CAPE CANAVERAL, Fla. — NASA Monday began its three-day countdown to the launch of space shuttle Atlantis on an assembly mission to the International Space Station, but stormy weather could delay liftoff by days or even weeks.
The rocket ship was poised on a seaside launch pad with a $164 million airlock stored in its cargo hold as launch engineers went through the preparation rituals for a 3:04 a.m. MDT liftoff on Thursday.
"So far everything is going extremely well with the countdown," National Aeronautics and Space Administration test director Jeff Spaulding said.
But the forecast from the shuttle weather office was causing some concern, mainly for Wednesday's fueling operation rather than Thursday's predawn liftoff. The space agency does not like more than a 10 percent chance of thunderstorms while 1.5 million pounds of highly explosive rocket fuel is loaded into the shuttle's external fuel tank. The current forecast is at 30 percent.
If the shuttle is fueled, there is a 60 percent chance of good weather for liftoff, but that forecast worsens for Friday and Saturday.
Sounding an optimistic note, shuttle commander Steven Lindsey said, "We're looking forward to an on-time launch," shortly after he and his crew arrived at the Kennedy Space Center late Sunday.
Lindsey and four fellow astronauts are to deliver the 6.5-ton airlock to the three-person Expedition Two crew living aboard the station.
The airlock will provide future station crews with the means to leave the orbiting outpost in their spacesuits when a shuttle is not docked to the station.
The $95 billion science complex is a partnership between space agencies in the United States, Russia, Japan, Canada and Europe.
If launch weather proves problematic, the shuttle can make four launch attempts in six days, but by July 18 the space station, orbiting some 250 miles above Earth, will have moved into an orbit that takes it beyond the shuttle's range.
If that were to happen, the Atlantis launch would slip until September. The space shuttle Discovery is already on a nearby launch pad awaiting an Aug. 5 departure for orbit. It is to carry a replacement crew to the station, the third to live there since November 2000.
Joining Lindsey, an Air Force lieutenant colonel, on the Atlantis flight crew are his second-in-command, pilot Charles Hobough, a U.S. Marine Corps major making his first flight, and flight engineer Janet Kavandi, a civilian making her third flight.
With them will be civilian astronauts Michael Gernhardt, making his fourth space flight, and James Reilly, on his second flight. They will make three spacewalks on the 11-day flight, the final one using the airlock they will help install.