For the second time in two days, NASA delayed the launch of the space shuttle Discovery due to stormy weather. Rain pelted the Kennedy Space Center, delaying an attempt at the second U.S. spaceflight since the Columbia disaster three years ago.

Only a day earlier, electrically charged clouds forced the space agency to call off Saturday's launch, which would have been the first shuttle flight in a year. The forecast for Sunday's 3:26 p.m. EDT launch wasn't much better, with a 70 percent chance that clouds and storms would prevent liftoff.

The next attempt was set for July 4.

"We knew we were going to get in a race against the weather," said launch director Michael Leinbach. "You can't control the weather, and we have very strict rules."

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Earlier, NASA had gone through with launch preparations, fueling the shuttle's external tank in just under three hours Sunday morning. Preparations Saturday had proceeded to the wire, with astronauts suited up and closed inside the shuttle before officials scrubbed the launch.

The only technical problem that popped up during the countdown Saturday 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 the astronauts could work around the problem in orbit.

NASA Administrator Michael Griffin approved launching the shuttle for the 12-day mission despite the concerns of two top agency managers who wanted additional foam repairs.

Griffin had said earlier that any risk would be only to the shuttle and not the crew, since the astronauts could take refuge in the international space station until a rescue vehicle is sent up.

Bryan O'Connor, the top safety officer, and chief engineer Christopher Scolese recommended at a flight readiness review meeting two weeks ago that the shuttle remain grounded until design changes were made to 34 areas on the fuel tank known as ice-frost ramps. These 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 these metal brackets once the tank is filled with super-cold fuel.

NASA engineers redesigned the external fuel tank after the Columbia accident, and again after a 1-pound piece of foam insulation came off the tank during the launch of Discovery last summer. The last foam loss caused Griffin to ground the shuttle fleet for almost a year while engineers worked on design changes. In the most recent change, more than 35 pounds of foam have been removed in what NASA describes as the biggest aerodynamic change ever made to the shuttle's launch system.

"Last year, I chose to delay the shuttle because I didn't think we were quite ready to go," Griffin told The Associated Press. "This year, I thought the statistics worked in our favor."

Discovery's seven-member crew will test shuttle inspection and repair techniques, bring supplies and equipment to the international space station and deliver the European Space Agency's Thomas Reiter for a six-month stay aboard the orbiting outpost.

Astronauts Piers Sellers and Mike Fossum will make two spacewalks and possibly a third, which would add a day to the mission. The crew also includes commander Steve Lindsey, pilot Mark Kelly and mission specialists Lisa Nowak and Stephanie Wilson.