Discovery, NASA's oldest surviving space shuttle, is set to make its last ever flight when it lifts off later today.
The US space agency reports that fuelling will begin at the Kennedy Space Center in Florida shortly after sunrise today, with lift-off scheduled for 16:50 EST (that's 21:50 GMT).
An earlier launch last November was abandoned after NASA detected cracks in Discovery's fuel tank and the leak of hydrogen gas during countdown.
Two earlier shuttle missions have ended in disaster, with the space shuttle Challenger breaking up 73 seconds after lift-off in 1986, killing all seven crew members. Columbia, the first shuttle ever launched by NASA, was destroyed shortly before landing in 2003 with the loss of a further seven astronauts.
Mission STS-133 will rendezvous with the International Space Station, where the shuttle's crew of six will deliver supplies and spare parts as well as a humanoid robot, Robonaut 2.
The flight will be the 39th and last for NASA's oldest surviving shuttle. Discovery first launched in 1984, and has since travelled a total of 143 million miles.
NASA expects crowds of up to 40,000 to arrive at the Space Center for the launch.
One further mission for the shuttle Endeavour will mark the swansong for the agency's space shuttle programme. No replacement has yet been announced, after the Obama administration shelved development of the Orion space vehicle as part of NASA's Constellation programme.