US space agency officials again defended their decision to go ahead with the mission despite lingering concerns over safety and said the shuttle was ready to rocket into orbit if weather permits.
Another space shuttle catastrophe could put an immediate end to the 25-year-old program, which is scheduled to retire in four years.
Discovery is currently scheduled to launch on Saturday at 3:49 pm local time (1949 GMT).
It will take a five-man and two-woman crew to the International Space Station (ISS) for a crucial mission that will determine whether NASA has improved the shuttle's safety.
"We are in good shape, we are just waiting for the weather to clear for us," Mike Leinbach, the shuttle launch director, told reporters at the Kennedy Space Center in Cape Canaveral, Florida.
John Shannon, chairman of the mission management team, added that there were "no constraints" to launch other than bad weather. We are ready to go for Saturday."
There is a 60 percent chance that a storm will scuttle the first launch attempt Saturday, weather officials said, adding that a similar probability was predicted for Sunday.
Vice President Dick Cheney will attend the launch. The shuttle has a July 1-19 launch window for the mission during which the shuttle will also deliver critical equipment and supplies for the ISS.