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NASA may fly shuttle even if sensor problem returns

By Thom Patterson
CNN

Programming Note: Miles O'Brien anchors from Kennedy Space Center on the shuttle's return to space. CNN, Tuesday, 10 a.m. ET.

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The space shuttle Discovery sits on the launch pad Monday at Kennedy Space Center.

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KENNEDY SPACE CENTER, Florida (CNN) -- NASA officials said Monday they might go ahead with Tuesday's launch of the space shuttle Discovery even if a fuel sensor problem recurs that scrubbed the flight nearly two weeks ago.

Discovery is set to launch from Kennedy Space Center at 10:39 a.m. ET Tuesday. It will be the first shuttle flight since the Columbia disaster in February 2003.

NASA scrubbed Discovery's launch July 13 just 2 1/2 hours before liftoff when a pre-launch test showed one of Discovery's four sensors in its hydrogen fuel tank was not working.

On Monday, after 12 days of troubleshooting, officials said they were again ready for liftoff but still weren't sure what caused the problem.

"We don't completely [know] because it looks like a grounding issue," said Discovery vehicle manager Scott Thurston.

He said stray voltage characteristics are difficult to predict: "That's like trying to predict lightning -- you know it's going to strike, you just don't know where."

Some of the work the engineers have done -- including redoing the grounding -- "should help with the issue, if that was the issue," Thurston said.

"But if we see anything other than what we're expecting, then we'll call it off."

Thurston said if a sensor problem arises that appears to be the same as the previous one, NASA could launch Discovery with only three working sensors.

To do that, however, officials would have to waive safety rules known as launch commit criteria, which require all four sensors operational, although only two are needed to fly.

Thurston suggested NASA might consider waiving the rules if engineers felt the problem was a sensor failure and not something more critical.

"We'd know we'd have the redundancy of the other three systems," Thurston said.

"When we fly with three out of four sensors working, all we're doing is going back to the original launch commit criteria that this design was based on," NASA Administrator Michael Griffin told CNN.

"We're two-failure tolerant," he said.

That disappoints former NASA shuttle engineer Randy Avera, a 14-year veteran of the orbiter program who participated in the investigation into the explosion that destroyed the shuttle Challenger in 1986.

"It disturbs me that in the first countdown attempt, a problem on the vehicle -- where it is known but not identified what the problem is -- that so quickly discussions about waiving the launch commit criteria surface," Avera said.

If a waiver is issued, and NASA launches Tuesday with the same indication of a problem it had last week, Avera wants NASA to release the documentation that details why it issued the waiver.

"What I'm looking for is NASA to come forward with the actual basis for that waiver," Avera said. "And that basis should be linked directly to design requirements and operational requirements."

Weather delay possible

As usual, the region's unpredictable weather also concerned flight controllers, with a 60 percent chance of favorable conditions predicted at launch time.

NASA's weather team is watching Tropical Storm Franklin in the Atlantic, although it's not expected to threaten the launch.

An array of cameras was positioned to videotape the launch, and cloud cover during Discovery's 10-minute launch window could prompt officials to scrub the liftoff.

Should Tuesday's planned launch be scrubbed, mission goals would require the spacecraft to fly by July 31 or wait until at least September 9.

The cameras are among many new safety measures implemented after the Columbia disaster.

Columbia disintegrated on re-entry when super-heated gases entered the spacecraft through a hole created during liftoff by a falling piece of insulation foam, according to an investigative panel. All seven crew members died.

Discovery's crew is scheduled to test a battery of tools and techniques that NASA engineers developed after the loss of Columbia to inspect the spacecraft's heat-resistant exterior tiles for any damage that might occur during liftoff.

Crew members are expected to board the spacecraft about 7 a.m. ET Tuesday.

First lady Laura Bush and several members of Congress are expected to watch Tuesday's launch from a VIP observation area.

The mission is scheduled to last 12 days. Besides safety issues, the focus will be on resupplying the international space station.

"We have had a great many challenges to get ready for a return to flight," NASA test director Jeff Spaulding said. "That is the cost of the safest shuttle to date.

"Our ground systems are ready; our flight crew is ready."

Asked to describe the mood of the team after the scrub July 13, Spaulding said members are "focused, excited, they are ready to go."

CNN's Marsha Walton contributed to this report.

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