Atlantis arrives at Kennedy Space Center's launch pad 39B on Aug. 14. Photo: NASA

Engineers are readying the shuttle Atlantis for fueling late tonight and blastoff early Friday on a critical mission to outfit and activate the international space station.

With no technical problems at pad 39B, the only question mark is the weather, with forecasters predicting a 40 percent chance of coastal showers during Atlantis's brief launch window that could delay liftoff. The odds are the same for Saturday if the flight is, in fact, delayed.

Early today, flight controllers at the Johnson Space Center in Houston updated Atlantis's projected launch times for Friday and Saturday.

The shuttle's theoretical 10-minute launch window will open at 8:40:28 a.m. EDT (1240 GMT) and close at 8:50:28 a.m. (1250 GMT). That defines the period when Atlantis can physically reach the station. NASA's preferred launch time, the moment Earth's rotation carries Atlantis into the plane of the space station's orbit, is 8:45:47 a.m. (1246 GMT).

NASA is targeting this "in-plane" launch time for Atlantis, rather than the opening of the theoretical 10-minute window, to minimize propellant consumption and to maximize the odds of reaching the station even if a main engine fails to perform at full power.

The in-plane launch time may change slightly based on final radar tracking of the space station. The actual duration of the launch window will not be known until after Atlantis is fueled for flight. But it is expected to last between four and five minutes.

In any case, engineers plan to begin pumping a half-million gallons of supercold liquid oxygen and hydrogen rocket fuel into Atlantis's external tank shortly before midnight. Fueling should be complete by 3 a.m. or so (0700 GMT).

Commander Terrence Wilcutt, pilot Scott Altman, Edward Lu, Richard Mastracchio, Daniel Burbank and two Russian cosmonauts, Mir-veteran Yuri Malenchenko and physician Boris Morukov are scheduled to make the trip from crew quarters to pad 39B at 5 a.m. (0900 GMT). The shuttle's hatch will be closed for flight by 6:45 a.m. (1045 GMT).

Atlantis' countdown is timed to end at the moment the shuttle's theoretical launch window opens, in this case 8:40:28 a.m. But a final 40-minute hold in the countdown at the T-minus nine-minute mark will be extended to hit the "in-plane" launch target of 8:45:47 a.m.

You can follow all of the countdown's events and the launch in our live Mission Status Center starting tonight.

Atlantis' launch on the 99th shuttle mission follows the successful launch in July of Russia's Zvezda command module, a critical space station component that ran two years behind schedule because of Russian funding problems.

Now that Zvezda is finally in orbit, NASA is poised to launch a backlog of U.S. components as station assembly finally begins in earnest.

An artist's concept of the current make up of the international space station. Photo: NASA

Atlantis' crew is charged with delivering critical supplies to the station and activating life support systems in the new Zvezda module (see Spaceflight Now's STS-106 mission preview for complete details). Another shuttle crew will deliver a set of stabilizing gyroscopes in early October and the lab's first full-time crew is scheduled to blast off Oct. 30 aboard a Russian Soyuz rocket.

Yet another shuttle crew will deliver a huge set of solar arrays in late November and the U.S. laboratory module Destiny - the scientific heart of the space station - is scheduled for delivery in late January.

Atlantis' seven-man crew will "kick off an incredible set of missions over the next year and indeed, if all goes well, for the next five years as we put this million pounds of hardware in orbit," said space station program manager Tommy Holloway.

"By this time next year, our plans would have us with a fully functional space station with adequate power to run our laboratory and keep the home running, which by the way is about the size of a three-room house, bigger than the Skylab in terms of internal volume and bigger than the Mir once we get the lab on board."

Shuttle program manager Ron Dittemore said the NASA team faces its sternest test in years as the agency gears up to launch seven to eight flights a year for the foreseeable future.

"Some time ago I talked to you about one of these days, the dam was going to burst and we would be ready to launch shuttles as fast as we could get them ready and that we would really embark on that journey to put the space station together," Dittemore said. "Well, that time is just about here. There's nothing that we see in our way that keeps us from pressing forward on that journey."

And that journey, Dittemore said, "is going to be a challenging one, it's going to be as complex and difficult as anything we've ever done at NASA, including landing a man on the moon."

Shuttle workers guide Discovery during its rollout from the Orbiter Processing Facility hangar last month. Photo: NASA

NASA and the shuttle prime contractor, United Space Alliance, spent the spring and summer preparing for the upcoming flight surge by hiring new engineers and managers to beef up the workforce, conducting tests to identify weaknesses and implementing plans to shore up areas that needed improvement.

As a result, the next three station assembly missions are generally on track and well ahead of schedule in terms of shuttle processing. But next year, as those shuttles return to Earth and are "turned around" for their next flights, the ground team will be put to the test.

"NASA is going to hire some 500 people to increase the civil servants throughout the agency, by the end of this month, by the end of this fiscal year, and the contractor here in Florida has increased its workforce on the order of 200 and that was completed some months ago," Dittemore said.

"We have spent this summer testing and training ourselves with a lot of that increased workforce in hand to try and understand exactly what our capabilities are," he said. "And we have found in some areas we didn't have enough people, in other areas our skills weren't quite what we needed and we have shored those areas up."

And that, he said, "puts us in an even better posture for processing vehicles in January and the spring of next year. That's where the real benefit will come from. The test has not been this summer, nor will it be this fall because we've had a lot of time to get ready.

"The test will be next spring, after we fly (assembly missions) 3A, 4A and 5A and turn these vehicles around. That's where the real test will be. So we still have some amount of time to get ready. But right now, I think we're in pretty good shape to fly that manifest."