WASHINGTON, June 5 (Xinhua) -- The launch countdown for the U.S. space shuttle Atlantis officially began at 9 p.m. EDT Tuesday (0100 GMT Wednesday), according to NASA TV.

During a morning briefing at the Kennedy Space Center in Florida, NASA Test Director Steve Payne announced that preparations for the liftoff, slated for 7:38 p.m. EDT Friday (2338 GMT), are underway and ongoing.

"All our systems right now are in great shape -- we're tracking no constraints that are show stoppers," said Payne. "Teams are ready and everybody's eager to launch -- we're looking forward to a successful assembly mission."

Launch Weather Officer Kathy Winters reported a 30 percent chance of weather prohibiting launch. Although a typical pattern of afternoon thunderstorms are expected to prevail, upper level winds would push the clouds away by launch time, he said.

From Friday on, NASA has four chances to launch Atlantis in five days. If the shuttle has failed to launch by June 12 the launch team will have to wait five days to allow an Atlas rocket to try to launch at the neighboring Cape Canaveral Air Force Base, according to Payne.

The STS-117 mission astronauts arrived at the Kennedy Space Center Monday evening.

Mission STS-117 is the U.S. Space Shuttle Program's 21st mission to the space station. The crew of Atlantis will deliver new additions to the International Space Station's truss system and make three spacewalks during the 11-day mission.

As this year's first shuttle flight, Atlantis was originally planned to leave for the ISS on March 15, but the launch was canceled after a freak storm pelted the orbiter's foam-covered fuel tank with golf ball-sized hail.