NASA planned Tuesday to start a countdown toward the first space shuttle flight of the year as engineers had no major technical problems and the weather forecast looked favorable for a Friday launch.

There was a 70 percent chance that the weather would be suitable at 7:38 p.m. EDT Friday, NASA's first opportunity to launch space shuttle Atlantis.

The shuttle will carry seven astronauts on an 11-day mission to deliver a third pair of solar wings, which help power the international space station. They also will take three spacewalks to continue construction of the orbiting outpost.

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There will be thunderstorms Friday afternoon — normal for a Florida summer day — but they should move inland by launch time, said Kathy Winters, shuttle weather officer.

"For the coast here, the weather looks reasonably promising for launch," Winters said.

NASA test director Steve Payne said engineers faced no major glitches.

Beginning Friday, NASA has four chances to launch Atlantis over five days.

If the shuttle hasn't gotten off the ground by June 12, the launch team will have to wait five days to allow an Atlas rocket to try to launch at neighboring Cape Canaveral Air Force Station, Payne said.

The forecast for Saturday and Sunday also was 70 percent favorable for a launch, although the weather wasn't as promising on those days at two emergency landing sites in Europe.

The astronauts arrived at Kennedy Space Center on Monday, and the countdown clock was to start ticking at 9 p.m. EDT on Tuesday.

About the only hiccup was the prospect of 569 workers at the Kennedy Space Center going on strike as early as this weekend. The workers rejected a contract offer last weekend from United Space Alliance, the space agency's primary contractor for preparing space shuttles for launch.

Kennedy Space Center has a 17,000-person work force, and few of the workers who might strike have any direct role in the final preparations for space shuttle launches.

Payne said a strike would not affect a launch and that there are other employees who could fill the roles of striking workers if necessary.

The launch originally was set for mid-March but was postponed after a freak hail storm caused thousands of dings in the insulating foam on Atlantis' external tank.

Technicians spent more than two months making painstaking repairs to the tank.

"We're relieved that we are finally here," Payne said. "It has been a long wait ... and it looks like everything is falling into place nicely."