With the countdown entering its final hours and a fuel gauge problem still unexplained, NASA (search) said it is prepared to bend its longstanding safety rules to launch the shuttle Tuesday on the first flight since Columbia's doomed mission 21/2 years ago.

Discovery and a crew of seven were set to blast off for the international space station at 10:39 a.m., after a two-week delay caused by a malfunctioning hydrogen fuel gauge in the spaceship's giant external tank.

Nature, rather than the fuel gauge, could ultimately decide whether Discovery takes off. Forecasters put the odds of good launch weather at 60 percent, with rain and storm clouds both posing threats.

• Watch live coverage of the Discovery countdown and launch beginning at 9 a.m. EDT Tuesday on FOX News Channel, and live streaming video here on FOXNews.com beginning at 9:30 a.m EDT.

NASA had the paperwork ready to go in case the equipment trouble reappeared and the space agency's managers decided to press ahead with the launch with just three of the four fuel gauges working. That would mean deviating from a rule instituted after the 1986 Challenger explosion (search).

"It's an acceptable risk and actually it's quite a low one," NASA Administrator Michael Griffin (search) said in an interview with The Associated Press late Monday afternoon. If the same fuel gauge problem occurs, "we would be good to go," he said. "If we see some other signature than what we saw before, then we're not going. We're absolutely not going."

The fuel gauges are designed to prevent the main engines from running too long or not long enough, in case the fuel tank is leaking or some other major breakdown occurs. An engine shutdown at the wrong time could prove catastrophic, forcing the astronauts to attempt a risky emergency landing overseas, or leading to a ruptured engine.

Griffin noted that multiple failures would have to occur in multiple systems for the worst-case scenario to come true.

Only two gauges, or sensors, are needed to do the job. But ever since NASA's return to space in 1988, the space agency has decreed that all four have to work to proceed with launch.

NASA test director Pete Nickolenko said he did not remember the last time one of these launch rules was waived. But he expressed confidence in NASA's game plan and said the space agency had done everything to understand the fuel gauge problem, which first cropped up during a test in April and resurfaced during the launch attempt July 13.

Over the past few days, NASA rewired two of the sensors to try to diagnose the trouble and repaired faulty electrical grounding aboard Discovery in hopes that would solve it.

"We have addressed everything we know on the shuttle that can go wrong that we have the technology to fix," Griffin told the AP.

But a retired agent in NASA's inspector general office, Joseph Gutheinz, said the space agency does not appear to have learned its lesson with Columbia.

"It is clear to me that NASA continues to put mission over safety," Gutheinz said. "I fear that if NASA is wrong this time, as they were for Apollo 1, Challenger and Columbia, manned space missions may be halted for a very long time in the United States."

Randy Avera, a former NASA engineer who helped develop the shuttle's inspection program, also questioned the space agency's willingness to bend the launch rule. He said it reminds him of the thinking that led to the Challenger accident, which was blamed on a cold-stiffened O-ring seal in a booster rocket and NASA inattention to safety.

Columbia was doomed by a a chunk of foam insulation that broke off the fuel tank at liftoff and damaged the wing. The shuttle disintegrated during its return to Earth on Feb. 1, 2003, killing all seven astronauts. The accident was blamed, in part, on NASA's "broken safety culture," or a tendency to downplay risks.

Some family members of the fallen Columbia astronauts planned to return for launch try No. 2. The VIP list was topped by first lady Laura Bush and Florida Gov. Jeb Bush, her brother-in-law.

Discovery has only until the beginning of August to fly to the space station on a 12-day supply and repair mission; the next launch opportunity will not come until Sept. 9.

The launch window is dictated by the space station's position and NASA's insistence on a daylight liftoff to provide good views for the more than 100 cameras that will be checking for any Columbia-type launch damage.

While in orbit, Discovery's crew will inspect the most vulnerable areas of the spacecraft, using a new 50-foot, laser-tipped boom, and practice repairing samples of deliberately damaged thermal tile and panels.

Griffin said only two other shuttle missions in the history of the program were as significant: the first one in 1981 and the 1988 return to flight after the loss of Challenger.

"If we were to lose another shuttle, I think obviously the shuttle program would be out of business and the United States would be years away from putting another crew of people in space," he said. "The initiation of President Bush's call to return the U.S. to the moon and go to Mars would be delayed. And of course -- of course, of course, of course -- seven people would be killed. So there's a lot riding on this launch."