NASA: Shuttle launch possible but unlikely Sunday
Days of troubleshooting ahead, manager says
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KENNEDY SPACE CENTER, Florida (CNN) -- The first space shuttle launch since the Columbia disaster could come as early as Sunday but probably will be delayed until later next week, NASA announced Thursday.
Launching Sunday would involve a "really optimistic good luck scenario," said Wayne Hale, deputy shuttle program manager. He said it is more likely that there are several days of troubleshooting ahead.
Just hours before the scheduled liftoff Wednesday, while the shuttle Discovery's crew was strapped in for their journey, controllers scrubbed the highly anticipated launch.
Discovery had been scheduled to begin its ascent at 3:51 p.m. ET.
After the spacecraft's external tank had been loaded, fuel sensors were in the "wet" position, indicating that the tanks were full, said Steve Poulos, NASA's orbiter project manager.
To test the sensors, commands were sent that should have changed their readings to the "dry," or empty, position. But one of the sensors didn't change, indicating a problem, Poulos said.
At that point, controllers decided to scrub the launch.
The sensor that malfunctioned is one of four that monitor hydrogen levels in the external fuel tank.
New testing Wednesday night -- as fuel was being drained from the orbiter's tanks -- showed one failure of the sensor and one case when it performed correctly.
Hale said the mission can't go forward until the problem is fixed because launch protocols require all four sensors to be working.
The redundancy is designed to keep the shuttle's engines from shutting down if more than one sensor fails in flight and mistakenly shows low fuel, he said.
About 10 miles from the launch site along the Banana River, scores of people gathered Wednesday afternoon to watch the shuttle's return to space --only to be disappointed when the launch was scrubbed.
"I am really bummed, but at least for their safety, they caught it before anything bad could happen," said Lacey Nielsen, a spectator from Alta, Iowa. (Full story)
Discovery's seven crew members, who were already on board the orbiter when their mission was postponed, were staying in Florida, rather than returning to their base in Houston, Texas, though they may return home if the delay goes on more than a few days.
A similar sensor problem cropped up during a test of the external fuel tanks in April, which engineers were never able to isolate.
However, Hale said NASA managers were comfortable with going ahead with Wednesday's launch because the tank had been changed, along with all of the wiring and boxes in the sensor system, and all of the components were successfully tested.
Working the problem
The still-unidentified technical problem appears to be somewhere along the path from the sensor in the fuel tank to computers on the orbiter which process the information, Hale said.
Mike Leinbach, NASA's shuttle launch director, said the repairs will likely require going inside the orbiter, which can be done while it is sitting on the launch pad. However, if repair crews have to go into the external fuel tank, Discovery might have to be wheeled back into the vehicle assembly building because it would be "an extremely large job," he said.
Leinbach said that although it would be possible to make repairs inside the fuel tank without moving Discovery from the launch pad, it has never been done before.
"We would have to take a hard look at that," he said, declining to speculate on how long that might delay the launch.
'Minor repair' among mishaps
A series of mishaps marked the last 24 hours before Discovery's scheduled launch.
On Wednesday morning, it appeared foul weather might postpone the high-profile mission. Repairing a ground heater earlier in the morning had delayed filling the massive external fuel tank.
On Tuesday, a cockpit window cover fell off and damaged two protective tiles near the orbiter's tail section.
But it was the fuel sensor that stopped the launch, a little more than three hours before the scheduled 3:51 p.m. ET launch.
NASA has committed to daytime launches for the next two missions to ensure ideal lighting conditions for cameras to scrutinize the shuttle's ascent into orbit.
Shuttle program gets overhaul
This is to be the first shuttle flight since February 1, 2003, when Columbia disintegrated over Texas while returning from orbit, killing all seven astronauts on board.
The loss of the Columbia was blamed on damage to a heat-resistant panel. The panels and insulating tiles make up the shuttle's thermal protection system.
NASA concluded a piece of foam from Columbia's external fuel tank hit the shuttle's wing during liftoff, punching a hole in the reinforced carbon-carbon panel and allowing super-hot gas into the wing during re-entry.
NASA shut down the program and made numerous safety improvements to the shuttle fleet recommended by a blue-ribbon panel that investigated the disaster.
For most of its scheduled mission, designated STS-114, the crew will devote time to inspecting and testing repairs. Discovery also plans to deliver much-needed supplies to the international space station.
CNN's Thom Patterson, Miles O'Brien, Geneen Pipher, Kate Tobin, and Marsha Walton contributed to this report.
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