A crew of American and Japanese astronauts climbed aboard the shuttle Discovery for launch on Sunday on a 13-day construction mission to the International Space Station.
Blastoff from the Kennedy Space Center in Florida was scheduled for 7:43 pm EDT (2343 GMT).
The seven-man crew includes Koichi Wakata, 45, who will be left behind on the space station as Japan's first resident crew member. NASA last year delivered and installed the main compartments of Japan's Kibo laboratory at the $100 billion orbital complex.
The station, a project of 16 nations, has been under construction for more than a decade.
"It is over 20 years since Japan started this endeavor, in participating in the International Space Station program," Wakata, a veteran of two previous shuttle missions, said in a preflight interview.
"To be able to conduct a variety of experiments, we need to be able to stay on board the space station on a long duration," he said. "I am very fortunate to be able to participate in a long-duration flight to fully utilize the Kibo module."
Joining Wakata aboard Discovery are commander Lee Archambault, 48; pilot Dominic "Tony" Antonelli, 41; flight engineer and lead spacewalker Steven Swanson, 48; spacewalkers Joseph Acaba, 41, and Richard Arnold, 45; and John Phillips, who already has served as a resident space station crewmember.
The primary job is to deliver and install a $300 million power module -- the 11th and final segment of the station's exterior backbone.
SEALS REPLACED AFTER LEAK
NASA has eight flights remaining to complete space station assembly before retiring the shuttle fleet next year. A proposal to fly a ninth mission to the station to deliver a dark matter experiment is pending before Congress.
The US space agency also plans to fly a final servicing call to the Hubble Space Telescope in May.
NASA had hoped to launch Discovery last month but a series of technical problems prompted delays until March. Its first launch attempt on Wednesday was called off due to a hydrogen fuel leak.
Two seals were replaced and the shuttle was filled with 500,000 gallons (1.9 million litres) of liquid oxygen and liquid hydrogen on Sunday without any issues.
Meteorologists predicted good weather for a launch attempt just after sunset.
NASA has until Tuesday to launch the shuttle or face a delay until April 7 to avoid a schedule conflict with Russia's Soyuz mission to replace the station's primary crew.