MARCIA DUNN,AP Aerospace Writer

CAPE CANAVERAL, Fla. (AP) - Discovery's astronauts are back at Kennedy Space Center for a launch attempt.

Good weather is forecast. Now all NASA needs this week is a smooth countdown all the way to zero.

NASA's countdown clocks will start ticking Monday afternoon. Liftoff is scheduled for 4:50 p.m. Thursday.

Discovery should have taken off on its final voyage in November. But fuel tank cracks kept the shuttle grounded for four months. The shuttle had to be returned to its hangar for the unprecedented repair work.

After returning from the International Space Station, Discovery will be retired and sent to a museum. Its final destination is expected to be the Smithsonian Institution.

Only three shuttle missions remain.