Cape Canavarel, Fla. ? Low clouds forced NASA to delay the launch of space shuttle Discovery late Thursday, and a forecast for strong winds prompted the space agency to postpone another attempt by at least two days.

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

“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.

Shuttle commander Mark Polansky responded, “Try not to be too disappointed.”

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

A new launch attempt was set for 8:47 p.m. Saturday, although weather forecasters gave the new time only a 30 percent chance of acceptable weather because of expected strong winds at the launch pad.

The space shuttle Discovery sits ready at the Kennedy Space Center in Cape Canaveral, Fla. Cloudy weather caused NASA to scrap its planned Thursday night launch.

Each launch scrub costs NASA $500,000.

Earlier, 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. Today’s forecast was even worse than Thursday’s, with only a 10 percent chance of decent weather.

The best opportunity for launching over the next several days is 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.