CAPE CANAVERAL, Florida — NASA is fueling space shuttle Discovery at the launch pad. But the shuttle won’t be flying anywhere, at least not anytime soon. Discovery is grounded until February because of cracks that popped up in its fuel tank during last month’s launch attempt. The cracks have been fixed. But engineers still do not understand why they occurred. So in a countdown test Friday morning, the launch team began pumping more than 500,000 gallons (1.9 million liters) of liquid hydrogen and oxygen into Discovery’s external fuel tank. The tank is rigged with dozens of strain gauges and temperature sensors to provide clues to the cracking. Whenever Discovery flies, it will be its last trip into orbit. It’s loaded with cargo for the International Space Station.