NASA engineers have loaded the shuttle with almost two million liters of liquid hydrogen and oxygen. Discovery's seven astronauts led by Commander Eileen Collins have also made their final preparations for the first shuttle launch in more than two years.
NASA Administrator Michael Griffin said Discovery's mission is essentially a test flight to evaluate extensive changes made since the shuttle Columbia broke apart on earth re-entry in February 2003.
"There is nothing that we know of that we have not addressed. Are there things out there that we do not know about, there may be, but we sure hope not," he said.
NASA engineers say their main concern is the weather. The shuttle has to have clear skies to launch, so in case something goes wrong at liftoff, Discovery's astronauts can carry out an emergency landing at the space center.
On Tuesday a plastic and foam window cover fell off the shuttle and caused minor damage to two heat resistant tiles, however NASA maintenance crews were able to quickly replace the tiles.
The shuttle Columbia was critically damaged shortly after liftoff when a large piece of insulation broke off an external fuel tank and damaged the shuttle's wing, allowing hot gases to enter and destroy the shuttle when it reentered earth's atmosphere.
A panel charged with investigating the incident in part blamed a lax attitude towards safety at NASA for contributing to the disaster, something NASA officials say has now changed.