Cape Canaveral, Florida - Nasa was completing fuelling of the space shuttle Discovery ahead of a Monday morning launch to rendezvous with the International Space Station.
Discovery's arrival at the space station, tentatively scheduled for Wednesday, would mark a first in space, with four women in orbit.
It also is also the first shuttle mission with three female crew members, out of a total of seven.
The Mission Management Team gave a "go" to begin loading Discovery's external fuel tank in preparation for the launch, the US space agency said.
Discovery's external fuel tanks were being filled with about 1.9 million litres of chilled liquid oxygen and liquid hydrogen at Launch Pad 39A.
The "tanking" operation began at 9.28pm on Sunday and was due to end roughly three hours later.
Although the fuelling operation began about half an hour later than planned, the delay was not expected to alter the blast-off, planned for 6.21am from Cape Canaveral, Florida.
"The weather forecast calls for an 80 percent chance of good weather for launch, and no weather issues are expected to prevent an on-time predawn liftoff," Nasa said. - Sapa-AFP