For the first time since the Columbia tragedy, NASA's countdown clocks were ticking down the hours, minutes and seconds to launch, heightening the anticipation for a Wednesday liftoff of Discovery.

The countdown for the first space shuttle (search) flight in 2½ years began Sunday evening as Hurricane Dennis (search) battered the Florida Panhandle off to the northwest. The mission's seven astronauts flew in from Houston on Saturday evening, a day early.

While Cape Canaveral (search) was spared, Dennis still threatened to interfere with NASA's plans, with a week of thunderstorms on the horizon. Forecasters were hoping a ridge of high pressure would provide a break in the weather, however, and offered up fairly decent 70 percent odds for an on-time afternoon launch.

"It sure does feel good to be back in the saddle again. It's been too long," said payload manager Scott Higginbotham.

Test director Jeff Spaulding said excitement had been "building and growing" ever since the space agency overcame fuel-tank difficulties that prompted a launch delay a few months ago.

"It's only recently, I think, that it's all come to fruition where we can see the light at the end of the tunnel," Spaulding said.

"There's some excitement for people to get back to launching again and also, I think, maybe a quiet reserve as well, just remembering where we've been. But we all do feel confident that we've done it right."

Discovery will be making its first flight in four years when it takes off for the international space station with much-needed supplies and replacement parts.

Even before Columbia broke up during re-entry on Feb. 1, 2003, Discovery had been undergoing an extensive overhaul. The catastrophe prompted nearly 50 additional modifications, all of which will be demonstrated for the first time on this 12-day test flight. Techniques for inspecting the shuttle's thermal shield and fixing any holes also will be tested by the crew.

The biggest change, by far, is the redesigned external fuel tank.

Columbia's fuel tank lost a large chunk of foam insulation at liftoff. The debris slammed into the left wing, smashing a hole that proved catastrophic during re-entry. All seven astronauts were killed.

NASA removed the responsible section of foam and installed heaters in its place to prevent ice buildup from the super-chilled fuel. Just 1½ months ago, engineers added a heater in another ice-prone spot on the tank; the work delayed the launch from May to July.

Managers also added extra checks for fuel-tank ice during the final portion of the countdown. Any significant patches of ice — which could be as lethal as flying foam — will mean a launch delay. Engineers considered putting infrared lamps at the pad to melt ice and covering vulnerable brackets with bags, but the ideas were scrapped, at least for now.

"With all the modifications, with all the improvements and changes and upgrades," Spaulding said, "certainly we can, without hesitation, say this will be the safest vehicle that we've ever had to launch."

That doesn't mean the tension level isn't high.

"It's a risky business so we're all sort of apprehensive," astronaut John Phillips said from the space station late last week. But he added, "I am fully confident that we've done what it takes to get this shuttle up here and I'm very anxious to see them come up here."