The US space agency has begun a three-day countdown to the launch of its next space shuttle mission.
At 2200 BST (1700 EDT) on Wednesday, launch team members at Kennedy Space Center in Florida set the clock running ahead of Saturday's planned lift-off.
The shuttle Discovery is set to visit the International Space Station on a 12-day mission to deliver supplies and equipment and test safety improvements.
But Nasa says there is a strong chance of weather delaying the launch.
Thunderstorms and anvil clouds bring the threat of lightning strikes, the agency said, estimating the chance of postponement at about 60%.
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