The countdown clocks began ticking again Saturday for NASA's return to space, as shuttle managers voiced optimism — but not certainty — that they had fixed the fuel gauge problem that thwarted the first launch attempt.

"No doubt there is some degree of finger crossing," NASA test director Pete Nickolenko (search) said before the start of the second countdown in two weeks.

"But the other side of the coin is that we have really performed a very thorough troubleshooting analysis to a great degree, an excruciating degree of detail with all the shuttle program experts and the contractors that we can get."

NASA (search) is aiming for a Tuesday morning liftoff of Discovery (search) on the first shuttle mission since Columbia's disastrous re-entry in 2003. The countdown was delayed by two weeks after one of four hydrogen fuel gauges in Discovery's big external tank failed a routine test on July 13, just two hours before the initial scheduled liftoff.

In the past few days, NASA has repaired three areas of spotty electrical grounding and scoured the spacecraft for any electromagnetic interference that might have exacerbated the fuel gauge problem.

Technicians switched the wiring between the troublesome fuel gauge and another one, in an attempt to better understand the sensor problem if it recurs.

The 14 engineering teams that have been working nonstop on this problem have eliminated more than 300 possible causes, Nickolenko said. All that remains in the so-called fault tree are faulty electrical grounding in the shuttle's aft fuselage, which has been fixed, and possible electromagnetic interference, which may still be out there. No interference has been found, but the true test will come when the shuttle is fueled and all its systems are running right before liftoff.

Nickolenko said he and others are confident the system will work the way it's supposed to come Tuesday. But he hastily added, "We were confident that we were going to be in that case for the first launch attempt, too."

Mission managers are considering launching Discovery and its crew of seven even if one of the fuel gauges malfunctions, as long as the problem is reminiscent of what happened two weeks earlier and is thought to be well understood. The same problem spoiled a fueling test back in April.

NASA's own launch rules require that all four fuel gauges be working, even though only two are needed to ensure that the main engines don't shut down too soon or too late, both potentially deadly situations. Any rule change at the last minute, to allow less than four good gauges, would almost certainly raise eyebrows.

Technical issues aside, the weather could end up interfering.

Forecasters are putting the odds of acceptable launch conditions at 60 percent because of the threat of rain.

NASA has until the beginning of August to launch Discovery to the international space station, or it must wait until September to ensure good lighting throughout the ascent. The space agency is insisting on a daylight liftoff for good camera views in case the shuttle is hit by fuel-tank foam insulation, ice or other debris.

Columbia was brought down by a 11/2-pound chunk of foam that pierced the left wing. The gaping hole led to the shuttle's destruction during re-entry on Feb. 1, 2003, and the deaths of all seven astronauts.