endeavor, has been grounded since the Columbia midair disintegration in Feb. 2003, in which all seven astronauts aboard were killed.

(Xinhua News Agency July 14, 2005)