Thunderstorms forced NASA to call off the launch of space shuttle Discovery on Saturday, a flight already tense over worries that foam could fall from the external fuel tank and endanger the mission.

Another attempt at what would be the first shuttle launch in a year was planned for Sunday, although bad weather was forecast to continue through the Independence Day weekend.

Storm clouds moved in and out of the launch zone throughout the morning and early afternoon, posing lightning threats. As the countdown held at the nine-minute mark, it became clear the weather would not improve, and launch director Mike Leinbach announced a 24-hour delay.

The seven astronauts aboard the fueled spaceship immediately halted their launch procedures. "It wasn't our time today," shuttle commander Steven Lindsey radioed from the cockpit. "We'll launch when we're ready and, hopefully, tomorrow will look better."

The delay was a disappointment for NASA, which last flew the shuttle last July and was eager to get flights to the international space station back on track.

The only technical problem that arose during the countdown was a failed heater for one of Discovery's thrusters, needed to keep the fuel from freezing. Mission managers decided to proceed with the launch, since the thruster was not needed during liftoff, and work around the problem in orbit.

As it has since the Columbia disaster, the overriding concern remained the foam insulation on the external fuel tank.

The space shuttle Discovery sits on the launch pad at the Kennedy Space Center in Cape Canaveral, Florida June 30, 2006. The launch for the crew of seven astronauts is scheduled for July 1. [Reuters]

NASA Administrator Michael Griffin decided to proceed with the 12-day mission despite the concerns of two top agency managers who wanted additional foam repairs.

"We now have a NASA in which senior officials feel free to discuss and debate openly complex, difficult and subtle technical topics that affect the flight," Griffin told The Associated Press on Saturday. "No matter what decision I made, I would have been disappointing somebody."

Bryan O'Connor, the top safety officer, and chief engineer Christopher Scolese recommended at a meeting two weeks ago that the shuttle remain grounded until design changes are made to 34 areas on the fuel tank known as ice-frost ramps. The wedge-shaped pieces of foam insulate brackets on the tank that hold long pressurization lines in place. The intent is to keep ice or frost from forming on the metal brackets once the tank is filled with super-cold fuel.

Griffin noted that the foam is important, "shame on us that we didn't realize it before" the Columbia tragedy. But he stressed that it is hardly the only thing that poses a flight risk.

If foam came off and struck Discovery, causing serious damage, the seven astronauts could move into the space station and await a rescue by shuttle Atlantis. But that would be risky, too, and something NASA would try to avoid if at all possible.

On Saturday, NASA had just five minutes to launch the shuttle because of the need to intercept the international space station in orbit. The next launch attempt was scheduled for 3:26 p.m. EDT Sunday.

Among the space agency's guests at the launch were Vice President Dick Cheney and several members of Congress.

"It's a great program and it's important that we keep going and keep our space program going," said Cheney, who was accompanied by his wife, Lynne, and three of their grandchildren. The family got to see shuttle Atlantis up-close in the hangar.