The 16-day mission to the International Space Station will be the second to last shuttle flight for the US program, which will end for good later this year after the final space mission by Atlantis.

"The trouble shooting done over the last two weeks is now complete," NASA shuttle test director Jeff Spaulding said Friday, describing extensive tests done to assure that the power problem was fixed.

"All the retests are complete, it's good and we are ready to go from that perspective," he said.

The initial April 29 launch attempt was scrubbed hours before liftoff when technicians discovered a power failure in a heating line that served to prevent fuel from freezing in orbit.

Countdown to Monday's launch at 8:56 am (1256 GMT) officially began on Friday at 7:00 am (1100 GMT) after the six-member crew of astronauts including five Americans and one Italian arrived at Kennedy Space Center on Thursday.

Weather conditions are expected to be 70 percent favorable for launch, according to NASA weather officer Kathy Winters, who said that if a 24-hour postponement is needed, the likelihood of good weather would slip to 60 percent.

The final rollback of the massive service structure around the shuttle is set for midday Sunday, marking the last major movement on launchpad 39A before the spacecraft is loaded with crew for take off.

Fueling of the external fuel tank is set to begin at 11:36 pm Sunday night (0336 GMT Monday) in anticipation of the early morning launch.

The key mission of Endeavour's trip to the orbiting research lab is to deliver a massive physics experiment, the Alpha Magnetic Spectrometer-2, which will be left behind to scour the universe for hints of dark matter and antimatter.

US congresswoman Gabrielle Giffords of Arizona, the wife of shuttle commander Mark Kelly, will watch the launch from Kennedy Space Center, her office said.

Giffords, who was allowed by her rehab doctors in Houston to fly to Florida to watch the planned April 29 launch, was shot in the head in January during a meeting with local voters.

The bullet tore through the left side of her brain, and she has been undergoing grueling rehabilitation to regain speech and movement on her right side.

Endeavour's delay has pushed back Atlantis's planned liftoff from June 28 to mid-July, but no final date has been set.

After the final shuttle missions, the three spacecraft in the flying fleet and the prototype Enterprise will be sent to different museums across the country.

Discovery was the first shuttle to retire after its last journey to the ISS ended in March.

Once the shuttle program formally ends, the world's astronauts will rely on Russia's space capsules for transit to and from the ISS until a new American spacecraft can be built by private enterprise, possibly by 2015.

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