The wake-up call came at 12:21 a.m. EDT, the space agency said. Discovery is scheduled to touch down at 8:48 a.m. at Kennedy Space Center but NASA said Sunday night weather may interfere with those plans -- with forecasters calling for high overcast and two layers of scattered clouds.
The backup landing site is Edwards Air Force Base, Calif.
The mission featured three spacewalks. NASA said Thursday a fourth had been considered to replace a jammed International Space Station nitrogen tank but officials decided it was not needed.
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