If problems prevent liftoff then, NASA will get a final try on Saturday at 11:15 a.m., the end of the launching window for September.

But so far, a eureka moment has eluded NASA, and officials are looking for ways to return to flight before the current launching window closes on July 31.

William Parsons, the shuttle program manager, said the shuttle team was also looking for ways to extend its launching window by as much as four days.

The next launching window would not be until about 2011, with an arrival date of 2019 or later.

The launching window is tight because Discovery has to go into a special orbit for the rendezvous.

With a loud report the shutters of the launching window were thrown open.

Instead, American companies are assigned a three-month launching "window" at Government installations.

In addition, NASA is talking with the Russian space agency about another possible extension in the current launching window.

But the length of the so-called launching window was, as planned, unusually short - only 10 minutes.

Scientists note that if a launching window of 2004 to 2006 is not met, any chance of visiting the planet will disappear for decades.