In this image made from NASA TV, German Astronaut Thomas Reiter exits the Space Shuttle Discovery at the Kennedy Space Center in Cape Canaveral

The space shuttle Discovery was set to lift off (at 1949 UTC) Saturday from Cape Canaveral, Florida, but thunderstorms moved in, causing the launch to be delayed until another try Sunday. NASA is launching Discovery on a 12-day mission to the International Space Station, despite continuing safety concerns.

Discovery's seven crew members, dressed in bright orange spacesuits, waved and gave a cheerful thumbs-up-sign as they headed to the launch pad. The astronauts were then strapped into the space shuttle, which was fueled and ready for take-off. But storm clouds lingered in the launch zone, prompting NASA officials to stop the countdown just minutes before the scheduled launch.

Launch Director Mike Leinbach announced the delay. "We're not going to make it today. So I appreciate your support, both from the crew and the whole launch team and the team worldwide for trying to get this vehicle off the ground today, but it's not a good day to launch the shuttle, so we're going to try again tomorrow," he said.

The delay was a disappointment for the U.S. space agency, which last launched the shuttle in July 2005, and is eager to resume flights to the International Space Station.

Vice President Dick Cheney and several members of Congress were at the Kennedy Space Center as guests to watch the launch.

Astronaut Steven Lindsey commands a crew of five American astronauts, and will bring German astronaut, Thomas Reiter, to expand the space station crew to three. The team will also deliver supplies to keep the space station running, and two crewmen will practice inspection and repair techniques during a spacewalk.

Discovery's mission will be only the second launch since the space shuttle Colombia disintegrated on reentry to the earth's atmosphere three years ago, killing all seven astronauts on board. The spacecraft was damaged on lift-off when a piece of foam insulation broke off the external fuel tank and hit the shuttle's left wing.

Since then, the U.S. space agency, NASA has spent hundreds of millions of dollars to modify the fuel tank to prevent a similar accident from happening again.

NASA Administrator Michael Griffin decided to go ahead with the mission, despite the safety concerns of two top agency managers, who said they believe foam debris could still be a threat. The two dissenting officials say they accept the decision to launch because expected foam loss would not endanger the crew, only the orbiter.