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The space shuttle Atlantis heads for the International Space Station after lifting off Friday evening at the Kennedy Space Center in Cape Canaveral, Fla. ((Chris O'Meara/Associated Press))

After a three-month delay, the space shuttle Atlantis, carrying seven astronauts, launched Friday from Floridaen route to the International Space Station.

The shuttle climbed into a still brightly lit sky at 7:38 p.m.after NASA reported no major technical problems or weather concerns.

The Atlantis shuttle was originally scheduled to launch in March before a freak hailstorm in February riddled the insulating foam of the spacecraft's external fuel tank with pockmarks and forced engineers to spend two months repairing it.

While at the station, astronauts will install a new truss segment, unfurl new solar arrays and fold up an old one during a 12-day mission.

Clayton Anderson will replace fellow U.S. astronaut Suni Williams as a member of the space station's three-person crew.

The fourth day of the mission will be crucial, as the astronauts will use the space station's Canadarm2 to install the 15.9-tonne S3/S4 truss segment, a new part of the station's backbone that also includes a new set of solar arrays.

Another important job will occur on the sixth day of the mission, when the astronauts will attempt to retract an old solar array. During the space shuttle Discovery mission in December, astronauts had to spend an extra few days working to retract the solar array after it became stuck.

The replacement of the old solar arrays with newer ones capable of rotating to face the sun should provide more energy and is an important step to the station's final assembly.

Push before shuttle's retirement

Both NASA and the European Space Agency are pushing to complete as much of the orbiting space station as they can before NASA retires the space shuttle program in 2010. NASA had originally scheduled five space flights this year, but had to cut that projection down to four because of the Atlantis delay.

The space shuttle will also be the first to operate with a new computer upgrade designed to monitor the performance of two critical turbo pumps of the shuttle's main engine and allow the engine to be shut down during launch if vibration levels exceed safe limits.

The Advanced Health Management System first flew aboard the space shuttle Discovery in December 2006 in a test mode that allowed NASA scientists to monitor the engine performance but did not allow for an emergency shutdown.

The system will operate on one of three Atlantis engines and should function on all three engines when the space shuttle Endeavour launches in August, NASA said.

With files from the Associated Press