The planned lift-off of space shuttle Discovery to the International Space Station (ISS) was postponed due to cloud cover over the launch pad in Cape Canaveral, Florida, said the US space agency NASA (National Aeronautics and Space Administration).
The launch had been scheduled for 9:35 p.m. (0235 GMT Friday).
NASA could try again as early as late Friday or over the weekend. The agency said immediately after the decision to scrub that it was "evaluating" when to reschedule the launch.
A forecast for severe crosswinds at the Kennedy Space Center was weighing strongly against a Friday launch. Conditions are expected to be less severe Saturday, but wind could still be a problem, while Sunday appears to present a clearer launch window.
Sunita Williams, an astronaut of Indian origin, is scheduled to replace German Thomas Reiter as a flight engineer on the space station.