The countdown began Saturday for the shuttle Discovery's launch Tuesday on a "Star Wars" research flight, just 12 days after Atlantis's return to Earth but more than a month behind schedule because of worrisome hinge cracks.

Discovery's carefully scripted countdown began on time at 8:45 a.m. Saturday for a launch attempt at 7:05 a.m. Tuesday, with forecasters predicting a 70 percent chance of stormy weather that could delay blastoff.If Discovery is not off the launch pad by Thursday, the flight will be delayed five days to top off coolant supplies needed by a "Star Wars" research satellite in the shuttle's cargo bay. But NASA managers were optimistic about getting the spaceplane into orbit at some point this week.