KENNEDY SPACE CENTER, FL--Countdown clocks began ticking Tuesday for the shuttle Atlantis' launch Friday on NASA's 135th and final shuttle mission, a flight to deliver more than 8,000 pounds of supplies and equipment to the International Space Station.

Credit: NASA

There are no technical problems of any significance at pad 39A, but forecasters are predicting a 60 percent chance of showers and possible thunderstorms that could trigger a delay.

"I wish I had a better weather briefing for you, but it does look like we are going to have some weather, at least potential for weather, in the area at launch time," said Kathy Winters, the shuttle weather officer at the Kennedy Space Center. "Right now, we're going with a 60 percent chance of KSC weather prohibiting launch due to the potential for showers and isolated thunderstorms in the area."

Launch is targeted for 11:26:46 a.m. EDT (GMT-4) Friday, the middle of a 10-minute window centered on the moment Earth's rotation carries the pad into the plane of the space station's orbit. Winters said conditions should improve to 60 percent acceptable on Saturday and to 70 percent "go" on Sunday. If Atlantis isn't off the pad by then, the flight will be delayed to July 16 to make way for launch of a military navigation satellite from the nearby Cape Canaveral Air Force Station.

"The next few days we're going to have some good weather, then an easterly wave comes into the area on Thursday, bringing a lot of shower activity in with it," Winters said. "Usually with easterly waves, we get a lot of showers, not necessarily a lot of thunderstorm activity, but some isolated activity. As it pushes in, it's going to really moisten up our atmosphere.

"So by Friday, we do expect the sea breeze to be developing around the time of the launch window and showers and even potentially an isolated storm to develop along that sea breeze as it forms. So our primary concern for launch will be showers and thunderstorms within 20 nautical miles of the shuttle landing facility ... and also cumulus cloud development within 10 nautical miles of the launch pad."

Weather aside, NASA Test Director Jeremy Graeber said Brevard County officials are expecting between 500,000 and 750,000 spectators Friday, a crowd that could have an impact on NASA's launch plans.

NASA managers have said the expected crowds could make it difficult for launch personnel to get home and then make it back to the spaceport in time for a second launch attempt Saturday. If the expected crowds actually materialize, and if Friday's launch attempt is called off within four hours of the planned liftoff, NASA managers may opt to delay a second attempt to Sunday.

"That launch-minus-four (-hour) timeframe is our decision point," Graeber said. "Again, it really is all dependent on what we see that day and the types of issues that come up, how the weather is developing and again, we'll talk will all the right folks and make that decision on launch day."

Countdown clocks began ticking backward at 1 p.m. Tuesday, setting the stage for NASA's last shuttle launch. Early Wednesday, engineers will pump liquid oxygen and hydrogen aboard Atlantis to power the ship's three electricity producing fuel cells, a remotely controlled procedure that should be completed by around 3:30 p.m.

Engineers then will move into detailed checks of the shuttle's three hydrogen-fueled main engines, the ship's communications gear and other critical systems. If all goes well, a protective gantry will be pulled away from the shuttle at 2 p.m. Thursday, exposing Atlantis to view and setting the stage for fueling early Friday.

Working by remote control, engineers plan to begin pumping a half-million gallons of supercold liquid oxygen and hydrogen into the ship's external tank starting at 2:01 a.m. Friday, a three-hour procedure that should be complete by 5:01 a.m.

Hoping for the best, Atlantis' crew -- commander Christopher Ferguson, pilot Douglas Hurley, Sandra Magnus and flight engineer Rex Walheim -- plans to begin strapping in around 8:06 a.m. to await liftoff.

"Our teams here at the Kennedy Space Center and all the NASA centers across the country have been working for over a year to prepare Atlantis, the external tank our solid rocket boosters, the payload and all of our ground systems for the STS-135 mission," Graeber said. "All of our vehicle and ground systems are ready, the STS-135 crew, Atlantis and the launch team are all ready to proceed and we're looking forward to ... a spectacular launch on Friday morning."

Additional coverage for subscribers:
VIDEO: COUNTDOWN PREVIEW AND WEATHER FORECAST BRIEFING PLAY
VIDEO: ASTRONAUTS ARRIVE AT LAUNCH SITE PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH CHRIS FERGUSON PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH DOUG HURLEY PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH SANDY MAGNUS PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH REX WALHEIM PLAY

VIDEO: SHUTTLE AND STATION PROGRAM BRIEFING PLAY
VIDEO: THE STS-135 MISSION OVERVIEW PRESENTATIONS PLAY
VIDEO: PREVIEW BRIEFING ON MISSION'S SPACEWALK PLAY
VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

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