Countdown begins

The countdown is on. The clock began ticking at 8 a.m. toward a Wednesday launch of the shuttle Atlantis and six astronauts on a mission to resume construction of the International Space Station.

Atlantis' crew returned to the Kennedy Space Center on Saturday morning to begin their final preparations for flight. The shuttle launch team reported to their stations at KSC at 7:30 a.m. this morning.

Among the tasks on the busy countdown schedule for this afternoon: preparing the orbiter's fuel cells with the liquid hydrogen and liquid oxygen necessary to generate electricity to power the spacecraft's systems. The loading of the fuel cells is set for Monday.

This marks the second time the countdown clock has started ticking toward this launch. NASA was into a countdown late last month when a powerful bolt of lightning struck Pad 39B, forcing engineers to evaluate whether or not shuttle systems or ground equipment were damaged.

As the launch team was about to clear the vehicle for flight, Tropical Storm Ernesto's forecast was changing enough to prompt fears that the shuttle could be in danger of facing hurricane conditions if left at the pad. That prompted talks of rollback.

NASA ultimately started that rollback on Tuesday morning, even as Ernesto began to weaken, but with a backup plan to turn the shuttle around if the forecast continued to improve. A little more than halfway through the trip, with the storm's strength fading fast, the shuttle launch team reversed course and sent the crawler-transporter back to the pad.

Work began Thursday to get ready to start the countdown clock today, and salvage the end of the September launch window. NASA has three shots Wednesday, Thursday or Friday to launch before it must stand down for a Russian Soyuz launch set for Sept. 18. What would happen next, if NASA can't launch this week, depends on decisions the agency would make regarding loosening of post-Columbia launch restrictions.

For now, the plan is to try to launch this week. The weather forecast for Wednesday calls for an 80 percent chance of acceptable launch weather. The chances of good launch weather are 70 percent on Thursday and Friday. Check back here for updates throughout the countdown.