Weather conditions appear favorable for Sunday's planned launch of the space shhuttle Atlantis from Cape Canaveral, Florida, a NASA official said Thursday. "Overall, the weather is looking pretty good for launch day," said shuttle weather officer Kathy Winters, though she added: "We have a 30 percent chance of weather prohibiting launch."

If the US space agency has to delay the launch, conditions would be even more favorable for liftoff on Monday or Tuesday, with just a 20 percent chance of inclement weather preventing a launch then, Winters told reporters at Kennedy Space Center in Florida.

The weather is also good at the sites where the shuttle would land in the event of an emergency, including the White Sands Space Harbor in New Mexico; Zaragoza, Spain; Moron, Spain; and Istres, France.

Countdown officially began at noon (1600 GMT) on Thursday ahead of Sunday's planned 4:30 pm (2030 GMT) launch. Sunday is the first day in a 19-day launch window set by the National Aeronautics and Space Administration.

Atlantis is set to blast off Sunday on the first in a series of major missions aimed at completing construction of the International Space Station (ISS) before NASA retires its shuttle fleet in 2010.

The six Atlantis astronauts will carry out the first major construction work on the half-finished ISS in nearly four years, as the February 2003 Columbia shuttle disaster forced a halt of the orbiting laboratory's assembly.

After the shuttle Discovery returned smoothly in July from a second post-Columbia mission focused on improving safety, NASA declared it was ready to start the first of 16 shuttle missions over four years to finish the ISS, which is key to US ambitions of sending humans to Mars.

STS-115 Crew Arrives

The Atlantis crew arrived at NASA's Kennedy Space Center in Florida today to begin final preparations for mission STS-115. Flying T-38 trainer jets, the astronauts landed at the Shuttle Landing Facility at 11:30 a.m. EDT.

Commander Brent Jett introduced the crew and spoke briefly about the upcoming mission to the International Space Station. "Speaking for myself and my fellow crewmates, I can assure you that we are ready for the challenge and we're ready to restart the assembly sequence." He added, "We have a saying back in Texas: 'It's time to walk the walk.' "

Mission Specialist Joe Tanner expressed the crew's appreciation to all the space workers who have worked tirelessly to prepare for the mission, adding, "When we climb aboard Atlantis on Sunday, we do so with confidence that the vehicle is ready, prepared by the best technicians, engineers and managers in the business."

Later today, Jett and Pilot Chris Ferguson will return to the runway to practice landings in the Shuttle Training Aircraft.

The countdown officially began at noon EDT today, at the T-43 hour mark. The countdown includes about 36 hours of built-in hold time prior to a targeted 4:30 p.m. EDT launch on Sunday; it is the middle point in the launch window that extends for 10 minutes.

At the 10 a.m. Countdown Status Briefing, NASA Test Director Steve Payne reported that the launch team is working no issues, saying, "Atlantis is in excellent shape and we're on track for the preparations for Sunday's launch to bring the P3 and P4 truss to the orbiting space station and return her crew safely home."

STS-115 Payload Manager Robbie Ashley expressed satisfaction that the P3/P4 segment would be joined to the space station soon, adding, "it's been a long time coming." He said, "We're anxious to see them get on orbit and perform the job they were intended to do."

Shuttle Weather Officer Kathy Winters provided a generally positive outlook for launch day conditions. "Overall the weather is looking pretty good for launch day. We do have a 30 percent chance of weather prohibiting tanking in the morning," she said. "We have a 30 percent chance of weather prohibiting launch." She added, "Generally, pretty good weather when it comes to an afternoon summertime day here in Florida."

Source: Agence France-Presse