A weather expert for the U.S. space agency says there is a 70 percent chance that the final U.S. space shuttle launch will be delayed Friday.

NASA Weather Officer Kathy Winters said in a news conference Wednesday that “nuisance weather” — clouds and thundershowers rather than a major weather event — could result in a one or two day delay of the launch of the space shuttle Atlantis.

She said the chance of a weather-related scrub of the launch is 60 percent on Saturday and only 40 percent on Sunday.

The countdown for Friday's launch began Tuesday. The 12-day mission will be the 135th and final shuttle flight in the 30-year U.S. shuttle program.

Hundreds of thousands of spectators are expected to gather for the launch Friday at Kennedy Space Center in the southeastern U.S. state of Florida.

Four astronauts will be on the 12-day shuttle mission to carry supplies to the International Space Station. In future, American astronauts will travel to the space station via Russian Soyuz capsules until the United States produces a new vehicle.

Two other shuttles have already completed their flights and are destined now to be displayed in science museums. The Discovery will go on display at the Smithsonian National Air and Space Museum and the Endeavour is headed to the California Science Center in Los Angeles.

After Atlantis completes its mission, it will be displayed at the Kennedy Space Center.