A pair of vultures perch on concrete poles near the space shuttle Endeavour Thursday morning July 9, 2009 at the Kennedy Space Center in Cape Canaveral, Fla. Seven astronauts are scheduled to liftoff Saturday evening on a trip to the international space station. (AP Photo/Chris O'Meara)
(AP) -- NASA is sailing through the countdown for Saturday's launch of space shuttle Endeavour, with weather the lone concern.
Forecasters said Thursday there is a 60 percent chance that thunderstorms could prevent Endeavour from flying to the international space station. The mission already has been delayed twice. Both of last month's postponements were caused by hydrogen gas leaks that have since been fixed.
NASA has four days to send up Endeavour. If the shuttle is not flying by Tuesday, it will have to wait for Russia to launch an unmanned craft with much-needed supplies. That would push the shuttle liftoff to July 27.
Endeavour and seven astronauts will deliver and install the last part of Japan's massive space station lab.
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