NASA has cleared the shuttle Endeavour for a Friday night launch to the international space station.
But the weather may stand in the way.
A cold front that's making its way toward Florida is expected to bring rain and thick clouds by the weekend.
Forecasters are putting the odds of acceptable conditions at 60 percent for a Friday evening liftoff and only 40 percent for Saturday.
The chance of success goes up dramatically on Sunday.
Countdown clocks at the Kennedy Space Center began ticking down on Tuesday toward the planned Friday launch; the liftoff is targeted for 7:55 p.m. EST. At a news conference on Tuesday, managers said the shuttle was in good shape for launch.
NASA had planned to fly a mission last month to service the Hubble Space Telescope but postponed the flight to allow engineers time to assemble components to fix a new problem with one of the observatory's computers.
The final flight to Hubble was retargeted for May 2009. The delay returned NASA's attention to completing construction and outfitting of the space station. Nine more flights are expected before the space shuttles are retired in 2010.
Endeavour's mission, slated to last 15 days, is devoted to preparing the station for a permanent six-member crew — double the current size. Among the new gear: a $250 million water recycling system that will enable the crew to turn urine and other wastewater into drinking water; two new sleeping berths; and perhaps most important, a second toilet.
"Our family in space is growing," said Joe Delai, the NASA manager in charge of Endeavour's cargo. "Putting in that second bathroom becomes important."
The space station crew, which includes newly arrived commander Michael Fincke, has been tidying up in preparation for the shuttle's visit. Fincke beamed down television images of cluttered areas and asked flight controllers for help figuring out where to stash gear left over from previous spacewalks and other activities.
"This is not how we were hoping to find things stowed," Fincke told flight controllers. "A place for everything and everything in its place."
Fincke and flight engineer Yury Lonchakov arrived at the station last month. The third crew member, astronaut Gregory Chamitoff, will be replaced by Endeavour astronaut Sandra Magnus. Chamitoff, who has been aboard the station since June, will return home on the shuttle.
Magnus and her crew mates — commander Chris Ferguson, pilot Eric Boe, Don Pettit and spacewalkers Heidemarie Stefanyshyn-Piper, Stephen Bowen and Shane Kimbrough — arrived at the Florida spaceport on Tuesday to prepare for Friday's planned liftoff.