CAPE CANAVERAL, Fla. -- Stymied once this week because of clouds, NASA is skipping a Friday launch attempt for space shuttle Discovery because of a dismal forecast and is looking toward the weekend.

Though the weather this weekend is predicted to be only slightly better, NASA is aiming for an 8:47 p.m. EST Saturday launch attempt.

Discovery couldn't lift off Thursday night because of thick, low clouds above the launch pad, which would have violated a NASA rule that requires clouds be high enough so engineers can track the shuttle's ascent. Each launch scrub costs NASA $500,000.

With only a 30 percent chance of acceptable weather Saturday, NASA managers late Thursday discussed the possibility of waiting until Sunday to try to launch, but opted to stay with Saturday, spokeswoman Jessica Rye said. Officials wanted "a little more flexibility in their options," she said.

The odds for good weather improve to 40 percent on Sunday and Monday. NASA gets the best opportunity for launching over the next several days on Tuesday, with a 60 percent chance of decent weather.

The space agency can make two launch attempts in a row before standing down a third day because of the need to refresh the fuel supply, Rye said.

While Thursday night's last-minute decision not to launch was a nail-biting tease, Friday's forecast proved anything but a close call with winds, rain and only a 10 percent chance of clear skies.

"It didn't look (like) it's a wise decision to even try it" on Friday, NASA chief spokesman David Mould told The Associated Press.

But the agency was hopeful about Thursday's planned launch. 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.

Less than an hour earlier, the skies had appeared to clear enough before the clouds built back up.

"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.

After more than four hours on the launch pad, the astronauts remained fairly upbeat. Sunita "Sunny" Williams, who mugged for the cameras on the way into Discovery, waved hello at the cameras on the way out of the shuttle.

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 move Williams into the station.

NASA Administrator Michael 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 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."