NASA began its official countdown to the Discovery shuttle's launch with a downbeat weather report showing that the critical mission could be delayed from Saturday.
Discovery's engines had been due to start on Saturday at 3:49 pm (0119 IST) from the the Kennedy Space Center, with a five-man and two-woman crew headed to the International Space Station (ISS) at 10 times the speed of a flying bullet.
NASA said initial preparations were going ahead smoothly despite earlier concerns from engineers about the safety of foam insulation that had come off the shuttle's fuel tank during its previous launch last year.
Space officials went ahead with the countdown but Kathy Winters of NASA's meteorological service said gathering storm clouds meant a 60-percent chance the flight -- only the second since the February 2003 Columbia disaster -- would be postponed.
With the Columbia's memory still fresh at NASA, another catastrophe could put a permanent end to the 25-year-old shuttle program.
The 115th shuttle flight will be headed by commander Steven Lindsey. His crew includes co-pilot Mark Kelly, and mission specialists Michael Fossum, Lisa Nowak, Stephanie Wilson, Piers Sellers and the European Space Agency's Thomas Reiter of Germany.
Reiter will remain in the ISS, joining American astronaut Jeffrey Williams and Russian cosmonaut Pavel Vinogradov.
NASA gave the green light for the launch despite calls from its top safety and engineering officials for a delay to ensure that potentially damaging foam insulation does not peel off the orbiter's external fuel tank yet again.