NASA begins countdown for space shuttle launch

CAPE CANAVERAL, Fla, Aug 24 (Reuters) Countdown clocks began ticking today for NASA's launch of the space shuttle Atlantis, which is due to blast off on Sunday on a mission to deliver and install power-generating solar arrays on the International Space Station.

The shuttle mission is only the third since the 2003 Columbia disaster. The previous flights were to test safety improvements undertaken by the US space agency.

Atlantis, making its 27th flight, will kick off a four-year marathon aimed at completing construction of the half-built, 100-billion dollar International Space Station before the shuttle fleet is retired in 2010.

Columbia's breakup over Texas on February 1, 2003, killed seven astronauts, destroyed a 2 billion spaceship and scuttled NASA's plan to fly the shuttles through 2015 and beyond.

The successful return last month of shuttle Discovery from the second post-Columbia flight cleared the way for the resumption of space station assembly.

Atlantis and its six astronauts are set to lift off at 4:30 pm (2030 GMT) on Sunday from the seaside Kennedy Space Center in central Florida.

The spaceship will be carrying the second of four sets of solar arrays that will power three station science laboratories, two living chambers and other systems.

More than a dozen truss segments and modules, designed to fly only on space shuttles, still await rides into orbit.

NASA needs to fly about 16 shuttle missions to complete station assembly before the aging spacecraft are retired.

Managers say the task will involve some of the most difficult and complex shuttle missions ever attempted.

'TIME TO WALK THE WALK' ''There's been a lot of talk in the press lately about NASA being back. But we have a saying back in Texas: 'It's time to walk the walk,''' Atlantis commander Brent Jett told reporters shortly after the crew arrived at Cape Canaveral on Thursday for final launch preparations.

They flew in NASA T-38 training jets from the astronauts' primary training center outside of Houston.

The goal of Atlantis' 11-day mission -- the 116th flight in shuttle programme history -- is to deliver and install the solar arrays. The glittery panels, which span 73 metres in length when fully extended, provide electricity to power station experiments and support systems.

The modules that house the arrays and a rotary joint so they can track the sun cost NASA 372 million dollar. The units arrived at the Kennedy Space Center more than six years ago.

They have been outfitted with new batteries in the downtime.

Two more sets of US-built arrays, as well as Russian solar panels, are expected to be installed on the outpost, generating about as much power as what would be needed for 55 average US homes.

''I can assure you that we are ready for the challenge,'' Jett said. ''All we need is a little good weather on Sunday and we'll be out of here.'' Meteorologists on Thursday said there was only a 30 percent chance that rain or thunderstorms would prevent Atlantis from lifting off on Sunday.

In addition to Jett, 47, the crew includes pilot Chris Ferguson, 44, flight engineer Dan Burbank, 45, and mission specialists Joe Tanner, 56, Heidemarie Stefanyshyn-Piper, 43, and Steve MacLean, 51, with the Canadian Space Agency.

REUTERS VJ BST0110