CAPE CANAVERAL, Fla. - The countdown is on once again for the launch of the space shuttle Discovery, NASA's first shuttle flight since the Columbia tragedy.

The clock began ticking down toward Discovery's July 26 launch, currently set for 10:39 a.m. EDT (1439 GMT), exactly at noon Saturday as engineers, astronauts and mission managers prepare for their second launch attempt in almost two weeks.

Discovery's STS-114 flight has been delayed since July 13, when launch controllers scrubbed the attempted space shot after a liquid hydrogen engine cut-off (ECO)sensor in the external tank failed a standard countdown test. A subsequent engineering investigation found that wiring grounding issues that have since been repaired, NASA officials said.

"Discovery is in excellent shape," said NASA test director Pete Nickolenko during a pre-launch status briefing here at Kennedy Space Center (KSC). "We are all very confident that all of the engine cut-off sensors will work as they're designed to in this next launch attempt."

Shuttle weather officer Kathy Winters said there is only a 43 percent chance that rain and thunderstorms could prevent Discovery's Tuesday launch attempt. While forecasters continue to watch Tropical Storm Franklin, it should not be an issue for the planned space shot, she added.

During Discovery's July 13 launch attempt, the No. 2 liquid hydrogen ECO sensor failed to comply with a standard test launch controllers run to make sure it and three other sensors are working properly. All four sensors are required to function properly under current shuttle flight rules, and serve as an engine cut-off back up to make sure Discovery's three main engines shut down before the external tank runs dry. Should the engines continue to fire without fuel, they could rip apart.

Orbiter engineers suspected that the grounding issue, as well as electromagnetic interference from other hardware on Discovery's launch stack, could lead to the sensor glitch that scrubbed the July 13 launch attempt. Tests to check eight potential sources of that interference, which included new heaters and cameras installed as safety measures after the Columbia accident, all turned up nominal, Nickolenko said.

Shuttle workers are now buttoning up Discovery's aft compartment, home to a sensor electronics box that commanded much attention during the engineering investigation, and are preparing to load the cryogenic reactants that power the orbiter's fuel cells on Sunday.

Other than standard system confidence checks, no additional troubleshooting measures are expected, Nickolenko said.

"It makes us feel good that we can go into another launch attempt," he added.

In addition to fine-tuning three suspect wire grounds, which failed shuttle specifications by just the slightest degree, pad engineers also switched wiring connections between the No. 2 and No. 4 liquid hydrogen ECO sensor to better understand the glitch.

"We think they have a great plan that they're going forward with," STS-114 mission commander Eileen Collins said of the troubleshooting efforts Friday. "We have a lot of confidence in what they're doing."

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