"Steve, happy Fourth of July!" a launch controller told Steve Lindsey, Discovery's commander.
The six U.S. astronauts waved small American flags, and European Space Agency astronaut Thomas Reiter gripped a tiny German banner, as they walked toward the launch pad to enter the shuttle. Astronaut Mike Fossum gave a thumbs up and called out: "Hi mom! I love you!"
Even though the sky was sunny at the Kennedy Space Center, NASA managers were watching the wind speed, slowly approaching the safety limit in the unlikely event of an emergency landing. But controllers remained optimistic for the 2:38 p.m. Eastern Time launch.
"The weather's looking good. Welcome aboard, Lisa!" a launch controller told astronaut Lisa Nowak.
Tuesday's launch was scheduled to be the first 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, damage that caused the shuttle to break apart over Texas on re-entry. The lingering memories of the Columbia disaster heightened concerns for Discovery when the 5-inch crack appeared Monday in the foam insulation on the shuttle's external fuel tank.
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. But after several hours of inspections and meetings, officials decided to continue with the launch as planned.
NASA administrator Michael Griffin said Tuesday that the crack wasn't 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. Photographs taken Monday night by an inspection team, using a tiny camera that can be manipulated into tight spaces, showed no further foam damage in the area.
"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 doesn't 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 at the joints on both solid rocket boosters. NASA considered sending crews out to repair the problem, but determined to go without the fix because the shuttle remained fully functional and the backups weren't expected to be needed.
NASA managers had decided to go ahead with the launch attempt because of three criteria: They were confident that enough foam was still on the bracket to prevent a large piece of ice from forming; they didn't believe the area would 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. 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 didn't 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 Reiter for a six-month stay.