After a clean countdown, US space shuttle Discovery was launched on Tuesday from NASA's Kennedy Space Center in Florida into one of its most complicated construction missions yet.

Less than one hour after the launch, Discovery has slipped into orbit.

Launch Director Mike Leinbach said the launch team at NASA's Kennedy Space Center was able to study a potential problem of ice buildup without jeopardizing the shuttle while still launching on time. "It was one of the cleanest countdowns we've had since I've been launch director," Leinbach said.

During the launch sequence, Commander Pamela Melroy kept her chit-chat with ground controllers to a minimum, saying only that she "copied" the instructions coming from Earth and at one point cheerily saying, "OK, we're going to load and press on."

The flight's mission specialists will be Scott E. Parazynski, Army Col. Douglas H. Wheelock, Stephanie D. Wilson and Paolo A. Nespoli, a European Space Agency astronaut from Italy. Zamka, Wheelock and Nespoli will be making their first spaceflight.

Melroy, 46, is the second woman commander in shuttle history and at the International Space Station, she will meet another female commander, Peggy Whitson, who is in charge of the orbiting three-astronaut crew.

The Discovery is expected to dock at the station on Thursday. STS-120 will be the twenty-third mission to the International Space Station and will deliver the U.S. Node 2 Harmony module expanding the space station's capability for future international laboratories.

Built in Italy for the United States, Harmony is a high-tech hallway and Tinkertoy-like hub. It is a 23- by 14-foot passageway that will connect the U.S. segment of the station to the European and Japanese modules, to be installed later this year and early next year, respectively.

Harmony will be the first new U.S. pressurized component to be added to the station since the Quest Airlock was attached to one of Unity's six berthing ports in 2001.

“STS-120 is such a cool mission,” said Commander Pam Melroy in a statement before the mission. “Node 2 is the expansion of the space station’s capability to bring international laboratories up. It’s the expansion of our capability to carry additional people. "It has additional life support equipment that will allow us to expand out beyond a three-person crew. It’s this big boost in the capability which is really exciting,” she added.

Until now, the space station has developed on a horizontal axis but the Harmony module will allow the station to grow in another axis and the international laboratory to be attached to the station.

The task of installing Harmony will be complicated, since the shuttle must dock at the port where Harmony is to be attached. That means Harmony must first be removed and installed in a temporary spot until the space station crew can move it into place after Discovery's departure.

The European module Columbus is scheduled for attachment to Harmony in December, and NASA officials said that Tuesday's  perfect take-off made the December 6 launch for Atlantis shuttle even more of a certainty.

The two-week mission includes a packed programme of five space walks, with huge construction projects including the addition of the first new room in six years to the ISS and repositioning of the giant solar panels built during recent shuttle missions.

The maneuvers will take about 24 hours of work, will involve robotic arms from both the space station and the shuttle and must be done under time pressure because of the delicate panels which refused to be collapsed earlier this year.

During the fifth spacewalk the astronauts will test a tile patch repair equipment that could be used to fix heat-shield damage that can happen to shuttles during launch.

The so-called T-RAD was due to only be tested next year, but the trial was moved up in light of tile damage discovered during September's Endeavour shuttle mission to the ISS.