Countdown begins for shuttle launch on Wednesday

CAPE CANAVERAL, Florida (Reuters) - Countdown clocks began ticking down on Sunday for NASA's first space shuttle launch of the year, a mission meant to complete the International Space Station's power system and exterior beams.

Shuttle Discovery is scheduled for liftoff at 9:20 p.m. EDT from the Kennedy Space Center in Florida. Astronauts plan to deliver and install a final set of solar wing panels and transport Japan's first live-aboard station crewmember to his new home.

"We're ready to get going," said Discovery commander Lee Archambault, who arrived at the Florida spaceport on Sunday afternoon along with his six crewmates.

The flight has been on hold since February 12 as engineers and managers reviewed a possible safety issue with valves needed to keep the fuel tank properly pressurized during the 8.5-minute ride into orbit. NASA replaced the valves and cleared the launch team to begin the three-day countdown.

A final review is scheduled for Monday. There were no outstanding technical issues, however, and meteorologists were forecasting a 90 percent likelihood the weather will be suitable for launching.

The mission, slated to last two weeks, will complete the station's 11-part exterior truss. The final segment contains a $300 million set of solar wing panels needed to bring the station up to full power.

Discovery's crew includes Archambault, pilot Dominic Antonelli, Japanese astronaut Koichi Wakata, spacewalkers Steven Swanson, Richard Arnold and Joseph Acaba, and mission specialist John Phillips, who has been a space station crewmember.

(Editing by Michael Connor in Miami)