National Aeronautics and Space Administration officials said the 11-day mission to boost power on the ISS entails the most complex work ever undertaken at the nearly eight-year-old, half-finished orbiting laboratory.

Completing the space station is central to US ambitions to fly humans to Mars.

Atlantis, carrying a six-member crew, lifted off from its seaside launch pad on time, at 11:15 am (1515 GMT).

"What you saw today is a flawless count, a majestic launch. This vehicle has not flown since 2001 and not everything in the count leading up to this day was easy," said NASA administrator Michael Griffin.

Atlantis's two rocket boosters successfully separated from the orbiter two minutes after the liftoff from the Kennedy Space Center in Cape Canaveral, Florida, and the shuttle later jettisoned its massive external fuel tank.

After settling into orbit 225 kilometers (140 miles) above Earth, the shuttle fired two small engines to power it to a 350-kilometer- (218 mile-) high orbit and the ISS, where it is expected to dock Monday at 6:46 am (1046 GMT).

Kyle Herring, the NASA spokesman for the flight, announced all systems were functioning normally after launch.

Five hours after liftoff, Wayne Hale, NASA space shuttle program manager, said that early HDTV footage showed several incidents of debris -- apparently foam and ice -- breaking off and possibly impacting the orbiter.

No damage to the spacecraft was visible, he said.

"We are looking at nits, nothing of any remote consequence," Hale said.

Hale noted the assessment was preliminary and a review of the data gathered by three radars, cameras, and a sensor on the orbiter would take about three to four days.

Foam shed during launch caused the 2003 space shuttle Columbia disaster. Columbia disintegrated upon re-entry into the Earth's atmosphere because a piece of loose foam insulation had pierced the shuttle's protective heat shield during take-off.

The two shuttle flights conducted since then were focused on correcting the problem and improving flight safety. After Discovery returned in July from its latest mission, NASA declared it was ready to resume construction of the ISS.

Atlantis was supposed to launch on August 27, but the mission was postponed five times due to a lightning strike, a looming tropical storm and technical glitches.

The Atlantis crew will begin their first full day in space at 1:15 am (0515 GMT) Sunday with a wake-up call from the Mission Control Center in Houston, Texas.

Their activities Sunday will focus on inspections of the shuttle's heat shield and preparations for its arrival at the International Space Station.

Atlantis is transporting a 16-tonne segment with two huge solar panels that will double the space station's ability to produce power from sunlight and ultimately provide a quarter of the power for the completed ISS.

Three spacewalks are planned for the construction work.

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

NASA plans 15 more shuttle trips to complete the orbiting laboratory by 2010, when the three-shuttle fleet is to be retired.

The last ISS construction mission was in November 2002.

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