US space shuttle Discovery fuel tank is loaded

NASA completed fueling the space shuttle Discovery Thursday, several hours ahead of its final launch to the International Space Station scheduled for 4:50 pm (2150 GMT).

The fueling process, which involves filling the external fuel tank with more than 535,000 gallons (2.0 million liters) of super cold liquid hydrogen and oxygen, finished at 10:19 am (1519 GMT), the US space agency said.

The bright orange external fuel tank contains enough fuel to power the shuttle's three main engines as Discovery makes the 8.5 minute climb into orbit.

The six astronauts were to board the shuttle later in the afternoon, a few hours ahead of launch time.

The weather forecast improved to 90 percent likelihood of favorable conditions for launch, up from 80 percent on Wednesday, with sunny skies and mild breezes expected throughout the day.

Discovery, the oldest shuttle in the fleet of three, will be retired after it completes this mission to the ISS. The remaining two American shuttles are scheduled for their last flights later this year.