Low clouds forced NASA to delay the launch of space shuttle Discovery Thursday night, and strong winds could delay another attempt for a day or two.

NASA managers waited until the end of the countdown before deciding to call off the launch scheduled for 9:35 p.m. EST. It would have been the first launch at night in four years.

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"We gave it the best shot and didn't get clear and convincing evidence that the cloud ceiling had cleared for us," launch director Mike Leinbach told Discovery's seven astronauts.

Commander Mark Polansky responded, "Try not to be too disappointed."

Earlier in the day, NASA Administrator Michael Griffin said if the space shuttle did not get off the ground Thursday night, NASA likely would wait until Saturday before trying again. The Friday forecast was even worse than Thursday's, with only a 10 percent chance for launching.

Too many clouds prevent the necessary observation of the shuttle during its ascent, and the shuttle commander needs visibility if an emergency landing is required.

The best opportunity for launching over the next several days was Tuesday, shuttle weather forecaster Matt Timmermann said.

At the beginning of next week, "we see an improving trend," he said. "The winds get lighter and it gets drier."

During the 12-day mission, Discovery's astronauts will rewire the space station, bring up a new 2-ton addition to the space lab and rotate out one of the three crew members at the space station.

NASA had required daylight liftoffs for the three launches after the 2003 Columbia accident to make sure the agency could get good daytime photos of the external fuel tank in case debris fell from it during launch. Foam breaking off the tank and striking Columbia's wing at liftoff caused the damage that led to the disaster that killed seven astronauts.

But NASA officials were comfortable with the acceptable levels of foam loss during the last two liftoffs and believe radar will be able to spot pieces falling from Discovery's tank.

Griffin said he felt no pressure to stick to the launch schedule, despite NASA's desire to go up before Dec. 17 so that Discovery is back on the ground for the new year. Shuttle computers are not designed to make the change from the 365th day of the old year to the first day of the new year while in flight. The space agency has figured out a solution for the New Year's Day problem, but managers are reluctant to try it if they don't have to do so.

If Discovery is still grounded by Dec. 18, NASA may decide to keep trying anyway through Dec. 26.

"We've got days and days, and we're not even worrying about the clock problem," Griffin said. "The clock problem is an annoyance, but it's not a real problem in the sense that we know how to deal with it."

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