The US space agency said it was on track to launch the Atlantis shuttle in late August despite lingering risks of foam break-off during the liftoff which worried agency officials during the two post-Columbia shuttle flights by Discovery in 2005 and 2006.

"This mission has been built as the return to assembly operations ... completing the construction of the International Space Station," Wayne Hale, Space Shuttle Program manager, said at a news conference.

National Aeronautics and Space Administration officials sounded more confident after Discovery's near-flawless shuttle mission last month.

"On this next flight, we do expect to have some foam loss from the tank," Hale said.

But he downplayed the risk of this type of debris, which had caused the Columbia shuttle disaster, saying that analysis of data from the Discovery mission had yielded some positive conclusions.

Only tiny pieces of insulation foam had peeled off the external tank during Discovery's launch on July 4 thanks to NASA tank modifications aimed at avoiding another disaster, he said.

In 2003, a 700-gram (1.5-pound) chunk of foam pealed off Columbia's external tank shortly after launch, damaging the shuttle's thermal shield and leading to the disintegration of the craft on reentry of the Earth's atmosphere, killing the seven astronauts aboard.

Discovery's safe return on July 18 after a smooth 13-day mission marked a milestone for the US space program, which has been plagued by continuing problems and safety concerns since the Columbia disaster.

"During the last flight we had what we would call an average number of foam losses from the external tank and we had an average amount of damage to the thermal protection system on board the shuttle. This is in fact good news," Hale said.

"What we are trying to do is to eliminate the outliers, the extremely dangerous conditions that might occur from those situations. We think we have made some great progress."

The Atlantis launch is expected to be the first of regularly scheduled shuttle missions to complete the building of the International Space Station (ISS), the orbiting space laboratory that is a cornerstone of US ambitions to send manned flights to the moon again, and eventually to Mars.

Hale said that NASA would continue to inspect Atlantis's thermal protection system during flight to "make sure that it is safe to reenter."

"We have achieved all our tests' objectives. We feel we are now prepared to go back into an assembly operations mode during the on orbit part of the flight," he said.

The Atlantis mission will be the first to attempt the assembly of a major portion of the ISS since December 2002. The crew will deliver and install a 17.5-tonne structure that will serve as a foundation for European and Japanese laboratories in future missions.

The astronauts also will bring a pair of giant solar antennas, with their batteries and electronic equipment, and install them on the 11-day mission, which includes three spacewalks.

The launch window for Atlantis from the Kennedy Space Center in Florida opens on August 28. It will be Atlantis's first mission in four years and the third shuttle flight since the Columbia disaster in 2003.

The six-astronaut crew will be commanded by Brent Jett and co-piloted by Chris Ferguson. The four mission specialists will be Daniel Burbank, Heide Stefanyshyn-Piper, Joe Tanner and Canadian Steve MacLean of the Canadian Space Agency.

NASA estimates it will take 16 more shuttle flights to finish assembling the ISS; Discovery is scheduled for the next mission, in December.

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