NASA will try to launch Discovery on the first shuttle mission in more than two years next Tuesday, after tracing last week's fuel gauge failure to, most likely, an electrical grounding problem lurking inside the spacecraft.

Shuttle program manager Bill Parsons (search) said the only way to thoroughly check the system is to fuel Discovery and have all its equipment running.

"We believe the best way to go through this is to do a countdown," he said. "If the sensors (gauges) work exactly like we think they will, then we'll launch on that day. If anything goes not per the plan that we've laid out in front of us, then we'll have a scrub and we'll have to talk about it."

In what would be an almost certainly controversial move in the wake of the 2003 Columbia tragedy, NASA (search) may also proceed with the liftoff if the fuel gauge problem recurs but is considered well understood. That would mean revoking a launch rule requiring all four hydrogen fuel gauges at the bottom of Discovery's external tank to be working properly, and instead relying on just three out of four.

That looser three-out-of-four rule was thrown out after the 1986 Challenger launch explosion.

The fuel gauges are intended to keep a shuttle's main engines from shutting down too early or too late after liftoff, both potentially disastrous situations. Only two of the four are needed to ensure safety, but ever since the Challenger accident (search), NASA has required all four to be operating.

Parsons said there are considerable "safety nets" to protect against launching a seriously flawed spacecraft, if an exception to the fuel gauge rule is made at the last minute.

"Right now, we think we have eliminated all the common causes that we believe could do this and we've done everything we possibly could on the vehicle," he told journalists at an evening news conference.

Technicians plan to swap some pins and wiring near the electronics box that is associated with the four hydrogen fuel gauges, to better understand what happened last week.

Discovery's countdown was halted with just two hours remaining before liftoff last Wednesday when one of the four fuel gauges malfunctioned. It was the same type of problem that marred a fueling test of Discovery back in April, with a different external tank.

Despite a week of exhaustive scrutiny by hundreds of engineers, NASA has been unable to pinpoint the precise cause or location of the failure, and an electrical grounding problem somewhere in the aft fuselage is considered the most probable cause. The space agency is holding out hope that the grounding problem can be traced to interference from shuttle equipment in the next few days, but will aim for a Tuesday launch even if the mystery persists.

"We have a great amount of work in front of us to get us through this and get us ready for this," Parsons said. "But we've all agreed that this work is doable and that it all takes us to a launch on the 26th."

The countdown is set to begin Saturday for a Tuesday morning launch.

Discovery and its crew of seven will fly to the international space station to drop off supplies and make repairs, and will test out inspection and patching techniques for the type of damage that doomed Columbia.

NASA's shuttles have been grounded ever since Columbia's catastrophic re-entry on Feb. 1, 2003, which killed all seven astronauts.