By Irene Klotz

CAPE CANAVERAL, Fla., Dec 4 (Reuters) - NASA hopes to launch its fourth shuttle mission in six months on Thursday, a pace that will keep the International Space Station on track for completion by 2010.

Europe's Columbus laboratory module is scheduled for liftoff aboard the space shuttle Atlantis at 4:31 p.m. EST (2131 GMT) on Thursday. Meteorologists on Tuesday predicted a 90 percent chance that weather at the Kennedy Space Center in Florida would be suitable for launch.

"It's been challenging for us to get ready for these flights so close together," LeRoy Cain, head of the shuttle Mission Management Team, said on Tuesday after Atlantis was cleared for launch.

Atlantis' flight is NASA's 24th shuttle mission to the space station, with nine more assembly flights and two resupply missions remaining before the shuttle fleet is retired in 2010.

NASA is developing new, capsule-style spaceships called Orion to go to the space station as well as to the moon and other destinations. Safely completing the remaining shuttle missions while preparing to close out the program is a complicated juggling act, especially since NASA is simultaneously beginning a new manned lunar exploration program.

For example, plans to retire Atlantis next year may be revised to give NASA more flexibility to complete station assembly, Cain said.

Atlantis is scheduled to fly its last mission, a servicing call to the Hubble Space Telescope, in August.

Space station components still fill a huge processing facility at the shuttle's launch site. More equipment is due to arrive next year, including two external payloads built by Japan as well as the station's third and final connecting hub.

"We've worked for very many years to get here and I'd like to think that now we're at the end of the beginning," said Alan Thirkettle, who heads the European Space Agency's human spaceflight programs. "We're very ready to go."

NASA had a slow start in 2007 after a freak hail storm damaged a shuttle fuel tank, delaying the first flight of the year until June. But the agency quickly recovered, dispatching shuttle crews in August and October to prepare the complex for its first new laboratories since the U.S. module Destiny arrived in 2001.

Japan's Kibo complex is scheduled to follow in 2008. (Editing by Jane Sutton and Eric Walsh)

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