The final U.S. space shuttle launch remains on schedule for Friday, but NASA mission managers are keeping a close eye on the weather.
The shuttle Atlantis is due to lift off Friday at 11:26 am Eastern Time (1526 UTC) from the Kennedy Space Center in the southeastern U.S. state of Florida. Its crew of four astronauts will deliver supplies, spare parts and some science experiments to the International Space Station (ISS) during a 12-day mission.
U.S. space agency weather experts said Thursday that there is a 70 percent chance that clouds and thunderstorms will delay the shuttle launch until Saturday or Sunday.
Hundreds of thousands of spectators are gathering to witness the launch.
The Atlantis mission will be the 135th and final flight of the 30-year-old U.S. space shuttle program, which has played a central role in the construction and operation of the ISS.
NASA says it is retiring its shuttle fleet to concentrate resources on deep-space exploration.
The retired U.S. space shuttles will be exhibited at museums in the United States. After its mission, Atlantis will remain on display at the Kennedy Space Center in Florida.
The shuttle Discovery will become part of the Smithsonian National Air and Space Museum in Washington, while the shuttle Endeavour is headed to the U.S. west coast to the California Science Center in Los Angeles.
NASA is working with several U.S. aerospace companies to develop vehicles to replace the shuttles. In the meantime, Russia's Soyuz spacecraft will take on sole responsibility for flying astronauts to and from the ISS, while Russian, European and Japanese cargo rockets will continue their resupply and waste disposal missions to the orbiting outpost.
Some information for this report was provided by AP, AFP and Reuters.