NASA has postponed the launch of the space shuttle Endeavour because of cloudy skies over the Kennedy Space Center in Florida.
Low clouds descended over the launch pad about two hours before the planned 4:39 a.m. liftoff on Sunday, violating safety rules for the flight.
The next launch opportunity would be on Monday at 4:14 a.m.
Endeavour and its six crew members are scheduled for a 13-day mission to install two new modules on the International Space Station.
Endeavour's crew of five men and one woman plan to conduct three spacewalks to hook up the new gear.
Five shuttle flights remain to complete assembly of the $100 billion International Space Station.
U.S. President Barack Obama wants NASA to seed development of commercial space taxis to ferry crew members to and from the station. He also wants NASA to develop technologies to prepare for eventual human missions to other destinations in the solar system.