Heavy thunderstorms in Florida are threatening the launch of the last space shuttle mission.
NASA forecasters say there is only a 30 per cent chance of favourable weather for the scheduled launch of the space shuttle Atlantis from Florida's Cape Canaveral on Friday (local time).
The 135th and final 12-day mission of America's 30-year space shuttle program will provide a year's supply of food and spare parts to the International Space Station.
Rolling thunderstorms have disrupted preparations and may keep crowd numbers below the forecast 1 million people expected to flock to vantage points around the Kennedy Space Centre.
Engineers attempted to do an early rollback of the rotating service structure around Atlantis on Thursday.
"The space shuttle launch team is evaluating a possible lightning strike within one third of a mile from launch pad 39A at NASA's Kennedy Space Centre," NASA said in a statement.
"Engineers will review data to determine if the lightning-affected space shuttle Atlantis or any of the pad's ground support equipment," it added.
"A continuing band of thunderstorms has prevented teams from conducting a detailed pad inspection, which must be performed before the rotating service structure can be rolled back from the shuttle."
The rollback marks the last major movement of equipment on the launch pad before the shuttle takes off.
If weather forces a delay, other launch opportunities open on Saturday and Sunday.