NASA postponed the launch of the space shuttle Discovery a second time Sunday because of bad weather.
The space agency said it will try for a launch Tuesday at 2:38 p.m. ET.
NASA had fueled the orbiter Sunday morning, and the crew of six Americans and a German began boarding just after noon.
But even early Sunday morning, NASA reported showers offshore and predicted a 70 percent chance that thunderstorms would prevent a launch.
The possibility of thunderstorms and lightning near the launch pad also postponed the liftoff of the shuttle Saturday afternoon.
"Steve, sorry to break your string. We're not going to make it today," mission control told STS-121 Commander Steve Lindsey on Saturday.
A launch Tuesday would be the first time a shuttle has taken off on the Fourth of July.
No launch attempt is scheduled for Monday because the weather forecast is even worse, and because NASA needs time to top off the fuel cells that provide power while the shuttle is in orbit.
NASA can keep trying until July 19. If the launch hasn't happened by then, the next window for the mission will open in late August.
Those windows of opportunity are determined by the path of the orbiting international space station, the shuttle's destination.
With each passing day, the time for a launch gets earlier by 22-1/2 minutes. That could be good news for NASA because summer thunderstorms are less likely to be a problem earlier in the day.
Plans call for a 12-day mission to deliver supplies to the space station and drop off European Space Agency astronaut Thomas Reiter, who will join the Expedition 13 crew members already there.
Two astronauts, Piers Sellers and Mike Fossum, will conduct two spacewalks, to test a new shuttle robotic arm and to repair a damaged piece of equipment outside the space station.
They might also do a third spacewalk, to test repair techniques on the shuttle's thermal protection system.
NASA's decision to resume shuttle flights this summer is not without controversy.
In the weeks leading up to the launch, two NASA officials, chief engineer Chris Scolese and chief safety officer Bryan O'Connor, gave a "no go" for the launch.
This launch would be only the second shuttle flight since Columbia disintegrated during re-entry in 2003, killing all seven astronauts on board.
In 2003, a 1.6-pound piece of insulating foam broke loose from Columbia's external tank during its launch and fatally damaged the wing, an accident investigation board concluded.
The spacecraft broke up when superheated gases entered the damaged wing upon re-entry into the atmosphere.
NASA spent nearly 30 months redesigning the tank.
Program manager Wayne Hale said NASA engineers have learned a lot about foam dynamics in the past year, but there is no way to stop the foam from flying off the tank.
"Foam will come off. There's no way around that. It is an expected event," said John Chapman, NASA's external tank project manager. "Our objective is to make sure if it does come off, it comes off in small enough pieces that it doesn't cause any harm."
O'Connor and Scolese are specifically concerned about three dozen pieces of foam on Discovery's external fuel tank known as ice-frost ramps.
Small pieces of foam have come off the ice-frost ramps on previous flights. And in a worst-case scenario, small pieces of foam debris could cause critical damage to the orbiter, according to wind tunnel test results conducted by NASA this spring. Even so, NASA officials have put off structural modifications until after the launch.
NASA Administrator Michael Griffin called the disagreements with the repairs a good sign that the culture at NASA has changed. The agency was faulted by the Columbia investigation board with having a conformity of opinion.
"I personally want every engineer to express the best opinion that they can give us," Griffin said.
He and top senior officials took into consideration O'Connor and Scolese's concerns but concluded that if falling foam damages Discovery, engineers will know about it, and the crew can take refuge on the space station and wait for a rescue mission.
Griffin said he wants to fly now because the shuttle program is slated to end in 2010 and NASA is committed to flying at least 16 missions to complete the space station. He said he worries that delays now will lead to dangerous schedule pressure later.
"I very strongly feel that we are not risking crew for foam in this case, or I won't feel comfortable launching," he said.
Lindsey, the commander, said he and his crew understand the risks and are ready to fly.
"We've been training for an awfully long time," Lindsey said. "We're as prepared as we're ever going to be."