The hours, minutes and seconds tick away on NASA's countdown clocks towards Wednesday's planned launch of Discovery -- NASA's first flight since the 2003 Columbia tragedy.

"Countdown for launch of Discovery is now under way," said NASA spokesperson George Diller Sunday night as the clocks began marking the march towards blastoff at the Florida spaceport -- set for 3:51 p.m. ET Wednesday.

The seven members of the Discovery crew arrived at the Kennedy Space Center in Florida on Saturday from Houston, a day early.

"It sure does feel good to be back in the saddle again. It's been too long," Discovery's payload manager, Scott Higginbotham, told reporters at a news briefing.

Test director Jeff Spaulding said Discovery is in "excellent shape," after NASA overcame fuel-tank problems that delayed the Discovery's launch a few months ago.

"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," said Spaulding.

Overhaul

An extensive overhaul of Discovery was started even before the break-up of shuttle Columbia over Texas during re-entry on Feb. 1, 2003, which killed all seven astronauts aboard.

The incident prompted nearly 50 additional modifications, all of which will be closely monitored during Discovery's 12-day test flight. The crew will also test techniques for inspecting the shuttle's thermal shield and fixing any holes that may develop.

The biggest change to Discovery is the redesigned external fuel tank. Columbia's fuel tank lost a large chunk of foam insulation during liftoff, which slammed a hole into the left wing that proved catastrophic during re-entry.

NASA removed the section of foam and installed heaters in its place to prevent ice buildup from the super-chilled fuel.

After the tank is filled with over two million litres of cryogenic propellants needed to get the shuttle in orbit, a special team will scrutinize Discovery's tank for ice buildups. Any significant patches of ice -- which could be as destructive as flying foam -- will delay the launch.

Engineers also added a heater in another ice-prone spot on the tank in May.

Canadian technology

While in orbit, a special Canadian-made laser system will be analyzing the shuttle, looking for any damage in the total blackness of space.

The laser, made by Ottawa-based Neptec, will be deployed on a special Canadarm extension manufactured by another Canadian company, MD Robotics. The arm enables engineers to scan even the most hard-to-reach corners of the spacecraft.

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

Weather factor

But even if Discovery passes all the safety standards, there's a chance Mother Nature could end up derailing the launch.

While Tropical Storm Dennis spared Cape Canaveral, there's still a week of thunderstorms in the forecast which could interfere with NASA's planned launch.

But a ridge of high pressure could provide a break in the weather, say forecasters, bumping up the odds for an on-time afternoon launch to 70 per cent from 60 per cent late last week.

NASA will also need clear skies at one of its three U.S. landing sites in Florida, California and New Mexico -- as well as at a transatlantic emergency landing site -- in case Discovery fails to reach orbit.

With files from The Associated Press