CAPE CANAVERAL - The countdown is on for the space shuttle Discovery and seven astronauts who, if all goes well, will be NASA's first shuttle astronauts to reach space in since January 2003.

At precisely 6:00 p.m. EDT (2200 GMT) Sunday, NASA began counting the days, minutes and seconds remaining before Discovery's STS-114 mission is expected to launch into space. The spaceflight is currently scheduled to liftoff on July 13 at 3:51 p.m. EDT (1951 GMT) from Launch Pad 39B here at NASA's Kennedy Space Center (KSC) in Cape Canaveral, Florida.

"We're getting excited about the upcoming flight," said STS-114 commander Eileen Collins after arriving here Saturday. "We're anticipating about what it's going to be like getting the shuttle back in space again."

Collins and her STS-114 crewmates are the first shuttle astronauts set to ride a NASA orbiter spaceward since the ill-fated flight of STS-107 aboard Columbia.

Columbia's mission, which launched on Jan. 16, 2003, ended in tragedy when the orbiter broke apart during reentry, leaving no survivors. Wing damage, caused at liftoff by a chunk of loose external tank foam, breached Columbia's protective thermal protection system and allowed hot gases into the orbiter's left wing, destroying the spacecraft.

Discovery's external tank has been modified not only to prevent the type of foam debris that struck Columbia, but also to reduce the formation of ice on the tank's exterior. NASA has added heaters to replace foam insulation on the external tank's bipod fitting - where the chunk that doomed Columbia originated - and along a bellows unit that flexes in response to the tank's supercold propellant.

"We are finally here, we're ready to go," said Sandy Coleman, NASA's external tank project manager, during a press briefing today. "The external tank is certified."

Discovery's STS-114 mission is a 12-day test flight bound for the International Space Station (ISS), where the shuttle crew will deliver a cargo module full of supplies and conduct three spacewalks. The shuttle astronauts will also test a new orbital boom to scan Discovery for damage and test repair techniques for fixing heat-resistant tiles and reinforced carbon carbon panels.

"Sheer excitement is the best way to explain it," Stephanie Stilson, Discovery's vehicle manager told "It's seems like it's a little unreal, I want to pinch myself to see if we really are as close [to launch] as we are now."

While Stilson and her entire vehicle team are excited for Discovery's spaceflight, they will keep a close eye on the orbiter's systems as launch day nears.

"We'll be watching things very closely," Stilson said. "I don't think we've ever had a launch countdown that didn't have something that you had to go in and fix or check out or take a second look at."

Over the next several days, launch officials will take a series of critical steps to prepare Discovery for flight.

On July 11 at about 2:00 p.m. EDT (1800 GMT), pad engineers will begin loading the hypergolic reactants Discovery will use during its spaceflight. The protective rotating service structure covering the orbiter is set to be swung back on July 12 at 7:00 p.m. EDT (2300 GMT). Technicians are expected to begin loading Discovery's external tank with the liquid hydrogen and liquid oxygen propellant at 6:00 a.m. EDT (1200 GMT) on July 13.

"Currently we're tracking no issues at all as far as preparations go," NASA test director Jeff Spaulding said during a morning countdown update.

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