The shuttle's seven astronauts are on a mission to rewire the international space station, one leg of a three-year race to finish construction on the orbiting outpost before shuttles are retired in 2010.
The illumination from the shuttle turned night into day for the spectators at the Kennedy Space Center on Saturday. A cloudy sky with blustery winds earlier in the day gave way to clear skies and a gentle breeze at launch time.
With the space shuttle facing no technical concerns, the biggest obstacle to launching Discovery on a mission to the international space station at 8:47 p.m. EST was weather, although the forecast improved as the clock ticked down. An improvement in the cloud cover at the Kennedy Space Center raised the chances of lifting off to 40 percent from 30 percent.
Discovery's astronauts ate a preflight meal, suited up in their orange spacesuits and then boarded the astro van for a trip to the launch pad. Astronaut Sunita Williams, who will be living at the space station for six months, made muscle poses for the camera while her crew mates finished putting on the spacesuits.
Later, on the launch pad, looking up at the 184-foot-long rocketship, the astronauts hugged, shook each others hands and gave thumbs up.
Before boarding the shuttle, Williams, who grew up outside Boston, home of the Boston Red Sox and New England Patriots, held up a sign that read, "Go Red Sox. Go Pats. Go for launch."
There were still concerns about crosswinds at an emergency landing site and isolated showers.
Forecasters continue to predict crosswinds up to 18 knots – 3 knots over NASA's safety limit – at the shuttle's emergency runway, reports CBS News space consultant Bill Harwood. While the limit can be extended to 17 knots based on a recommendation from NASA's chief astronaut, the winds need to die down a bit for a launch tonight.
Low clouds block views of the space shuttle during launch and make an emergency landing more difficult.
Low clouds forced the space agency to scrub an attempt Thursday night during a countdown that ran down to the wire. Managers decided not to try again Friday because the forecast looked even worse.
Unusually cold weather at the Kennedy Space Center caused delays in removing fuel from the shuttle's external tank from Thursday night's attempt, which then caused a two-hour postponement in starting fueling for Saturday's try.
There was enough flexibility in the schedule to make the launch time.
"We are running a pretty tight line right now," said NASA launch commentator Bruce Buckingham. "We can catch up with a lot of activities over the next few hours."
The chances for good weather was 40 percent Sunday and Monday. The best opportunity over the next several days is Tuesday, with a 60 percent chance of decent weather.
During a 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 station crew members.
NASA wants Discovery back from its 12-day mission by New Year's Eve because 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 agency has developed a fix, but would prefer not to try it.
NASA had required daylight launches for the three flights 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. 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 spot pieces falling from Discovery's tank.