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

After 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 missions over four years to finish the ISS, which is key to US ambitions of sending humans to Mars.

But safety remains a priority, and NASA will not take any chances, as it will use the same thorough shuttle inspection techniques used in the Discovery missions of 2005 and 2006.

Atlantis will launch from the Kennedy Space Center in Cape Canaveral, Florida, at around 4:30 pm (2030 GMT), carrying a 16-tonne segment with two huge solar panels that will double the station's ability to generate power from sunlight. Its launch window closes on September 13.

"It has been quite a while since we've added on to the mass of the International Space Station," ISS deputy program manager Kirk Shireman told reporters this week.

Three spacewalks are planned during the 11-day mission, which will be followed by another shuttle flight planned for December.

"This is probably the most complicated assembly sequence that has been undertaken, and the record will not last, because the next flight will be more complicated than this one," shuttle program manager Wayne Hale said recently.

The ISS currently weighs 197 tonnes (434,000 pounds) and will mushroom to a massive, 454-tonne (one-million-pound) structure once it is completed.

The station, launched in November 1998, allows scientists to understand the effects of living in zero gravity, but its scientific value has been questioned.

Two leading US newspapers wondered if launching Discovery to the ISS in July was worth the risk three years after the Columbia tragedy.

"The aging shuttles are a diminishing asset, required primarily to finish the space station, which is itself of only marginal scientific value," The New York Times said in an editorial.

But NASA officials insist that the ISS is a crucial part of plans to return astronauts to the moon by 2020 and eventually send humans to Mars.

"Fundamentally, we are building the space station as a permanent toehold off of Earth for our expansion into space," Griffin said in late June.

Griffin warned prior to Discovery's last mission that failure to complete the ISS would hurt US credibility as it attempts to convince its international partners to share its goal of sending humans to Mars.

The United States, Russia, Japan, Canada, Brazil and 11 countries from the European Space Agency are involved in the ISS.

Sunday's launch was unanimously approved by a dozen NASA officials following a two-day review last week, although two of them urged the space agency to redesign foam insulation on the external fuel tank as soon as possible.

NASA has modified several foam sections to limit the amount and size of debris that falls during liftoff since a large chunk pierced Columbia's heat shield, dooming its return home.

Smaller pieces of foam fell off during Discovery's last launch, and NASA officials were pleased with the tank's overall performance.

Griffin said last week that officials did not believe they were putting the astronauts at risk and that the crew could take refuge in the ISS while waiting for a rescue mission should the shuttle suffer irreparable damage.

The Atlantis crew will commanded by Brent Jett. Chris Ferguson will be the co-pilot, and the four mission specialists will be Dan Burbank, Heidemarie Stefanyshyn-Piper, Joe Tanner and Steve MacLean of the Canadian Space Agency.

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