Cape Canaveral: Countdown clocks at the Kennedy Space Center began ticking on Tuesday toward the final flight in the 30-year-old US space shuttle program, a cargo run to the International Space Station.
Liftoff of shuttle Atlantis carrying astronauts is set for 11:26 am EDT (1526 GMT) on Friday, though meteorologists are concerned about the weather.
An approaching front is expected to cloud central Florida's skies beginning on Thursday, stirring up thunderstorms right around Friday's launch time.
"I wish I had a better weather briefing for you," Air Force meteorologist Kathy Winters told reporters on Tuesday.
Overall, the chance of an on-time liftoff is 40 percent, Winters said.
Conditions improve for Atlantis launch opportunities on Saturday and Sunday. An unmanned Delta rocket launch from the nearby Cape Canaveral Air Force Station, which uses the same launch support personnel and equipment, would bump the next flight opportunity for Atlantis after Sunday to July 16.
Whenever Atlantis is launched, it will be the last launch from Kennedy Space Center for a while.
NASA is in the midst of remodeling one of the shuttle's two launch pads for a variety of commercial uses. The agency hopes to use the second shuttle launch pad for a future heavy-lift rocket, capable of sending astronauts and cargo to destinations beyond the space station's 220-mile-high (354 km) orbit.
But the shuttle program is ending with no firm details of NASA's human space ventures beyond the station, which will be serviced by a mix of commercial and partner countries' launch vehicles.
For now, the Kennedy Space Center launch team has one goal in mind: getting Atlantis safely into orbit.
"The team gets into the mode of 'this is launch countdown' and that's really the focus that everybody has," said NASA official Jeremy Graeber. "To do it one more time is a great feeling."