美宇航局确认奋进号故障

NASA says engineers at Kennedy Space Center believe they have identified the cause of the heater failure that forced the postponement of the launch of the shuttle Endeavour.

NASA technicians say the fuel heater failure likely stems from a problem in a box of switches that controls power feeds. Engineers say it will take several days to replace and test that switch box to ensure fuel line heaters are functioning properly. 

The shuttle Endeavour's launch was scrubbed last Friday only hours before liftoff. Shuttle Launch Director Mike Leinbach explained the heaters are needed to keep fuel from freezing in each of three auxiliary power units during orbit.

The units provide hydraulic power to steer the shuttle during ascent and re-entry.

"We did not want to commit to flight with only one of two heaters on the auxiliary power unit fuel line, because of the chance that if you lost that one good one on orbit, then you run the very high risk of freezing the fuel in that line and therefore the auxiliary power unit would not function and therefore you would not have full hydraulic power," he said.

Leinbach said a loss of hydraulic power could cause problems during re-entry.

Managers have yet to set a date for Endeavour's next launch attempt.

"And as we always say in this business, we will not fly before we are ready," said Leinbach.

NASA officials say they do not expect to be ready to launch before Sunday, May 8.  

This mission will be Endeavour's 25th and final flight. NASA is retiring its 30-year-old shuttle program this year.