Showers and thunderstorms are likely to force a delay to this week's planned final launch of the space shuttle, NASA's weather officer said on Tuesday.

"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 ahead of Friday's planned liftoff.

The turbulent weather and clouds within 20 nautical miles of the launch pad were likely to move in "near the end of the countdown, right around that 11:00 am time period," she added.

Atlantis is set for liftoff July 8 at 11:26 am (1526 GMT) from Kennedy Space Center with four US astronauts on board for a 12-day mission to the International Space Station.

The flight will be the last by a US shuttle, ending the 30-year program and leaving a gap in US human spaceflight. Until a new crew vehicle can be built, the world's astronauts will have to rely on Russia's space capsules for transit to the ISS.

As the shuttle program comes to an end, a Pew poll released Tuesday found most Americans believe the United States should maintain its dominance in space exploration.

Almost six out of 10 people polled, or 58 percent, said they felt it was essential that the US should remain the world leader in space flight, while about four of 10 or 38 percent said it was not necessary.

And 55 percent also agreed that the shuttle program had been a good investment for the United States, saying it had encouraged greater interest in science, led to scientific advances and fueled patriotic sentiment.

NASA has said that a shuttle replacement could emerge sometime between 2015 and 2021.

If weather forces a delay in Friday's liftoff, other weekend opportunities will arise. Conditions improve to a 40 percent chance of weather prohibiting launch on Saturday, and a 30 percent chance on Sunday.

"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 Jeremy Graeber, NASA test director.

Graeber added that the US space agency was expecting crowds in the area of 500,000 to 750,000 on launch day.

If the shuttle departs on time, NASA has the option of adding an extra day to the 12-day mission, but that scenario is not likely if the launch is delayed.

The countdown to launch officially begins at 1:00 pm (1700 GMT) on Tuesday.

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