But the seven astronauts arrived Tuesday ahead of the countdown start and hopeful for an on-time liftoff.
During the 15-day mission, the astronauts will deliver a new bathroom, kitchenette, two bedrooms and exercise machine, as well as a water recycling system - and a new resident for the space station. A new astronaut will replace one of the three space station residents.
The plan is to expand the living quarters of the space station so the crew can be doubled to six by next June.
"On the inside of the space station, the walls are largely up," Ferguson said. "... Well, it's moving day. It's time to fill them up."
Ferguson also noted the never-before-attempted repairs that are planned for outside the space station. Three of the crew will take turns going outside to clean and lubricate a clogged joint that is preventing one set of solar wings from turning automatically toward the sun, and they'll replace its bearings.
Six hours after the astronauts' arrival, the countdown clocks began ticking late Tuesday night.
This will be NASA's first shuttle launch since the end of May.
"We haven't had a launch for a while, so we're really excited to be back in the saddle again," said test director Jeff Spaulding.
The threatening cold front was moving across the central part of the nation Tuesday and was expected to bring rain and thick clouds to the launch site by week's end.
Shuttle weather officer Kathy Winters said there was a 60 percent chance of acceptable conditions at the 7:55 p.m. Friday liftoff time and only a 40 percent chance on Saturday.
"The timing of the front will be critical," she said.
NASA has a shuttle launch window until Nov. 25.
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