Countdown Clock on for Shuttle Endeavour's Launch

NASA has begun counting down toward the planned predawn launch of the space shuttle Endeavour on Sunday.

Endeavour is poised to blast off from the Kennedy Space Center in Florida on Sunday morning at 4:39 a.m. EST to deliver a brand-new room and observation portal to the International Space Station.  The clocks began ticking down toward launch time early this morning.

"We're right on schedule, where we're supposed to be," said NASA test director Jeff Spaulding during a Thursday status briefing.

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There is a 70 percent chance of good weather for Endeavour's launch. The potential for high winds is the only concern, shuttle weather officials said.

Commanded by veteran spaceflyer George Zamka, Endeavour's six-astronaut crew plans to fly a 13-day mission to deliver the space station's new Tranquility module and a seven-window portal called the Cupola.

Tranquility is named in honor of NASA's historic Apollo 11 moon landing site and will house vital life support equipment, exercise gear and a robotic arm work station. The Cupola is a long-awaited window fixture that promises to give astronauts their best view yet on space and the Earth below.

Three spacewalks are planned to install the station's new additions, which will leave the $100 billion orbiting laboratory nearly complete after more than 11 years of construction.

NASA hopes to loft the shuttle mission on schedule to clear the way for the planned Feb. 9 launch of an unmanned rocket carrying the new Solar Dynamics Observatory, a sun-watching probe, into orbit.

Endeavour's upcoming launch is the first of NASA's five final shuttle missions before the shuttles are retired later this year. It is expected to be the last shuttle flight ever to lift off at night since the orbiter fleet began launching in 1981.