Shuttle Endeavour is fueled up and ready for launch. However the weather remains the real challenge, with skies over the Kennedy Space Center growing increasingly overcast.

Forecasters say there's a 60 percent chance of stormy weather similar to the conditions that forced yesterday's postponement at the nine-minute mark of the countdown. That occurred after lightning struck the launch tower.

In his latest Twitter update from crew quarters, commander Mark Polansky says while he and his fellow astronauts hope this will be the day, "we have a saying that you never know for sure until the solid rockets light off."

NASA has until tomorrow, or possibly Wednesday, to launch Endeavour with the final piece of Japan's space station lab. Otherwise, it must wait until the end of July because of a Russian supply ship that's awaiting liftoff.