CAPE CANAVERAL, Fla. -- A three-day countdown picked up at Kennedy Space Center Monday as NASA pressed ahead with plans to launch shuttle Atlantis on an International Space Station construction mission.
But seasonal showers and thunderstorms could keep Atlantis on the ground this week, and the mission must be launched by July 17 to avoid a two-month flight delay.
As it stands, Atlantis and a five-member astronaut crew remain scheduled to blast off at 5:04 a.m. EDT (0904 GMT) Thursday on a mission to deliver a $164 million airlock to the station.
Countdown clocks began ticking at 8 a.m. EDT (1200 GMT) Monday and launch preparations were proceeding without any technical problems.
"The vehicle is in excellent shape right now," NASA Test Director Jeff Spaulding said. "And thus far, everything is going extremely well with the countdown."
Launch managers, however, will be keeping close tabs on a tropical weather system that could bring showers, thunderstorms and lightning into the Central Florida area, preventing NASA from fueling the shuttle's 15-story external tank the night before launch.
Forecasters say there is a 30 percent chance that stormy weather might force NASA to cancel a three-hour fuel-loading operation now scheduled to begin around 8 p.m. EDT Wednesday (0000 GMT Thursday).
NASA safety rules call for a fuel-loading operation to be called off if rain or lightning is forecast to come within 34.5 statute miles (55.2 kilometers) of the agency's coastal spaceport.The forecast for launch, meanwhile, is similar. Forecasters say there is a 40 percent chance that coastal rain showers will force NASA to delay launch on Thursday, and the weather is expected to get worse Friday and Saturday.
Shuttle Weather Officer Ed Priselac said there is a 60 percent chance that rain showers or thunderstorms would prevent a launch on either of those days.
Any delay beyond July 17, meanwhile, stands to trigger a launch postponement until Sept. 22 at the earliest.
The reason: Between July 18 and Aug. 3 the station will be flying in an orbit that would expose a docked shuttle to extremely high temperatures that could foul its docking mechanism.
Consequently, if the Atlantis flight is delayed more than six days, NASA managers say they would opt to press ahead first with the planned launch in early August of shuttle Discovery on a mission to ferry a new crew to the station and return to Earth with its current tenants.
In that case, the Discovery mission -- now scheduled for launch Aug. 7 -- would take precedence because the station crewmates Yuri Usachev, Susan Helms and Jim Voss already face an extra month in space.
The additional time in orbit was prompted by a one-month delay in the Atlantis flight, which had been slated for launch in mid-June but was postponed to give engineers time to sort out start-up problems with the station's new robot arm.
The $600 million construction crane is required to install the airlock on the station.
Complicating matters is a scheduled month-long shut down for the Air Force's Eastern Range, a widespread network of ground stations that provide range safety and tracking services for all launches from Florida's Space Coast.
The range shutdown will extend from Aug. 18 to Sept. 21, making Sept. 22 the first available launch date for Atlantis in the event of a lengthy flight delay.
In any case, the Atlantis crew -- which includes shuttle commander Steve Lindsey, pilot Charles Hobaugh and mission specialists Janet Kavandi, Michael Gernhardt and Jim Reilly -- arrived at KSC late Sunday and are finishing up flight preparations.
An on-time launch Thursday would lead to a July 23 landing here at KSC.
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