WASHINGTON, July 5 (Xinhua) -- The countdown for the final space shuttle launch began Tuesday afternoon, but unfavorable weather may delay shuttle Atlantis' launch attempt on Friday, NASA announced.

"We are going with a 60 percent chance of KSC (Kennedy Space Center) weather prohibiting launch due to the potential for showers and isolated thunderstorms in the area," shuttle weather officer Kathy Winters told reporters Tuesday at a precountdown status briefing. "I wish I had a better weather briefing for you."

Atlantis is scheduled to lift off at 11:26 am (1526 GMT) on Friday from Kennedy Space Center with four U.S. astronauts on board for a 12-day mission to the International Space Station. Overall, the chance of an on-time liftoff is 40 percent, Winters said.

"We will do everything we can to launch on Friday but if things don't work out so that we can do that we have plenty of options... Saturday and Sunday," said NASA Test Director Jeremy Graeber. Conditions improve for Atlantis launch opportunities on Saturday and Sunday.

NASA said it must launch Atlantis by Sunday, otherwise it will have to wait until at least July 16. That's because of an unmanned rocket due to lift off next week, using the same launch support personnel and equipment.

NASA's 30-year-old shuttle program is ending due to high operating costs. The Obama administration wants to spur private companies to get into the space taxi business, freeing NASA to focus on deep space exploration and new technology development.

When the U.S. space shuttle program officially ends later this year, the Russian space program's Soyuz capsule will be the only method for transporting astronauts to and from the station.