The space shuttle Discovery was cleared for a Thursday launch on its final mission before being retired, NASA managers said.
NASA said Wednesday Discovery is ready for its planned lift-off at 2150 GMT from Cape Canaveral for an 11-day mission to the International Space Station (ISS), dpa reported.
"Everything is on track and going beautifully with the countdown," said Mike Moses, mission management team chair. "We're really looking forward to a very action-packed, successful mission and everything is on track."
There were no apparent problems with the shuttle's external fuel tank, where cracks discovered prior to an earlier launch attempt in October had prompted months of delays.
Weather also appeared good for the launch, with just a 20-per-cent chance of poor conditions for a launch, forecasts said.
The mission is to deliver the last major US contribution to the ISS - an extra room - along with supplies, including a human-like robot, known as Robonaut 2 (R2), the first-such robot ever sent to space.
The oldest vehicle in the operating space shuttle fleet, Discovery entered construction in 1979 and blasted off into space for the first time in 1984.
The shuttle fleet is due to be retired later this year.