NASA began fueling its most traveled shuttle shortly after sunrise Thursday. The weather outlook has improved to 90 percent "go" for the 4:50 p.m. liftoff.
This time, NASA is confident no hydrogen gas will leak and no cracks will develop in the fuel tank. Both problems cropped up during the initial countdown in November. The repairs took almost four months.
Discovery will head to the International Space Station with a crew of six, as well as a load of supplies and a humanoid robot.
This will be the 39th flight for Discovery. It first blasted off in 1984, and has since logged 143 million miles.
A huge crowd is expected in Florida for Discovery's farewell launch.