Still perplexed by a fuel-gauge problem, NASA said today it is prepared to bend its safety rules to launch Discovery on Tuesday on the first shuttle flight since the Columbia tragedy 2 1/2 years ago.
Space agency officials said they may approve a waiver or an exception to the rules if the problem crops up again during fueling.
Pete Nickolenko, a NASA test director, said waivers of the launch-safety rules are rare, and he could not immediately recall the last time one was granted. At the same time, he expressed confidence the sensor that has proved so troublesome would work properly.
"Personally, I think that we've done an extensive degree of troubleshooting and analysis ... to best understand what we've got," he said. "We fully expect that it should work as designed."
Engineers still do not fully understand why one of the four hydrogen sensors in the big external fuel tank gave a faulty reading July 13, forcing NASA to scrub the flight as the astronauts were boarding Discovery. The rescheduled launch is set for 10:39 a.m. EDT (1439 GMT) Tuesday.
The fuel sensors are designed to prevent the main engines from shutting down too soon or too late during liftoff in case of a leaky tank or some other major problem. An engine shutdown at the wrong time could be tragic too early, and it could force a risky, never-before-attempted emergency landing; too late, and it could cause the engines to rupture and even destroy the spacecraft.
A NASA launch rule that was put place after the 1986 Challenger disaster requires that all four sensors be working properly, though only two are actually needed.
Randy Avera, a former NASA engineer who helped develop the shuttle's inspection program, said he is troubled by the agency's willingness to bend the rule. He said it reminds him of the thinking that led to the problems that brought down Challenger in 1986 and killed all seven astronauts aboard.
"I'm skeptical," he said. "What we need to hear from NASA is the ... basis from which they make this decision."
Although the focus of NASA's attention has been on the sensor, rain and clouds may end up causing more concern around launch time. Forecasters put the chances of good launch weather Tuesday at 60 percent. The forecast at some of the emergency landing sites also is spotty.
"In general, we'll be watching the weather closely," said Kathy Winters, a shuttle weather officer.
NASA has just one week to launch Discovery and its crew of seven to the international space station before putting off the mission until September. The launch window is dictated by the position of the space station and NASA's insistence on a daylight liftoff in order to photograph any signs of the kind of launch damage that doomed Columbia.
Thousands of spectators, including first lady Laura Bush and dozens of other dignitaries, are expected to come to the Kennedy Space Center to watch the launch.
Columbia and its seven astronauts were brought down by a piece of foam insulation that broke off the fuel tank during liftoff and gashed the wing. The spacecraft disintegrated during re-entry two weeks later, on Feb. 1, 2003.
Workers last week repaired faulty electrical grounding inside Discovery and rewired some of the sensors in hopes that would solve the problem that cropped up during the previous launch attempt. The same type of problem occurred back in April during a fueling test and was written off then as an "unexplained anomaly."
NASA had 14 teams around the country studying the problem. They have eliminated possible explanations one by one but have been unable to arrive at a definitive answer.
But Nickolenko said that after extensive troubleshooting, "I think we're smarter in understanding exactly what we have."
"The teams are ready. We're certainly very eager," he said. "We're looking forward to launch", AP reports.