"No doubt, there is some degree of finger crossing," NASA test director Pete Nickolenko 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 programme experts and the contractors that we can get."

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

Electrical grounding

In the past few days, NASA has repaired three areas of spotty electrical grounding.

Scientists have been working
around the clock for the launch

They 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 eliminated more than 300 possible causes, Nickolenko said. All that remains are faulty electrical grounding in the shuttle's aft fuselage, which has been fixed, and possible electromagnetic interference.

True test

No interference has been found, but the true test will come when the shuttle is fueled and all its systems are running right before lift-off.

Nickolenko said he and others were confident the system would work the way it's supposed to come Tuesday. But he 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.

Launch conditions

Even if everything else goes well,
weather can play the spoiler

NASA's 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% because of the threat of rain and clouds.
 
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 lift-off 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 675-gram chunk of foam that pierced the left wing. The gaping hole led to the shuttle's destruction during re-entry on 1 February, 2003, and the deaths of all seven astronauts.