Discovery is set to launch with a seven-member crew at 3:49 p.m. ET Saturday.
"We're tracking no issues in our preparation at this point," NASA Test Director Jeff Spaulding said at a countdown status briefing.
"Our teams have been working tirelessly during this last year to help make this flight and all shuttle flights as safe as possible for the crews," he said.
The mission, designated STS-121, will deliver 2 tons of supplies and equipment to the International Space Station.
The flight also will test a remodeled fuel tank aimed at avoiding the problem of foam insulation peeling off and hitting the shuttle, which caused Columbia to disintegrate during re-entry into the Earth's atmosphere in 2003.
NASA grounded its fleet of shuttles after the accident, adjusted its foam application process and made numerous other safety upgrades before launching Discovery last summer. A piece of foam again fell off the tank during liftoff, but this time it missed the orbiter and caused no damage.
The agency's top safety official and the chief engineer said at a flight-readiness conference earlier this month that they did not believe the shuttle should launch until more fixes were made to the fuel tank's foam insulation. But NASA Administrator Michael Griffin decided to go ahead with the launch.
The crew of the shuttle Discovery -- commander Steve Lindsey, pilot Mark Kelly, and mission specialists Mike Fossum, Thomas Reiter, Piers Sellers, Lisa Nowak and Stephanie Wilson -- arrived at the Kennedy Space Center on Tuesday to begin training for the launch.
"We're as prepared as we're ever going to be," Lindsey said, according to Reuters.
The space agency reports a 60 percent chance that weather will prevent the launch.