Countdown to the launch of the space shuttle Discovery moved smoothly toward its final stages Monday, fueling a growing sense of anticipation among NASA personnel on the threshold of the first shuttle mission in nearly 21/2 years.

"It's great to be here, and it's great to be at this point," Bill Parsons, the space shuttle program manager, told reporters at a news conference after a three-hour meeting of NASA engineers. "I think we're on our way."

With no technical glitches arising, the lone concern was whether a mid-afternoon thunderstorm -- almost a meteorological fixture in central Florida in July -- would disrupt the launch scheduled for 3:51 p.m. Eastern time Wednesday.

Air Force First Lt. Mindy Chavez, the launch weather officer, said there was a 30 percent chance of rain Wednesday, but a high pressure zone to the north and a sea breeze coming in from the Atlantic would probably keep the foul weather inland, leaving the Kennedy Space Center with the required 20-mile radius of clear skies.

Discovery, being readied for a 13-day mission to the international space station, is the product of nearly 21/2 years of safety improvements by NASA since the shuttle Columbia disintegrated on reentry, grounding the shuttle fleet.

A chunk of foam insulation from the external fuel tank punched a hole in Columbia's heat shielding during launch, and NASA has concentrated its efforts since then on improving the tank's safety features and preparing contingency plans in the event of damage.

"There are risks in spaceflight," Wayne Hale, deputy shuttle program manager, told reporters, acknowledging that the engineers had a "spirited" discussion yesterday about Discovery's readiness. But "there comes a point when you decide you have reached an acceptable level of risk."

Discovery, mated to two solid-rocket boosters and the 154-foot external fuel tank, is poised at Launch Pad 39B undergoing final checks during a three-day countdown that began Sunday evening.

The seven Discovery crew members, led by mission commander Eileen Collins, were relaxing with friends and family but have been kept in a relatively clean environment since arriving Sunday.

"They don't have much to do," astronaut Richard Mastracchio said in an interview. "You've done your studying, you've passed the final exam and everything is stowed on the vehicle. Now all you want to do is protect the crew from catching a last-minute virus."

NASA is committed to launching during daylight to ensure it can get top-to-bottom imaging of the shuttle during the first stressful 81/2 minutes after liftoff, when the craft generates 7 million pounds of thrust as it flings itself into orbit at 17,500 miles per hour. For the shuttle, the weather concerns reach beyond limiting the imaging capability. Raindrops become dangerous missiles as the shuttle blasts past them at several times the speed of sound.

Planners also worry about crosswinds and thunderstorms that could prevent the shuttle from turning around for an "abort landing" if there is trouble during launch. And at least one emergency landing site downrange in Europe must have clear weather before NASA will allow a launch.

"You worry about all kinds of things," Mastracchio said. "You're watching the weather and waiting. You're just looking for some little hiccup somewhere."