The American shuttle Endeavour will try again on Monday to launch to the International Space Station (ISS).
Engineers believe they have fixed a heater problem in the aft of the ship that kept the orbiter from making an ascent two weeks ago.
Endeavour is undertaking its final mission before retirement to a Los Angeles museum.
It will carry to the ISS a $2bn particle physics experiment known as the Alpha Magnetic Spectrometer (AMS).
Also in the shuttle's payload bay will be a tray of critical spare parts for the station.
"Endeavour is pretty close to my heart as it's the first space shuttle I flew on in 2001," said shuttle commander Mark Kelly. "So I'm glad it's the one I'm going to fly in last.
"It's the baby of the fleet; it's coming up on 19 years in service - the 25th flight. Twenty-five is a good round number to end on."
Lift-off from the Kennedy Space Center in Florida is timed for 0856 local time (1256 GMT; 1356 BST).
Local police say they expect perhaps half a million people to gather around the spaceport to try to get a good view of the orbiter climbing into the morning sky.
