CAPE CANAVERAL, Fla. -- The most ambitious space construction mission of all time is about to unfold in low Earth orbit, and for NASA, the stakes are sky-high.

With the agencys long-stalled International Space Station (ISS) project finally back on track, shuttle Discovery and seven astronauts are set to blast off Thursday on an outpost assembly mission that is unprecedented in its degree of difficulty.

During four consecutive days of dangerous spacewalking construction work, the astronauts are to mount the first piece of the stations girder-like backbone, add a new shuttle docking port and deploy the outposts main communications antenna.

Shuttle Discovery is moved to the Vehicle Assembly Building in August to be mated with an external tank and set of solid rocket boosters. Photo by Todd Halvorson.

And if something goes badly amiss, an impending six-year string of highly intertwined assembly flights suddenly could be derailed, bringing NASAs cornerstone project for the 21st century to yet another screeching halt.

"I cannot underestimate the importance of this mission," Discovery astronaut Pam Melroy, who will become only the third woman to pilot a shuttle, said in an interview with SPACE.com.

"If we screw it up," said mission specialist Leroy Chiao, "we screw it up not only for ourselves, but we screw it up for everyone else through the rest of the assembly sequence."

Added crewmate Bill McArthur: "Its like that long chain -- you break one link and youre in trouble."

Mission Discovery

Look here for the latest news from NASA's STS-92.

Discovery and its crew -- which includes six American astronauts and a Japanese mission specialist -- are scheduled to lift off on NASAs historic 100th shuttle flight at 9:38 p.m. Eastern Daylight Time Thursday (Friday, 01:38 GMT).

Coming just two weeks after NASAs last station visit, the mission represents an encore to the outfitting of new Russian-made crew quarters that finally showed up at the station in late July after a show-stopping two-year delay.

The long-awaited flight also will herald the arrival in early November of the stations first full-time tenants and set the stage for a daunting flood of seven more assembly missions that NASA plans to launch on shuttles during the coming 12 months.

"Its kind of a jump-start at the beginning of a new push -- hopefully a rally for station assembly," said Chiao. "And were looking forward to starting the new ripple."

Discoverys launch from Kennedy Space Center -- which will be broadcast live on SPACE.com -- is being precisely timed to put the shuttle crew on course for a docking at the 67-ton station around 4:30 p.m. EDT (20:30 GMT) Saturday.

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Ambitious schedule

A six-day flurry of construction work then will begin in earnest just before noon EDT (16:00 GMT) next Sunday.

The two most critical chores:

• Securing NASAs so-called Z-1 truss to the top of an American docking module dubbed "Unity," one of three permanent wings that now make up the international station.

  Weighing in at 18,400 pounds (8,280 kilograms), the cube-shaped truss is the first of 10 skeletal vertebras that eventually will connect together to form the stations 356-foot (108-meter) metal spine.

  Housed within it: Four stabilizing gyroscopes and a dish-shaped communications antenna.

  The domed-shaped gyroscopes will reduce the amount fuel needed to keep the station properly positioned in orbit and the antenna will be used to beam television and voice communications between the outpost and ground controllers.

• Attaching a conical docking adapter to the bottom of the Unity module.

  Eight feet (2.5 meters) in length, the tunnel-like adapter will serve as the primary shuttle docking port -- and a pressurized gateway into the station -- for future outpost construction crews.

Now nestled in Discoverys expansive cargo bay, the two new station additions are to be put in place by Koichi Wakata, the first Japanese astronaut to take part in station construction. Japan is part of the international partnership building and paying for the ISS.

To do the job, Wakata will have to operate the shuttles 50-foot (15-meter) robot arm as if it is a construction crane, plucking the hefty pieces from cargo-bay carriers and then maneuvering them to their attach points atop the station.

The failure to do so, meanwhile, would bring NASAs intricate station assembly plan to an abrupt standstill.

The Z-1 truss is meant to serve as a temporary mounting platform for a pair of power-producing solar arrays to be launched aboard shuttle Endeavour on November 30.

Whats more, Endeavour wont have a place to park unless Wakata can successfully attach the new docking port to the outpost.

Parking at the stations current shuttle docking port wont be an option because Endeavours tail then would block installation of the two massive solar wings, which will stretch 240 feet (73 meters) from tip to tip once unfurled in orbit.

"I think what Koichi is going to do with the robot arm [represents] the single-most difficult tasks on the flight," said Discovery commander Brian Duffy. "Were really asking a lot of him."

Thats not to say, however, that the construction work outside the station will be any easier.

Four back-to-back spacewalks -- each lasting about 6.5 hours -- will be staged on consecutive days to wire up the Z-1 truss and the new docking port. In addition, the communications dish will be deployed at the end of a long boom, electric power converters will be installed on the station and toolboxes will be stowed outside the outpost for future construction crews.

The spacewalks will be carried out by a quartet of astronauts working in two-man teams.

Chiao and McArthur will perform the first and third spacewalks. Mission specialists Jeff Wisoff and Michael Lopez-Alegria take on the second and fourth forays.

The highlight of the mission likely will come on the last jaunt when Wisoff and Lopez-Alegria test-fly "Buck Rogers" jet backpacks that would enable astronauts to rescue themselves during spacewalking emergencies.

Powered by a cluster of small nitrogen gas thrusters, the $7 million jetpacks give spacewalkers a fighting chance to fly back to a shuttle or the station should their braided steel safety tethers snap, casting them adrift in the deadly vacuum of space.

A second emergency drill also will be staged to see if a spacewalking construction worker could carry an unconscious partner back to the shuttle for medical attention if necessary.

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Playing dead

"We call it the 'dead-guy test,'" said Daryl Schuck, an engineer in NASAs spacewalk projects office at Johnson Space Center in Houston.

Wisoff and Lopez-Alegria will take turns "playing dead," or acting as if they are suffering from an orbital version of "the bends" -- the type of decompression sickness that can make scuba diving deadly.

The idea then is to haul the "incapacitated" spacewalker back to the shuttles airlock just to make sure it could be done in a real emergency.

"You can have the same type of decompression sickness on orbit doing a spacewalk, and if we ever had a crew member have that happen to them while theyre outside, they might go unconscious," said Wisoff.

"So we want to make sure its a reasonable expectation that one crew member can drag the other person into the airlock so we can get him some medical attention if necessary."

Once the spacewalks are done, Discoverys crew will enter the space station to deliver supplies and finish up some electrical connections to the Z-1 truss. And as it stands now, the shuttle is scheduled to depart the outpost October 14 for a planned October 16 landing at KSC.

Given such a work load, NASA officials expect the mission to rival a Hubble Space Telescope repair flight in sheer complexity. But at the same time, after an extraordinary three and a half years of training, Discoverys astronauts say they are up to the challenge.

"Were looking forward to just a very rich, varied mission. Its just got anything you could ever want to do in space, and if youre a real space cadet, its a dream come true," McArthur said.

Added Lopez-Alegria: "If youre in the space business, it doesnt get any better than this."