The crew of space shuttle Discovery are preparing to lift off on what Nasa says will be a "truly complex mission".
The mission may be one of the hardest yet for Nasa astronauts
Early on Friday, seven astronauts will embark on a tricky and hazardous bid to rewire electrics on the International Space Station (ISS).
Discovery will launch from Kennedy Space Center in Florida at 2136 EST (0236 GMT) - making it the first night lift-off since the Columbia disaster.
The countdown clock started as scheduled at 0400 GMT this morning.
"This is a truly, truly, truly complex mission," said Nasa's space operations chief Bill Gerstenmaier.
In three spacewalks, the astronauts will rewire the orbiting station, replacing its eight-year-old temporary power cable system with a permanent one. They will need to do this without interrupting the station's life-support and other critical systems.
The job was made possible after the crew of the previous mission installed two huge electricity-generating solar array panels on the ISS in September.
Discovery will also transport a new $11m (£5.6m) girder weighing two tonnes to the ISS and install it during a spacewalk.
"You have to prepare yourself for quite a number of very ugly contingencies or failures," Florida Today quoted Nasa's deputy station programme manager Kirk Shireman as saying.
The mission, which Nasa says is one of the hardest yet for its astronauts, is vital for getting ISS construction back on pace.
Construction of the station is well behind schedule following the two-year effort to return the space shuttle to flight following the Columbia disaster in 2003.
The US space agency says it would prefer to launch Discovery during the daytime. But it must resume night launches in order to finish building the ISS before the shuttles are retired in four years.
Daytime launches give cameras a better view of the tank as the shuttle climbs to orbit.
Managers believe the shuttle's fuel tank has been improved to the point that foam debris shedding from the tank during lift-off - which triggered the Columbia accident - is no longer a threat.
A briefcase-sized chunk of foam damaged Columbia's wing on lift-off, allowing superheated gasses to penetrate the heat shield as it returned to Earth in February 2003. All seven astronauts were killed in the disaster.
For Discovery's lift-off on Thursday, Nasa will rely on backlighting from the shuttle's solid rocket booster to illuminate the tank for onboard cameras and radar systems which are set up to track debris.
The crew - five men and two women - arrived at Kennedy Space Center on Sunday for final mission preparations.
One astronaut, Sunita Williams, will be making a one-way trip. She is to remain aboard the space station, replacing Germany's Thomas Reiter, who will return with the rest of the Discovery crew on 19 December.