Space shuttle Discovery was cleared for Thursday's liftoff after NASA resolved technical concerns, but possible clouds and rain threatened to delay the launch.
If the sky opens up above Cape Canaveral, Florida, Discovery will blast off at 0235 GMT tomorrow (0805 IST) in the first night launch since the 2003 Columbia disaster.
The shuttle is to take seven astronauts on the second International Space Station (ISS) construction mission since the Columbia tragedy as NASA races to complete the orbiting laboratory's assembly by 2010.
The National Aeronautics and Space Administration has described the construction mission as the most complex to date with three spacewalks to rewire the station and install a new USD 11-million truss segment.
"Discovery and her crew are set to embark on one of the most complicated missions ever performed, and I'm happy to say our vehicle is ready," said NASA Test Director Jeff Spaulding.
But the launch could be delayed until at least tomorrow, as the forecast shows a possibility of light rain and low clouds in the launch area at the Kennedy Space Center in Cape Canaveral, Florida.
Engineers had worried late on Tuesday that a brief power surge damaged the shuttle's circuits and launch system. A routine test of an adhesive used in the two solid rocket boosters also had raised concern. But NASA on Wednesday said the shuttle was ready to go.
"Everything has been cleared, now they (NASA officials) are watching the weather," June Malone, a NASA spokeswoman, told reporters at the Kennedy Space Center.