CAPE CANAVERAL, Fla. – After a four-month grounding of the space shuttle fleet, NASA's countdown clocks started ticking again Monday for Discovery's final ride into orbit. Discovery is scheduled to blast off Thursday afternoon to the International Space Station. Forecasters put the odds of good weather at 80 percent. When NASA tried to launch Discovery in early November with supplies and a humanoid robot for the space station, the countdown never got past the fueling phase. A hydrogen gas leak halted everything, then a more insidious problem cropped up: cracks in the external fuel tank.