Inspectors spotted the crack Monday. A 3-inch piece of foam also popped out of the area, which covers an expandable bracket holding a liquid oxygen feed line against the huge external fuel tank.
After several hours of inspections and meetings, officials decided to continue with the launch as planned at 2:38 p.m. Eastern time. NASA administrator Michael Griffin said Tuesday that the crack was not a major concern and that cameras were trained on the area so if ice built up after the super-cold fuel was added, the launch could be delayed.
"We establish days, weeks, months in advance the criteria that we have to meet in order to be able to launch. If we don't meet them, we stand down," meaning the shuttle does not launch, Griffin told NBC's "Today" show.
"If we get through the list without a problem, we'll see a nice fireworks display," he said.
Another technical problem sprung up Tuesday morning when inspectors discovered they lost backup circuit breakers that control heaters on both solid rocket boosters. Crews were being sent to repair the problem, though NASA said the shuttle remained fully functional and could fly without the fix.
If Discovery gets off the ground Tuesday, it would be the first manned launch by the United States on the nation's birthday, the first launch in almost a year, and only the second launch since the Columbia disaster killed seven astronauts in 2003.
Columbia's wing had been hit by a chunk of flyaway foam during liftoff. That damage caused the shuttle to break apart over Texas on reentry Feb. 1, 2003.
NASA managers decided to go ahead with the attempt Tuesday for three reasons: They are confident enough that the foam is still on the bracket to prevent a large piece of ice from forming; they do not believe the area will be exposed to extreme heat during ascent; and the area of foam where the piece dropped was still intact.
"They fully have shown that the foam is acceptable and ready to fly," said Bill Gerstenmaier, a NASA associate administrator. "There were no dissenters when we went around the room ... no concerns raised."
Early inspections Tuesday were promising, with no ice buildup overnight in the area where the piece fell off. Discovery's was fueling was completed in just under three hours, and the forecast improved to an 80 percent chance weather would be favorable for launch — the best odds so far. NASA had scrubbed launch plans Saturday and Sunday as electrically charged clouds loomed over the area.
The 3-inch triangular piece of foam that dropped from the tank is far smaller than the 1-pound foam chunk that brought down Columbia. Gerstenmaier showed reporters the piece of foam, which looked like a wedge of toast.
NASA has spent millions of dollars trying to prevent foam from breaking off at liftoff. Engineers were startled when it broke off Discovery during last year's mission — the first shuttle flight after the Columbia disaster — but it did not harm the shuttle.
The external tank expanded when the super-cold fuel was drained after Sunday's launch was canceled because of the weather. The ice that formed "pinched" some of that foam, causing the quarter-inch-wide crack and the piece of foam to drop off, officials said.
Griffin decided two weeks ago that the shuttle should go into orbit as planned, despite the concerns of two top agency managers — the top safety officer and chief engineer — who wanted additional repairs to the foam insulation.
But the two agency officials said the foam loss will not threaten the crew because NASA has a plan for the astronauts to move into the international space station if in-orbit inspections find serious damage to the spacecraft. The crew would await rescue 81 days later by a second space shuttle.
The mission for Discovery's crew this time is to test shuttle-inspection techniques, deliver supplies to the international space station and drop off European Space Agency astronaut Thomas Reiter for a six-month stay.