NASA chief Michael Griffin is taking a calculated gamble by going ahead with the launch of Discovery, overruling two top managers who fear foam flying off the fuel tank might harm the space shuttle.
The world will soon know if his gamble pays off.
Discovery was set to blast off from Kennedy Space Center at 3:49 p.m. EDT (1949 GMT) Saturday, the first launch of a space shuttle in almost a year and only the second since the Columbia disaster in 2003.
Storm clouds forecast for the afternoon remained the chief obstacle to launch. The shuttle faced no technical problems a day before launch.