|The Space Shuttle Discovery sits on Launch Pad 39B at the Kennedy Space Center at Cape Canaveral, Florida|
The countdown for Discovery's flight to the International Space Station was halted on July 13 shortly after technicians discovered a malfunctioning hydrogen fuel sensor.
The flight would end a two and a half year hiatus for the space shuttle program, which was grounded after the Columbia disintegrated upon reentering earth's atmosphere in February 2003, killing it's seven member crew.
On Saturday, mission controllers began counting down anew for lift off on Tuesday, but space agency officials are still not sure why the hydrogen sensor gave a low reading despite the fact that the fuel tank was full.
Space shuttle program manager Wayne Hale said Sunday technicians have run through every possible check they could think of to see if they could duplicate the failure, but without success.
NASA technicians have also rewired two of the four sensors to see if the problem is in the wiring or the master sensor itself. He says there's still no word.
"I don't want anybody to go away with the idea that we're indicting the sensors," Mr. Hale says. "As a matter of fact, while it could always be that we had a sensor failure, that is just one of the things that we have got on our list of topics to look at."
Mr. Hale says technicians are looking for unusual causes for the faulty sensor reading, including unexpected ice formation.
NASA Administrator Michael Griffin says he's comfortable that space agency officials are doing what's necessary to ensure the safety of the mission.
"Look, I think what you want coming out the Columbia accident and the loss of Columbia and the soul searching examination that NASA has undertaken since then is that we make the right technical decisions, that we do the right thing, to the extent that we can figure that out, which is hard," Mr. Griffin indicated.
NASA officials say there's a 40 percent chance Tuesday's lift-off could be postponed due to weather. If that happens, the next possible launch date would be August 4.